HV Cable Test Voltage Calculator

Test voltage is a multiple of U₀, the phase-to-earth rating, and the multiple depends on the method and why you are testing. A new cable given a VLF or AC site test is normally taken to 2 U₀ for 60 minutes, a maintenance test to 1.5 U₀ for 30 minutes, and a test after a repair to 1.73 U₀. Factory routine testing goes higher, at 2.5 U₀ for XLPE. A 6 kV U₀ cable given a VLF test after installation is therefore tested at 12 kV.

A direct voltage injects and traps space charge in the polymer, which does not dissipate when the test ends. When the cable is re-energised on alternating voltage, that trapped charge adds to the peak of the AC waveform and produces local field enhancement far above the design stress. The result is a cable that passes its test and then fails weeks or months later. This is why IEEE 400 treats DC as a legacy method for extruded insulation and why VLF at 0.1 Hz has replaced it. DC remains acceptable for paper-insulated lead-covered cable.

Very low frequency testing applies an alternating voltage at around 0.1 Hz instead of 50 or 60 Hz. Because the capacitive charging current of a cable is proportional to frequency, a 0.1 Hz source needs roughly one five-hundredth of the reactive power that a mains-frequency set would, so the equipment is small enough to carry to site. It stresses the insulation in a way that resembles service conditions without the space charge problem of DC, and it can be combined with tan δ and partial discharge measurement in the same connection.

Insulation resistance roughly halves for every 10 °C rise, so readings must be corrected to a common reference, normally 20 °C, before they can be compared with earlier tests. The correction multiplies the reference value by 0.5 raised to the power of the temperature difference divided by ten. A 3.5 MΩ limit at 20 °C becomes 1.75 MΩ at 30 °C and 14 MΩ at 0 °C. Comparing an uncorrected summer reading against an uncorrected winter one is the most common way a healthy cable appears to have deteriorated.

Cable ratings are written as U₀/U, for example 6/10 kV. U₀ is the rated power frequency voltage between one conductor and earth or the metallic screen, and U is the voltage between two phase conductors. In a three-phase system U = √3 × U₀, so a 6/10 kV cable has 6 kV to earth and 10 kV between phases. Withstand test voltages are always specified as a multiple of U₀, because the insulation wall between conductor and screen is what the test stresses.