The insulation resistance must be at least Cable insulation resistance

Wiring in a wooden house
26.07.2019

1. Purpose of the measurement .

Measurements are carried out in order to verify that the insulation resistance complies with established standards.

2. Security measures.

2.1 Technical measures.

Before and during measurements, it is necessary to carry out technical measures in accordance with the “Safety Rules” (PTB). When working with a megohmmeter, it is necessary to be guided by paragraphs B 3.7.17-B 3.7.22 PTB.

2.2 Organizational events.

Measurements with a megohmmeter are allowed to be performed in installations with a voltage above 1000V by two persons, one of whom must have an electrical safety group of at least IV. The work is done in sequence. In installations with voltage up to 1000V, measurements are performed by two persons, one of whom must have a group of at least III. The work is carried out in the order of current operation with subsequent recording in the operational log.

3. Normalized values .

The frequency of tests and the minimum allowable value of insulation resistance must comply with those specified in the test standards for electrical equipment and apparatus of the “Rules technical operation electrical installations of consumers”. As a rule, the insulation resistance of BSSN and FSSN systems measured with a 250 V megger must be at least 0.25 MΩ, power circuits up to 500 V (except for BSSN and FSSN systems) measured with a 500 V megohmmeter must be at least 0.5 MΩ, and secondary circuits - not less than 1 MΩ. Insulation resistance power circuits above 500 V measured with a megohmmeter at 1000 V must be at least 1.0 MΩ, (GOST R50571.16-99). The insulation resistance of electrical wiring, including lighting networks, measured with a megohmmeter at 1000 V must be at least 0.5 MΩ, (PTEEP clause 28.1)

4.
Applied devices.

To measure insulation resistance, megohmmeters of types are used: MI 3102H (for voltage 100 V, 250 V, 500 V 1000 V and 2500 V) and, E6-24 (for voltage 500 V 1000 V and 2500 V). These devices have their own power source - generator direct current and allow direct reading of readings in megaohms and gigaohms.

5. Measurement of insulation resistance of electrical equipment.

5.1. Measuring the insulation resistance of power cables and wiring

When measuring insulation resistance, the following must be taken into account:

Measurement of the insulation resistance of cables (with the exception of armored cables) with a cross section of up to 16 mm2 is carried out with a megohmmeter for 1000 V, and above 16 mm2 and armored - with a megometer for 2500 V; measurement of the insulation resistance of wires of all sections is carried out with a 1000 V megameter.

In this case, it is necessary to make the following measurements:

On 2- and 3-wire lines - three measurements: L-N, N-PE, L-PE;

On 4-wire lines - 4 measurements: L1-L2L3PEN, L2 - L3L1PEN, L3-L1L2PEN, PEN-L1L2L3, or 6 measurements: L1-L2, L2-L3 ,
L1-L3, L1-PEN, L2-PEN, LЗ-PEN- on 5-wire lines - 5 measurements: L1-L2L3 NPE, L2-L1L3NPE, LЗ-L1L2PE, N-L1L2L3PE, PE-NL1L2L3, or

10 measurements: L1-L2, L2-L3, L1-L3, L1-N, L2-N, L3-N, L1-PE, L2-PE, LZ-PE, N-PE.

It is allowed not to measure the insulation resistance in lighting networks that are in operation, if this requires significant work to dismantle the circuit, in this case, at least once a year, it is required to perform visual inspection together with checking the reliability of operation of overcurrent protective equipment (determination of currents single-phase short circuits in accordance with paragraph 1.7.79 of the PUE).

If the electrical wiring in operation has an insulation resistance of less than 0.5 MΩ, then the conclusion about their suitability is made after testing them with alternating current of industrial frequency with a voltage of 1 kV in accordance with the recommendations given in this publication.

5.2. Measurement of insulation resistance of power electrical equipment

The value of the insulation resistance of electrical machines and apparatuses depends to a large extent on temperature. Measurements should be made at an insulation temperature of at least +5°C, unless otherwise specified in special instructions. At lower temperatures, the measurement results, due to the unstable state of moisture, do not reflect the true performance of the insulation. If there are significant differences between the measurement results at the installation site and the manufacturer's data due to the temperature difference at which the measurements were taken, these results should be corrected according to the manufacturer's instructions.

The moisture content of the insulation is characterized by an absorption coefficient equal to the ratio of the measured insulation resistance 60 seconds after applying the megger voltage (R60) to the measured insulation resistance 15 seconds later (R15),

Cubs = R60/ R15

When measuring the insulation resistance of power transformers, meggers with an output voltage of 2500 V are used.

Measurements are taken between each winding and the frame and between the windings of the transformer.

In this case, R60 must be brought to the results of factory tests, depending on the temperature difference at which the tests were carried out.

The value of the absorption coefficient should differ (in the direction of decrease) from the factory data by no more than 20%, and its value should not be lower than 1.3 at a temperature of 10-30°C. If these conditions are not met, the transformer must be dried.

Minimum allowable resistance insulation for installations in operation are given in Appendix 3 of PTEEP, table 9, and for installations put into operation - in Ch. 1.8. PUE, table 8. The insulation resistance of hand-held electrical machines is measured relative to the housing and external metal parts with the switch on.

The body of the power tool and the parts connected to it, made of dielectric material, must be wrapped in metal foil connected to the ground loop for the duration of the test.

If the insulation resistance in this case is at least 10 MΩ, then the insulation test with increased voltage can be replaced by measuring its resistance with a megohmmeter with an output voltage of 2500 V for 1 minute.

For portable transformers, the insulation resistance is measured between all windings, as well as between the windings and the housing. When measuring the insulation resistance of the primary winding, the secondary must be closed and connected to the case.

Insulation resistance of circuit breakers and RCDs are produced:

1. Between each pole terminal and opposite pole terminals connected to each other when the circuit breaker or RCD is open.

2. Between each opposite pole and the remaining poles connected to each other when the switch or RCD is closed.

3. Between all interconnected poles and the body wrapped with metal foil.

At the same time, for circuit breakers for household and similar purposes (GOST R50345-99) and RCDs when measured according to p.p. 1, 2 insulation resistance must be at least 2 MΩ, according to paragraph 3 - at least 5 MΩ.

For other circuit breakers (GOST R50030.2-99), in all cases, the insulation resistance must be at least 0.5 MΩ.

6. Measurement of insulation resistance with the E6-24 device

6.1.
The appearance of the device is shown in Figure 1

Picture 1

1, 2, 3 - sockets for connecting cables

4 - indicator

5 - indicator of units of measurement (from top to bottom, respectively:

Voltage, V

Gom resistance

Mohm resistance

6 - test voltage indicator (from left to right, respectively: 500V, 1000V, 2500V)

7 - battery charge indicator

8 - switch on and off device status

9 - test voltage setting button

10 - button for displaying results from memory

11 - resistance measurement button

6.2.
Before starting measurements, make sure that there is no voltage on the object under test, carefully clean the insulation near the measuring point from dust and dirt and for 2-3 minutes. Ground the object to remove possible residual charges from it. After the end of the measurements, the object under test must be discharged by short-term grounding.

To connect the megaohmmeter to the device or line under test, separate wires with a high insulation resistance (usually not less than 100 MΩ) should be used.

Before use, the megohmmeter should be subjected to a control check, which consists in checking the reading on the scale with open and short-circuited wires. In the first case, the arrow should be at the “infinity” scale mark, in the second - at zero.

In order to prevent leakage currents on the insulation surface from affecting the readings of the megohmmeter, especially when measuring in wet weather, the megohmmeter is connected to the object being measured using the clamp E (screen) of the megohmmeter. With this connection, leakage currents on the insulation surface are diverted to the ground, bypassing the winding of the device.

The insulation resistance value is highly temperature dependent. Insulation resistance should be measured at an insulation temperature of at least +5°C, unless otherwise specified in special instructions. At lower temperatures, the measurement results, due to the unstable state of moisture, do not reflect the true performance of the insulation.

When measuring the insulation resistance to ground using a megohmmeter, it is recommended to connect the “+” terminal to the current-carrying part of the installation under test, and the “-” (ground) terminal to its body. When measuring insulation resistance electrical circuits, not

connected to ground, the connection of the megohmmeter clamps can be any.

The use of the “E” clamp (screen) significantly increases the measurement accuracy at high insulation resistances, eliminates the influence of surface leakage currents and thus does not distort the measurement results.

To connect the megaohmmeter to the object under test, it is necessary to have flexible wires with insulated handles and restrictive rings at the ends. The length of the wires should be as short as possible.

Before starting the measurement, it is necessary to measure the insulation resistance of the connecting wires. The value of this resistance must be at least the upper limit of the megohmmeter.

The insulation resistance is taken as the 60-second resistance value R-60, fixed on the megohmmeter indicator after 60 s, which is counted automatically.

Before starting measurements, you must make sure: that there is no voltage on the object under test, that the equipment under test, wires, cable funnels, etc. are clean, and that all parts with reduced insulation or reduced test voltage are disconnected and shorted. If available on site AC voltage the megaohmmeter will detect it automatically. In the absence of voltage, you can begin to take measurements.

6.3. Switching the value of the test voltage of 500 V, 1000 V and 2500 V is done by briefly pressing the "UR" button.

6.4. To take a measurement, press and hold the "RX" button. When the button is released, the measurement process will stop. Double pressing the "RX" button leads to its capture, and the measurement process will occur for a specified time interval without holding it (from 1 to 10 minutes), which can be set using the UR and MRx / K buttons after turning on the megohmmeter while pressing the "RX" button . If it is necessary to terminate the measurement process early, press the “RX” button again.

6.5. Lighting up on the indicator of the symbol "P" (overflow) indicates that the resistance of the object of measurement exceeds the limit of the instrument reading of 99.9 Gom. Also, the “P” indication may appear during transients, therefore, in this case, the measurement should be continued for another 10 seconds.

6.6. Unplugging the cables from the object should be carried out no earlier than 10 seconds after the end of the test voltage supply.

7.1. Procedure for measuring insulation resistance

Step 1 Use the rotary switch to select the function Insulation.

Use the and buttons to select between the "R ISO" and "DIAGNOSTICS" functions. Select the option " R ISO". Connect the measuring cable to the EurotestXE 2.5 kV instrument.

Step 2 set the values ​​of the following parameters and measurement limits:

Rated measuring voltage,

Minimum limit permissible value resistance.

Step 3 P Connect the test cable to the object under test. To carry out an insulation resistance measurement, follow the connection diagram shown in figure 2. If necessary, refer to the help menu. For insulation resistance measurements at voltage UN= 2.5 kV, special test leads must be used, since the test signal is applied to different measuring terminals than for measurements at UN ≤ 1 kV! The standard 3-wire measuring cable, cable with euro plug and "commander" probes can only be used for resistance measurements at UN≤ 1 kV!

Figure.2 : Connecting the 3-wire measuring cable and test probe with

ferrule (UN ≤1 kV)

For insulation resistance measurements at voltage UN= 2.5 kV, a two-wire 2.5 kV measuring cable must be used. Connection in accordance with the connection diagram shown in Figure 3

Figure 3 : Connecting two-wire 2.5 kV measurement cable (UN =2.5 kV)

Step 4 P Before starting measurements, check the displayed warnings and the operating voltage / output monitor. If the measurement is enabled, press and hold the TEST button until the result stabilizes. During measurements, the display shows the actual resistance value. After the TEST button is released, the last measured value is displayed, accompanied by a pass/fail rating (if applicable).

Displayed results:

R… … … … Insulation resistance,

um… … … Measuring voltage.

Save the measurement results for future documentation.

7.2. Classification of insulation resistance measurement results while saving

When saving, after pressing the button Memory, ten insulation resistance subfunctions are available:

The procedure for measuring the insulation resistance is the same regardless of which sub-function is selected. However, it is important to select the appropriate sub-function in order to correctly classify the measurement results in the future for their correct entry in the measurement protocols.

8. Registration of measurement results .

The results of measuring the insulation resistance of wires, cables, windings of machines and devices are recorded in a protocol, the final part of which characterizes the quality of the insulation. The completed protocol is attached to the report on the commissioning of electrical equipment.

DEVELOPED:

Head of electrical laboratory

Getting Started cable insulation resistance measurement it is important to take into account the temperature indicators environment. Why is that?

This is due to the fact that at sub-zero temperatures in the cable mass, water molecules will be in a frozen state, in fact, in the form of ice. And as you know, ice is a dielectric and does not conduct current.

So when determining the insulation resistance at sub-zero temperatures, it is these particles of frozen water that will not be detected.

To calculate the conductor resistance, you can use the conductor resistance calculator.

Instruments and means for measuring cable insulation resistance.

The next step in the measurement of insulation resistance cable lines, will be the measuring instruments themselves.

The most popular device for measuring insulation resistance among employees of our electrical laboratory is the MIC-2500.

With this device manufactured by Sonel, you can not only take measurements of the resistance of cable lines, cords, wires, electrical equipment (transformers, switches, motors, etc.), but also determine the level of deterioration and moisture content of the insulation.

It is worth noting that it is the MIC-2500 device that is included in the state register of approved insulation resistance measurements.

According to the instructions, the MIC-2500 device must undergo an annual state verification. After the verification procedure, a hologram and a stamp are applied to the device, which confirm the verification. The stamp contains information about the date of the scheduled verification and the serial number of the measuring device.


Only serviceable and verified devices are allowed to work with insulation resistance measurements.

Insulation resistance standards for various cables.

To determine the norm of cable insulation resistance, it is necessary to classify them. Cables according to their functional purpose are divided into:

  • above 1000 (V) - high-voltage power
  • below 1000 (V) - low-voltage power
  • control cables - (protection and automation circuits, switchgear secondary circuits, control circuits, power circuits for electric drives of switches, separators, short circuiters, etc.)

Measurement of insulation resistance, both for high-voltage cables and for low-voltage cables, is carried out with a megohmmeter for a voltage of 2500 (V). And control cables are measured at a voltage of 500-2500 (V).

Each cable has its own insulation resistance standards. According to PTEEP and PUE.

High voltage power cables above 1000 (V) - insulation resistance must be at least 10 (MΩ)

Low voltage power cables below 1000 (V) - insulation resistance should not fall below 0.5 (MΩ)

Control cables - insulation resistance must not fall below 1 (MΩ)

Algorithm for measuring the insulation resistance of high-voltage power cables.

To understand and simplify the process of performing measurement work insulation resistance in high voltage power cables, we recommend the procedure for measurements.

1. We check the absence of voltage on the cable using a high voltage indicator

2. We put a test ground using special clamps on the cable cores on the side where we will carry out the measurement.

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

4. We place warning information posters. It is advisable to put a person on the other side to monitor safety during measurement with a megohmmeter.


5. We measure each core for 1 minute with a megohmmeter at 2500 (V) to obtain indicators insulation resistance power cable.

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

Visually it looks like this:

6. We write down the measurement data in the process of work in a notebook.

Method for measuring the insulation resistance of low-voltage power cables.

With regard to measuring the insulation of low-voltage power cables, the measurement technique differs slightly from that described above.

Similarly:

1. We check the absence of voltage on the cable using protective equipment designed for work in electrical installations.

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

3. We place prohibitory and warning posters. We leave a person on the other side to monitor security.

4. Measurement insulation resistance of low voltage power cable we carry out a megaohmmeter at 2500 (V) for 1 minute:

  • between phase conductors (А-В, В-С, А-С)
  • between phase conductors and zero (A-N, B-N, C-N)
  • between the phase conductors and earth (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

6. The obtained indicators of insulation resistance measurements are recorded in a notebook.

Method for measuring the insulation resistance of control cables.


A feature of measuring the insulation resistance of control cables is that the cable cores can not be disconnected from the circuit and measurements can be taken together with electrical equipment.

Measurement of the insulation resistance of the control cable is performed according to a familiar algorithm.

1. We check the absence of voltage on the cable using protective equipment that is designed for work in electrical installations.

2. We measure the insulation resistance of the control cable with a megohmmeter for 500-2500 (V) in the following sequence.

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

1 minute we measure the test core. Then we return this core to the rest of the cable cores and measure each core in turn.

3. We fix all the obtained indicators for measuring the insulation resistance of the control cable in a notebook.

Cable insulation resistance measurement protocol.

All of the above electrical measurements, after receiving the cable insulation resistance data, must be subjected to a comparative analysis with the requirements and norms of the PUE and PTEEP. Based on the comparison, it is necessary to formulate a conclusion about the suitability of the cable for subsequent operation and draw up a protocol for measuring the insulation resistance.

Each electrical equipment, including conductors, has its own specific parameters. Sometimes, to make the installation of electrical wiring, it is necessary to clarify each characteristic. It is imperative to know the value of the cable insulation resistance norm, since without this parameter it makes no sense to carry out installation work.

The conductor device is made in this way: two metal conductors are designed to transmit current, but they cannot exist on their own, as they are afraid of factors affecting the external environment, and they themselves are an unfavorable phenomenon. First of all, metal conductors will interfere with each other until an emergency situation is created. Therefore, insulating materials are needed. To prevent accidents, fires or electric shocks, materials that do not conduct current are used in the production of conductive cables.

To make such isolation and, accordingly, protect a person from negative situations, rubber, plastic and other consistencies are taken as a basis. Sometimes they are mixed, creating an insulating combination with added strength. To date, there are all kinds of cables with effective insulation on the industry market, using which in installation, it is important to calculate the resistance.

Insulation resistance: standards table and calculation principle

Table of wire insulation resistance standards

We ask you to pay attention to the tabular data, here the norm of the indicator for each individual class of voltage of the electrical network is indicated.

So, let's start considering some rules regarding resistance measurements. Unfortunately, it is impossible to directly check the required dielectric value. For the further procedure, we need a megaohmmeter.

  1. The first thing we will do is make sure that the wiring under test is not connected to voltage;
  2. then all current-carrying conductors should be grounded for literally a few minutes. This is done in order to remove the remaining charge;
  3. the insulating layer must be dry and clean, therefore, we eliminate all impurities of dust and dirt, if any.

In the future, we work with the device. To do this, select the measurement chapel. For example, for home electrical networks, it is enough to supply 1000 volts. At the same time, you need to check the device. This method is performed strictly with closed and open conductors. In the open state, the arrow shows zero, and when closed, it should go to "infinity".

The norm of the PUE of the cable insulation resistance must confirm the value on the ohmmeter dial.

Important! It is necessary to take readings from the device after measurements only when its arrow is at rest.

Remember that resistance cable insulation should be checked at the time of the 15th and 60th second from the beginning of the rotation of the ohmmeter knob.

It is not uncommon to find insulated wires with grounding, in this situation three measurements are performed:

  • between zero and phase;
  • between ground and phase;
  • between ground and zero.

Important! When working with the megger, wear overalls and non-conductive gloves.

To find out what insulation resistance the cable should have, you need to refer to the PUE standards for each individual section.

Insulation coating control measurements


Checking the insulation resistance of wires in the electrical panel

Cable insulation resistance is a very important indicator that indicates the performance of the conductor. To avoid unexpected breakdowns and not be in danger of electric shock, it is recommended to carry out regular inspection. What does it mean, we will tell below.

Please note that the check electrical networks and installations with current consumption must be carried out without fail, this is stated by state regulations and a number of regulatory documents.

Therefore, you need to control:

  • resistance of numerous mobile installations strictly once every six months;
  • resistance of wires and electric cables in production, as well as external electrical installations no later than every year;
  • the resistance of the remaining elements with the consumption of electric current is sufficient once every two and a half years.

Important! It is customary to record all measurement results in a state act, and in case of a discrepancy, be sure to indicate it.

In conclusion, we want to note that only timely monitoring and taking into account all the regulations increases the chances for the duration of the work of conductors.

With metal conductive cores is made in order to determine its performance. The quality of the signal transmitted through the conductors also depends on this indicator. Reduced insulation resistance typically results in noise on the line, which in turn leads to audible noise (telephone line), reduced bandwidth (digital data systems), or a complete loss of communication.

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

Communication cable insulation resistance standards

The electrical standards of communication cables determine the minimum values ​​of the resistance of external insulation and core insulation, at which cable products are allowed to be used. The resistance value depends on the type and purpose of the cable.

Requirements for 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 commonly used for the construction of primary networks, public transport networks and other lines (values ​​per 1 km of cable length, without terminals / with terminals):

Cables with tubular-paper and porous-paper insulation (, etc.) - 8000/1000 MΩ.
. Polyethylene insulation (brands -, and others) - 6500/1000 MΩ.
. Cordless-paper insulation (, etc.) - 10000/3000 MΩ.

Communication cable testing

Communication cable insulation resistance measurements are also made 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 manufacturers subject to testing at a temperature of +20 °C and a cable product length of 1 km. The 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

Measurement of the insulation resistance of a communication cable is carried out with a special device called a megohmmeter. To determine the desired electrical value, these devices generate a certain voltage (from 100 V or more).

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

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

Short-wave electrical impulses are applied to the cores of the cable under test.
. If there are any defects in the cable, the applied pulse is reflected from the obstacle and returns back to the device.
. The returned signal is captured by the reflectometer sensors, measured, analyzed, after which the measurement result is displayed on the display.

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

Requirements and test methods for communication cables

Measuring the parameters of communication cables (insulation) is a simple process, but it 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 cores are disconnected from the terminals of the switchboards).
. It is impossible to 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.
. It is possible to disconnect the conductor under test from the "ground" only after it is connected to the corresponding terminals of the megohmmeter (i.e., the device is connected first, and only then the wires are disconnected from the "ground").
. Before and after measurements, the conductor must be freed from residual current by means of a short circuit. This operation is also performed on the measuring probes of the megohmmeter.
. To obtain an accurate result, the current is passed through the tested conductor 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 figures are given in the corresponding 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 screen (we will measure the insulation resistance of the cores). According to GOST 2990-78, the conditional scheme for applying voltage to the cable cores is as follows:

Core "1" is connected to the input "R-" (the input can also be designated as "-", "Earth" or "Z") of the megohmmeter.
. Core “1” and the 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 scheme:


Measurement process:

First, the output voltage level is set on the megaohmmeter, 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 this is a pointer device, it is necessary to wait for it to stop 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 strongly 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 ° С (indicated in the passport for the cable brand);

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

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

For example, consider a PE-insulated cable that has an initial resistance (without terminations) of 5000 MΩ. After measuring the resistance of the cores at a temperature of 15 ° C, we got a result of, say, 11,500 MΩ. According to GOST 3345-76, the correction factor "K" in the case of polyethylene core insulation 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 length of the cable:

R=R_(20)*l, where:

R_(20) - insulation resistance at +20 °С;

l is the length of the tested cable;

Take the same brand of cable 1.5 km long. We know the initial insulation resistance of the conductors under normal conditions - 5000 MΩ. From here:

R=6500* 1.5=7500 MΩ

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METHODOLOGY

measuring the insulation resistance of electrical equipment

multifunctional electrical tester (type MET-5035)

1. INTRODUCTION.

DC insulation resistance measurement is the most common type of insulation monitoring. The essence of the method is to measure the ratio of the DC voltage applied to the insulation U current flowing through it i

R=

U

i

Taking into account the dielectric equivalent circuit, the total current flowing through the insulation

i=i well +i abs+i o ,

i well- through conduction current;

i abs- absorption current due to slow polarization processes;

i o- current. due to the processes of fast polarization.

Since the current io flows only for 10–12 ... 10–14 s, then its influence on the measurement results does not affect, while the value of the absorption component iabs plays a very significant role, i.e., in the measurement circuit until the completion of the dielectric polarization processes, a current will flow, decreasing in time at a rate depending on the constant τ abs =R abs *C abs

Therefore, the measured resistance value during this period will depend on the duration of the applied voltage.

With an increase in the time from the beginning of the measurement to the moment of counting, the measured resistance value increases.

To ensure the uniformity of measurements, it is customary to read the instrument readings after 60 seconds. after applying the measuring voltage to the insulation.

2. NORM, PERIODICITY AND MEASUREMENT ERRORS

2.1. According to the PUE and PTEEP:

2.1.1. Insulation resistance of electrical wiring and cable lines with voltage up to 0.4 kV. inclusive must be at least 0.5 mΩ (Table 1.8.39. PUE, Table 37 Appendix 3.1. PTEEP).

2.1.2. The insulation resistance of switchgear, switchboards and conductors must be at least 1 mΩ (Table 37, Appendix 3.1. PTEEP).

2.1.3. The insulation resistance of stationary electric stoves must be at least

1 mΩ (Table 37 App. 3.1. PTEEP).

2.1.4. The insulation resistance of cranes and elevators must be at least 0.5 mOhm (Table 37 appendix 3.1. PTEEP).

2.1.5. The insulation resistance of the electrode boiler without water must be at least 0.5 mΩ, unless higher requirements are specified by the manufacturer. (clause 25.4. appendix 3. PTEEP).

2.1.6. Insulation resistance of stator windings for electric motors alternating current for voltage up to 1000 V must be at least 1 mΩ at a temperature of 10 ... 30 ° C, and at a temperature of 60 ° C - 0.5 mΩ (Table 1.8.8. PUE, clause 23.1.2. Appendix 3. PTEEP).

2.1.7. The insulation resistance of the rotor windings for electric motors with a phase rotor for voltages up to 1000 V must be at least 0.2 mOhm (Table 1.8.8. PUE, clause 23.1.4. App. 3. PTEEP).

2.1.8. The insulation resistance of the windings of DC electrical machines for voltages up to 1000 V depends on the temperature of the winding and the lowest allowable value is determined according to Table 32 of Appendix 3. PTEEP.

2.1.9. If insulating rooms are used as a protective measure, in which simultaneous contact with parts under different potentials is prevented, if the main insulation of the current-carrying parts of the resistance insulating floor in such rooms is damaged, it should not be lower relative to the local ground (clause 1.7.86. PUE):

50 kOhm at a rated voltage of the electrical installation not higher than 500 V;

100 kOhm at a rated voltage of the electrical installation above 500 V.

2.2. Measurement of insulation resistance is carried out within 1 minute with a megohmmeter for voltage:

Power cable lines up to 1 kV. -2500 V,

Distribution devices, shields and conductors - 1000 ... 2500 V,

Electrode boilers - 2500 V,

Wiring, cranes and elevators -1000 V.

Electric motors and DC machines up to 500 V - 500 V,

Insulating floors at rated voltage up to 500 V inclusive - 500 V,

Insulating floors with a rated voltage of more than 500 V - 1000 V.

2.3. If the insulation resistance of power and lighting wiring is below 1 mΩ, being tested increased voltage industrial frequency 1000 V for 1 min. (clause 28.3.2. appendix 3. PTEEP), which can be replaced by a test with a megohmmeter with a voltage of 2500 V (clause 3.6.22. PTEEP).

2.4. The measurement of the insulation resistance of electrical wiring, including lighting networks, is carried out at least once every 3 years, and for electrical wiring in especially dangerous rooms and outdoor installations of stationary, electric stoves, cranes and elevators, at least once a year (Table 37 Appendix 3.1 PTEEP).

Tests of electrode boilers, AC motors and electrical machines up to 1000 V are carried out within the time limits established by the PPR system.

2.5. The measurement technique provides an error of no more than

+ 0.05% of the scale length when measured with the MET 5035 instrument

3.METHOD OF MEASUREMENTS

3.1. The measurement of the insulation resistance is carried out with a megaohmmeter.

The megohmmeter consists of a DC generator or an AC generator with a rectifier, a ratiometer and additional resistance R1, designed to protect the device in case of insulation breakdown. The generator is rotated by hand or with the help of a converter

and outputs a voltage at the terminals, the value of which corresponds to the rated voltage of the megohmmeter. The current flowing through the device is inversely proportional to the value of the measured resistance Rx, so the scale of the device is calibrated directly in megaohms. In megohmmeters, a logometer is most often used, in which the uneven rotation of the generator has practically no effect on the readings of the device. This is due to the fact that the role of the counteracting spring in the ratiometers is played by a parallel winding connected to the output voltage of the generator through the resistor R2.

When measuring low resistances, the voltage applied to the measured insulation may be significantly lower than the nominal value.

3.2. To measure the resistance of the insulating floor, a square metal plate with a side of 250 mm is used. A wet cloth is placed between the metal plate and the surface to be measured. The plate is pressed against the surface of the floor or wall with a force of 25 kg. The insulation resistance is measured between the measuring plate and the protective conductor of the electrical installation.

4.SAFETY REQUIREMENTS

4.1. Before starting the tests, it is necessary to make sure that there are no people working on that part of the electrical installation to which the test device is connected and, if necessary, set up an observer.

4.2. The test site, as well as the connecting wires, which are under test voltage during the test, are fenced off.

4.3. A poster is posted on the fences and equipment "Trial. Dangerous for life"

4.4. After the end of the test, it is necessary to remove the residual charge from the equipment under test by means of its short-term (about 1 min.) grounding.

4.5. Connecting wires must have standard terminations and insulation resistance of at least 10 mΩ.

4.6. When measuring the insulation of the floor and walls in the measurement area, be in dielectric galoshes or boots. The plate is pressed against the wall with dielectric gloves.
5. PERSONNEL QUALIFICATION REQUIREMENTS

5.1. Tests are carried out by a team of at least two people, of which the foreman must have an electrical safety group not lower than IV, and the rest - not lower than III.

5.2. Tests can be carried out by personnel who have undergone special training and who have a mark on the admission to testing in the certificate of industrial safety.

5.3. The team conducting the tests may include persons from the repair personnel with the electrical safety group II to perform preparatory work, observation, as well as to disconnect and connect tires.

6. MEASUREMENT CONDITIONS

6.1. Measurement of insulation resistance should be carried out:

Between current-carrying conductors, taken in turn;

Between each current-carrying conductor and "ground".

(clause 612.3 GOST 50571.16-99)

6.2. Measurements should be made with electrical appliances disconnected, fuses removed.

6.3. When measuring insulation resistance in lighting circuits, the lamps must be unscrewed and the switches turned on.

Attention The norm for the replacement of the test without dismantling the lamp for measuring short-circuit currents made of PTEEP is excluded!

6.4. When measuring the insulation of half-stands, 3 measurements should be made (clause 612.5 of GOST 50571.16-99). One of the measurements should be made approximately 1 m from the external conductive parts.

6.5. The insulation resistance of floors, walls is measured before applying coatings (lacquer, paint, etc.) to the tested surfaces.

6.6. For boilers, the insulation resistance is measured in the position of the electrodes at maximum and minimum power.

6.7. Motor windings that are tightly interconnected and do not have the output of the ends of each phase or branch must be tested relative to the housing without disconnection (clause 3.6.17. PTEEP).

6.8. In operation, the insulation resistance of the windings of DC electrical machines is measured together with the circuits and cables connected to them (clause 24.2.1. appendix 3. PTEEP).

6.9. The insulation resistance of electric stoves is produced when they are heated.