High voltage testing
High voltage testing is preventative in nature (proof testing). Also known as withstand testing, this is a go/no-go test, although in some cases other attributes are monitored during the test (a.k.a., a monitored withstand test). If these other tests are implemented the overall test becomes diagnostic/predictive as well. In a high voltage cable test, a voltage that is typically larger than the rated voltage of the cable is applied to the cable for a prescribed amount of time. If the insulation does not break down, the cable passes. High voltage testing is a check for local faults. For example, in a XLPE cable, it looks to grow critical water trees into electrical trees and subsequently cause a failure in order to avert imminent in-service failures.
A cable testing failure indicates that the cable is already in a highly compromised condition. Crews should be prepared to make repairs to the cable if it becomes necessary due to the test. After repair, the cable is tested again until it passes the withstand test.
This testing may include a DC hi-pot (though not to be used on XLPE cables), a line frequency AC hi-pot, a 0.1 Hz AC (VLF) hi-pot, or a damped AC (DAC), whereby electrical stresses are created at a frequency ranging from approximately 50 Hz to several hundred Hertz depending on the test object capacitance.
A central application of cable testing is to verify the cable’s insulation and ensure safety requirements are met before installation in the field. Another use is to verify the integrity of a cable after a new cable or circuit has been installed or after repairs, before making the circuit operational.