Cable insulation resistance standard. Insulation standard for cable communication line

And cables have 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 electric current, 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.

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 electric 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 of existing ones. at the moment types of cables. 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 isolate when high temperatures conductors, 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 resistance value DC, occurring in a circuit between the conductors 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 anyone physical impact they may 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 of length, and the temperature environment for these 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 evaluate a 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 those used in networks with voltages over 1000 V power cables the value of such a parameter is not standardized, but cannot be less than 10 OM;
• 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 cannot be less than 1 ohm.

Those who are constantly engaged in measuring the insulation of new cables over time develop a disregard for the exact value of this parameter. Life teaches. Today the insulation of the cable line is more than 30,000 MOhm; tomorrow in the morning on the same cable it is 800 MOhm, and in the evening it is 16,000.

The insulation value is highly dependent on air temperature and humidity. I left the distribution cabinet open for several hours on a damp morning, and voila, the insulation dropped to 400 megohms. That is, the figure floats within very wide limits, and management often does not want to understand how unstable the insulation is and requires accurate values.

As a rule, smart meters quickly realize that it is better to estimate several pairs from one cable, and write any numbers that correspond to the standard in the protocol. It is better to judge the integrity of the cable sheath by the shield-to-ground insulation, but you cannot check the correct assembly of the boxes by measuring the insulation. In fact, there is a self-filling protocol for them on the website.

How to achieve good insulation of a new cable line

Several times I came across a situation where, during acceptance measurements, the receiving side was not satisfied with an insulation of, say, 800 MOhm; after all, this is “not the norm” and, as a rule, the young meter began to be indignant. In this case, experienced solders usually carry out emergency drying. In the distribution cabinet, the supplied cable plinths are carefully heated with a blowtorch or gas torch.

The insulation is quickly restored to several thousand megohms, the solders call the meter, he measures it and sometimes is even surprised how the guys quickly repaired the damage.

Poor insulation at terminal units usually indicates a leak in the bottom seal of the control cabinet. About the reasons for baseboards getting damp, see the page “Why do baseboards get damp in ShR, how to dry them, how to increase insulation”

To find out more precisely what a decrease in insulation gives, you can disconnect the core from the plinth and separately measure it relative to the “ground”

In operation, the insulation of terminal devices can even drop to several kilo-ohms and at the same time the green oxides on the plinths become noticeable

Insulation standard for a new cable line

During acceptance measurements, the standard for a TPP cable with terminal devices is considered to be an insulation resistance of 1000 MOhm for lines less than 1 km. That is, for 20 meters and 1 kilometer of cable the norm is the same, and usually no one climbs into the jungle described below. They check the isolation of several couples and, without further ado, sign protocols and acts. More attention is paid to continuity, screen insulation and correct assembly of plinths.

However, several times I have encountered electromechanics and engineers who read the regulatory and technical documentation more carefully and notice that the insulation standard is indicated for 1 km of circuit. From this they conclude that a cable line 500 meters long should have an insulation of 2000 MOhm, and a cable line 50 meters long should have an insulation of 20000 MOhm. It’s difficult to argue with them, and trying to somehow reason with these “nerds” I asked the question, how much transmission insulation should be between cabinets 5 meters long? The figure of 200,000 MOhm usually casts doubt on the logic of such calculations.

Responding to one of the letters about the insulation standard, I came up with a formula for calculating this standard. And although the data for the calculation were taken from an official document, the derived formula should be treated as a joke and considered the norm in new line less than a kilometer long 1000 MOhm.

By the way, for some reason this is not specified in some instructions “from above” sent to polling stations.

Derivation of the formula for calculating the cable line insulation standard

A cable line with terminal devices can be represented as three parallel resistance, Where
R i1p And R i2p - these are the insulation resistances of the first and second plinths,
R IR - insulation resistance of the cable core

R and Kl - the resistance of the entire cable line will be derived from the formula for calculating parallel resistances:

R i1p could be taken from the "Manual for the operation of linear cable structures local networks communications, 1998" (Appendix 6. Electrical insulation resistance of terminal cable devices and elements) but there the insulation resistance of the plinth of 3500 MOhm is given only for the insulation standard short lines- 1000 MOhm.

GOST 3345-76

Group E49

INTERSTATE STANDARD

CABLES, WIRES AND CORDS

Determination method electrical resistance isolation

Cables, wires and cords.
Determination of insulation electric resistance

ISS 29.060.01

Date of introduction 1978-01-01

INFORMATION DATA

1. DEVELOPED AND INTRODUCED by the Ministry of Electrical Engineering of the USSR

2. APPROVED AND ENTERED INTO EFFECT by the Resolution State Committee standards of the Council of Ministers of the USSR dated June 23, 1976 N 1508

3. The standard fully complies with ST SEV 2784-80

4. INSTEAD GOST 3345-67

5. The validity period was lifted according to Protocol No. 3-93 of the Interstate Council for Standardization, Metrology and Certification (IUS No. 5-6, 1993)

6. EDITION with Amendments No. 1, 2, approved in September 1981, June 1988 (IUS 11-81, 10-88)


This standard applies to cables, wires and cords (hereinafter referred to as products) and establishes a method for determining their electrical insulation resistance at DC voltage.

1. SAMPLING METHOD

1. SAMPLING METHOD

1.1. For measurement, construction lengths of cables, wires and cords wound on drums or coils, or samples with a length of at least 10 m, excluding the length of end cuts, if in the standards or technical conditions No other length is specified for cables, wires and cords.

1.2. The number of construction lengths and samples for measurement must be specified in the standards or technical specifications for cables, wires and cords.

2. EQUIPMENT

2.1. Measurement of electrical insulation resistance is carried out at a voltage from 100 to 1000 V, unless other conditions are specified in the standards or technical specifications for cables, wires and cords.

The measurement is carried out using measuring circuits and instruments that provide measurements with an error of no more than 10% of the measured values ​​from 1·10 to 1·10 Ohm, no more than 20% of the measured values ​​above 1·10 to 1·10 Ohm and no more than 25% of the measured values. values ​​above 1·10 Ohm. If standards or technical specifications for cables, wires and cords allow measurements to be taken on short (less than 10 m) product samples, then the error of such measurements should not be more than 10% for any measured insulation resistance values.

(Changed edition, Amendment No. 1, 2).

2.2. Electrical insulation resistance value of connecting wires measuring circuit must exceed at least 20 times the minimum permissible value of the electrical insulation resistance of the product being tested.

2.3. The measurement installation must comply with the requirements for installations with voltages up to 1000 V and must ensure the safety of measurements.

3. PREPARATION AND CARRYING OUT MEASUREMENTS

3.1. If necessary, the ends of the test product must be cut before measurement.

To increase the measurement accuracy, it is allowed to install guard rings on the end grooves, which must be grounded during measurement or connected to the screen of the measuring circuit.

3.2. The measurement is carried out at an ambient temperature (20±15) °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 and cords, or in water.

3.3. The ambient temperature is measured with an error of no more than ±0.5 °C at a distance of no more than 1 m from the product being tested.

The error in measuring the water temperature throughout the entire volume should be no more than ±2 °C if measurements are carried out at a temperature of St. 20 °C, and no more ± 1 °C if measurements are carried out at a temperature of 20 °C.

The water temperature during measurement should be the same throughout the entire volume.

3.4. The holding time of samples before testing at ambient temperature must be at least 1 hour, unless a different holding time is specified in the standards or technical specifications for specific cable products.

3.3, 3.4. (Changed edition, Amendment No. 1).

3.5. When measuring the electrical insulation resistance of cables, wires and cords at construction lengths wound on drums or coils, the diameters of the necks of the drums or coils must comply with those specified in the standards or technical specifications for cables, wires and cords.

3.6. If the measurement of electrical insulation resistance is provided on a metal rod, then the test sample must be wound with turns tightly adjacent to each other and the rod with a tension force of at least 20 N per 1 mm of the nominal cross-section of the core.

The diameter of the rod must be specified in the standards or specifications for cables, wires and cords.

3.7. If the measurement of electrical insulation resistance is carried out in water, then the ends of the test sample must protrude above the water by at least 200 mm, including the length of the insulated part by at least 100 mm, and the length of the metal shell, screens and armor - by at least 50 mm.

3.8. The electrical insulation resistance of individual cores and single-core cables, wires and cords must be measured:

- for products without a metal sheath, screen and armor - between a conductor and a metal rod or between a conductor and water;

- for products with a metal shell, screen and armor - between the conductive conductor and the metal shell or screen, or armor.

3.9. 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 conductor and the remaining conductors connected to each other or between each conductor and the remaining conductors connected to each other and to water;

- 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.

3.10. For repeated measurements, the product under test must be discharged for at least 2 minutes by connecting conductor with a grounding device (subject to safety regulations).

3.11. Readings of the electrical insulation resistance values ​​during measurement are 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 specifications for specific cable products.

Before re-measurement, all metal elements of the cable product must be grounded for at least 2 minutes.

4. PROCESSING RESULTS

4.1. If the measurement was carried out at a temperature different from 20 °C, and the value of electrical insulation resistance required by standards or technical specifications for specific cable products is normalized at a temperature of 20 °C, then the measured value of electrical insulation resistance is recalculated to a temperature of 20 °C using the formula

- electrical insulation resistance at measurement temperature, MOhm;

- coefficient for bringing the electrical insulation resistance to a temperature of 20 °C, the values ​​of which are adjusted 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.

(Changed edition, Amendment No. 1).

4.2. Recalculation of the electrical insulation resistance for a length of 1 km should be carried out according to the formula

where is the electrical insulation resistance at a temperature of 20 °C, MOhm;

- length of the tested product excluding end sections, km.

The length of the product must be determined with an accuracy of 1%.

(Changed edition, Amendment No. 2).

APPENDIX (required). Coefficient K for reducing the electrical insulation resistance to a temperature of 20 °C

APPLICATION
Mandatory

Coefficient of reduction of electrical insulation resistance to a temperature of 20 °C

The text of the document is verified according to:
official publication
Cables, wires and cords.
Test methods: Sat.GOSTov.-
M.: IPK Standards Publishing House, 2003

CABLES, WIRES AND CORDS

METHOD FOR DETERMINING ELECTRICAL INSULATION RESISTANCE

GOST 3345-76
(ST SEV 2784-80)

COMMITTEE OF STANDARDIZATION AND METROLOGY OF THE USSR

Moscow

STATE STANDARD OF THE USSR UNION

Validity period from 01/01/78

until 01/01/94

This standard applies to cables, wires and cords (hereinafter referred to as “products”) and establishes a method for determining their electrical insulation resistance at direct current voltage.

1. SAMPLING METHOD

1.1. For measurement, construction lengths of cables, wires and cords, wound on drums or in coils, or samples with a length of at least 10 m should be selected, excluding the length of the end cuts, unless a different length is specified in the standards or technical specifications for cables, wires and cords.

1.2. The number of construction lengths and samples for measurement must be specified in the standards or technical specifications for cables, wires and cords.

2. EQUIPMENT

2.1. Measurement of electrical insulation resistance is carried out at a voltage from 100 to 1000 V, unless other conditions are specified in the standards or technical specifications for cables, wires and cords.

The measurement is carried out using measuring circuits and instruments that provide measurements with an error of no more than 10% of the measured values ​​from 1× 10 5 to 1 × 10 10 Ohm, no more than 20% of the measured values ​​of light. 1× 10 10 to 1 × 10 14 Ohm and no more than 25% of the measured values ​​of St. 1× 10 14 Ohm. If standards or technical specifications for cables, wires and cords allow measurements to be made on short (less than 10 m) product samples, then the error of such measurements should not be more than 10% for any measured insulation resistance values.

(Changed edition, Amendment No. 1, 2).

2.2. 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.

2.3. The measurement installation must comply with the requirements for installations with voltages up to 1000 V and must ensure the safety of measurements.

3. PREPARATION AND CARRYING OUT MEASUREMENTS

3.1. If necessary, the ends of the test product must be cut before measurement.

To increase the measurement accuracy, it is allowed to install guard rings on the end grooves, which must be grounded during measurement or connected to the screen of the measuring circuit.

3.2. The measurement is carried out at an ambient temperature (20±15) °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 and cords, or in water.

3.3. The ambient temperature is measured with an error of no more than ±0.5 °C at a distance of no more than 1 m from the product being tested.

The error in measuring the water temperature throughout the entire volume should be no more than ±2 °C if measurements are carried out at a temperature of St. 20 °C and no more than ±1 °C if measurements are carried out at a temperature of 20 °C.

The water temperature during measurement should be the same throughout the entire volume.

3.4. The holding time of samples before testing at ambient temperature must be at least 1 hour, unless a different holding time is specified in the standards or technical specifications for specific cable products.

3.3, 3.4.

3.5. When measuring the electrical insulation resistance of cables, wires and cords at construction lengths wound on drums or coils, the diameters of the necks of the drums or coils must comply with those specified in the standards or technical specifications for cables, wires and cords.

3.6. If the measurement of electrical insulation resistance is provided on a metal rod, then the test sample must be wound with turns tightly adjacent to each other and the rod with a tension of at least 20 N per 1 mm 2 of the nominal cross-section of the core.

The diameter of the rod must be specified in the standards or specifications for cables, wires and cords.

3.7. If the measurement of electrical insulation resistance is carried out in water, then the ends of the test sample must protrude above the water by at least 200 mm, including the length of the insulated part by at least 100 mm, and the length of the metal shell, screens and armor - at least by 50 mm.

3.8. The electrical insulation resistance of individual cores and 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 water;

for products with a metal shell, screen and armor - between the conductive conductor and the metal shell or screen, or armor.

3.9. 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 conductor and the remaining conductors connected to each other or between each conductor and the remaining conductors connected to each other and to water;

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.

3.10. For repeated measurements, the test product must be discharged for at least 2 minutes by connecting the current-carrying conductor to a grounding device (subject to safety regulations).

3.11. Readings of the electrical insulation resistance values ​​during measurement are 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 specifications for specific cable products.

Before re-use, all metal elements of the cable product must be grounded for at least 2 minutes.

(Changed edition, Amendment No. 1).

4. PROCESSING RESULTS

4.1. If the measurement was carried out at a temperature different from 20 °C, and the value of electrical insulation resistance required by standards or technical specifications for specific cable products is normalized at a temperature of 20 °C, then the measured value of electrical insulation resistance is recalculated to a temperature of 20 °C using the formula

R 20 = KR t,

Where R 20

R t - electrical insulation resistance at measurement temperature, MOhm;

TO- 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 insulation resistance at a temperature (20± 1) °C.

(Changed edition, Amendment No. 1).

4.2. Recalculation of electrical insulation resistanceRfor a length of 1 km should be carried out according to the formula

R = R 20 l,

Where R 20 - electrical insulation resistance at a temperature of 20 °C, MOhm,

l- length of the tested product excluding end sections, km

The length of the product must be determined with an accuracy of 1%.

(Changed edition, Amendment No. 2).

APPLICATION
Mandatory

Coefficient K bringing the electrical insulation resistance to a temperature of 20 ° WITH

INFORMATION DATA

1. DEVELOPED AND INTRODUCED by the Ministry of Electrical Engineering of the USSR

2. DEVELOPERS

Yu. V. Obraztsov,Ph.D. tech. Sciences (topic leader); V. S. Turutin, Ph.D. tech. sciences, A. I. Balashov; I. E. Kushnir

3. APPROVED AND ENTERED INTO EFFECT by Resolution of the State Committee of Standards of the Council of Ministers of the USSR dated June 23, 1976 No. 1508

Inspection frequency 5 years

4. The standard fully complies with ST SEV 2784-80

5. INSTEAD GOST 3345-67

6. Validity period extended until 01/01/94 by Decree of the USSR State Standard dated 06/21/88 No. 2033

7. Reissue (January 1992) with Amendments No. 1, 2, approved in September 1981, June 1988 (IUS 11-81, 10-88)