FAQ - Service Power grid

Sheath testing

Do I need to use a special sheath testing equipment?

No, a 5 kV DC insulation tester typically works for 12-24 kV cables (10 kV test voltage for 36 kV cables and higher), but it cannot be used for troubleshooting.
 

I can't reach the desired test voltage. What could be wrong?

It could obviously be due to a sheath fault, but it could also be an issue with a part of the screen potential in a termination that is in contact with the ground.
 

I get a leakage current of 10 µA/km to 1.0 mA/km. What should I do?

Dry the terminations with propane if they are in a damp environment. It could also be a splice or sheath repair that is not properly sealed. Sheath damage in pipes can also cause similar leakage currents.
 

Can I test the sheath of cables in pipes?

Yes and no. If the sheath fault is in contact with the ground, it can be detected. However, if the fault is in a dry pipe or suspended in the air, potential damage may be missed. If the fault is far from the end of the pipe, pinpointing the location might be impossible.
 

Is it enough to have access to one end of the cable?

Yes, if the screen at the other end is not in contact with the ground. However, for rough localization of sheath faults, access to both ends is needed.
 

Can I see how many sheath faults a cable has?

No, the equipment will find the fault with the lowest resistance to ground. With multiple sheath faults, the rough localization might be misleading.
 

Can I test the sheath of a cable before it is connected in a substation or termination is installed?

Yes, but ensure that no one has access to the other end during testing and that there is no fire risk.
 

I have found damage to the outer sheath. What should I do?

Check that only the outer sheath is damaged. Build up a new sheath using, for example, vulcanized tape and finish with a winding sleeve with melt glue.
 

How do I test the sheath of a cable with a conductive layer on the outer sheath (TTCL)?

The conductive layer must be removed at least 100 mm from the edge of the sheath at the splice and termination. In the case of sheath damage, it must be removed at least 100 mm in all directions.

Insulation testing

What is an acceptable value for insulation testing (megger testing)?

There is no fixed standard, as it is up to the asset owner, but many use the following guidelines:

Low voltage cables: At least 1 MΩ at 500-1000 V test voltage.

Medium voltage cables: Often at least 10 MΩ/kV test voltage.
 

What is VLF testing?

VLF (Very Low Frequency) is an alternating voltage test with a low frequency (0.01-0.10 Hz) that replaces insulation testing with DC voltage (megger testing).
 

What voltage is used for VLF testing?

1.7-2.0 times U0 for aged cables.

Up to 3.0 times U0 for new cables.

Test duration: 30-60 minutes.
 

Can VLF testing damage my cable?

Yes, if the insulation is already weakened, it may be negatively affected. However, "healthy" insulation will not be affected.

Longitudinal ground control - (Ymer-, screen or return current measurement.)

What does the earth connection test measure?

It compares the resistance in the cable screen with external earth connections between two points, often between substations.
 

What is a normal measurement value (M-value)?

0.3-0.9 is normal.

Below 0.3 suggests that too much of the return current is passing through the cable screen, e.g., due to a break in the earth line.

Above 0.9 suggests that too little of the return current is passing through the cable screen, e.g., due to a corroded cable screen or strong external earth connections.
 

When should I perform an earth connection test?

When newly installed or when changes are made to the network, such as after a splice.
 

Can I locate a break in an earth line?

Not with any measuring instrument. By measuring the resistance between two points where the earth line is exposed, you can determine if it's OK or not. After that, the halving method is used to locate the break.

Other test

What is PD measurement?

PD (Partial Discharge) measures partial discharges, flashes that can be caused by installation errors, insulation damage, or advanced treeing. Nexans offers PD measurement on de-energized cables using VLF as the voltage source.
 

What is Tangent Delta measurement (TD)?

A measurement that detects weaknesses, such as treeing, in cable insulation. The test voltage is gradually increased to avoid affecting already weakened insulation. Nexans performs TD measurement as part of the condition assessment of older MV cables.
 

What is screen resistance measurement?

A method that can be used if the cable does not pass the earth connection test with an acceptable value. At Nexans, the measurement is part of the condition assessment of older MV cables.