Getting started cable insulation resistance measurement It is important to take into account the temperature indicators of the environment. Why is that?
This is due to the fact that at sub-zero temperatures, water molecules in the cable mass will be in a frozen state, actually 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 subzero temperature 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 point when measuring the insulation resistance of cable lines will be the measuring instruments themselves.
The most popular device for measuring insulation resistance among our electrical laboratory workers is the MIC-2500 device.
Using 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 wear and moisture level 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 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 scheduled verification and the serial number of the measuring device.
Only serviceable and verified instruments are allowed to work with insulation resistance measurements.
Insulation resistance standards for various cables.
To determine the standard cable insulation resistance, you need 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, secondary switchgear circuits, control circuits, power supply circuits for electric drives of switches, separators, short circuiters, etc.)
Insulation resistance measurement for both high-voltage cables and 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) - the insulation resistance must reach at least 10 (MOhm)
Low voltage power cables below 1000 (V) - insulation resistance should not fall below 0.5 (MΩ)
Control cables - insulation resistance should 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 taking measurements.
1. Check the absence of voltage on the cable using a high voltage indicator
2. We install a test ground using special clamps on the cable conductors on the side where we will carry out the measurement.
3. On the other side of the cable we leave free conductors, while we separate them at a sufficient distance from each other.
4. We place warning information posters. It is advisable to place a person on the other side to monitor safety while measuring with a megohmmeter.
5. We measure each core for 1 minute with a 2500 (V) megohmmeter to obtain the indicators insulation resistance of the power cable.
For example, we measure the insulation resistance on the conductor of phase “C”. At the same time, we place the grounding on the conductors of phases “B” and “A”. We connect one end of the megohmmeter to grounding, or, more simply, to “ground”. The second end is to the core of phase “C”.
Visually it looks like this:
6. We record measurement data in the process of work in a notebook.
Methodology for measuring insulation resistance of low-voltage power cables.
As for measuring the insulation of low-voltage power cables, the measurement technique differs slightly from that described above.
Likewise:
1. We check that there is no voltage on the cable using protective equipment designed for work in electrical installations.
2. On the other side of the cable, we separate the conductors at a sufficient distance from each other and leave them free.
3. We place prohibition and warning posters. We leave a person on the other side to monitor safety.
4.Measurement insulation resistance of low voltage power cable run a megohmmeter at 2500 (V) 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
6. We record the obtained insulation resistance measurements in a notebook.
Methodology for measuring the insulation resistance of control cables.
A special 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 the electrical equipment.
The insulation resistance of the control cable is measured using a familiar algorithm.
1. We check that there is no 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 of 500-2500 (V) in the following sequence.
First, 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. To the second terminal of the megohmmeter we connect either the ground or any other non-tested conductor.
We take 1 minute to measure the core being tested. Then we return this core to the rest of the cable cores and measure each core one by one.
3. We record all the obtained indicators for measuring the insulation resistance of the control cable in a notebook.
Protocol for measuring cable insulation resistance.
All of the above electrical measurements, after obtaining cable insulation resistance data, must be subjected to comparative analysis with the requirements and standards of 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 insulation resistance.
It is carried out with metal conductors in order to determine its performance. The quality of the signal transmitted through the conductors also depends on this indicator. The result of a decrease in insulation resistance, as a rule, is the appearance of interference on the line, which, in turn, leads to the appearance of audible noise (telephone line), a decrease in bandwidth(digital data transmission systems) or a complete interruption of the message.
According to GOST 15125-92, measurement of the insulation resistance of a communication cable should be carried out once every 6 months.
Communication cable insulation resistance standards
Electrical standards for communication cables determine the minimum resistance values of external insulation and core insulation at which cable products are allowed for use. The amount of resistance depends on the type and purpose of the cable.
Requirements for the insulation resistance values of cables put into operation are given in GOST 15125-92, OST 45.01-98, OST 45.83-96 and other regulatory and technical documentation. Let's look at a few examples.
Insulation resistance standards for communication cables most often used for the construction of primary networks, GTS and other lines (values per 1 km of cable length, without terminals / with terminal devices):
Cables with tubular-paper and porous-paper insulation (, etc.) - 8000/1000 MOhm.
. Polyethylene insulation (brands -, and others) - 6500/1000 MOhm.
. Cord-paper insulation (, etc.) - 10000/3000 MOhm.
Testing of communication cables
Measuring the insulation resistance of a communication cable is also carried out according to regulatory requirements. When performing this task, it is important to take into account the current temperature and humidity. All electrical parameters of communication cables are given by the manufacturers, subject to testing at a temperature of +20 °C and a cable product length of 1 km. Deviation of these parameters from the norm leads to an increase or decrease in readings. However, there are simple formulas that allow you to recalculate the resistance depending on temperature and length.
Equipment
The insulation resistance of a communication cable is measured using a special device called a megohmmeter. To determine the required electrical quantity, these devices generate a certain voltage (from 100 V or more).
Currently, two types of megohmmeters are used - digital and analog. In the first case, electromechanical (manual) generators and dial indicators are used to generate voltage. Digital megohmmeters usually use galvanic cells or rechargeable batteries. The measurement results are displayed on a digital display. Also, some megohmmeter models do not have their own current generator and require an external power source.
To test cable lines, reflectometers are also widely used, capable of detecting various cable defects using the location (reflectometric) method. The operating principle of the devices is as follows:
Short-wave electrical pulses are applied to the cores of the cable being tested.
. If there are any defects in the cable, the supplied pulse is reflected from the obstacle and returns back to the device.
. The returned signal is captured by the reflectometer sensors, measured, analyzed, and then the measurement result is displayed on the display.
Thus, with the help of reflectometers, it is possible to detect breaks, short circuits, mixed-up pairs, dense ground and other defects that occur, including when the cable insulation is damaged.
Requirements and testing methods for communication cables
Measuring the parameters of communication cables (insulation) is a simple process, but requires compliance with the requirements established by regulatory documentation (in particular, GOST 3345-76, GOST 2990-78). In short:
Before carrying out work, the cable must be de-energized and disconnected from all terminal devices and conductors (if it is, for example, a GTS cable, the tested conductors are disconnected from the terminals of the distribution boards).
. You cannot test with a megohmmeter over cables located in close proximity to other electrical systems, since the voltage generated by the device can create powerful electromagnetic fields that can disrupt the operation of these systems.
. It is impossible to test overhead communication lines during a thunderstorm.
. The tested conductors (cores) must be grounded.
. The test conductor can be disconnected from the ground only after it has been connected to the corresponding terminals of the megohmmeter (i.e., the device is first connected, and only then the wires are disconnected from the ground).
. Before and after measurements, the conductor must be cleared of residual current by short circuiting. This operation is also performed on the measuring probes of the megohmmeter.
. To obtain an accurate result, current is passed through the conductor under test for (and no more than!) 1 minute. After testing, the device and the tested conductor are allowed to “cool down” for 2 or more minutes, unless other numbers are given in the relevant documentation for the megohmmeter and/or cable.
. All other safety requirements are given in GOST 2990-78.
Now let's consider the process of measuring the insulation resistance of a communication cable using the example of a coaxial pair without a protective shield (we will measure the insulation resistance of the cores). According to GOST 2990-78, the conditional diagram for applying voltage to the cable cores is as follows:
Core “1” is connected to the “R-” input (the input can also be designated as “-”, “Ground” or “3”) of the megohmmeter.
. Core “1” and the “R-” input of the megohmmeter are grounded.
. Core “2” is connected to the voltage source input “R+” (“+”, “Rx”, “Line” or “L”) of the megohmmeter.
Conditional working diagram:
Measurement process:
First, the output voltage level is set on the megohmmeter, which depends on the brand of the cable being tested (usually, to test communication cables, it is enough to apply a voltage of 500 V).
. After applying voltage to the circuit, the megohmmeter will take about 1 minute to take measurements. If it is a pointer device, you must wait until it stops completely; for this, the megohmmeter must be stationary. In the case of digital devices, this is not necessary.
. If necessary, measurements are carried out several times. As mentioned above, before each procedure the device is allowed to “cool down” for about 2 minutes (plus or minus - depends on the characteristics of the megohmmeter).
The readings are greatly influenced by the ambient temperature (the higher it is, the lower the resistance and vice versa). If its value is different from +20 degrees, you must use the following “correction” formula:
R_(20)=K*R_1, where:
R_(20) - cable insulation resistance (in our case, core insulation resistance) at +20 °C (indicated in the data sheet for the cable brand);
R_1 - resistance obtained as a result of measurements at a temperature other than +20 ° C;
K is a “correction” coefficient that allows you to determine the value of insulation resistance that would occur at +20 °C (coefficients are given in the appendix to GOST 3345-76).
For example, let's take a polyethylene-insulated cable whose initial resistance (without termination devices) is 5000 MOhm. After measuring the resistance of the conductors at a temperature of 15 °C, we obtained a result of, say, 11,500 MOhm. According to GOST 3345-76, the correction factor “K” in the case of polyethylene insulation of cores is 0.48. Substituting this value into the formula, we have:
R_(20)=0.48*12500=5520 (resistance under normal conditions)
Using the following formula, you can determine the insulation resistance depending on the cable length:
R=R_(20)* l, where:
R_(20) - insulation resistance at +20 °C;
l is the length of the cable being tested;
Let's take the same brand of cable 1.5 km long. We know the initial insulation resistance of the cores under normal conditions - 5000 MOhm. From here:
R=6500* 1.5=7500 MOhm
The company "Kable.RF ®" is one of the leaders in the sale of cable products and has warehouses located in almost all regions Russian Federation. By consulting with the company’s specialists, you can purchase the brand you need at competitive prices.
Measurements of insulation resistance of wires, cables, power electrical equipment and devices.
1. Purpose of measurements.Measurements are carried out to verify compliance of insulation resistance with established standards.
2. Security measures.
2.1. Organizational events.
Insulation resistance measurements with a megohmmeter can be carried out in electrical installations with voltages above 1000 V by a team of at least two people, one of whom must have an electrical safety group of at least IV.
In electrical installations with voltages up to 1000 V, measurements are carried out by order of two workers, one of whom must have an electrical safety group of at least III.
In electrical installations up to 1000 V, located in premises, except those that are especially dangerous in terms of electric shock, an employee who has group III and the right to be a work performer can carry out measurements alone.
Measurements of the insulation resistance of the rotor of a running generator are allowed to be carried out by order of two workers with electrical safety groups IV and III.
2.2. Technical events.
The list of necessary technical measures is determined by the person issuing the order or order in accordance with Section 3 and Chapter 5.4. Interindustry rules on labor protection during the operation of electrical installations (IPBEE). Insulation resistance measurements with a megohmmeter should be carried out on disconnected live parts from which the charge has been removed by first grounding them. Grounding from live parts should be removed only after connecting the megohmmeter.
3. Standardized values.
The frequency of tests and the minimum permissible value of insulation resistance must comply with those specified in the testing standards for electrical equipment and devices of the Rules for the Construction of Electrical Installations (PUE), the Rules for the Technical Operation of Consumer Electrical Installations (PTEEP). In accordance with GOST R 50571.16-99, the standardized values of insulation resistance of electrical installations of buildings are given in Table 1.
Table 1.
*Resistance of stationary household electric stoves must be at least 1 MOhm.
In accordance with Ch. 1.8 PUE for electrical installations with voltages up to 1000 V, the permissible insulation resistance values are presented in Table 2.
Table 2.
| Test element | Megger voltage, V | Lowest permissible value of insulation resistance, MOhm |
|---|---|---|
| 1. DC buses on control panels and switchgears (with disconnected circuits) | 500-1000 | 10 |
| 2. Secondary circuits of each connection and power supply circuits for drives of switches and disconnectors * | 500-1000 | 1 |
| 3. Control, protection, automation and measurement circuits, as well as excitation circuits of DC machines connected to power circuits | 500-1000 | 1 |
| 4. Secondary circuits and elements when powered from a separate source or through an isolation transformer, designed for an operating voltage of 60 V and below ** | 500 | 0,5 |
| 5. Electrical wiring, including lighting networks *** | 1000 | 0,5 |
| 6. Switchgears ****, switchboards and busbars (busbars) | 500-1000 | 0,5 |
* The measurement is carried out with all connected devices (coils, wires, contactors, starters, circuit breakers, relays, devices, secondary windings of current and voltage transformers, etc.).
**Precautions must be taken to prevent damage to devices, especially microelectronic and semiconductor components.
*** Insulation resistance is measured between each wire and ground, as well as between every two wires.
**** The insulation resistance of each section of the switchgear is measured.
MIC-10 Electrical insulation parameter meter
|
Analysis of these requirements shows contradictions in terms of testing voltage and insulation resistance for secondary circuits with voltages up to 60 V (PUE, Chapter 1.8) and BSSN and FSSN systems included in this range (50 V and below), according to GOST 50571.16-99.
In addition, the resistance of the internal circuits of input distribution devices, floor and apartment panels of residential and public buildings in a cold state, in accordance with the requirements of GOST 51732-2001 and GOST 51628-2000, it must be at least 10 MOhm (according to the PUE, Chapter 1.8 - at least 0.5 MOhm).
4. Appliances used.
To measure insulation resistance, generator-type megohmmeters or digital meters with a voltage converter are used. Monitoring the accuracy of measurement results is ensured by annual verification of instruments in the bodies of the State Standard of the Russian Federation. Devices must have valid state verification certificates. Carrying out measurements with a device whose verification period has expired is not allowed.
5. Measuring the insulation resistance of electrical equipment.
5.1. Measuring the insulation resistance of power cables and electrical wiring.
When measuring insulation resistance, the following must be taken into account: measurement of the insulation resistance of cables (except for armored cables) with a cross-section of up to 16 mm² is carried out with a 1000 V megohmmeter, and above 16 mm² and armored cables - with a 2500 V megohmmeter; The insulation resistance of wires of all sections is measured with a 1000 V megohmmeter.
If electrical wiring in operation has an insulation resistance of less than 1 MOhm, then a conclusion about their unsuitability is made after testing them with alternating current of industrial frequency voltage of 1 kV.
5.2. Measuring the insulation resistance of power equipment.
The insulation resistance value of electrical machines and devices largely depends on temperature. Measurements should be made at an insulation temperature not lower than +5°C, except in cases specified in special instructions. At lower temperatures, the measurement results due to unstable moisture conditions do not reflect the true insulation performance. If there are significant differences between the measurement results at the installation site and the manufacturer's data due to the difference in temperature at which the measurements were taken, these results should be corrected according to the manufacturer's instructions.
The degree of insulation moisture is characterized by an absorption coefficient equal to the ratio of the measured insulation resistance 60 seconds after applying the megohmmeter voltage (R 60) to the measured insulation resistance after 15 seconds (R 15), while:
K abs =R 60 /R 15
When measuring the insulation resistance of power transformers, megohmmeters with an output voltage of 2500 V are used. Measurements are taken between each winding and the housing and between the windings of the transformer. In this case, R 60 must be adjusted 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 (downwards) 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.
Insulation resistance circuit breakers and RCDs are produced:
1. Between each pole terminal and the opposite pole terminals connected to each other when the switch 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 in metal foil.
At the same time, for circuit breakers for household or similar purposes (GOST R 50345-99) and RCDs when measuring according to paragraphs. 1, 2 the insulation resistance must be at least 2 MOhm, for 3 - at least 5 MOhm.
For other circuit breakers (GOST R 50030.2-99), in all cases the insulation resistance must be at least 0.5 MOhm.
5.3. The procedure for carrying out measurements.
When measuring insulation resistance, it should be taken into account that to connect the megohmmeter to the object being tested, it is necessary to use flexible wires with insulating handles at the ends and restrictive rings in front of the contact probes. The length of the connecting wires must be minimal based on the measurement conditions, and their insulation resistance must be at least 10 MOhm.
Megohmmeter measurements are carried out in the following sequence:
- check the absence of voltage on the tested object;
- clean the insulation from dust and dirt near the connection of the megohmmeter to the test object;
- connect the tested object to the sockets;
- select the output voltage corresponding to the object being tested;
- to carry out measurements, rotate the generator handle at a speed of 120-140 revolutions per minute (generator-type megohmmeter) or press the measurement start button (digital meter);
- take megohmmeter readings.
Attention! After each measurement, it is necessary to remove the capacitive charge by briefly grounding the parts of the test object to which the megohmmeter output voltage was applied.
The measurement results are documented in protocols.
Any type of wires and cables has specific, primary and secondary electrical parameters that characterize these products. One of the main parameters of a cable is its insulation resistance. The insulation resistance standard is considered to be the data that is used to guide the construction, operation and maintenance of cables.
Flows through two metal wires electricity, and they are constantly exposed to various environmental influences, in some cases even dangerous. In addition, these veins themselves influence each other. As a result, metal wires that do not have protection suffer colossal losses due to various leaks, even leading to emergency situations.
What is insulation of conductors
To ensure that such negative situations are minimized or significantly reduced, the conductive cores in the cables should be protected with an insulating coating made of a material that does not conduct electrical current.
Material to create insulating shells are considered:
- plastics;
- paper;
- rubber.
These materials can also be combined. Insulation that is used for different types cables, has quite a significant difference both in the materials used and in the principles of using insulating covers. Today, a large number of cable products are produced, which are used for a variety of needs.
Variety of cable products
Cables are distinguished:
These products may differ from each other not only in their functions, but also design and physical characteristics designed for the environment in which it will be used. The great need for wire materials needed for a variety of needs has led to the creation of various modifications currently existing cable types. For example, if underground telephone distribution networks are laid directly in the ground, the cable structure used in the telephone sewer system is further strengthened by enclosing their core in metal armor tapes. And also to protect the cable cores from external currents, its core is encased in an aluminum sheath.
What is insulation resistance
The type of insulating material depends on the environment and conditions in which the manufactured conductor products will be used. For example, to insulate conductors at high temperatures, it is better to use rubber than other materials. Rubber is resistant to such temperature influences than, for example, conventional plastic.
Thus, the use of insulating materials for cable products is necessary to protect its conductors from external and mutual electrical influences. The value of this parameter for an individual core and the entire core as a whole is determined by the amount of direct current resistance that occurs in the circuit between the cores and some source, for example, the ground. To determine the performance and security of cable products, the term “insulation resistance” is used.
Materials used in cables as insulation, over time grow old and begin to lose their properties. Therefore, even from any physical impact they can collapse. In order to clarify how and within what limits the parameters of the insulating material could change, it is required for comparison to know the standard for the product parameter, which is set by the manufacturer.
Insulation resistance standard
What is the specific value of the product insulation resistance for different brands of cable? laid down in GOST or TU for the production of certain cable products. Such products supplied for sale must have a passport with electrical parameters. For example, the insulation resistance standard for a communication cable is reduced to 1 km in length, and the ambient temperature for this data should be +20 degrees.
For urban low-frequency communication cables, the resistance standard must be at least 5000 MOhm/km; for coaxial and trunk symmetrical cables, the resistance standard can reach 10000 Mohm/km. When assessing the condition of the cable being tested, the insulation resistance rating data is used only when it is necessary to recalculate them to the length of the actual piece of cable. If the cable section is more than a kilometer, the norm should be divided by this length. If it is less than a kilometer, then multiply accordingly.
The resulting estimated figures are often used to estimate cable line. It should be remembered that the passport data is taken into account for a temperature of +20 degrees, so it is necessary to make corrections by taking control measurements for humidity and temperature.
There are brands of cable products that have an aluminum sheath and a polyethylene hose covering. For them, the standard of insulation resistance between the ground and the shell is determined. It is usually 20 Mohm/km. To use this standard in work, it must be recalculated to the actual length of the section.
For power cable The following provisions are provided for DC insulation resistance:
- for power cables used in networks with voltages of more than 1000 V, the value of this parameter is not standardized, but cannot be less than 10 ohms;
- for power cables used in networks with voltages less than 1000 V, the parameter value should not be higher than 0.5 Ohm.
For control cables the norm is cannot be less than 1 ohm.
This document was developed for electrical technical personnel of electrical laboratories, electrical technical sections of industrial facilities, carrying out work to measure the insulation resistance of electrical equipment, wires and cables in existing and reconstructed electrical installations for all electricity consumers, regardless of their departmental affiliation.
2. REGULATORY REFERENCES
- Rules for the technical operation of consumer electrical installations, 1992;
- Safety regulations for the operation of consumer electrical installations, 1994;
- Electrical Installation Regulations 1986;
- Standards for testing electrical equipment and devices of consumer electrical installations, 1982;
- Electrical Test Code 1978;
- GOST 26567-85. Semiconductor power converters. Test methods;
- GOST 3345-76. Cables, wires and cords. Determination method electrical resistance isolation;
- GOST 3484-88. Power transformers. Electromagnetic testing methods;
- GOST 3484.3-83. Power transformers. Methods for measuring dielectric parameters of insulation.
3.DEFINITIONS
3.1. This methodology uses the terms established in GOST 3345-76, GOST 3484.3-83, GOST 3484.1-88, GOST 16504, GOST 23875.
Switchgear - a switchgear of the generator voltage of a power plant or the secondary voltage of a step-down substation of a district (enterprise) to which the networks of the district (enterprise) are connected.
Designations and abbreviations:
- HV - high voltage windings;
- MV - medium voltage windings;
- LV - low voltage windings;
- NN1, NN2 - low voltage windings of transformers with split windings;
- R15 - fifteen second value of insulation resistance in MOhm;
- R60 - one-minute insulation resistance value in MOhm;
- PEEP - operating rules for consumer electrical installations;
- PTBEEP - safety regulations for the operation of consumer electrical installations;
- PUE - Rules for electrical installations.
4. MEASUREMENT PROCEDURE
4.1 Measurable indicators
Insulation resistance is measured with megohm meters (100-2500V) with measured values in Ohm, kOhm and MOhm.
4.2 Measuring instruments
Insulation measuring instruments include megohmmeters: ESO 202, F4100, M4100/1-M4100/5, M4107/1, M4107/2, F4101. F4102/1, F4102/2, BM200/G and others, produced by domestic and foreign companies.
4.3 Qualification requirements
Trained electrical personnel who have a certificate of knowledge testing and a qualification group for electrical safety of at least 3rd, when performing measurements in installations up to 1000 V, and not lower than 4th, when measuring in installations above 1000 V, are allowed to perform insulation resistance measurements.
Persons from electrical engineering personnel with secondary or higher specialized education may be allowed to process measurement results.
Analysis of measurement results should be carried out by personnel involved in the insulation of electrical equipment, cables and wires.
5. SAFETY REQUIREMENTS
- When performing insulation resistance measurements, safety requirements must be met in accordance with GOST 12.3.019.80, GOST 12.2.007-75, Rules for the operation of consumer electrical installations and Safety rules for the operation of consumer electrical installations.
- The premises used for insulation measurements must meet explosion and fire safety according to GOST 12.01.004-91.
- Measuring instruments must meet the safety requirements in accordance with GOST 2226182.
- Megger measurements may only be carried out by trained electrical personnel. In installations with voltages above 1000 V, measurements are carried out by two persons at a time, one of whom must have electrical safety ratings of at least IV. Carrying out measurements during installation or repair is specified in the work order in the line “Entrusted”. In installations with voltages up to 1000 V, measurements are carried out by order of two persons, one of whom must have a group of at least III. An exception is the tests specified in clause BZ.7.20.
- Measuring the insulation of a line that can receive voltage from both sides is permitted only if a message is received from the responsible person of the electrical installation that is connected to the other end of this line by telephone, by messenger, etc. (with a reverse check) that the line disconnectors and switch are turned off and a poster “Do not turn on. People are working” is posted.
- Before starting 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, to prohibit persons located near it from touching live parts and, if necessary, to set up security.
- To monitor the insulation condition electric machines in accordance with methodological instructions or measurement programs with a megohmmeter on a stopped or rotating, but not excited machine, can be carried out by operating personnel or, by their order, in the order of routine operation by employees of the electrical laboratory. Under the supervision of operating personnel, these measurements can also be performed by maintenance personnel. Tests of insulation of rotors, armatures and excitation circuits can be carried out by one person with an electrical safety group of at least III, tests of stator insulation - by at least two persons, one of whom must have a group of at least IV, and the second - not lower than III.
- When working with a megger, touching the live parts to which it is connected is prohibited. After completion of work, it is necessary to remove the residual charge from the equipment being tested by briefly grounding it. The person removing the residual charge must wear dielectric gloves and stand on an insulated base.
- It is prohibited to take measurements with a megger: on one circuit of double-circuit lines with a voltage above 1000 V, while the other circuit is energized; on a single-circuit line, if it runs in parallel with a working line with a voltage above 1000 V; during a thunderstorm or when it is approaching.
- Measuring the insulation resistance with a megger is carried out on disconnected current-carrying parts from which the charge has been removed by first grounding them. Grounding from live parts should be removed only after connecting the megger. When removing grounding, you must use dielectric gloves.
6. CONDITIONS FOR PERFORMING MEASUREMENTS
- Insulation measurements must be carried out under normal climatic conditions in accordance with GOST 15150-85 and under normal power supply conditions or as specified in the manufacturer's data sheet - technical description for megohmmeters.
- The value of the electrical insulation resistance of the connecting wires of the measuring circuit must exceed at least 20 times the minimum permissible value of the electrical insulation resistance of the product under test.
- The measurement is carried out indoors at a temperature of 25±10 °C and a relative air humidity of no more than 80%, unless other conditions are provided for in the standards or technical specifications for cables, wires, cords and equipment.
7. PREPARATION FOR MEASUREMENTS
- They check the climatic conditions at the place where the insulation resistance is measured with the measurement of temperature and humidity and the compliance of the room with explosion and fire hazards to select a megger for the appropriate conditions.
- The condition of the selected megohmmeter, connecting conductors, and the operability of the megohmmeter are checked by external inspection in accordance with the technical description for the megohmmeter.
- Check the validity period of the state verification on the megohmmeter.
- The preparation of measurements of cable and wire samples is carried out in accordance with GOST 3345-76.
- When performing periodic preventive work in electrical installations, as well as when performing work on reconstructed facilities in electrical installations, the preparation of the workplace is carried out by the electrical technical personnel of the enterprise where the work is performed in accordance with the rules of PTBEEEP and PEEP.
8. TAKE MEASUREMENTS
The reading of the electrical insulation resistance values during measurement is carried out after 1 minute from the moment the measuring voltage is applied to the sample, but not more than 5 minutes, unless other requirements are provided for in the standards or technical conditions for specific cable products or other equipment being measured.
Before re-measuring everything metal elements cable product must be grounded for at least 2 minutes.
The electrical insulation resistance of individual cores of single-core cables, wires and cords must be measured:
- for products without a metal sheath, screen and armor - between the conductor and the metal rod or between the conductor and grounding;
- for products with a metal shell, screen and armor - between the conductive conductor and the metal shell or screen, or armor.
The electrical insulation resistance of multi-core cables, wires and cords must be measured:
- for products without a metal sheath, screen and armor - between each current-carrying conductor and the remaining conductors connected to each other or between each conductive conductor; residential and other conductors connected to each other and grounding;
- for products with a metal shell, screen and armor - between each current-carrying conductor and the remaining conductors connected to each other and to the metal shell or screen, or armor.
If the insulation resistance of cables, wires and cords is lower than the normative rules of PUE, PEEP, GOST, it is necessary to perform repeated measurements by disconnecting the cables, wires and cords from the consumer terminals and separating the current-carrying conductors.
When measuring the insulation resistance of individual samples of cables, wires and cords, they must be selected for construction lengths, wound on drums or in coils, or samples with a length of at least 10 m, excluding the length of end cuts, if in the standards or technical specifications for cables, wires and cords are not specified in other lengths. The number of construction lengths and samples for measurement must be specified in the standards or technical specifications for cables, wires and cords.
9. CONVERTER ISOLATION MEASUREMENT
9.1. Measurement of electrical resistance and insulation of converters is carried out in accordance with the requirements of this standard, and when exposed to climatic factors, measurement of insulation resistance is carried out taking into account GOST/16962-71.
Measuring instruments: megohmmeters and ohmmeters according to GOST 16862-71.
Electrical insulation resistance is measured:
- in normal climatic conditions; at the upper value of the ambient temperature after thermal equilibrium has been established in the converter;
- at the upper value of relative humidity.
Insulation resistance is measured between electrically unconnected circuits, electrical circuits and the housing. In the specifications or design documentation for converters of specific series and types, pins are indicated between which the resistance and value should be measured DC voltage, at which this measurement is carried out. If one of the terminals or elements according to the circuit is connected to the housing, then this circuit must be disconnected for the duration of the tests.
When measuring the insulation resistance of converters, the following conditions must be met:
Table 1.
- before testing, the converter must be disconnected from external power supplies and load;
- the input (output) terminals of the converter, capacitors connected to power circuits, as well as anode, cathode and control terminals of power semiconductor devices must be connected to each other or shunted;
- contacts of switching equipment of power circuits must be closed or bypassed;
- electrical circuits containing semiconductor devices and microcircuits must be disconnected and, if necessary, tested separately;
- The voltage of the measuring device when measuring insulation resistance, depending on the nominal (amplitude) value of the circuit voltage, is selected according to the table. 1.
If necessary, the insulation resistance is measured at higher voltages, but not exceeding the test voltage of the circuit.
Measuring the insulation resistance of converters consisting of several cabinets can be carried out separately for each cabinet.
If the insulation resistance of each cabinet and (or) structural unit of the converter is measured, then the value of the insulation resistance of each cabinet and (or) structural unit must be indicated in the specifications for converters of specific series and types.
The values of the minimum permissible insulation resistance for power cables, switches, load switches, disconnectors, valve arresters, dry-type reactors, instrument transformers, 6-10 kV indoor switchgear, AC motors, stationary, mobile and complete testing devices are given in Table. 2.
10. PROCESSING OF MEASUREMENT RESULTS
10.1. If the measurement for cable products was carried out at a temperature other than 20 °C, and required by standards or technical specifications for specific cable products, the value of the electrical insulation resistance is normalized at a temperature of 20 °C, then the measured value of the electrical insulation resistance is recalculated to a temperature of 20 °C using the formula:
where R20 is the electrical insulation resistance at a temperature of 20 °C, MOhm;
Rt - electrical insulation resistance at measurement temperature, MOhm;
K is the coefficient for bringing the electrical insulation resistance to a temperature of 20 °C, the values of which are given in the appendix to this standard.
In the absence of conversion factors, the arbitration method is to measure the electrical resistance of the insulation at a temperature of (20±1)°C.
10.2. The recalculation of the electrical insulation resistance R for a length of 1 km should be carried out according to the formula:
R=R20L,
where R20 is the electrical insulation resistance at a temperature of 20 °C, MOhm;
L is the length of the tested product without taking into account the end sections, km.
Coefficient K for reducing the electrical resistance of insulation to a temperature of 20 °C.
The error in the insulation resistance value is calculated according to the recommendations specified in technical descriptions and operating instructions for megohmmeters, taking into account external influencing factors.
11. REGISTRATION OF MEASUREMENT RESULTS
The measurement results are included in test reports for cables up to and above 1000 V, as well as in protocols for preventive adjustment work on relay protection and electrical equipment.
Table 2.
Name of insulation resistance measurements |
Standardized value, Mohm, not less | Megger voltage, V | Directions |
| Power cables above 1000 V | Not standardized | 2500 | When testing with increased voltage, the insulation resistance R60 must be the same before and after the test |
| Power cables up to 1000V | 1 | 1000 | |
| Oil switches: | |||
| 1. Movable and guides | |||
| parts made of organic material. 3-10kV, | 300 | 2500 | |
| 15-150kV | 1000 | ||
| 220kV | 3000 | ||
| 2. Secondary circuits, including turning on and off coils. |
1 | 1000 | |
| H.Load switches: measuring the insulation resistance of the closing and disconnecting coils | 1 | 500-1000 | The insulation resistance of the power section is not measured, but is tested with increased power frequency voltage |
| 4. Disconnectors, short circuiters and separators: | Produced only at positive ambient temperatures | ||
| 1.Leash rods made | |||
| from organic materials | |||
| 3-10kV | 300 | 2500 | |
| 15-150kV | 1000 | 2500 | |
| 220kV | 3000 | 2500 | |
| Element resistance measurement valve arrester for voltage: |
Arrester resistance or its element should differ by no more than 30% of measurement results |
||
| above 3 kV and above | 2500 | ||
| less than 3 kV | 1000 | at the manufacturer or previous measurements during operation | |
| Dry reactors. Winding resistance measurement relative to fastening bolts |
0,5 | 1000-500 | After major renovation. |
| 0,1 | 1000-500 | In use | |
| Instrument transformers voltage above 1000V: |
Not standardized. | 2 500 | When assessing the condition of the secondary windings, you can focus on the following average resistance values of a serviceable winding: for built-in CTs - 10 MOhm, for remote CTs - 50 MOhm |
| primary windings, secondary windings |
Not lower than 1 together with connected chains |
1000 | |
| Switchgear 3-10 kV: primary circuits secondary circuits |
300 | 2 500 | The measurement is performed at fully assembled chains |
| 1 | 500-1000 V | ||
| AC electric motors current above 660 V |
Not | Must be taken into account when drying is required. | |
| normalized | 2500 | ||
| exchange stator. up to 660 V | 1 | 1000 | |
| The stator windings of the electric engines for voltages above 3000 V or power more than 3000 kW |
R60/R15 | 2500 | Produced at synchronous engines and asynchronous motors with a wound rotor voltage of 3000 V and above or power above 1000 kW |
| I don’t standardize | 1000V | ||
| Rotor windings | yes | ||
| Stationary, mobile, portable complete testing installations. | Not standardized | 2500 | |
| Circuit insulation measurements and equipment e.g. above 1000V. |
|||
| Circuits and equipment for voltage up to 1000 V |
1 | 1000 | |
| DC machines: | Winding insulation resistance | ||
| measurement of insulation of windings and bands up to 500V, | 0,5 | 500 | measured relative to the body, and bandages - relative to the body and |
| above 500V | 1 000 | windings held by it together with the circuits and cables connected to them | |
| Power and lighting wiring | 0,5 | 1000 | |
| Distribution devices, boards and conductors |
0,5 | 1000 | |
| Secondary control circuits, protection and automation DC buses |
1 | 500-1000 | |
| 10 | 500-1000 | ||
| Each connection of secondary circuits and power supply circuits of drives switches |
1 | 500-1000 | |
| Control, protection, automation, telemechanics, excitation circuits machines post. current to voltage 500-1000V, connected to the main switchgear circuits |
1 | 500-1000 | Circuit insulation resistance voltage up to 60 V, normal but feeding from separate sources, measured meg- meter at 500 V and must be at least 0.5 MOhm |
| Circuits containing devices with microelectronic elements: |
|||
| above 60 V | 0,5 | 500 | |
| 60 and below | 0,5 | 100 |