FAQ - Service Power grid

Sheath testing

Do I have to use special sheath testing equipment?

• No, it is almost always possible to test with a 5 kV DC insulation tester (megger) if it is 12-24 kV cable (10 kV test voltage from 36 kV cable and up). However, you can´t search sheath faults with it.

 

The insulation/sheath tester does not come up to the desired voltage.

• You have one or more sheath faults.
• Something on the cable/terminations with shield potential (eg shield wire, outer conductive layer, conductive crotch) has contact with earth.

 

I get a leakage current of 10 µA/km to 1.0 mA/km.

• One or both terminations are located in a humid environment near earthed parts. Increase the distance/insulate and dry with heat/LPG.
• Cable repairs, joints or other accessories have not become tight during assembly. Leakage current that will "creep" a bit in moisture before reaching  contact with earth becomes weak and thus difficult to find.
• The sheath fault does not have a good contact with earth, e.g. in pipes.

 

How long cables can I sheath test?

• It is usually possible to test longer cables than many people think, but it takes longer to reach the desired test voltage.
• For longer distances, it is wise to test the sheath before each splicing. If a sheath fault is detected or suspected, you can wait with a joint until the damage has been repaired. That way you do not have to troubleshoot a very long cable with possible several scattered damages, which can be time consuming. As a final step, the entire distance is tested.

 

Is it possible to sheath test cables lying in pipes?

• Both yes and no.
  • Sheath testing is based on contact between cable shield and earth. If a sheath fault does not have that contact, e.g. if the fault is in a dry pipe or hanging in the air, it will not be detected.
  • Many pipes are filled with water and soil, which means that sheath faults are detected.
  • Locating a sheath fault in pipes is more difficult. If the pipe is tight it is at the ends of it that the signal is found. Depending on how far into the pipe the fault is, that signal can be very weak. Sometimes it is not noticeable at all.
  • Longitudinal earth connector in the same pipe can affect testing and troubleshooting.
  • Pipes with many holes, e.g. Snip-snap or pipes with leaky joints, can facilitate testing and pinpointing.
  • Sheath faults in pipes on cables with outer conductive layer on the outer sheath (TTCL) are detected during the test but have the same problems at pinpointing as "ordinary" cables.

 

 

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

• If the shield at the other cable end is not in contact with earth, the cable can be sheath tested.
• If the other cable end is buried, the end seal must be completely tight.
• If the test indicates a sheath fault, both ends of the cable must be accessible.
  • To ensure that the other end is tight or does not have contact with earth.
  • To short-circuit two conductors with the cable screen to be able to prelocate. Therefore, it is good if the other cable end is available and e.g. not mounted in a pole, tucked in an energized compartment or lying far in on a cable drum.

 

 

Can I see how many sheath faults a cable has?

• No, you cannot see how many sheath faults a cable has.
• At pre-locating, the equipment points to the sheath fault with the lowest transition resistance from the cable shield to earth at the measuring equipment.
• If there are several faults with approximately the same resistance, the equipment "bakes" them together into one fault. Once you pinpointing without finding something where it should, but far from it, there is likely to be further damage.
  • Example: There is damage 10 m from each end of a cable. They have about the same transition resistance to the earth. The measuring equipment will say that the fault is in the middle of the distance. Once there, nothing is found and you have to start searching in one direction or another until you find something. If the cable is 300 m long, it goes quite fast, if it is 10 km it can take a while.

 

 

If the cable has several sheath faults, which one do I find first?

• The sheath fault with the lowest transition resistance from the cable shield to earth at the measuring equipment is found first. However, this does not have to be the most serious injury. There is much else that comes into play; type of soil, fault´s contact with soil, soil moisture, distance to possible earth connector etc.
• The measuring equipment does not find the nearest fault first.

 

Is it possible to sheath test a cable before it is mounted in the station or the terminations are in place?

• Yes, it works but there are a few things to keep in mind:
  • Is there a risk that someone or something will reach the other cable end during the test?
  • Is there a fire risk at the other cable end? For example dry grass.
  • Are the conductors grounded at the measuring cable end (or elsewhere if the conductors are connected)? At a cut cable end, the test voltage in the screen can reach a grounded conductor due to the short distance between the screen and the conductor. Open the grounding switch, release connections or remove outer conductive layer a few cm at the cut cable end.
  • Make sure that no part with screen potential (incl. conductive crotch or slacks) on a possible termination has contact with ground, branches, grass etc.
  • The parts of the cable that do not have contact with the ground are not sheathed, e.g. if it is not backfilled around a station.

 

 

I have found a damage to the outer sheath. What am I doing now?

• Make sure that only the outer sheath is damaged. Open and check. The outer conductive layer on the conductors must be intact. Splice if you are unsure. Do not take a chance!
• Build up a new outer sheath with e.g. thick self-fusing sheet.
• Finish with a heat shrink wraparound with hot melt glue as sealant - no matter how small the damage has been.
  • Thoroughly insulate between the screen and the wraparound, especially if the wraparound contains an aluminum foil.
  • Grind roughly all surfaces that the hot melt glue of the wraparound should attach to, even at the overlap.

 

 

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

• The thin, conductive layer must be removed at least 100 mm from the sheath edge at the joints and at least 100 mm from the creep current catcher on the cable (see picture) at the termination.
• In the event of sheath fault, the conductive layer must be removed at least 100 mm in all directions.
• The layer can be removed with a lathe (if the cable is round), glass, planer or similar.

Insulation testing

What is the approved value for meggering?

• There is no standard answer to it, but the following limits are often mentioned:
• Low voltage cable:
  • Tested with 500 or 1000 V DC and at least 1 MΩ insulation resistance.

• Medium voltage cable:
  • Many have a limit of 10 MΩ/kV test voltage. This means 50 MΩ at 5 kV DC test voltage, which in practice is not particularly good for this type of cable.
• External factors affect, e.g. humidity, dirt and temperature.
• Several conductors can be tested simultaneously if there is earth potential between them, e.g. through an outer conductive layer, like in PEX-insulated medium voltage cables.

 

 

What is VLF testing?

• VLF = Very Low Frequency, alternating current (AC) at a very low frequency, 0.01-0.10 Hz.
• Replaces insulation testing with direct current (DC, meggering).

 

 

How high voltage do I test with VLF testing?

• The test voltage is based on the grids operating voltage (U0). 1.7 or 2,0xU0 on aged cable, 3xU0 on new.
• The test time is 30-60 min.

 

 

How long cables can I test?

• The capacitance of the cable (depending on length, area, insulation, construction) affects how long cables can be tested. 0.1 Hz is preferably used, but the capacity of the measuring equipment may limit so a lower frequency is required. This means increased test time.

 

 

What results do I get from the measurement?

• The test does not give a measured value. It is a "Go/No go test".
• Several conductors can be tested simultaneously if there is earth potential between them, e.g. through an outer conductive layer, like in PEX-insulated medium voltage cables.

 

 

Can the test destroy my cable?

• Yes, VLF voltage can adversely affect already weakened insulation. However, good insulation is not affected.

 

 

What is PD measuring?

• PD stands for Partial Discharge.
• PD can occur in the event of assembly defects, damage to conductive layers, severe treeing (water trees that have developed into electrical trees) etc.
• PD measurement in operation (on-line):
  • Equipment is mounted  on the terminations and monitors the cable in real time.
  • Only test at operating voltage.
  • Nexans does not have this service.
• PD measurement on non-energized cable (off-line):
  • VLF test instrument is used as a voltage source.
  • Test voltage up to 1.7 or 2.0xU0 is the most common.
  • The test is monitored and can be interrupted if necessary.
  • Nexans offers this service.

 

 

What is Tangens Delta measuring (TD)?

• Measurement that detects the presence of treeing (water trees, small tree-like weakening’s in the insulation caused by moisture and electrical stress).
• VLF test instrument is used as a voltage source.
• Test voltage 0.5xU0 to a maximum of 2xU0 in several steps.
• The test is monitored and interrupted if necessary.

Longitudinal ground control - Also called Ymer, screen or return current measurement.

What am I really measuring?

• The measurement compares the resistance in the cable shield with all other earth connections between two points, often two stations.
  • Outside densely populated areas, the outer earth connection usually consists of a longitudinal copper earth connector.
  • In densely populated areas, several parallel external earth connections can affect the measurement.

 

 

What do I get for measured values?

• By dividing the current over the entire cable (I-outer) by the total measuring current (I-test), a so-called M-value is calculated. This should be between 0.3-0.9.
  • M-value below 0.3 shows that too much of the return current flows through the cable shield.

Common causes:

• • • Missed to short circuit conductor + screen + ground at the other end.
    • The earth connector is not connected to earth at one or both ends.
    • Interruption of the earth connector.
  • M-value above 0.9 shows that a too small part of the return current flows through the cable shield.

Common causes:

• • • Missed to short circuit conductor + screen + ground at the other end.
    • The cable shield is not connected to ground at one or both ends.
    • Strong external earth connections. Several parallel.
    • Interruption or high resistance in the cable shield.

 

 

When should I do a longitudinal ground control?

• The test must be carried out on newly laid cables or if changes have been made to the existing grid. For example: If newly laid cable is spliced ​​to an existing.
  • When new cables are to be joined to existing ones, you should start with a check of only the new cable. Then the contractor receives a "receipt" that their work has been carried out correctly. After splicing against existing cable and earth connector, the test is performed again, from station to station.

 

 

Can I measure where an earth connector is interrupted?

• We do not know of any instrument for this. Instead, we usually dig down to the earth connector in two places and lay an isolated cord on the ground between them, connect the cord to the earth connector in one pit and measure the resistance between the cable and the earth connector in the other.
  • If the earth connector is intact, you get an even, low resistance. Then continue forward with another hole and repeat the resistance measurement.
  • If you get an uneven, high resistance (often around a few hundred kΩ) in the circuit, it shows that the earth connector is interrupted. Then you move on to the halving principle until the interruption is found. There can be many pits.