Test period for cable lines 0.4 sq. What is cable insulation resistance and its standards. Presentation of test results

Insulation resistance is one of the most important parameters cables and wires, because during operation, power and signal cables are always subject to various external influences. In addition, in addition to external influences, there is also a constant influence of the cores inside the cable on each other, their electrical interaction, which certainly leads to the appearance of leaks. By adding here the factors that influence the quality of insulation, we get a more complete picture.

For these reasons, cables are always protected with dielectric insulation, which includes: rubber, PVC, paper, oil, etc. - depending on the purpose of the cable, the operating voltage, the type of current, etc. For example, underground distribution telephone lines are carried by tape-armored cable, and some telecommunications cables are encased in aluminum to protect against external current interference.

As for the dielectric properties of insulation, they are not the only ones that influence the choice of a specific material for a particular cable. Heat resistance is no less important: rubber is more resistant to high temperatures than plastic, plastic is better than paper, etc.

Thus, cable insulation is the protection of the cores from their influence on each other, from short circuits, from leaks, and from external influences from the environment. And the insulation resistance is determined by the value between the conductors and between the conductor and the outer surface of the insulating shell (or between the conductor and the screen).

Of course, the insulation material during cable operation loses its former qualities, ages, and is destroyed. And one of the indicators of these unfavorable changes is a decrease in DC insulation resistance.

The DC insulation resistance for various cables and wires is standardized according to their GOST, which is indicated in the passport for specific cable products: in laboratory conditions, the normal insulation resistance is recorded at an ambient temperature of +20°C, after which the resistance is reduced to a cable length of 1 km , as indicated in the technical documentation.

Thus, low-frequency communication cables have a minimum standardized resistance of 5 GOhm/km, and coaxial cables - up to 10 GOhm/km. When taking measurements, it is taken into account that this is the given length for 1 km of cable; accordingly, a piece twice as long will have half the insulation resistance, and a piece twice as short will have twice as much. In addition, temperature and humidity during measurements have a significant impact on the current value, so it is necessary to introduce corrections, experts know this.

Speaking about power cables, take into account the provisions of the PUE clause 1.8.40. Thus, power cables of secondary switching circuits and lighting wiring with voltages up to 1000 V are assigned a norm of 0.5 MOhm for each core between the phase wires and between the phase and neutral wires and the protective grounding wire. And for lines with voltages of 1000 V and above, the resistance standard is not indicated, but the leakage current in mA is indicated.

Special tests are carried out in which the test voltage is normalized. In accordance with the type of current of the testing equipment and the purpose of the cable being tested, taking into account the material of its insulation -. This is how the quality of insulation of high-voltage cables is assessed using a megohmmeter.

An insulation resistance of 1 MOhm per kilovolt of cable operating voltage is considered acceptable, that is, for a cable operating at a voltage of 10 kV, a resistance of 10 MOhm will be accepted as normal after testing with a megohmmeter with a test voltage of 2.5 kV.

Insulation resistance measurements are carried out regularly with a megohmmeter: on mobile installations - once every six months, on high-risk facilities - once a year, on other facilities - once every three years. These measurements are carried out by qualified specialists. As a result of the measurements, the specialist draws up a document - an act in the form established by Rostechnadzor.

Based on the results of the inspection, a conclusion is made about whether the object needs repair or whether its performance meets the inspection requirements. If repairs are required, repairs are carried out in order to restore the insulation resistance to normal. A protocol is also drawn up based on the results of the repair, after regular measurements with a megohmmeter.

Andrey Povny

Any electrical product is characterized by a number of parameters. For cables, one of the main ones is insulation resistance. There are certain standards that must be taken into account during design and installation, as well as during operation and maintenance of communication routes.

What are the standards for cable insulation resistance? The fact is that there are often discrepancies on this issue. This is caused, according to the author, by several factors.

Firstly, cable is a general concept. This group of products includes samples used for laying power, signal and telephone lines. Cables can be coaxial (radio frequency), control, distribution and general purpose. That is, there are many options for the design of protective shells, differing, among other things, in thickness.

Secondly, the most different materials– rubber, plastics, even paper impregnated in a special way. Although in more modern cables the protection is usually complex, that is, combining various dielectric layers.

Thirdly, what kind of insulation resistance are we talking about - the outer shell or the surface coating of the cores?

Fourthly, the specifics of installation and further operation of a particular cable should be taken into account. For example, the route laying method is open or closed. Where it is laid - in the ground, in trays (there are plenty of options). What characterizes the environment - the maximum value and changes in temperature, humidity, aggressiveness, and so on.

Insulation resistance - standards for cables

All values are in MOhm.

Power cables

High voltage (more than 1,000 V). There are no norms for them. That is, the higher the insulation resistance, the better. It is generally accepted that its value should not be less than 10.
Low voltage (up to 1,000 V). Essentially, we are talking about electrical wiring and secondary circuits various installations. The minimum limit for the insulation resistance value is 0.5. More detailed information on this issue can be found in the 7th edition of the PUE (Table 1.8.34 and clause 1.8.37).

Control, signal, general purpose cables

It's pretty large group products. This includes cables installed for control circuits, automation, power supply of electric drives, connection of protective, distribution devices and so on. For them, it is considered normal if the insulation resistance is not lower than 1. But this is a generally accepted indicator. The exact meaning, depending on, should be found in its accompanying documentation.

For communication cables, the resistance standards are somewhat different, more “strict”. For city low-speed lines – at least 5, trunk lines – 10 (MOhm/km).

If the cable has an outer sheath made of aluminum coated with PVC, then the resistance standard is higher and equal to 20.

Note. The PUE stipulates that the measurement of insulation resistance is carried out with a megohmmeter with the inductor voltage:

for cables in circuits not exceeding 500 V – 500;
up to 1,000 V – 1,000;
all others – 2,500.

Specialists do not need to explain that all requirements for insulation resistance are specified in technical specifications, GOST and SNiP for a certain type of work. Its value can be easily found out from the cable passport, and if it is necessary to monitor the condition of the product, make the appropriate measurement. The specifics of this operation are specified in clause 1.8.7. PUE (7th edition).

In everyday life, to assess the degree of wear of the power cable insulation, you can use the following table, which reflects the approximate average standards.

Since a non-professional is not able to take into account all the nuances of the design of the product and its use, this, as a rule, is quite enough to understand whether a given sample is worth laying down or whether it is no longer suitable for use. That is, reject it. Well, if there are certain doubts, then it is a good idea to consult with a specialized specialist.

2016-08-22

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 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 electronic devices, battery-powered, they are compact in 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 more external source high voltage. 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, carrying out measurements of 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 megohmmeter 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 that are usually exposed to high voltage, after disconnection, may carry significant residual charges or interference charges from surrounding high-voltage equipment. 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 everything possible options 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 for different distances from 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 serial number 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 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 exactly what it is main reason, what you need to do is measure the insulation resistance of the cables.

To avoid people electric shock, fires and others unpleasant situations etc., it is necessary to constantly take electrical measurements of 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 that 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 alternating current to it industrial frequency, 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, voltage 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 ( resistivity soil 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.

And today we will talk about testing cables with impregnated paper, plastic and rubber insulation with increased rectified current voltage.

Insulation monitoring of power cables with voltages above 1000 (V) is carried out using the applied voltage method, which makes it possible to detect defects that may, during further operation of the cable, reduce the electrical strength of its insulation.

Preparation for high voltage cable testing

Let me remind you right away that testing with increased voltage (high-voltage tests) is permitted to an employee over 18 years of age who has undergone special training and knowledge testing (reflected in the table for carrying out special work on his or her certificate). It looks something like this.

By the way, I specially created an online for you — you can test your knowledge.

Before testing the power cable with increased voltage of rectified current, it is necessary to inspect it and wipe the funnels from dust and dirt. If during inspection any defects in insulation or outside surface If the cable is heavily contaminated, it is prohibited to proceed with testing.

It is also worth paying attention to the ambient temperature.

The ambient air temperature should only be positive, because at a negative air temperature and if there are water particles inside the cable, they will be in a frozen state (ice is a dielectric), and such a defect will not appear during a high-voltage test.

Immediately before testing the cable with increased voltage, it is necessary to measure its insulation resistance. You can read more about this in the article .

As I said above, power cable lines are tested with increased rectified current voltage.

The increased rectified voltage is applied to each core of the power cable in turn. During testing, other cable cores and metal sheaths (armor, screens) must be grounded. In this case, we immediately check the insulation strength between the conductor and the ground, as well as in relation to other phases.

If the power cable is made without a metal sheath (armor, screen), then an increased rectified current voltage is applied between the core and other cores, which we first connect to each other and to the ground.

It is allowed to test all the cores of the power cable with increased voltage at once, but in this case it is necessary to measure the leakage currents for each phase.

We completely disconnect the power cable from the busbar, and separate the wires at a distance of more than 15 (cm) from each other.

We have figured out the circuit for testing rectified voltage power cables. Now we need to decide on the size and duration of the tests. To do this, open the reference books: PTEEP and PUE.

You can also use the electronic version of these books. I suggest you download right now and completely free electronic version.

I made the task a little easier for you and compiled a general table taking into account the requirements of the PUE (chapter 1.8, clause 1.8.40) and PTEEP (Appendix 3.1., table 10).

The duration of testing cable lines with voltage up to 10 (kV) with paper and plastic insulation after installation is 10 minutes, and during operation - 5 minutes.

The duration of testing cable lines with voltage up to 10 (kV) with rubber insulation is 5 minutes.

Now we will consider the standardized values of leakage currents and asymmetry coefficients when testing cable lines with increased rectified current voltage.

There are slight disagreements here between the PUE and PTEEP (values from PTEEP are indicated in parentheses).

If the power cable has cross-linked polyethylene insulation, for example, PvVng-LS(B)-10, then it is not recommended to test it with direct (rectified) voltage; moreover, the value of the test voltage differs significantly. I talked about this in more detail in a separate article about.

Power cable testing apparatus

Well, we have smoothly moved on to what is used to test cables with increased rectified voltage. In ours, we use either the AII-70, AID-70, or IVK-5 testing apparatus. The last two devices are used most often on the road.

We will talk about these devices in more detail in the following articles, and if you do not want to miss the release of new articles on the site, then subscribe to receive notifications by email.

Method of testing cables with increased voltage

Let's say we need to conduct operational tests of a 10 (kV) power cable of the AAShv brand (3x95).

Using an AII-70 or IVK-5 apparatus, we raise the test voltage to a value of 60 (kV) at a speed of 1-2 (kV) per second. From this moment the time countdown begins. During the entire 5 minutes, we closely monitor the magnitude of the leakage current. After the time has elapsed, we record the resulting leakage current and compare it with the values in the table above. Next, we calculate the asymmetry coefficient of leakage currents by phase - it should be no more than 2, but sometimes it can be more (see table).

The asymmetry coefficient is determined by dividing the maximum leakage current by the minimum leakage current.

After high-voltage testing of the cable, it is necessary to test it again.

The cable is considered to have passed the test when:

During the test, no breakdown, surface flashover or surface discharges occurred
There was no increase in leakage current during the test
the cable insulation resistance has not decreased

It happens in practice that leakage currents exceed the values indicated in the tables. In this case, the cable is put into operation, but the period of its next test is reduced.

If during testing the leakage current begins to increase, but breakdown does not occur, then the test must be carried out for more than 5 minutes. If after this the breakdown does not occur, then the cable is put into operation, but the period of its next test is reduced.

Results and protocol for high voltage cable testing

After testing the cable with increased rectified voltage, it is necessary to draw up a protocol. Below I will give you the protocol form (example) used by our electrical laboratory (click on the picture to enlarge).

P.S. This concludes the article on testing cables with increased voltage. If you have questions about the material, ask them in the comments.