GOST 3345-76

Group E49

INTERSTATE STANDARD

CABLES, WIRES AND CORDS

Method for determining the electrical resistance of insulation

Cables, wires and cords.
Determination of electrical resistance

MKS 29.060.01

Introduction date 1978-01-01

INFORMATION DATA

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

2. APPROVED AND INTRODUCED BY Decree State Committee standards of the Council of Ministers of the USSR dated 06/23/76 N 1508

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

4. REPLACE GOST 3345-67

5. The limitation of the validity period was removed according to protocol N 3-93 of the Interstate Council for Standardization, Metrology and Certification (IUS N 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, the construction lengths of cables, wires and cords wound on drums or in coils, or samples with a length of at least 10 m, excluding the length of the end grooves, should be selected, unless another length is specified in the standards or technical specifications for cables, wires and cords.

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

2. EQUIPMENT

2.1. The measurement of electrical insulation resistance is carried out at a voltage of 100 to 1000 V, unless other conditions are specified in the standards or 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 ​​over 1 10 to 1 10 Ohm and no more than 25% of the measured values ​​over 1 10 Ohm. If the standards or specifications for cables, wires and cords allow measurements 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, Rev. N 1, 2).

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

2.3. The installation for measurements must be made taking into account the requirements for installations with voltage up to 1000 V, and must ensure the safety of measurements.

3. PREPARATION AND MEASUREMENTS

3.1. Where necessary, the ends of the test article shall be cut before measurement.

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

3.2. The measurement is carried out at a temperature environment(20 ± 15) ° С and relative air humidity not 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. Measurement of the ambient temperature is carried out with an error of not more than ±0.5 °C at a distance of not more than 1 m from the product under test.

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

The temperature of the water during the measurement must be the same throughout the volume.

3.4. The holding time of samples before testing at ambient temperature should be at least 1 hour, unless other holding times are specified in the standards or specifications for specific cable products.

3.3, 3.4. (Changed edition, Rev. N 1).

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

3.6. If the measurement of the electrical resistance of the insulation is provided for 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 the electrical resistance of the insulation 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 sheath, screens and armor - by at least 50 mm.

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

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

- for products with a metal sheath, screen and armor - between a conductive core and a metal sheath 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 conductive core and the rest of the cores connected to each other or between each conductive core and the rest of the cores connected to each other and to water;

- for products with a metal sheath, screen and armor - between each conductive core and the rest of the cores connected to each other and to the metal sheath or screen or armor.

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

3.11. The readings of the values ​​of the electrical resistance of the insulation during the measurement are carried out after 1 min from the moment the measuring voltage is applied to the sample, but not more than 5 min, unless other requirements are provided for in the standards or 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 THE RESULTS

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

- electrical insulation resistance at the measurement temperature, MΩ;

- coefficient for bringing the electrical resistance of the insulation to a temperature of 20 ° C, the values ​​\u200b\u200bof which are given in this standard.

In the absence of conversion factors, the arbitration method is the measurement of the electrical resistance of the insulation at a temperature of (20 ± 1) ° C.

(Changed edition, Rev. N 1).

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

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

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

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

(Changed edition, Rev. N 2).

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

APPENDIX
Mandatory

The coefficient of reduction of the electrical resistance of the insulation to a temperature of 20 ° C

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

CABLES, WIRES AND CORDS

METHOD FOR DETERMINING THE ELECTRICAL INSULATION RESISTANCE

GOST 3345-76
(ST SEV 2784-80)

USSR STANDARDIZATION AND METROLOGY COMMITTEE

Moscow

STATE STANDARD OF THE UNION OF THE SSR

Validity 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 dc voltage.

1. SAMPLING METHOD

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

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

2. EQUIPMENT

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

The measurement is carried out using measuring circuits and instruments that provide measurements with an error of not more than 10% of the measured values ​​from 1× 10 5 to 1 × 10 10 Ohm, no more than 20% of the measured values ​​St. one× 10 10 to 1 × 10 14 Ohm and not more than 25% of the measured values ​​of St. one× 10 14 Ohm. If the standards or specifications for cables, wires and cords allow measurements 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, Rev. No. 1, 2).

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

2.3. The installation for measurements must be made taking into account the requirements for installations with voltage up to 1000 V, and must ensure the safety of measurements.

3. PREPARATION AND MEASUREMENTS

3.1. Where necessary, the ends of the test article shall be cut before measurement.

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

3.2. The measurement is carried out at an ambient temperature of (20 ± 15) ° C and a relative air humidity of not 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. Measurement of the ambient temperature is carried out with an error of not more than ±0.5 °C at a distance of not more than 1 m from the product under test.

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

The temperature of the water during the measurement must be the same throughout the volume.

3.4. The holding time of samples before testing at ambient temperature should be at least 1 hour, unless other holding times are specified in the standards or specifications for specific cable products.

3.3, 3.4.

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

3.6. If the measurement of the electrical resistance of the insulation is provided for 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 the electrical resistance of the insulation 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 sheath, screens and armor - by at least 50 mm.

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

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

for products with a metal sheath, screen and armor - between a conductive core and a metal sheath 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 conductive core and the rest of the cores connected to each other or between each conductive core and the rest of the cores connected to each other and to water;

for products with a metal sheath, screen and armor - between each conductive core and the rest of the cores connected to each other and to the metal sheath or screen or armor.

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

3.11. The readings of the values ​​of the electrical resistance of the insulation during the measurement are carried out after 1 min from the moment the measuring voltage is applied to the sample, but not more than 5 min, unless other requirements are provided for in the standards or specifications for specific cable products.

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

(Revised edition, Rev. No. 1).

4. PROCESSING THE RESULTS

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

R 20 = KR t ,

where R 20

R t - electrical insulation resistance at the measurement temperature, MΩ;

To- coefficient for bringing the electrical resistance of the insulation to a temperature of 20 ° C, the values ​​\u200b\u200bof which are given in the appendix to this standard

In the absence of conversion factors, the arbitration method is the measurement of the electrical resistance of the insulation at a temperature of (20± 1) °C.

(Revised edition, Rev. No. 1).

4.2. Calculation 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, MΩ,

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

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

(Revised edition, Rev. No. 2).

APPENDIX
Mandatory

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

INFORMATION DATA

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

2. DEVELOPERS

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

3. APPROVED AND INTRODUCED BY Decree of the State Committee for Standards of the Council of Ministers of the USSR dated June 23, 1976 No. 1508

Inspection interval 5 years

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

5. REPLACE GOST 3345-67

6. The validity period was extended until 01/01/94 by the Decree of the State Standard of the USSR 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)

Those who constantly measure the insulation of new cables eventually develop a disregard for the exact value of this parameter. Life teaches. Today the insulation of the cable line is more than 30,000 MΩ tomorrow morning on the same cable 800 MΩ, and in the evening 16000.

The insulation value is highly dependent on temperature and humidity. The switch cabinet stood open for several hours on a damp morning, and that's it, the insulation dropped to 400 MΩ. That is, the figure floats within very large limits, and management often does not want to understand how unstable the isolation is and requires exact values.

As a rule, intelligent meters quickly realize that it is better to figure out several pairs from one cable, and write any numbers corresponding to the norm in the protocol. It is better to judge the integrity of the cable sheath by the screen-to-ground insulation, and you cannot check the correct assembly of the boxes by measuring the insulation. Actually for them on the site there is self-filled protocol.

How to achieve good insulation of a new cable line

Several times I came across a situation when, during acceptance measurements, the receiving side was not satisfied with, say, 800 MΩ insulation, after all, this is "not the norm" and, as a rule, the young meter began to resent. In this case, experienced solders usually carry out emergency drying. AT switch cabinet with a blowtorch or a gas burner, the plinths of the delivered cable are carefully heated.

The insulation is quickly restored to several thousand megohms, the splicers call the meter, it measures and sometimes even wonders how the guys quickly fixed the damage.

Poor insulation at the terminations usually indicates a leak in the sealing of the switch cabinet bottom. About the reasons for dampness of plinths page " Why do plinths dampen in SHR, how to dry, how to increase insulation "

It is more accurate to find out what gives a decrease in insulation, allows you to disconnect the core from the plinth and separate its measurement relative to the "ground"

In operation, the insulation of terminal devices can even drop to several kiloohms, and at the same time, green oxides on the plinths are already becoming noticeable.

Insulation rate for a new cable line

In acceptance measurements, it is customary to consider the insulation resistance of 1000 MΩ for lines less than 1 km as the norm for a CCI cable with terminal devices. That is, the norm is the same for 20 meters and 1 kilometer of cable, and usually no one climbs into the jungle described below. They check the isolation of several couples and sign protocols and acts without further ado. More attention is paid to dialing, screen insulation and correct assembly of plinths.

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

Responding to one of the letters about the norm for isolation, derived a formula for calculating this norm. And although the data for the calculation are taken from an official document, the derived formula should be treated as a joke and considered the norm for a new line less than a kilometer long 1000 MΩ.

By the way, in some instructions "from above", sent to the sites, for some reason this is not prescribed.

Derivation of the formula for calculating the insulation norm of a cable line

A cable line with terminations can be represented as three parallel resistances, where
R u1p and R u2p are the insulation resistances of the first and second plinths,
R IR - cable core insulation resistance

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

R u1p could be taken from the "Guidelines for the operation of linear cable structures of local communication networks, 1998" ( Appendix 6. Electrical insulation resistance of terminal cable devices and elements) but there the insulation resistance of the plinth of 3500 MΩ is given only for the norm for the insulation of short lines - 1000 MΩ.

And the cable has specific, primary and secondary electrical parameters that characterize these products. One of the main parameters of the cable is the insulation resistance. The norm of insulation resistance is considered to be data that are guided by when performing work on the construction, operation and maintenance of cables.

An electric current flows through two metal conductors, and they are constantly affected by various environmental influences, in some cases even dangerous ones. In addition, these veins themselves influence each other. As a result of this, metal wires that do not have protection, suffer huge losses due to various leaks, up to the formation of emergencies.

In order to minimize or significantly reduce such negative situations, the conductive cores in the cables should be protected with an insulating coating of a non-conductive material.

material to create insulating sheaths are considered:

• plastic masses;
• paper;
• rubber.

Also, these materials can be combined. The insulation that is used for different types of cables has a rather significant difference both in the materials used and in the principles of the use of insulating covers. Today, a large number of cable products are produced, which are used for various needs.

Variety of cable products

There are cables:

These products may differ from each other not only in their functions, but also structural and physical characteristics designed for the environment in which it will be used. The great need for wire materials necessary for a variety of needs has led to the creation of various modifications of currently existing types of cables. For example, if underground distribution telephone networks are laid directly in the ground, the design of cables used in telephone ducts is additionally strengthened by dressing their core in metal armor tapes. And also, in order to protect the cable cores from external currents, its core is dressed in an aluminum sheath.

What is insulation resistance

The type of insulating material depends on the environment and under what conditions the manufactured conductor products will be used. For example, to isolate high temperatures conductive wires, it is best to use rubber than other materials. Rubber resistant to such temperature influences than, for example, ordinary plastic.

Thus, the use of insulating materials for cable products is necessary to protect its conductive cores from external and mutual electrical influences. The value of this parameter for a single core and the entire core as a whole is determined by the resistance value direct current, arising 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 that are used as insulation in cables over time get old and begin to lose their properties. Therefore, even from any physical impact, they can collapse. To clarify how and within what limits the parameters of the insulating material could change, it is required for comparison to know the norm for the parameter of the product, which is set by the manufacturer.

Insulation resistance rate

As a specific value of the product insulation resistance for different brands of cable laid in GOST or TU for the manufacture of certain cable products. Such products supplied for sale must have a passport with electrical parameters. For example, the norm of insulation resistance for a communication cable is given to 1 km of length, and the ambient temperature for these data should be +20 degrees.

For urban low-frequency communication cables, the resistance rate should be at least 5000 MΩ / km, for coaxial and trunk symmetrical cables, the resistance rate can reach 10000 MΩ/km. When evaluating the condition of the cable being tested, the passport data of the insulation resistance are used only when it is necessary to recalculate them to the length of the actual piece of cable. With a cable section of more than a kilometer, the norm should be divided by this length. If it is less than a kilometer, then, accordingly, multiply.

The resulting calculated figures are often used to evaluate the cable line. It should be remembered that passport data are taken into account for a temperature of +20 degrees, so it is necessary to make adjustments by conducting control measurements for humidity and temperature.

There are such brands of cable products that have an aluminum sheath and a hose polyethylene coating. For them, the norm of insulation resistance between the ground and the shell is determined. It is usually 20 MΩ/km. To use this standard in work, it must be recalculated for the actual length of the section.

For power cable the following provisions for DC insulation resistance are provided:

• for those used in networks with a voltage of more than 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 a voltage of less than 1000 V, the value of the parameter should not exceed 0.5 Ohm.

For control cables norm cannot be less than 1 ohm.