DC HV Testing

When you are doing a DC HV test, you are applying more than 2 time of rated voltage and this will provide over stress for cable. If it is a new cable, slowly we are destroying the insulation by applying HV DC. For example consider the case of cables. If we are applying positive DC Voltage, the stress because of the electric field will be radially outwards as shown below. Simply radial tensile force.

But if you are using negative DC voltage the stress devoloped due to electric field will be radially inwards. That is radial compressive force.

All materials having a capacity to withstand more compressive stress than tensile stress. In this way we can protect the good cable. But anyway week points will get break, if it is there.

This is the simple reason that negative DC is good for High Voltage testing.