Low Voltage and High Voltage Cable Testing

Low Voltage and High Voltage Cable Testing

Low Voltage XLPE Distribution Cables:

Insulation Resistance:

Cables shall be tested for insulation resistance with an insulation tester (i.e. Megger) at 1000 Volts for 1 minute.
The minimum insulation resistance to earth or between phases shall be 100 meg-ohms.
The instrument used for this measurement shall have a minimum resolution of 10 meg-ohms on the 0 to 500 meg-ohm range.
At the conclusion of LV insulation resistance testing, the neutrals must be connected to the earth stakes.

Phasing Test:

The correct phasing of all LV circuits shall be checked at all positions where the LV cables are terminated into fuse bases and where any LV cable is run from point to point.
This test shall be performed with an instrument designed for the purpose. Mains frequency voltage of 240 Volts is not acceptable for this test.
The neutral conductor shall be connected to the earth stake for this test.

Continuity Test (resistance of bolted connections):

For loop LV systems, a continuity test shall be carried out on each LV circuit to ensure that all bolted connections are complete and adequate. The test shall be carried out as follows:
(1) At the transformer firmly bond all 4 conductors together
(2) Undertake a continuity test at every point where there is a service provision or open point. In a fused service pillar the bottom row of fuses bases must be the point at which the test is undertaken as that is the furthest extent of the network.
The difference between the readings of each phase conductor and the neutral for each individual test shall not be greater than 10% of each other. Any difference greater than this may indicate a loose or dirty connection and will require further investigation.
The instrument used for this measurement should have a resolution to the second decimal point in the 0 to 5 ohm range.
A typical instrument would be the earth “Megger” type and taking into account the resistance values of the test leads.

Earth Resistance Test:

In any overhead or underground network the earth resistance at any point along the length of a LV feeder is to have a maximum resistance of 10 ohms prior to connection to the existing network.
In any overhead or underground network the overall resistance to earth Shall be less than 1 ohm prior to connection to the existing network.

11 KV AND 33 KV XLPE Cables:

Phasing Test

The correct phasing of all HV circuits shall be checked at all positions where the HV cables have been terminated.
This test shall be performed with an instrument designed for the purpose. 240 Volt mains frequency is not acceptable for the performance of this test. The test may be conducted on either the wire screens or the aluminum conductors.
Where the test is performed on the wire screens, they shall be disconnected from earth.

Outer Sheath Insulation Resistance (Screen wire test)

The purpose of the test is to determine soundness of the outer polyethylene sheath against water ingress, mechanical damage and termite attack.
Values below 0.5 meg-ohms (500 kΩ) can indicate sheath damage. Values between 1.0 and 10 meg-ohms may not indicate damage in a single location. Fault finding can often be very difficult. In new cables, values of greater than 100 mega ohms are required.
The integrity of the outer sheath shall be checked after cables have been buried by an insulation tester (Megger) at 1000 Volts.
The test shall be conducted for 1 minute between each wire screen and earth after the cable has been jointed and terminations installed.
For cables after repairs, the resistance must not be less than 10 meg-ohms.
Where HV cable circuits are cut and joined to new circuits, sheath testing must be carried out on the existing old circuit prior to joining to the new cable.

HV test on XLPE cables already in service or previously energized

Except for New Cables, Testing at Voltage greater than 5.0KV is not permitted

Studies carried out on DC high voltage testing of XLPE cables now conclude that;
DC testing above 5kV of field aged XLPE cables generally increases water tree growth and reduces service life.
5kV is not considered a “High Voltage DC Test”. The test voltages for tests on XLPE cables is now limited to 5kV after in service repairs and 10kV for new installations.
A 5kV Megger is suitable for a 5kV test on cables after repairs.
The changes to this section will also make it possible for a repaired cable to be tested by repair crews and made available for immediate return to service.

HV test on new XLPE cable:

Prior to the performance of this test, the screen wires must be connected to the permanent earth position.
The cable shall be tested at the test voltage and the pass criteria shall be in accordance with the following table:

If further repair works are undertaken, and they require additional joints to be installed, the complete HV testing procedure shall be repeated.

Alternative HV Test Requirement on Insulation for 11kV Cables

Where it is not practical to conduct a high voltage test, the test requirements for insulation (core to screen wire) may be limited to testing for the condition of “safe to energize”. The following list of circumstances and conditions must be met as a minimum requirement:
The cable circuit voltage shall be 11kV,
The circuit outage duration shall be not more than 48 hours,
The work shall involve extending or repairs to existing circuits,
The insulation test shall be applied for 1 minute between each
phase core and screen with a 1000 Volt minimum insulation tester (Megger),
Typically the test result should be in the order of 1000 meg-ohms.

Paper Insulated Cables:

Tests on LV Cables

An insulation resistance test shall be conducted with a 1000 Volt megger. Test results as low as 10 meg-ohms on old cable circuits are common and therefore considered safe to energies.

Test on 11kV and 33kV Cables between Cores and Earth

For three core belted cables, the test on any core shall be conducted between the core and lead sheath with the remaining two cores connected to earth.
The test voltages and pass criteria shall be in accordance with the table below.

66kV XLPE CABLES

Core to Sheath Test after Repairs:

After repairs have been carried out, the 66kV XLPE cable shall be energized at power frequency for 24 hours without load. DC testing is not permitted.
The cable sheath link box/cross bonding system shall be put into its normal condition.

Outer Sheath Integrity Test:

An insulation resistance test between the metallic sheath and earth shall be conducted. The anti-termite barrier must be connected to the metallic sheath and the insulation test performed to earth.
The test voltage applied for 1 minute shall be 5kV DC applied with either a high voltage test set or insulation resistance tester (Megger).

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