GB/T 3048.5-2007Test methods for electrical properties of electric cables and wires - Part 5: Test of insulation resistance (English PDF)

5.1

The effective length of the sample shall not be less than 10m, unless

otherwise specified in the product standard. The coverings on the insulation at the both ends of the sample shall be peeled carefully and avoid damage to the insulating surface.

5.2

The sample shall be kept in the test environment for enough time to make the sample temperature and the test temperature in balance. Keep the temperature stable.

5.3

When the sample is tested in the water, the exposed length of the two ends of the sample shall not be less than 250mm; the exposed length of the insulating part shall not be less than 150mm.

When the sample is tested in the air, the exposed length of the insulating part at the sample ends shall not be less than 100mm.

The exposed insulating surface shall be kept dry and clean.

6.1 Connection of the sample

6.1.1 Cable samples with metal sheath, shielding layer or armor

For single-core cables, the insulation resistance of the conductor to the metallic sheath or shielding layers or armor layers shall be measured; as for multi-core cables, each wire core shall be connected to the metallic sheath or shielding layer or armor layer for testing against the rest wire cores. If it is required to measure the insulation resistance between the shield and the wire cores of multi-core cables, it shall measure the shield after all of the wire cores are in parallel connection.

6.1.2 Non-metallic sheathed, non-shielded or unarmored cable samples

The single-core cable shall be submerged in the water; the insulation resistance of the conductor to the water shall be measured; as for multi-core cables, each wire core shall be tested against the rest wire cores.

The sample may also be wound around the metallic test bar. For single-core cables, the insulation resistance of the conductor to the test bar shall be measured; as for multi-core cables, each wire core shall be connected to the test bar for testing against the rest. The external diameter of the test bar shall meet […]

6.6 Test charging time

In order to keep the measured insulation resistance stable, the test charging time shall be adequate, not less than 1min and does not exceed 5min. Generally, 1min reading is recommended.

6.7 Measuring error of the test system

The measuring error of the test system shall meet the following requirements:

-- If the insulation resistance value of the tested sample is (1×10

10

)ohm or below, the measuring error shall not be greater than ±10%;

-- If the insulation resistance value of the tested sample is above (1×10

10

)ohm, the measuring error shall not be greater than 20%.

7.1

When DC comparison method is adopted in the test, the insulation resistance value shall be calculated according to the formula provided by the instrument instructions.

When voltage - current method or digital instrument is adopted in the test, the insulation resistance value shall be read according to the requirements of the instrument instructions.

7.2

The insulation resistance of per kilometer length shall be calculated according to Formula (1):

RL=RX L (1)

Where,

RL -- Insulation resistance of per kilometer length; unit: Mohm·km;

RX -- Insulation resistance of the sample; unit: Mohm;

L -- Actual measurement length of the sample; unit: km.

7.3

The insulation resistance of per kilometer length at 20°C shall be calculated according to Formula (2):

R20=KRL (2)

Where,

R20 -- Insulation resistance of per kilometer length at 20°C; unit: Mohm·km;

8.4

When the insulation resistance is measured by adopting a megger whose

output end is suspended over the ground, it is recommended that the measuring end (low voltage end) of the megger to be connected to the conductor of the tested insulated core, and the high voltage end to be connected to the other pole (water, it is permitted to be grounded) of the sample. When a general megger is adopted to measure the insulation resistance, the submerged sample must be of ground insulation; otherwise, it may cause damage to the megger because of the short circuit of the output high voltage side TO the ground; or it may enlarge the test error due to the effects from the heating power supply.

8.5

Attention shall be drawn to the connection methods for the internal and external shield of the whole set apparatus used by the DC comparison method, so as to avoid to enlarge the measuring error.