AC & DC HV Testing

Both are good. Please explain the reason for the query.

Some things to consider:

If you use DC for high voltage testing, there will be a small amount of current flow initially as the two insulated conductors build up charge. This could cause a false trigger of the leakage detection circuit if you push it to the limit of the equipment. In addition, the item under test may store that charge briefly after you disconnect so it must dissipate in some way. Nearly everything has some slight capacitive and inductive characteristics including the test leads you use.

If using AC it gets a little more complicated in that a sinusoidal waveform has a zero to peak voltage that is 1.414 times that of the RMS value of that voltage. The charge storage problem can be eliminated by turning off the test voltage when it crosses through zero.

An if your AC is more like a square wave or spiked then there will be high frequency components that more easily penetrate the insulator. So you must spend some time matching the test up to the characteristic you want to check out.

Ideally you will need to test your test. Once you understand the capability and limitations you can do more reliable testing by avoiding any equipment based issues.

Hope that helps!

See below note that I got from one website. It tells for proof testing DC is good.

So for a installed and used cable whether AC is good? (for creating the real working environment)

"When an insulated cable arrives on the job site, the recipient should be able to confidently assume it will attain the designed service life. This means it must arrive free of internal discontinuities in the dielectric such as voids or inclusions, as well as freedom from air pockets at the interfaces between the shielding systems and the dielectric's surfaces. It is, however, the specter of mechanical damage, or substandard splicing and terminating that could cause the engineers responsible for continuity of service to desire a field applied proof test to establish the cable's serviceability. The time-honored methods of proof testing in the field involve high potential direct current (dc). The advantage of the dc test is obvious. Since the dc potential does not produce harmful discharge as readily as the ac, it can be applied at higher levels without risk or injuring good insulation. This higher potential can literally "sweep-out" far more local defects. The simple series circuit path of a local defect is more easily carbonized or reduced in resistance by the dc leakage current than by ac, and the lower the fault path resistance becomes, the more the leakage current increased, thus producing a "snow balling" effect which leads to the small visible dielectric puncture usually observed. Since the dc is free of capacitive division, it is more effective in picking out mechanical damage as well as inclusions or areas in the dielectric which have lower resistance.

Field tests should be utilized to assure freedom of electrical weakness in the circuit caused by such things as mechanical damage, unexpected environmental factors, etc. Field tests should not be used to seek out minute internal discontinuities in the dielectric or faulty shielding systems, all of which should have been eliminated at the factory, nor should the dc potential be excessive such that it would initiate punctures in otherwise good insulation.

For low voltage power and control cables it is general practice to use a megger for checking the reliability of the circuit. This consists essentially of measuring the insulation resistance of the circuit to determine whether or not it is high enough for satisfactory operation. For higher voltage cables, the megger is not usually satisfactory and the use of high voltage testing equipment is more common. Even at the lower voltages, high voltage dc tests are finding increasing favor. The use of high voltage dc has many advantages over other types of testing procedure. "