The rule of thumb is less than the rated voltage or the maximum capacity of the test equipment.
There are several test techniques available in literature and standards (IEEE400). All these tests have advantages and disadvantages. Sometimes the client or the utility may have specific test requirements. However selection of tests must generally be done with the following requirements in mind:
- If it's a new installation, is it safe to energize the cables?
- If it's an existing installation and some remediation work has been carried out, is it safe to energize the cables?
- If the cable has been in service, how much longer can it be in service?
- Will testing damage the new/existing cable installation?
I usually use a single or a combination of these tests, before making a decision to energize the distribution assets. The test shot values given below are for a brand new installation:
- Insulation Resistance: This is the test you are mentioning about. For 30-36 kV assets, you apply 10kV DC for approximately 1 minute and measure the insulation resistance. This is fairly standard irrespective of the age of installation for 33 kV.
- Power Frequency Withstand Test (also called AC hipot): In this test you apply a power frequency voltage (46kV~56kV at 50/60 Hz for equipment rated at 36kV) to the equipment for 1 minute.
- Very Low Frequency Test (VLF): For 36kV equipment apply approximately (46kV~56kV) for 15 minutes and measure the leakage current.
The exact test shots must be carefully evaluated depending on the installation. There are standard test kits available in the market. The most suitable test will depend on the type of cable you have.
For example, if you have old paper insulated cables, you should not use power frequency withstand test.
Alternatively if there is an inductive Voltage Transformer or distribution transformer connected to the network, you can test the whole installation in one go (instead of isolating each section of cable), by using the power frequency withstand test (56kV for 1 minute for equipment rated to work at 36kV) at 50 Hz. This saves time and money. Obviously a DC resistance test or VLF will fail in these cases.