What should the electrical insulation resistance be? Measuring cable insulation resistance. What will the regulatory authorities require?

Like any equipment or technique, over time, electrical cables of various types begin to fail. One of the methods for determining the safety margin of a cable and identifying defects is to measure the insulation resistance. This article explains what it is, when and how it is done.

Electrical wiring inspection

Each organization that manages electrical installations must have a person responsible for electrical equipment. His responsibilities include drawing up scheduled maintenance work for the repair of this equipment, as well as conducting periodic tests and measurements, and inspecting electrical wiring. The frequency of such measurements, as a rule, is based on the requirements of PTEEP. For example, regarding the measurement of insulation resistance, it says that tests should be carried out once every 3 years.

What is insulation resistance measurement

This is a measurement with a special device (megaohmmeter) of the resistance between two points of an electrical installation, which characterizes the leakage current between these points when DC voltage is applied. The result of the measurement is a value expressed in MOhm (megaOhm). The measurement is carried out by a device - a megohmmeter, the principle of which is to measure the leakage current that occurs under the influence of a constant pulsating voltage on an electrical installation. Modern megohmmeters provide different voltage levels for testing different equipment.

Allowable resistance for various equipment

The main guiding document is the PTEEP, which provides the frequency of tests, the magnitude of the test voltage and the standard resistance value for each type of electrical equipment (PTEEP Appendix 3.1, Table 37). Below is an excerpt from the document.

Do not confuse the resistance of electrical cables with the resistance of a coaxial cable and the characteristic impedance of the cable, because This applies to radio engineering and there are different principles of approach to permissible values.

Electrical safety issue

Insulation resistance measurement is carried out in order to protect a person from electric shock and for fire safety purposes. Hence the minimum resistance value is 500 kOhm. It is taken from a simple calculation:

U – phase voltage of the electrical installation;

RIZ – insulation resistance of electrical equipment;

RF is the resistance of the human body; for electrical safety calculations, RF = 1000 Ohm is taken.

Substituting known values (U=220 V, RIZ=500 kOhm), a leakage current of 0.43 mA is obtained. Sensible current threshold is 0.5 mA. Thus, 0.5 MOhm is the minimum insulation resistance at which the average person will not feel any leakage current.

When measuring with a megohmmeter, you should also pay attention to safety, because the device produces up to 2500 V on its probes, it can be fatal to humans. Therefore, only specially trained personnel can carry out measurements. The connection of the megohmmeter and measurements must be carried out at an electrical installation disconnected from the electrical network. It is necessary to check the electrical wiring for lack of voltage. If testing is carried out on a cable, the area should be protected from accidental contact with bare parts of the cable at the opposite end from the test site.

Method for measuring cable insulation resistance

First, personnel must determine that there is no voltage on the cable using a voltage indicator. At the opposite end, the cable cores must be separated at a sufficient distance so that there is no accidental short circuit. Then prohibition signs are posted in the testing area. You should also conduct a visual inspection of the cable, if possible, to determine if there are hot spots or exposed areas. After this, you can start measuring. It is necessary to measure the insulation resistance between phases (A-B, A-C, B-C), between phases and zero (A-N. B-N, C-N), between zero and the ground wire. The time of each measurement is 1 minute. After each test, it is necessary to ground the cable core, although modern megohmmeters can carry out independent discharge. The results obtained are recorded in the protocol. It is worth remembering that if the data obtained is made for some inspection commission, only a specialized electrical laboratory has the right to make the protocol.

Instruments for measurements

For testing with constant pulsating voltage, the best choice is a megohmmeter. In devices of older designs, a built-in mechanical generator operating on the principle of a dynamo was used to obtain voltages. To produce the required voltage, it was necessary to twist the knob hard. Currently, megohmmeters are made in the form of electronic devices powered by batteries; they have a compact size and convenient software.
temporary megohmmeters have a memory where several tests are stored. With each measurement, the absorption coefficient is automatically calculated. Its value is determined by the ratio of the polarization current to the leakage current through the dielectric - the winding insulation. With wet insulation, the absorption coefficient is close to 1. With dry insulation, R60 (insulation resistance 60 seconds after the start of the test) is 30-50% greater than R15 (after 15 seconds).

amperof.ru

How to check insulation

When wiring is done, they talk about the cross-section of the conductor. When creating an electrical contact, they think about the contact area of the conductors and whether it will be sufficient for reliable contact. But the area of contact between the insulation and the conductor in wires, cables or insulating substrates is never considered. How then to talk about this, and in general, how to measure insulation resistance?

Illustration 1

To measure the resistance of various materials, you can take a sample of a material of a certain shape and size and, by applying some voltage to the two ends, obtain some current. Measure it and get the resistance using Ohm's law

Formula

The resistivity will be equal to

Formula 2

It, unlike R, does not depend on either the length (thickness) of the material or the contact area.

According to this principle, resistivities are measured for various materials and can be found in reference tables. And for insulators too.

That is, for work, you could simply choose a better insulator and use it. Yes, this does not need to happen, because usually the word “isolator” speaks for itself. Electrical materials are produced by industry taking into account all standards. The task of the insulator is not to pass current, providing resistance (as we see from the table - the resistance is huge), but simply to isolate some conductors from others.

But reference values for insulator resistance may change over time. All materials age, collapse, decompose under the influence of temperature changes, light, vibrations, and their structure is disrupted. Microcracks, peeling, and peeling appear. They become thinner, water penetrates into the pores, and can decompose chemically. Dust occurs, and not all dust is an insulator. That is, the insulating properties of dielectrics deteriorate over time.

Therefore, I would like to be sure that this particular insulator on a given wire or electrical bus will play its role well.

Then they check the insulation resistance of the cable (or wires and cables, cords, and so on). And at the same time they check for electrical strength at a certain measuring voltage. All this is done in electrical power circuits, where such characteristics are vital.

Cable insulation resistance standard

There are Rules for the Operation of Consumer Electrical Installations (PEEP, ed. 5, 1997, MinTopEnergo of the Russian Federation, Moscow), which set out standards regarding the safe operation of electrical installations, as well as power lines and premises where electrical equipment operates. Table 43 of Appendix 1 describes what voltages should be used to test insulation on various electrical installations up to 1000 volts. Specifically, in which places to measure and what standard resistance the insulation should have.

I present part of the table here (without extensive instructions on where exactly the insulation resistance is measured for many of the types of installations given in it).

As you can see, the insulation resistance should generally be no higher than 0.5 MOhm*m.

And measurements (tests) are carried out with voltages of up to 1000 volts, and this is a life-threatening voltage. The methodology is such that the test is carried out in installations at their locations. To prevent the test from damaging the circuit elements, they are first shunted.

Cables are tested by applying voltage to one of their wires and measuring the insulation resistance between it and the other wires of the cable.

Instruments for measuring insulation resistance

Any device for measuring electrical resistance uses a reference voltage source in its design. Some multimeters allow you to connect an external high voltage source to measure high resistances. There are only instruments specifically designed to measure cable insulation resistance. They are called megohmmeters. They carry out: measuring the insulation resistance of electrical wiring, checking the insulation resistance for breakdown by high voltage, measuring the insulation resistance in various devices, measuring the insulation resistance of power electrical equipment, and so on.

Megger Measuring device Cables

To operate, the megger must meet the following characteristics:

be in good working order - from the point of view of external inspection;
officially verified in a metrological laboratory, the period for the next verification must not be completed;
it must have an unbroken metrologists’ seal;
the high-voltage part must be tested in an electrical laboratory for proper insulation; the kit must contain high-voltage wires with a measured insulation resistance that is sufficient for work with high voltage;
A control measurement of the insulation of a sample with a known resistance must be carried out on it.

Please keep in mind that:

Any work with a megger is classified as dangerous. The danger concerns both the people directly carrying out the measurement and anyone who may be in the testing area. Equipment that may be damaged by the test voltage is also at risk.

The danger comes from the high voltage under which installation conductors, cables, and grounding bars are placed during testing.

Preparing for an Insulation Resistance Test

Much of the preparation for taking measurements concerns work safety. All actions must be carried out carefully to avoid accidents. Particular attention should be paid to alerting people who are not involved in the measurements, but who for some reason may find themselves near the work sites.

Insulation resistance measurements with a megger should be carried out on conductors disconnected from the supply voltage. Surrounding equipment must also be de-energized to avoid electrical fields influencing the measurement results.

Although the test voltage when measuring the insulation resistance of electrical wiring is high, the measurement itself is subtle and subject to very little interference. This is explained by the fact that currents of microampere values penetrate through the insulation, even at high voltage, due to the extremely high specific resistances of the insulators. Measuring these currents ultimately gives a resistance value of the order of several megohms.

The cable being tested, which is part of the working wiring of the equipment, must be completely disconnected from the rest of the wiring before measurements are taken.

Preparation diagram for measuring insulation resistance

Preparation diagram for measuring insulation resistance:

It is necessary to take into account the configuration and length of the cable being tested, since all of it will be under high test voltage. It is necessary to exclude the impact of this voltage on people along the entire length of its presence. This is achieved by posting warning signs and monitoring the testing area.
Long cables, which are typically exposed to high voltages, may carry significant residual charges or interference charges from surrounding high-voltage equipment when disconnected. This is dangerous for people and can damage equipment if discharged. This may affect the measurement results. For all these reasons, the cable under test, as well as all electrically conductive parts of the circuits, must be discharged through grounding.

How to use a megohmmeter

Use protective equipment and install portable grounding before starting work at a specific measurement location.

Protective attributes Protected tool Device

Method for measuring insulation resistance

There are several tests on cable lines; they cover all possible options for line breakdowns in different directions. Similar measurements of cable insulation with a megger are periodically carried out at places where electrical equipment is installed.

The insulation resistance of the wires relative to the ground is measured.

The sequence is:

First, portable grounding is installed.
One end is connected to the ground wire.
At the other end, all wires of the cable line are connected in turn to discharge them from residual charges. All cable cores are shorted together.
Without removing the grounding from them, the grounding wire is connected to the device.
The cores of the cable lines are disconnected from grounding.
The second wire of the megger is connected to the cores.
The test voltage is turned on - about 1000 V. It must be applied to the cable for about a minute so that all transient processes in the line wires are completed.
A measurement is made on the device, and the results are entered into the test table.

Measuring the insulation resistance of wires in a cable line relative to each other

The difference from the previous test is that the measurement is made sequentially in the cable conductors relative to the grounding conductor.

Preparation for core insulation measurement Continuation of measurement

In the same way, you can measure the resistance of the core insulators relative to the neutral wire and relative to each other.

Between different tests, the test voltage is turned off, and the cable line conductors participating in the test are discharged through grounding.

Measurements of the insulating properties of power equipment dielectrics relative to ground.

Equipment insulation measurements are carried out relative to grounding. Work of this kind should be carried out only after a thorough study of the equipment diagrams. First, all equipment is disconnected from external networks, then discharged through grounding, after which its insulation is tested at the terminals of the main buses supplying the equipment.

Equipment insulation measurement

Checking floors and walls for insulation resistance with a megger.

Wiring diagram for walls and floors

Floors and walls are checked several times at different distances from the equipment. First in the immediate vicinity, then after a few meters. One wire of the megger is connected to ground, the other to an electrode made of a piece of flat metal measuring at least 250x250 mm. The electrode, under which wet paper or cloth is placed, is pressed against the wall (floor) for the duration of the measurement. For pressing, a minimum force is used: 750 N - to the floor, 250 N - to the wall.

All work is carried out wearing rubber protective gloves and protective boots.

After all activities are completed, the results are documented in a protocol.

domelectrik.ru

Hello, readers of the Electrician's Notes blog.

In the previous article about testing cable lines, I told you that one of the points of testing cable lines is measuring the cable insulation resistance.

This is what we will talk to you about in detail. Let's consider how to correctly measure the insulation resistance of both power and control cables. We will also get acquainted with the methodology for carrying out these measurements.

Preparing to measure cable insulation resistance

Before starting work on measuring cable insulation resistance, it is necessary to accurately know the ambient temperature.

What is this connected with?

This is due to the fact that at negative temperatures, if there are water particles in the cable mass, these particles will be in a frozen state, i.e. in the form of pieces of ice. You all know that ice is a dielectric, i.e. has no conductivity.

Therefore, when measuring insulation resistance at subzero temperatures, these particles of frozen water will not be detected.

Instruments and measuring instruments

The second thing we need to measure the insulation resistance of cable lines is the availability of instruments and measuring instruments.

To measure the insulation resistance of cables for various purposes, I and the workers of our electrical laboratory use the MIC-2500 device. There are other devices, but we use them somewhat less frequently.

This device is manufactured by Sonel and can be used to measure the insulation resistance of cable lines, wires, cords, electrical equipment (motors, transformers, switches, etc.), as well as measure the degree of aging and moisture content of the insulation.

I would like to note that the MIC-2500 device is included in the state register of devices that are approved for measuring insulation resistance.

The MIC-2500 device must be subject to state verification annually. After passing verification, a hologram and a stamp indicating verification completion are placed on the device. The stamp indicates the serial number of the device and the date of the next verification.

Accordingly, it is necessary to measure insulation resistance only with a serviceable and verified device.

Insulation resistance standards for various cables

Before moving on to the standards for cable insulation resistance, it is necessary to classify them somehow.

I offer you my simplified classification of cables.

Cables according to their intended purpose are divided into:

high-voltage power above 1000 (V)
low-voltage power below 1000 (V)
control and control cables, we will simply call them control cables (this includes secondary switchgear circuits, power supply circuits for electric drives of switches, separators, short circuiters, control circuits, protection and automation circuits, etc.)

Insulation resistance measurement for both high-voltage cables and low-voltage power cables is carried out with a megohmmeter for a voltage of 2500 (V). And control cables are measured with a megohmmeter for a voltage of 500-2500 (V).

Accordingly, each cable has its own insulation resistance standards. According to PTEEP (clause 6.2. and table 37) and PUE (clause 1.8.37 and table 1.8.34):

High-voltage power cables above 1000 (V) - not standardized, but the insulation resistance must be at least 10 (MOhm)
Low voltage power cables below 1000 (V) - insulation resistance should not be less than 0.5 (MΩ)
Control cables - insulation resistance should not be less than 1 (MΩ)

Methodology for measuring insulation resistance of high-voltage power cables

For a clearer picture of the work on measuring the insulation resistance of high-voltage power cables, I will give you a visual diagram and procedure.

1. Check the absence of voltage on the cable with a high voltage indicator

2. We install a test grounding with special alligator clips on the cable cores from the side where we will measure the insulation resistance.

3. On the other side of the cable, leave the cores free and separate them at a sufficient distance from each other.

4. We hang prohibition and warning posters. On the other hand, I recommend leaving a person who will observe that when measuring the insulation resistance with a megohmmeter, no one comes under the test voltage.

5. We measure the insulation resistance of a high-voltage power cable with a 2500 (V) megohmmeter, alternately on each core for 1 minute.

For example, we measure the insulation resistance on the conductor of phase “C”. At the same time, we install test grounding on the conductors of phases “B” and “A”. We connect one end of the megohmmeter to a grounding device, or, more simply, to “ground”. The second end goes to the core of phase “C”.

In an example it looks like this:

6. We write down the readings obtained during the measurement of the insulation resistance of the high-voltage cable in a notebook.

Methodology for measuring insulation resistance of low-voltage power cables

The method for measuring the insulation resistance of low-voltage power cables differs from the previous one (described above), but only slightly.

Likewise:

2. On the other side of the cable, leave the cores free and separate them at a sufficient distance from each other.

3. We hang prohibition and warning posters. On the other hand, I recommend leaving a person who will observe that when measuring the insulation resistance with a megohmmeter, no one comes under the test voltage.

4. We measure the insulation resistance of a low-voltage power cable with a 2500 (V) megohmmeter for 1 minute:

between phase conductors (A-B, B-C, A-C)
between phase conductors and zero (A-N, B-N, C-N)
between phase conductors and ground (A-PE, B-PE, C-PE), if the cable is five-core
between zero and ground (N-PE), having previously disconnected zero from the zero bus

5. We write down the readings obtained during the measurement of the insulation resistance of the low-voltage cable in a notebook.

Methodology for measuring insulation resistance of control cables

Well, now we have reached the point of measuring the insulation resistance of control cables.

The peculiarity of their measurement is that the cable cores can not be disconnected from the circuit and measurements can be taken together with the installed electrical equipment.

Measuring the insulation resistance of the control cable is performed in the same way.

1. We check that there is no voltage on the cable using protective equipment designed for work in electrical installations.

2. We measure the insulation resistance of the control cable with a 500-2500 (V) megohmmeter as follows.

We connect one terminal of the megohmmeter to the core being tested. We connect the remaining cores of the control cable to each other and to the ground. We connect the second terminal of the megohmmeter either to ground or to any other non-tested conductor.

For clarity, see the photo:

Within 1 minute we measure the core being tested. Next, we return the measured core to the rest of the cable cores and proceed to measuring the next core.

So every vein.

3. We write down all the obtained readings of the insulation resistance of the control cable in a notebook.

Cable insulation resistance measurement protocol

In all of the above electrical measurements, after receiving readings of the cable insulation resistance, it is necessary to compare them with the requirements and standards of PUE and PTEEP. Based on the comparison, it is necessary to draw a conclusion about the suitability of the cable for further operation and draw up a protocol for measuring the insulation resistance.

P.S. This concludes the article. If you have any questions, feel free to ask them. And also don’t forget to subscribe to new articles from my website.

zametkielectrika.ru

Measuring cable insulation resistance is one of the most important points in cable testing. For example, if the sheath, which has properties that protect the cable, is damaged, then unpleasant consequences are possible, among them various violations in the energy saving system are common. This is the main reason why it is necessary to measure the insulation resistance of cables.

To avoid electric shock, fires and other unpleasant situations, etc., it is necessary to constantly measure the insulation resistance of VVG cables in order to identify faulty areas in the electrical wiring.

In order to measure resistance, you need to start by inspecting the electrical wiring and wires. Particular attention should be paid to those cables that have connections to protection devices. There should be no melted ends so that the cable does not heat up during operation, as this can significantly complicate the work. For example, the cable may heat up due to improper connection of the cores to the terminals; it may also be due to the fact that the circuit breaker is in a faulty state.

In order to take a measurement, you need:

First, turn off all electrical appliances and all cables and wires that are subject to electrical measurements.
Before taking measurements, you need to remove all light bulbs from the lighting fixtures. At the same time, all lighting switches must be turned on.
It is necessary to turn off the power supply to cables and wires.

After following all the above instructions, the power system will be completely ready to measure insulation resistance.

The permissible cable insulation resistance reading must be above 0.5 mOhm. If these indicators do not meet, then this cable must be dismantled.

It is also necessary to take into account that the determination of resistance is carried out only after its phasing, as well as an integrity check. You need to measure the cable resistance using a megohmmeter. (Figure 1)

If you are taking a measurement with a large value, it is best to take it when the needle that is oscillating has completely calmed down. It is also necessary that all electrical appliances be unplugged from the network.

It is prohibited to determine the resistance of lines that are close to other similar lines.

Fig 1. Megaohmmeter

The resistance is determined using a megohmmeter with a voltage of 2500 (V) for 1 minute.

Measurements:

(A – B; B – C; C – A), that is, between phase conductors;
(A – N; B – N; C – N), also between neutral and phase conductors;
(A – PE; B – PE; C – PE), also between the ground and phase conductors;
(N – PE), and finally between ground and neutral conductors.

There are some rules to consider when measuring cable insulation resistance:

Firstly, in order to take a measurement, you need to know the exact ambient temperature. Because if there is a negative temperature, and there is water in the cable mass (even in small quantities), then it will turn into pieces of ice. And ice itself is a dielectric, that is, it does not have conductivity abilities. Moreover, when carrying out insulation, you will not be able to identify these pieces of ice, so you need to immediately take care of an acceptable temperature. The optimal temperature should not be lower than +5°C (exceptions are cases specified in special instructions.).
Secondly, if the resistance of the electrical wiring, which is in working condition, is less than 1 MOhm, then a conclusion about their suitability is given after a special check of this electrical wiring is first carried out, which consists of applying an alternating current of industrial frequency to it, but with a voltage of 1 kV, and then conclusions are drawn about their suitability.
Thirdly, we must not forget that only flexible wires should be used when measuring (they have special insulating handles at the ends, and they also have restrictive rings in front of the contact probes). The wires that connect have a minimum length.
Fourthly, a megohmmeter of 1000 V and above is used for determination. Devices that have not passed annual government inspections are not allowed for use.

If the voltage in electrical installations is above 1000 (V), measuring the cable resistance should be carried out wearing dielectric gloves.

In order to determine the standards for cable insulation resistance, you must first classify these cables:

Cable classification:

above 1000 (V), that is, high-voltage power;
below 1000 (V), that is, high-voltage power;
as well as control cables.

Accordingly, the insulation resistance standards are different for each type of cable, for example:

For cables above 1000 (V), high-voltage, there is no specific standard, but the resistance will be higher than 10 (MOhm).
For cables below 1000 (V), low voltage - the resistance should be above 0.5 (MOhm).

Whether high or low voltage is used, it all depends on the voltage of your electrical installation.

myfta.ru

Power cable lines

Power cable lines with voltage up to 1 kV are tested according to paragraphs 1, 2, 7, 13, voltages above 1 kV and up to 35 kV - according to paragraphs 1-3, 6, 7, 11, 13, voltage 110 kV and above - in to the full extent provided for in this paragraph.

1. Checking the integrity and phasing of the cable cores. The integrity and coincidence of the phase designations of the connected cable cores are checked.

2. Insulation resistance measurement. Produced with a megohmmeter for a voltage of 2.5 kV. For power cables up to 1 kV, the insulation resistance must be at least 0.5 MOhm. For power cables above 1 kV, the insulation resistance is not standardized. The measurement should be made before and after testing the cable with increased voltage.

3. Test with increased voltage of rectified current.

The test voltage is taken in accordance with Table 1.8.39.

Table 1.8.39 Rectified current test voltage for power cables

________________

* Rectified voltage tests of single-core cables with plastic insulation without armor (screens) laid in air are not carried out.

For cables for voltages up to 35 kV with paper and plastic insulation, the duration of application of the full test voltage is 10 minutes.

For rubber-insulated cables with a voltage of 3-10 kV, the duration of application of the full test voltage is 5 minutes. Cables with rubber insulation for voltages up to 1 kV are not subjected to high voltage tests.

For cables with a voltage of 110-500 kV, the duration of application of the full test voltage is 15 minutes.

Permissible leakage currents depending on the test voltage and permissible values of the asymmetry coefficient when measuring leakage current are given in Table 1.8.40. The absolute value of the leakage current is not a rejection indicator. Cable lines with satisfactory insulation must have stable leakage current values. During the test, the leakage current should decrease. If there is no decrease in the leakage current value, or if it increases or the current is unstable, the test should be carried out until a defect is identified, but not more than 15 minutes.

Table 1.8.40 Leakage currents and asymmetry coefficients for power cables

Cables voltage, kV	Test voltage, kV	Permissible values of leakage currents, mA	Acceptable values of the asymmetry coefficient ()
6	36	0.2	8
10	60	0.5	8
20	100	1.5	10
35	175	2.5	10
110	285	Not standardized	Not standardized
150	347	Same	Same
220	610	"	"
330	670	"	"
500	865	"	"

When laying mixed cables, take the lowest test voltage according to Table 1.8.39 as the test voltage for the entire cable line.

4. Test with AC voltage frequency 50 Hz.

This test is allowed for cable lines for voltages of 110-500 kV instead of the rectified voltage test.

The test is carried out with voltage (1.00-1.73). It is allowed to carry out tests by switching on the cable line to the rated voltage. The duration of the test is according to the manufacturer's instructions.

5. Determination of the active resistance of the cores. Produced for lines 20 kV and above. The active resistance of the cable line conductors to direct current, reduced to 1 mm cross-section, 1 m length and temperature +20 ° C, should be no more than 0.0179 Ohm for a copper conductor and no more than 0.0294 Ohm for an aluminum conductor. The measured resistance (reduced to specific value) may differ from the specified values by no more than 5%.

6. Determination of the electrical working capacitance of the cores.

Produced for lines 20 kV and above. The measured capacity should not differ from the factory test results by more than 5%.

7. Checking protection against stray currents.

The operation of the installed cathodic protection is checked.

8. Test for the presence of undissolved air (impregnation test).

Produced for oil-filled cable lines 110-500 kV. The content of undissolved air in the oil should be no more than 0.1%.

9. Testing of feeding units and automatic heating of end couplings.

Produced for oil-filled cable lines 110-500 kV.

10. Checking anti-corrosion protection.

When accepting lines into operation and during operation, the operation of anti-corrosion protection is checked for:

— cables with a metal sheath laid in soils with medium and low corrosive activity (soil resistivity above 20 Ohm/m), with an average daily leakage current density into the ground above 0.15 mA/dm;

— cables with a metal sheath laid in soils with high corrosive activity (soil resistivity less than 20 Ohm/m) at any average daily current density into the ground;

— cables with an unprotected sheath and destroyed armor and protective coverings;

— steel pipeline of high-pressure cables, regardless of the aggressiveness of the soil and types of insulating coatings.

During the test, potentials and currents in the cable sheaths and electrical protection parameters (current and voltage of the cathode station, drainage current) are measured in accordance with the guidelines for the electrochemical protection of underground energy structures from corrosion.

The assessment of the corrosive activity of soils and natural waters should be carried out in accordance with the requirements of GOST 9.602-89.

11. Determination of characteristics of oil and insulating liquid.

The determination is made for all elements of oil-filled cable lines for a voltage of 110-500 kV and for end joints (inputs into transformers and switchgear) of plastic-insulated cables for a voltage of 110 kV.

Samples of oils of grades S-220, MN-3 and MN-4 and insulating liquid of grade PMS must meet the requirements of the standards of tables 1.8.41 and 1.8.42.

Table 1.8.41 Standards for quality indicators of oils of grades S-220, MN-3 and MN-4 and insulating liquid of grade PMS

Note. Test oils not listed in Table 1.8.39 in accordance with the manufacturer's requirements.

Table 1.8.42 Tangent of the dielectric loss angle of oil and insulating liquid (at 100,%, no more, for voltage cables, kV)

110	150-220	330-500
0,5/0,8*	0,5/0,8*	0,5/-

________________

* The numerator indicates the value for S-220 grade oils, the denominator for MN-3, MN-4 and PMS

If the values of electrical strength and degree of degassing of MN-4 oil meet the standards, and the values of tg δ, measured according to the GOST 6581-75 method, exceed those indicated in Table 1.8.42, the oil sample is additionally kept at a temperature of 100 ° C for 2 hours, periodically measuring. When the tg δ value decreases, the oil sample is kept at a temperature of 100 °C until a steady value is obtained, which is taken as the control value.

12. Ground resistance measurement.

Produced on lines of all voltages for terminations, and on lines 110-500 kV, in addition, for metal structures of cable wells and make-up points.

About the company » Questions and answers » What is the standard for cable insulation resistance

Thanks for the question! If we stick to the rules, then for a cable with a voltage of 380 V, the insulation resistance of the cable cores is 2540 kOhm, which is quite a sufficient figure. The regulatory technical documentation states that to insulate cable cores with voltages up to 1000 V, the resistance should not be less than 500 kOhm or 0.5 MOhm. In your case, there is a five-fold reserve. But, taking into account the experience and the huge number of experiments carried out, as a rule, the insulation resistance of the cores of a new cable up to 1000 Volts is about 10,000-15,000 kOhm, that is, we can assume that the service life of your cable will not be long.

There is also one more nuance, namely, what voltage did you test the cable with? The testing rules and standards say (PUE Ch. 1.8. Clause 1.8.37, RD 34.45-51.300-97) that tests are carried out with increased rectified voltage if the cable is not insulated with rubber. And at the same time, each such test reduces the service life of the cable by almost half! This factor is also worth taking into account, namely, you should not torture the cable often, otherwise there will be nothing to test.

I hope the answer was complete! In any case, call us; our electrical laboratory specialists will help answer all your questions.

1. PUE Ch. 1.8. clause 1.8.37
2. PTEEP – Table 37

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Communication cable insulation resistance standards | Useful articles - Kabel.RF

Measuring the insulation resistance of a communication cable with metal conductors is carried out in order to determine its performance. The quality of the signal transmitted through the conductors also depends on this indicator. The result of a decrease in insulation resistance, as a rule, is the appearance of interference on the line, which, in turn, leads to audible noise (telephone line), a decrease in throughput (digital data transmission systems), or a complete interruption of the message.

According to GOST 15125-92, measurement of the insulation resistance of a communication cable should be carried out once every 6 months.

Communication cable insulation resistance standards

Electrical standards for communication cables determine the minimum resistance values of external insulation and core insulation at which cable products are allowed for use. The amount of resistance depends on the type and purpose of the cable.

Requirements for the insulation resistance values of cables put into operation are given in GOST 15125-92, OST 45.01-98, OST 45.83-96 and other regulatory and technical documentation. Let's look at a few examples.

Insulation resistance standards for communication cables most often used for the construction of primary networks, GTS and other lines (values per 1 km of cable length, without terminals / with terminal devices):

Cables with tubular-paper and porous-paper insulation (TGShp, TBpShp, TKpShp, TStShp, etc.) - 8000/1000 MOhm. Polyethylene insulation (brands - TPPep, TPPepB, TPVBG, STPAPP, STPAPPBG and others) - 6500/1000 MOhm. Cord-paper insulation (ТЗБ, ТЗБГ, ТЗКл, ТЗБн, etc.) - 10000/3000 MOhm.

Testing of communication cables

Measuring the insulation resistance of a communication cable is also carried out in accordance with regulatory requirements. When performing this task, it is important to take into account the current temperature and humidity. All electrical parameters of communication cables are given by the manufacturers, subject to testing at a temperature of +20 °C and a cable product length of 1 km. Deviation of these parameters from the norm leads to an increase or decrease in readings. However, there are simple formulas that allow you to recalculate the resistance depending on temperature and length.

Equipment

The insulation resistance of a communication cable is measured using a special device called a megohmmeter. To determine the required electrical quantity, these devices generate a certain voltage (from 100 V or more).

Currently, two types of megohmmeters are used - digital and analog. In the first case, electromechanical (manual) generators and dial indicators are used to generate voltage. Digital megohmmeters usually use galvanic cells or batteries to generate voltage. The measurement results are displayed on a digital display. Also, some megohmmeter models do not have their own current generator and require an external power source.

To test cable lines, reflectometers are also widely used, capable of detecting various cable defects using the location (reflectometric) method. The operating principle of the devices is as follows:

Short-wave electrical pulses are applied to the cores of the cable being tested. If there are any defects in the cable, the supplied pulse is reflected from the obstacle and returns back to the device.

The returned signal is captured by the reflectometer sensors, measured, analyzed, and then the measurement result is displayed on the display.

Thus, with the help of reflectometers, it is possible to detect breaks, short circuits, mixed-up pairs, dense ground and other defects that occur, including when the cable insulation is damaged.

Requirements and testing methods for communication cables

Measuring the parameters of communication cables (insulation) is a simple process, but requires compliance with the requirements established by regulatory documentation (in particular, GOST 3345-76, GOST 2990-78). In short:

Before carrying out work, the cable must be de-energized and disconnected from all terminal devices and conductors (if it is, for example, a GTS cable, the tested conductors are disconnected from the terminals of the distribution boards). You cannot test with a megohmmeter over cables located in close proximity to other electrical systems, since the voltage generated by the device can create powerful electromagnetic fields that can disrupt the operation of these systems. It is impossible to test overhead communication lines during a thunderstorm. The tested conductors (cores) must be grounded. The test conductor can be disconnected from the ground only after it has been connected to the corresponding terminals of the megohmmeter (i.e., the device is first connected, and only then the wires are disconnected from the ground). Before and after measurements, the conductor must be cleared of residual current by short circuiting. This operation is also performed on the measuring probes of the megohmmeter. To obtain an accurate result, current is passed through the conductor under test for (and no more than!) 1 minute. After testing, the device and the tested conductor are allowed to “cool down” for 2 or more minutes, unless other numbers are given in the relevant documentation for the megohmmeter and/or cable.

All other safety requirements are given in GOST 2990-78.

Now let's consider the process of measuring the insulation resistance of a communication cable using the example of a coaxial pair without a protective shield (we will measure the insulation resistance of the cores). According to GOST 2990-78, the conditional diagram for applying voltage to the cable cores is as follows:

Core “1” is connected to the “R–” input (the input can also be designated as “–”, “Ground” or “3”) of the megohmmeter. Core “1” and input “R–” of the megohmmeter are grounded.

Core “2” is connected to the voltage source input “R+” (“+”, “Rx”, “Line” or “L”) of the megohmmeter.

Conditional working diagram:

Measurement process:

First, the output voltage level is set on the megohmmeter, which depends on the brand of the cable being tested (usually, to test communication cables, it is enough to apply a voltage of 500 V). After applying voltage to the circuit, the megohmmeter will take about 1 minute to take measurements. If it is a pointer device, you must wait until it stops completely; for this, the megohmmeter must be stationary. In the case of digital devices, this is not necessary.

If necessary, measurements are carried out several times. As mentioned above, before each procedure the device is allowed to “cool down” for about 2 minutes (plus or minus - depends on the characteristics of the megohmmeter).

The readings are greatly influenced by the ambient temperature (the higher it is, the lower the resistance and vice versa). If its value is different from +20 degrees, you must use the following “corrective” formula:

R_(20)=K*R_1, where:

R_(20) – cable insulation resistance (in our case, core insulation resistance) at +20 °C (indicated in the data sheet for the cable brand);

R_1 - resistance obtained as a result of measurements at a temperature other than +20 °C;

K is a “correction” coefficient that allows you to determine the value of insulation resistance that would occur at +20 °C (coefficients are given in the appendix to GOST 3345-76).

For example, let's take a KTPZBBbShp cable with polyethylene insulation, the initial resistance of which (without terminal devices) is 5000 MOhm. After measuring the resistance of the conductors at a temperature of 15 °C, we obtained a result of, say, 11,500 MOhm. According to GOST 3345-76, the correction factor “K” in the case of polyethylene insulation of cores is 0.48. Substituting this value into the formula, we have:

R_(20)=0.48*12500=5520 (resistance under normal conditions)

Using the following formula, you can determine the insulation resistance depending on the cable length:

R=R_(20)* l, where:

R_(20) – insulation resistance at +20 °C;

l is the length of the cable being tested;

Let's take the same brand of cable TPPepBbShp with a length of 1.5 km. We know the initial insulation resistance of the cores under normal conditions - 5000 MOhm. From here:

R=6500* 1.5=7500 MOhm

The Kabel.RF company is one of the leaders in the sale of cable products and has warehouses located in almost all regions of the Russian Federation. After consulting with the company’s specialists, you can purchase the brand of communication cable you need at competitive prices.

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Cable insulation resistance | Purpose, stages of work, results, measurement rules - on the industrial portal Myfta.Ru

In order to take a measurement, you need:

First, turn off all electrical appliances and all cables and wires that are subject to electrical measurements.
Before taking measurements, you need to remove all light bulbs from the lighting fixtures. At the same time, all lighting switches must be turned on.
It is necessary to turn off the power supply to cables and wires.

After following all the above instructions, the power system will be completely ready to measure insulation resistance.

The permissible cable insulation resistance reading must be above 0.5 mOhm. If these indicators do not meet, then this cable must be dismantled.

It is prohibited to determine the resistance of lines that are close to other similar lines.

Fig 1. Megaohmmeter

The resistance is determined using a megohmmeter with a voltage of 2500 (V) for 1 minute.

(A – B; B – C; C – A), that is, between phase conductors;
(A – N; B – N; C – N), also between neutral and phase conductors;
(A – PE; B – PE; C – PE), also between the ground and phase conductors;
(N – PE), and finally between ground and neutral conductors.

There are some rules to consider when measuring cable insulation resistance:

Firstly, in order to take a measurement, you need to know the exact ambient temperature. Because if there is a negative temperature, and there is water in the cable mass (even in small quantities), then it will turn into pieces of ice. And ice itself is a dielectric, that is, it does not have conductivity abilities. Moreover, when carrying out insulation, you will not be able to identify these pieces of ice, so you need to immediately take care of an acceptable temperature. The optimal temperature should not be lower than +5°C (exceptions are cases specified in special instructions.).
Secondly, if the resistance of the electrical wiring, which is in working condition, is less than 1 MOhm, then a conclusion about their suitability is given after a special check of this electrical wiring is first carried out, which consists of applying an alternating current of industrial frequency to it, but with a voltage of 1 kV, and then conclusions are drawn about their suitability.
Thirdly, we must not forget that only flexible wires should be used when measuring (they have special insulating handles at the ends, and they also have restrictive rings in front of the contact probes). The wires that connect have a minimum length.
Fourthly, a megohmmeter of 1000 V and above is used for determination. Devices that have not passed annual government inspections are not allowed for use.

If the voltage in electrical installations is above 1000 (V), measuring the cable resistance should be carried out wearing dielectric gloves.

In order to determine the standards for cable insulation resistance, you must first classify these cables:

Cable classification:

above 1000 (V), that is, high-voltage power;
below 1000 (V), that is, high-voltage power;
as well as control cables.

Accordingly, the insulation resistance standards are different for each type of cable, for example:

For cables above 1000 (V), high voltage, there is no specific standard, but the resistance will be higher than 10 (MOhm).
For cables below 1000 (V), low voltage - the resistance should be above 0.5 (MOhm).

Whether high or low voltage is used, it all depends on the voltage of your electrical installation.

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Why measure cable insulation resistance?

Resistance is a value that reflects the ability of materials to resist the passage of electric current. The lower it is, the less electricity is lost on the conductors and the more current can be transmitted safely. The cable insulation resistance allows you to evaluate the integrity of the sheaths, and therefore determine whether the product is suitable for use.

The safety and durability of the wire depends on the integrity of the insulation of electrical wiring. Modern products have several shells for different purposes, located one below the other: protection from electromagnetic interference, electric shock to people, rupture, moisture ingress, exposure to aggressive environments. To ensure the integrity of all layers, tests must be carried out. Their purpose is to ensure that the shells are not damaged along the entire length of the product. Therefore the test must be non-destructive. The only option is to measure the cable insulation resistance.

Conductor resistance is calculated using the formula:

where R is the desired value, the resistivity of the material (tabular value), l is the length of the conductor, S is the cross-sectional area.

The formula shows that the larger the area of the conductor, the lower its resistance will be. This is the basis for integrity testing through cable insulation measurements. If it is damaged, the area through which current flows will decrease, and as a result, the resistance will increase. The cable insulation test results and the permissible insulation resistance should be equal or slightly different. Specific figures are provided in the technical documentation accompanying the products. You can also determine how much resistance should be using the formula presented above. Take the value from the table below, the length of the product is measured in meters, area - in mm2.

Cable insulation resistance standards: table of material resistivities under normal conditions

The work will require a special tool. There are also several procedures that need to be carried out before starting an insulation test.

Conditions for the test

Before measuring insulation resistance, you need to know about the microclimate of the room. The table above shows the resistivities of materials at normal temperature (+20°C). As this value increases, the resistivity of the materials increases, and with it the insulation resistance of wires and cables. A decrease in temperature affects the indicator slightly. But if there is ice between the layers, it cannot be detected, since the substance does not conduct electricity.

The change in resistivity is calculated using the formula

where is the resistivity at a temperature of +20°C, and is the temperature coefficient (tabular value), t is the air temperature.

The value of a is small, for example, for copper it is 0.0068, and for aluminum it is 0.00429.

The ideal air temperature for testing is +20°C. With it, all results will be as close as possible to the table values. If it is not possible to create normal thermal conditions, then care must be taken to ensure that the room temperature is above 0°C, otherwise it will not be possible to detect the presence of moisture under the shells.

Equipment

Insulation resistance is measured using a megohmmeter. There is equipment for different types of wiring and to determine different characteristics. Some devices are able to provide simple values, others determine the presence of water and the humidity of the shells.

Measuring cable insulation resistance is such an important procedure that government agencies monitor it. Tests can only be carried out using equipment that is included in a special register. Every year, the devices are submitted for performance testing, after which a hologram and a stamp with information about the expiration date are applied to them.

When choosing devices for measuring wire insulation resistance, you should be guided by the following:

Type of conductors to be tested. Depending on it, the range in which the megohmmeter is capable of operating is selected.
Indication type. There are analog (with a pointer and a dial), light, and graphic devices. The accuracy of each of them is guaranteed by the state control body (if the product is included in the appropriate register) - the type of indication only affects the simplicity and speed of operation. It is most convenient to use products with a display. But all other things being equal, they are more expensive than others.
Climatic performance. To measure insulation resistance in the Far North, special models are needed.
Compactness. Depends on the power source - electric generator, battery, galvanic cell.
Additional features. There are multimeters that are designed with a megohmmeter. With them you can not only check the insulation resistance, but also measure voltage, current, absorption coefficient (moisture absorption strength).

A megohmmeter with a display allows you to measure insulation resistance many times faster

Wire classification

Cable types are important when making resistance measurements. There are different classifications. For these purposes, the voltage that can be passed through the product is important. Depending on it, products are divided into the following types:

High voltage - for currents over 1000 Volts.
Low voltage - for voltages up to 1000 Volts;
Control wires are wires that are used in equipment. These include secondary circuits of the switchgear, power supply circuits for separators, control elements, protection, and automation.

Depending on the type of wiring, the appropriate testing device is used.

Norms

There are standards that determine the suitability of products for use depending on the results of measuring insulation resistance (per 1000 meters):

for high voltage - not lower than 10 MOhm;
for low voltage - at least 0.5 MOhm;
control - not lower than 1 MOhm.

More details about cable insulation resistance standards can be found in clause 6.2. PTEEP and clause 1.8.37 PUE.

All conductors are subject to testing. The time intervals with which the insulation resistance of electrical wiring is measured differ:

measurements of conductors of mobile electrical installations - at least once every six months;
The electrical wiring of external electrical installations, as well as equipment installed in hazardous areas, is checked for compliance with standards once a year.
the insulation resistance of the rest is checked every three years.

Carrying out such tests is necessary, first of all, to ensure network security. This is not just a requirement of control authorities that needs to be carried out “for show”. Therefore, the intervals at which testing is carried out may vary. Extraordinary tests should be carried out if there is any suspicion that the insulation may have been damaged.

Working with different types of conductors

The procedure for checking the protection of products depends on their type. The algorithm for working with each type of conductor is slightly different. Therefore, you need to consider instructions for working with different wiring options.

The rule common to all cases is to check the presence of voltage in the network using special instruments. If the state of the cable is not reliably known, it is considered active.

Shell resistance is measured as follows:

Install test grounding on untested conductors. The clamps are mounted on the side from which testing will be carried out.
The cable cores located on the side opposite to the grounding are separated from each other.
Install/turn on warning and prohibition signs - posters, cones, illuminated signs. For greater safety, it is recommended that someone be assigned to guard the area where the insulation test is being carried out.
Check cable products using a 2.5 kV megohmmeter for 1 minute.
Write down the measurement results in a notepad.

When working with high-voltage wires, tests are carried out on each core. If you need to check the insulation on low-voltage cables, test the following pairs:

A-PE;
V-PE;
zero and ground, having previously disconnected the first from the zero bus.

Features of working with control wiring

Control wiring can be tested on the equipment without disconnecting the wires from the circuit. The way to connect the equipment is slightly different:

One terminal of the megohmmeter is connected to the core being tested.
The second probe is connected either to grounding or to an untested conductor.
The remaining conductors are connected to each other and grounded.

What will the regulatory authorities require?

State control authorities, in particular the fire inspectorate, may require insulation resistance measurement protocols. They contain information about the data obtained, the conditions under which the test was carried out, the device, and the performer. Therefore, such work can only be entrusted to an organization that has permission to carry out such research. If the measurements are taken by an ordinary electrician, the protocol will not be valid.

It’s good if an employee of the organization knows how to do such work. It is worth monitoring the insulation resistance for yourself in order to be confident in the quality of the conductors used and their safety for property and others.

Based on the article "Measurement of insulation resistance (IR) - 2", http://electrical-engineering-portal.com

1. Insulation resistance values for electrical equipment and systems

(PEARL/NETA MTS-1997 Standard Table 10.1)

Rated maximum equipment voltage	Megger class

1 MΩ Rule for Equipment Insulation Resistance Value

Depending on the rated voltage of the equipment:

< 1 кВ = не менее 1 МОм
> 1 kV = 1 MΩ per 1 kV

According to IE Rules - 1956

When 1000 V is present between each live conductor and ground for one minute, the insulation resistance of high voltage installations shall be not less than 1 MΩ or as specified by the Bureau of Indian Standards. Medium Voltage and Low Voltage Installations - If 500 V is present between each live conductor and earth for one minute, the insulation resistance of Medium Voltage and Low Voltage Installations shall be not less than 1 MΩ or as specified by the Bureau of Indian Standards. According to CBIP specifications, acceptable values are 2 MΩ per kV.

Medium Voltage and Low Voltage Installations - If 500 V is present between each live conductor and earth for one minute, the insulation resistance of Medium Voltage and Low Voltage Installations shall be not less than 1 MΩ or as specified by the Bureau of Indian Standards.

According to CBIP specifications, acceptable values are 2 MΩ per kV

2. Insulation resistance value for transformer

Insulation resistance testing is necessary to determine the insulation resistance of individual windings to ground or between individual windings. In this type of testing, insulation resistance is usually either measured directly in MΩ or calculated from the applied voltage and the magnitude of the leakage current.

When measuring insulation resistance, it is recommended to always ground the frame (and core). Short-circuit each transformer winding to the bushing terminals. After this, measure the resistance between each winding and all other grounded windings.

Insulation resistance testing: between high voltage side and ground, and between high voltage side and low voltage side.
HV1 (2, 3) - Low voltage 1 (2, 3); LV1 (2, 3) - High voltage 1 (2, 3))

When measuring insulation resistance, never leave the transformer windings ungrounded. To measure the resistance of a grounded winding, it is necessary to remove solid grounding from it. If it is not possible to remove the ground, as is the case with some windings with solidly grounded neutrals, the insulation resistance of such a winding will not be measurable. Consider them part of the grounded section of the circuit.

Testing must be done between windings and between winding and ground (E). On three-phase transformers, it is necessary to test the winding (L1, L2, L3) minus the ground for transformers with a delta connection or the winding (L1, L2, L3) with ground (E) and neutral (N) for transformers with a star connection.

Insulation resistance value for transformer

Where C = 1.5 for oil-filled transformers with an oil tank, 30 for oil-filled transformers without an oil tank or for dry transformers.

Temperature correction factor (relative to 20°C)

Example for a three-phase transformer 1600 KVA, 20 kV / 400 V:

insulation resistance value on high voltage side = (1.5 x 20000) / √1600 = 16000 / 40 = 750 MOhm at 20°C;
insulation resistance value on low voltage side = (1.5 x 400) / √1600 = 320 / 40 = 15 MOhm at 20°C;
insulation resistance value at 30°C = 15 x 1.98 = 29.7 MOhm.

Transformer winding insulation resistance

Transformer insulation resistance value

Voltage	Test voltage (DC), low voltage side	Test voltage (DC), high voltage side	Minimum insulation resistance value


6.6 kV - 11 kV
11 kV - 33 kV
33 kV - 66 kV
66 kV - 132 kV
132 kV - 220 kV

Measuring the insulation resistance of a transformer:

turn off the transformer and disconnect jumpers and lightning rods;
discharge the interturn capacitance;
completely clean all bushings;
short-circuit the windings;
Protect the terminals to prevent surface leakage across the terminal insulators;
record the ambient temperature;
connect test leads (avoid additional connections);
Apply test voltage and record readings. The insulation resistance value 60 seconds after applying the test voltage is taken as the insulation resistance of the transformer at the testing temperature;
The neutral terminal of the transformer must be disconnected from ground during testing;
Also, during testing, all connections to the ground of the lightning rod on the low voltage side must be disconnected;
due to the inductive characteristics of the transformer, insulation resistance readings must be taken only after the test current has stabilized;
Do not take resistance readings while the transformer is under vacuum.

Transformer connections when testing insulation resistance (at least 200 MOhm)

Transformer with two windings

2. High voltage winding - (low voltage winding + ground)
3. Low voltage winding - (high voltage winding + ground)

Transformer with three windings
1. High voltage winding - (low voltage winding + tap winding + ground)
2. Low voltage winding - (high voltage winding + tap winding + ground)
3. (High voltage winding + low voltage winding + tap winding) - ground
4. Branch winding - (high voltage winding + low voltage winding + ground)

Autotransformer (two windings)
1. (High voltage winding + low voltage winding) - ground

Autotransformer (three windings)
1. (High voltage winding + low voltage winding) - (tap winding + ground)
2. (High voltage winding + low voltage winding + tap winding) - ground
3. Branch winding - (high voltage winding + low voltage winding + ground)

For any insulation, the measured insulation resistance should not be less than:

high-voltage winding - ground 200 MOhm;
low-voltage winding - ground 100 MOhm;
high-voltage winding - low-voltage winding 200 MOhm.

Factors affecting the transformer insulation resistance value

The insulation resistance value of transformers is affected by the following:

condition of the surface of the terminal bushing;
oil quality;
winding insulation quality;
oil temperature;
duration of use and test voltage value.

3. Insulation resistance value for output winding switch

insulation resistance between high-voltage and low-voltage windings, as well as between windings and ground;
The minimum resistance value for the output winding switch is 1000 ohms per volt of operating voltage.

An insulation tester is used to measure the grounded motor winding resistance (E).

for rated voltages below 1 kV, measurement is carried out with a 500 V DC megger;
for rated voltages above 1 kV, measurement is carried out with a 1000 V DC megger;
In accordance with IEEE 43, Article 9.3, the following formula should be applied:
minimum insulation resistance value (for a rotating machine) = (Rated voltage (V) / 1000) +1.

In accordance with IEEE 43 1974, 2000

Example 1: For a three-phase electric motor 11 kV

insulation resistance value = 11 + 1 = 12 MΩ, but according to IEEE43 should be 100 MΩ.

Example 2: For a 415V three-phase motor

insulation resistance value = 0.415 + 1 = 1.41 MΩ, but according to IEEE43 should be 5 MΩ;
as per IS 732 minimum insulation resistance value for electric motor = (20 x Voltage (p-p)) / (1000 + 2 x kW).

Motor insulation resistance value according to NETA ATS 2007 Section 7.15.1

Motor nameplate (B)	Test voltage	Minimum insulation resistance value
	500 V DC
	1000 V DC
	1000 V DC
	1000 V DC
	2500 V DC
	2500 V DC
	2500 V DC
	5000 V DC
	15000 V DC

Submersible motor insulation resistance value

5. Insulation resistance value for electrical cables and wiring

Insulation testing requires disconnecting cables from the panel or equipment and from the power source. Wiring and cables should be tested relative to each other (phase to phase) with the ground cable (E). The IPCEA (Insulated Power Cable Engineers Association) offers a formula for determining the minimum insulation resistance values.

R = K x Log 10 (D/d)

R= Insulation resistance value in MOhm for 305 meters of cable
TO= Constant of the insulating material. (Electrical insulating lacquer fabric = 2460, thermoplastic polyethylene = 50000, composite polyethylene = 30000)
D= Outer diameter of conductor insulation for solid wire or cable (D = d + 2c + 2b diameter of solid cable)
d= Conductor diameter
c= Conductor insulation thickness
b= Thickness of insulating sheath

High voltage testing of new XLPE cable (according to ETSA standard)

11 kV and 33 kV cables between core and earth (according to ETSA standard

Measuring the insulation resistance value (between conductors (cross insulation))

The first conductor to be measured for cross-insulation must be connected to the Line terminal of the megger. The other conductors are connected together (using alligator clips) and connected to the Earth terminal of the megger. At the other end, the conductors are not connected;
then turn the knob or press the megger button. The meter display will show the insulation resistance between conductor 1 and the remaining conductors. Insulation resistance readings should be recorded;
then connect another conductor to the Line terminal of the megger, and connect the other conductors to the ground terminal of the megger. Take a measurement.

Measuring insulation resistance value (insulation between conductor and ground)

connect the conductor under test to the Line terminal of the megger;
connect the Earth terminal of the megohmmeter to ground.;
turn the knob or press the megohmmeter button. The meter display will show the insulation resistance of the conductors. After maintaining the test voltage for a minute until a stable reading is obtained, record the insulation resistance value.

Measured values:

If, during periodic testing, the insulation resistance of an underground cable at the corresponding temperature is from 5 MΩ to 1 MΩ per kilometer, this cable must be included in the replacement program;
if the measured insulation resistance of an underground cable at the corresponding temperature is from 1000 kOhm to 100 kOhm per kilometer, this cable should be replaced urgently, within a year;
If the measured cable insulation resistance is less than 100 kOhm per kilometer, this cable should be replaced immediately as an emergency cable.

6. Insulation resistance value for transmission line/distribution line

7. Insulation resistance value for panel bus

Insulation resistance value for panel = 2 x rated voltage of panel in kV
For example, for a 5 kV panel the minimum insulation resistance is 2 x 5 = 10 MOhm.

8. Insulation resistance value for substation equipment

Typical resistance values for substation equipment are:

Typical insulation resistance value for substation equipment

Equipment		Megger class	Minimum insulation resistance value
Circuit breaker	(Phase - Earth)
	(Phase - Phase)
	Control circuit
	(Primary - Earth)
	(Secondary - Phase)
	Control circuit
Insulator	(Phase - Earth)
	(Phase - Phase)
	Control circuit
	(Phase - Earth)
Electric motor	(Phase - Earth)
Switchgear LT	(Phase - Earth)
Transformer LT	(Phase - Earth)

Insulation resistance value of substation equipment in accordance with DEP standard:

Equipment	Measurement	Insulation resistance value at the time of commissioning (MOhm)	Insulation resistance value at the time of service (MOhm)
Switchgear	High voltage bus
	Low voltage bus
	Low voltage wiring
Cable (minimum 100 meters)		(10 x kV)/km
Electric motor and generator	Phase - Earth
Transformer immersed in oil	High voltage and low voltage
Transformer, dry type	High voltage
Transformer, dry type	Low voltage
Stationary equipment/tools	Phase - Earth	5 kOhm per volt	1 kOhm per volt
Removable equipment	Phase - Earth
Distribution equipment	Phase - Earth
Circuit breaker	Power circuit	2 MΩ per kV
Circuit breaker	Control circuit
	DC Circuit - Ground
	Circuit LT - Ground
	LT - DC circuit

9. Insulation resistance value for household/industrial wiring

Low resistance between phase and neutral conductors or between live conductors and earth will result in leakage current. This leads to deterioration of insulation, as well as energy losses, which will result in increased operating costs for the installed system.
At normal supply voltages, the phase-to-phase-neutral-to-earth resistance should never be less than 0.5 MΩ.

In addition to the leakage current due to the active resistance of the insulation, there is also a leakage current due to its reactance, since it acts as the dielectric of a capacitor. This current does not dissipate any energy and is not harmful, but we need to measure insulation resistance, so DC voltage is used to prevent reactance measurement from being included in the test.

Single phase wiring

Insulation resistance testing between phase-neutral and ground should be performed on the entire installation with the power switch turned off, with the line and neutral connected together, with lamps and other equipment disconnected but with circuit breakers closed, and with all circuit breakers closed.

If bidirectional switching is used, only one of the two wires will be tested. To test a different wire, you must operate both bi-directional switches and retest the system. If necessary, the installation can be tested as a whole, but then a value of at least 0.5 MΩ must be obtained.

Three-phase wiring

In the case of a very large installation with many parallel connections to ground, lower readings can be expected. In this case, it is necessary to repeat the testing after partitioning the system. Each of these parts must meet minimum requirements.

Insulation resistance testing should be performed between phase-phase-neutral-ground. The minimum acceptable value for each test is 0.5 MΩ.

Low Voltage Insulation Resistance Testing

Minimum insulation resistance value= 50 MOhm / number of electrical outlets (all electrical points with installation elements and plugs)

Minimum insulation resistance value= 100 MOhm / number of electrical outlets (all electrical outlets without installation elements and plugs)

Safety precautions when measuring insulation resistance

High test voltage can cause damage to electronic equipment such as electronic fluorescent lamp starters, touch switches, dimmer switches, and power controllers. Therefore, such equipment should be disconnected.

Capacitors and indicator or test lamps should also be disconnected because they may cause inaccurate test results.

If any equipment is disconnected for testing, it must be subject to its own insulation test using a voltage that will not damage them. The result must be as specified in the UK standard or be at least 0.5 MΩ if not specified in the standard.

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Method for measuring insulation resistance

The purpose of this methodology is to ensure high-quality and safe work during the electrical laboratory (hereinafter referred to as EL) tests (measurements).

The methodology is based on:

GOST R 8.563-96 “Measurement methods”;
interindustry rules on labor protection (safety rules) during the operation of electrical installations. POT R M-016-2001;
documentation from manufacturers of instruments used in the work.

Purpose

The purpose of the methodology is to describe procedures for the organization, implementation and registration of work carried out by electric power plants to measure insulation resistance.

Name and characteristics of the measured value

The measured value is the insulation resistance. DC insulation resistance is the main indicator of the insulation condition and its measurement is an integral part of testing all types of electrical equipment and circuits.

Composition of instruments used in measurement

Insulation resistance is measured with a megger. Currently, the most common types of megohmmeters are M-4100, ESO202/2G, MIC-1000, MIC-2500.

Description of megohmmeters

A megohmmeter is a device consisting of a voltage source (a constant or alternating generator with a current rectifier) and a measuring mechanism.

Meggers are divided according to rated operating voltage up to 1000 V and up to 2500 V.

Megohm meters are equipped with flexible copper wires up to 2-3 m long with an insulation resistance of at least 100 MOhm. The ends of the wires connected to the megohmmeter must have terminations, and the opposite ends must have alligator clips with insulated handles.

Measurement procedure

The procedure for carrying out measurements with megohmmeters type M-4100 and ESO202/2G. Before starting measurements you must:

Before starting the measurement, the megohmmeter must be subjected to a control test, which consists of checking the readings of the device with open wires (the arrow of the device should be at the infinity mark -?) and closed wires (the arrow of the device should be at the 0 mark).
Make sure that there is no voltage on the cable under test (it is necessary to check the absence of voltage using a tested voltage indicator, the serviceability of which must be checked on parts of the electrical installation that are known to be under voltage - clause 3.3.1 of the “Inter-industry rules on labor protection” POT R M-016-2001) .
Ground the current-carrying conductors of the cable being tested (grounding from live parts can only be removed after connecting the megger).

The connected megohmmeter wires must have clamps with insulated handles; in electrical installations above 1000 V, in addition, dielectric gloves should be used.

When working with a megger, touching the live parts to which it is connected is not allowed.

As a rule, the insulation resistance of each phase of the cable is measured relative to the remaining grounded phases. If measurements using this abbreviated version give an unsatisfactory result, then it is necessary to measure the insulation resistance between each two phases and each phase to ground.

When making measurements on cables above 1000 V (when the measurement results can be distorted by leakage points along the insulation surface), an electrode (screen rings) connected to terminal “E” (screen) is placed on the insulation of the measurement object (end funnel, etc.).

When measuring the insulation resistance of cables for voltages up to 1000 V with zero cores, remember the following:

neutral working and protective conductors must have insulation equal to the insulation of phase conductors;
Both on the power supply side and on the receiver side, the neutral conductors must be disconnected from the grounded parts.

Scheme for measuring insulation resistance: a - electric motor; 6 - cable; 1 - terminal board; 2 - coil terminals; 3 - metal protection (shell); 4 - insulation; 5 - screen; 6 - conductive core.

Measurement (taking readings) should be carried out with the instrument needle in a stable position. To do this, you need to rotate the device handle at a speed of 120 rpm.

The insulation resistance is determined by the reading of the instrument arrow 15 seconds and 60 seconds after the start of rotation. If determination of the cable absorption coefficient is not required, the readings are taken after the pointer has calmed down, but not earlier than 60 seconds from the start of rotation.

If the measurement limit is incorrectly selected, you must:

remove the charge from the test phase by applying grounding;
switch the limit and repeat the measurement at the new limit.

When applying and removing grounding, you must use dielectric gloves

At the end of the measurements, before disconnecting the ends of the device, it is necessary to remove the accumulated charge by applying grounding.

Measuring the insulation resistance of lighting networks is carried out with a 1000 V megohmmeter and includes:

Measuring the insulation resistance of main lines - from 0.4 kV assemblies (main switchboards, ASU) to automatic switchboards (SC) or group circuit breakers (depending on the circuit);
Measurement of insulation resistance from distribution (floor) switchboards to local control (apartment) group switchboards.
Measuring the insulation resistance of the lighting network from local circuit breakers (fuses), group control panels (SC) to lamps (including the insulation of the lamp itself). At the same time, in lighting networks in luminaires with incandescent lamps, insulation resistance is measured with the voltage removed, switches turned on, fuses removed (or switches turned off), neutral working and protective wires disconnected, electrical receivers turned off and electric lamps turned out. In lighting networks with gas-discharge lamps, measurements can be taken both with the lamps installed and without them, but with the starters removed.
The insulation resistance value in each section of the lighting network, starting from the circuit breaker (fuse) panel and including the luminaire wiring, must be at least 0.5 MOhm.

Processing and registration of measurement results

Data on the instruments used in the process of measuring work, as well as the measurement results, are recorded in protocols.

Requirements for safe work performance

Table 1. Permissible distances to live parts that are energized.

In accordance with Chapter 12 of the Interindustry Rules for Labor Protection (Safety Rules) for the Operation of Electrical Installations. POT R M-016-2001" EL workers (as employees of organizations sent to perform work in existing, under construction, technically re-equipped, reconstructed electrical installations and who are not on the staff of the organizations that own the electrical installation) are classified as seconded personnel.

Seconded workers must have certificates of the established form for testing knowledge of norms and rules of work in electrical installations with a mark on the group assigned by the commission of the sending organization. The sending organization is responsible for the compliance of the groups assigned to posted workers, as well as for the staff’s compliance with regulatory documents for the safe performance of work.

The organization of work for traveling personnel involves the following procedures performed before the start of work:

notifying the organization that owns the electrical installation in a letter about the purpose of the business trip, as well as the composition and qualifications of the traveling electric personnel;
determination and provision by the owner organization to seconded workers of the right to work in existing electrical installations (as issuers of work orders, responsible managers and work producers, team members);
Conducting introductory and initial briefings on electrical safety with seconded personnel upon their arrival;
familiarization of seconded personnel with the electrical circuit and features of the electrical installation in which they will work (moreover, the employee who is granted the right to perform the duties of a work supervisor must undergo training on the electrical installation's electrical supply diagram);
preparation of the workplace by employees of the owner organization and admission of seconded personnel to work.

The organization in whose electrical installations work is carried out by seconded personnel is responsible for the implementation of the prescribed safety measures and permission to work.

Work is carried out on the basis of a permit, order or in the order of routine operation in accordance with the requirements of Chapter 5 of the Interindustry Rules for Labor Protection (Safety Rules) for the Operation of Electrical Installations. POT R M-016-2001". In addition, when carrying out tests and measurements you should:

Be guided by the instructions in the passports (operating instructions) of the instruments used and safety instructions (in force at the enterprise where the measurements are performed), as well as additional safety requirements specified in permits, orders, and briefings.
Check the absence of voltage (it is necessary to check the absence of voltage with a tested voltage indicator, the serviceability of which must be checked on parts of the electrical installation that are known to be energized - clause 3.3.1 of the “Inter-industry rules on labor protection” POT R M-016-2001). The absence of voltage should be checked both between all phases and between phase and ground. Moreover, in electrical installations with the TN-C system, at least six measurements should be made, and in electrical installations with the TN-S system - at least ten measurements.
Connect and disconnect everything with the voltage removed.
Ensure the use of protective equipment and tools with insulating handles, tested in accordance with the “Instructions for the use and testing of protective equipment used in electrical installations”, approved by Order of the Ministry of Energy of Russia dated June 30, 2003 No. 261.

The team performing the work must consist of at least two people, including a work performer with an electrical safety group of at least IV and a team member with an electrical safety group of at least III. When carrying out measurements, it is prohibited to approach live parts at distances less than those specified in Table 1.

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