GOST 3345-76
Group E49
INTERSTATE STANDARD
CABLES, WIRES AND CORDS
Method for determining electrical insulation resistance
Cables, wires and cords.
Determination of insulation electric resistance
ISS 29.060.01
Date introduced 1978-01-01
INFORMATION DATA
1. DEVELOPED AND INTRODUCED by the USSR Ministry of Electrotechnical Industry
2. APPROVED AND INTRODUCED BY THE RESOLUTION of the State Committee of Standards of the Council of Ministers of the USSR of 23.06.76 N 1508
3. The standard is fully consistent with ST SEV 2784-80
4. REPLACEMENT of GOST 3345-67
5. The validity period has been lifted according to the protocol N 3-93 of the Interstate Council for Standardization, Metrology and Certification (IMS N 5-6, 1993)
6. EDITION with Changes N 1, 2, approved in September 1981, June 1988 (IMS 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 coils, or samples of at least 10 m in length, excluding the length of the end cuts, should be selected, unless a different length is specified in the standards or specifications for cables, wires and cords.
1.2. The number of building lengths and samples for measurement should be indicated in the standards or specifications for cables, wires and cords.
2. APPARATUS
2.1. The insulation resistance is measured 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 providing measurements with an error of not more than 10% of the measured values \u200b\u200bfrom 1 · 10 to 1 · 10 Ohms, not more than 20% of the measured values \u200b\u200babove 1 · 10 to 1 · 10 Ohms and not more than 25% of the measured values \u200b\u200bover 1 · 10 Ohm. If standards or specifications for cables, wires and cords are allowed to measure on short (less than 10 m) product samples, then the error of such measurements should not be more than 10% for any measured values \u200b\u200bof insulation resistance.
(Amended edition, Amendment N 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 tested product.
2.3. The installation for measurements should be carried out taking into account the requirements relating to installations with voltage up to 1000 V, and should ensure the safety of measurements.
3. PREPARATION AND MEASUREMENT
3.1. If necessary, the ends of the test product must be cut before measurement.
To increase the accuracy of the measurement, it is allowed to install guard rings at the end cuts, which should 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) ° С and a relative humidity of not more than 80%, if other conditions are not provided for in the standards or technical conditions for cables, wires and cords, or in water.
3.3. The ambient temperature is measured with an error of not more than ± 0.5 ° C at a distance of not more than 1 m from the test product.
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 ° C, and no more ±
1 ° C, if the measurements are carried out at a temperature of 20 ° C.
The temperature of the water during measurement should be the same throughout the volume.
3.4. The exposure time of the samples before testing at ambient temperature should be at least 1 hour, unless another exposure time is specified in the standards or specifications for specific cable products.
3.3, 3.4. (Amended edition, Amendment No. 1).
3.5. When measuring the electrical insulation resistance of cables, wires and cords on building lengths wound around drums or bays, the diameters of the necks of drums or bays must comply with those specified in the standards or 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 should be wound tightly adjacent to each other and the rod with turns with a 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 should 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 - not less than 50 mm.
3.8. The electrical insulation resistance of individual cores and single-core cables, wires and cords should be measured:
- for products without a metal shell, shield and armor - between the conductive core and the metal rod or between the core and water;
- for products with a metal sheath, shield and armor - between a conductive core and a metal sheath or shield, or armor.
3.9. The electrical insulation resistance of multicore cables, wires and cords should be measured:
- for products without a metal sheath, shield and armor - between each conductive core and other conductors interconnected or between each conductive conductive and other conductors interconnected and with water;
- for products with a metal sheath, a shield and armor - between each conductive core and the remaining cores connected to each other and with a metal sheath or shield, or armor.
3.10. During repeated measurements, the test product must be discharged for at least 2 minutes by connecting a conductive core with a grounding device (subject to safety regulations).
3.11. The readings of the values \u200b\u200bof the electrical insulation resistance during the measurement are carried out after 1 minute from the moment of application of the measuring voltage to the sample, but not more than 5 minutes, unless other requirements are specified in the standards or specifications for specific cable products.
Before re-measurement, all metal elements of the cable product must be grounded in at least 2 minutes.
4. PROCESSING RESULTS
4.1. If the measurement was carried out at a temperature other than 20 ° C, and the insulation resistance value 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 converted to a temperature of 20 ° C according to the formula
- electrical insulation resistance at measurement temperature, MΩ;
- coefficient for bringing the electrical resistance of insulation to a temperature of 20 ° C, the values \u200b\u200bof which are given to this standard.
In the absence of conversion factors, the arbitration method is to measure the electrical insulation resistance at a temperature of (20 ± 1) ° С.
(Amended edition, Amendment No. 1).
4.2. Recalculation of electrical insulation resistance to 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;
- the length of the test product excluding the end sections, km.
The length of the product must be determined with an accuracy of 1%.
(Amended edition, Amendment No. 2).
ANNEX (mandatory). Coefficient K of bringing the electrical resistance of insulation to a temperature of 20 ° C
APPENDIX
Mandatory
The coefficient of reduction of electrical insulation resistance to a temperature of 20 ° C
Temperature ° C | Insulation material |
||
Impregnated paper | PVC compound and polyethylene | Rubber |
|
The text of the document is verified by:
official publication
Cables, wires and cords.
Test methods: Sat. GOST.-
M .: IPK Standards Publishing House, 2003
CABLES, WIRES AND CORDS
METHOD FOR DETERMINING ELECTRICAL RESISTANCE OF INSULATION
GOST 3345-76
(ST SEV 2784-80)
COMMITTEE OF STANDARDIZATION AND METROLOGY OF THE USSR
Moscow
STATE STANDARD OF THE USSR
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 resistance to insulation at DC voltage.
1. SAMPLING METHOD
1.1. For measurement, the construction lengths of cables, wires and cords wound on drums or coils, or samples of at least 10 m in length, excluding the length of the end cuts, should be selected, unless a different length is specified in the standards or specifications for cables, wires and cords.
1.2. The number of building lengths and samples for measurement should be indicated in the standards or specifications for cables, wires and cords.
2. APPARATUS
2.1. The insulation resistance is measured 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 devices that provide measurements with an error of not more than 10% of the measured values \u200b\u200bfrom 1× 10 5 to 1 × 10 10 Ohm, not more than 20% of the measured values \u200b\u200bof St. 1× 10 10 to 1 × 10 14 Ohms and not more than 25% of the measured values \u200b\u200bof St. 1× 10 14 ohms. If standards or specifications for cables, wires and cords are allowed to measure on short (less than 10 m) product samples, then the error of such measurements should not be more than 10% for any measured values \u200b\u200bof insulation resistance.
(Amended 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 tested product.
2.3. The installation for measurements should be carried out taking into account the requirements relating to installations with voltage up to 1000 V, and should ensure the safety of measurements.
3. PREPARATION AND MEASUREMENT
3.1. If necessary, the ends of the test product must be cut before measurement.
To increase the accuracy of the measurement, it is allowed to install guard rings at the end cuts, which should 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) ° С and a relative humidity of not more than 80%, if other conditions are not provided for in the standards or technical conditions for cables, wires and cords, or in water.
3.3. The ambient temperature is measured with an error of not more than ± 0.5 ° C at a distance of not more than 1 m from the test product.
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 not more than ± 1 ° С, if measurements are carried out at a temperature of 20 ° С.
The temperature of the water during measurement should be the same throughout the volume.
3.4. The exposure time of the samples before testing at ambient temperature should be at least 1 hour, unless another exposure time is specified in the standards or specifications for specific cable products.
3.3, 3.4.
3.5. When measuring the electrical insulation resistance of cables, wires and cords on building lengths wound around drums or bays, the diameters of the necks of drums or bays must comply with those specified in the standards or 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 should be wound tightly adjacent to each other and the rod by turns 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 should 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 - not less than 50 mm.
3.8. The electrical insulation resistance of individual cores and single-core cables, wires and cords should be measured:
for products without a metal shell, shield and armor - between the conductive core and the metal rod or between the core and water;
for products with a metal sheath, shield and armor - between a conductive core and a metal sheath or shield, or armor.
3.9. The electrical insulation resistance of multicore cables, wires and cords should be measured:
for products without a metal shell, shield and armor - between each conductive core and other conductors interconnected or between each conductive conductive and other conductors interconnected and with water;
for products with a metal sheath, a shield and armor - between each conductive core and the remaining cores connected to each other and with a metal sheath or shield, or armor.
3.10. During repeated measurements, the test product must be discharged for at least 2 minutes by connecting a conductive core with a grounding device (subject to safety regulations).
3.11. The readings of the values \u200b\u200bof the electrical insulation resistance during the measurement are carried out after 1 minute from the moment of application of the measuring voltage to the sample, but not more than 5 minutes, unless other requirements are specified in the standards or specifications for specific cable products.
Before re-intention, all metal elements of the cable product must be grounded in at least 2 minutes.
(Amended edition, Amendment No. 1).
4. PROCESSING RESULTS
4.1. If the measurement was carried out at a temperature other than 20 ° C, and the insulation resistance value 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 converted to a temperature of 20 ° C according to the formula
R 20 = KR t
where R 20
R t - electrical insulation resistance at measurement temperature, MΩ;
TO - coefficient to bring the electrical insulation resistance 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 to measure the electrical insulation resistance at temperature (20± 1) ° C.
(Amended edition, Amendment No. 1).
4.2. Conversion of electrical insulation resistanceR for 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 - the length of the test product excluding the end sections, km
The length of the product must be determined with an accuracy of 1%.
(Amended edition, Amendment No. 2).
APPENDIX
Mandatory
Coefficient K reduction of electrical insulation resistance to a temperature of 20 ° WITH
|
Temperature ° C |
Insulation material |
||
|
Impregnated paper |
PVC compound and polyethylene |
Rubber |
|
|
0,58 |
0,10 |
0,50 |
|
|
0,60 |
0,12 |
0,53 |
|
|
0,64 |
0,15 |
0,55 |
|
|
0,67 |
0,17 |
0,58 |
|
|
0,69 |
0,19 |
0,61 |
|
|
0,72 |
0,22 |
0,64 |
|
|
0,74 |
0,26 |
0,68 |
|
|
0,76 |
0,30 |
0,70 |
|
|
0,79 |
0,35 |
0,73 |
|
|
0,82 |
0,42 |
0,76 |
|
|
0,85 |
0,48 |
0,80 |
|
|
0,87 |
0,56 |
0,84 |
|
|
0,90 |
0,64 |
0,88 |
|
|
0,93 |
0,75 |
0,91 |
|
|
0,97 |
0,87 |
0,96 |
|
|
1,00 |
1,00 |
1,00 |
|
|
1,03 |
1,17 |
1,05 |
|
|
1,07 |
1,35 |
1,13 |
|
|
1,10 |
1,57 |
1,20 |
|
|
1,14 |
1,82 |
1,27 |
|
|
1,18 |
2,10 |
1,35 |
|
|
1,22 |
2,42 |
1,43 |
|
|
1,27 |
2,83 |
1,52 |
|
|
1,32 |
3,30 |
1,61 |
|
|
1,38 |
3,82 |
1,71 |
|
|
1,44 |
4,45 |
1,82 |
|
|
1,52 |
5,20 |
1,93 |
|
|
1,59 |
6,00 |
2,05 |
|
|
1,67 |
6,82 |
2,18 |
|
|
1,77 |
7,75 |
2,31 |
|
|
1,87 |
8,80 |
2,46 |
|
INFORMATION DATA
1. DEVELOPED AND INTRODUCED by the USSR Ministry of Electrotechnical Industry
2. DEVELOPERS
Yu. V. Obraztsov,cand. tech. sciences (head of the topic); V.S. Turutin,cand. tech. sciences A.I. Balashov; I. E. Kushnir
3. APPROVED AND IMPLEMENTED BY Decree No. 1508 of the State Committee of Standards of the USSR Council of Ministers on June 23, 1976
Inspection frequency 5 years
4. The standard is fully consistent with ST SEV 2784-80
5. REPLACE GOST 3345-67
6. The validity period has been extended until 01.01.94 by the Decree of the USSR State Standard of 06.21.88 No. 2033
7. Reprint (January 1992) with Amendments No. 1, 2 approved in September 1981, June 1988 (IMS 11-81, 10-88)
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 the data on which they are guided during the construction, operation and maintenance of cables.
Electric current flows through two metal cores, and the environment all the time has various effects on them, in some cases even dangerous. In addition, these veins themselves affect each other. As a result, metal wires that have no protection suffer huge losses due to a variety of leaks, up to the formation of emergency situations.
In order to minimize such negative situations or to significantly reduce them, conductive conductors in cables should be protected with an insulating coating made of a material that does not conduct electric current.
Material to create insulation shells are considered:
- plastics;
- paper;
- rubber.
These materials can also be combined. Insulation, which is used for different types of cables, has a rather significant difference both in the materials used and in the principles of applying insulating covers. Today, they produce a large number of cable products, which are used for a variety of 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 characteristicsdesigned 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 the currently existing cable types. For example, if underground distribution telephone networks are laid directly in the ground, the cable construction used in telephone conduits is additionally strengthened by wrapping their core in metal armor bands. And also to protect the cable conductors from external currents, its core is clad 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 insulate conductive cores at high temperatures, it is best to use rubber than other materials. Rubber resistant to such temperature effects than, for example, ordinary plastic.
Thus, the use of insulating materials for cable products is necessary to protect its conductive conductors from external and mutual electrical influences. The magnitude of such a parameter for an individual core and the entire core as a whole is determined by the value of direct current resistance that arises in the circuit between the wires and any source, for example, ground. To determine the operability 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 be destroyed. 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 norm on the parameter of the product, which is set by the manufacturer.
Insulation rate
As a specific product value, insulation resistance for different cable brands 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 reduced to 1 km of length, and the ambient temperature for this 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 balanced cables, the resistance rate can reach 10,000 Mohm / km. Assessing the condition of the cable under test, the passport data of the insulation resistance is used only when it is necessary to recount them to the length of the actual piece of cable. When the cable section is more than a kilometer, the norm should be divided by this length. If it is less than a kilometer, then, respectively, multiply.
The resulting calculated numbers are often used to evaluate the cable line. It should be remembered that the passport data are taken into account for a temperature of +20 degrees, so it is necessary to make corrections by making control measurements for humidity and temperature.
There are such brands of cable products in which the aluminum sheath and hose polyethylene coating. For them, the norm of insulation resistance between the ground and the shell is determined. It is usually 20 Mom / km. To use this standard in work, it must be recalculated to the actual length of the plot.
For power cable The following provisions for DC insulation resistance are provided:
- for power cables used in networks with a voltage of more than 1000 V, the value of this parameter is not standardized, but cannot be less than 10 Ohm;
- for power cables used in networks with voltage 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 MΩ tomorrow morning on the same cable 800 MΩ, and in the evening 16,000.
The value of insulation is highly dependent on temperature and humidity. The control cabinet stood open several hours on a damp morning, and done, the insulation dropped to 400 megohms. That is, the figure floats in very large limits, and the management often does not want to understand how isolation is unstable and requires accurate values.
As a rule, sensible meters quickly wry that it is better to figure out several pairs from one cable, and write any numbers that correspond to the norm in the protocol. It is better to judge the integrity of the cable sheath by the shield-to-ground insulation, and you won’t check the correctness of the assembly of the boxes by measuring the insulation. Actually for them on the site there is self-filling protocol.
How to achieve good insulation of the new cable line
Several times I came across a situation when, for acceptance tests, the isolation of the receiving side, say, at 800 megohms did not suit, yet this is "not the norm" and, as a rule, the young meter began to resent. In this case experienced seasoners usually carry out emergency drying. In the control cabinet, with the help of a blowtorch or gas burner, carefully glue the skirtings of the cable to be handed over.
Isolation is quickly restored to several thousand megohms, spikers call the meter, it measures and sometimes even wonders how the guys quickly fixed the damage.
Low insulation on terminal devices usually indicates a leak in the bottom of the control cabinet. About the reasons for damping skirtings page " Why do skirting boards in SHR damp, how to dry, how to increase insulation "
It is more precise to find out what the lowering of insulation gives, allows the detachment of the core from the plinth and a separate measurement of it relative to the "ground"
In operation, the insulation of terminal devices can even drop to a few kilo-ohms and at the same time, the greenery of oxides on the skirtings becomes already noticeable
Insulation rate for the new cable line
For acceptance tests, the norm for cable CCI with terminal devices is considered to be for insulation lines less than 1 km insulation resistance of 1000 MΩ. That is, the norm is the same for 20 meters and 1 kilometer of cable, and usually nobody climbs into the jungle described below. They check the isolation of several pairs and without further ado sign the protocols and acts. More attention is paid to dialing, screen isolation and the correct assembly of skirtings.
Nevertheless, several times I came across electrical engineers and engineers who read the technical documentation more carefully and notice that the insulation norm is indicated for 1 km of the circuit. From this they conclude that a cable line 500 meters long should have an insulation of 2000 MΩ, and at 50 meters, respectively, 20,000 MΩ. It is difficult to argue with them, and trying to somehow reason with these "nerds" asked the question, how much should the transmission isolation between cabinets 5 meters long be? The figure of 200,000 megohms usually raises doubts about the consistency of such calculations.
Replying to one of the isolation norm letters , derived a formula for calculating this norm. And although the calculation data is taken from an official document, the derived formula should be treated as a joke and considered the norm for a new line with a length of less than a kilometer 1000 MΩ.
By the way, in some instructions "from above" sent to the sites, for some reason this is not prescribed.
The conclusion of the formula for calculating the insulation rate of the cable line
A cable line with terminal devices can be represented as three parallel resistances, where
R and 1p
and R and 2p
is the insulation resistance of the first and second skirtings,
R IR
- insulation resistance of the cable core
R IC - the resistance of the entire cable line is derived from the formula for calculating parallel resistances:
R and 1p could be taken from the "Operation Guide for 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 3500 MΩ baseboard is given only for the standard for insulation of short lines - 1000 MΩ.