test method

The required test voltage level, the duration of the test, the type of test, and the maximum permissible leakage current are clearly defined in the corresponding standards.

1. High Voltage Test with Focus on Excess Current and Breakdown

The classic purpose of the high voltage test is to verify the dielectric strength of the insulation and to detect dangerous faults such as breakdowns or unacceptably high leakage currents. In this approach, the exact insulation resistance is not determined; instead, it is exclusively monitored whether the flowing test current remains below a specified limit value.

During every high voltage test, the test current must not exceed a defined maximum value. If this limit value is exceeded, an automatic and rapid shutdown of the test voltage occurs for safety reasons. This type of evaluation is typical for high voltage tests and primarily serves to prove electrical safety.

Additionally, a minimum current can be specified. This is particularly useful for devices under test with significant capacitive components. As soon as a minimum current is measured, it is ensured that the device under test is actually under high voltage. During the DC high voltage test, when the test voltage is switched on, the capacitance of the device under test is briefly charged. The resulting charging current surge can be used for the detection and monitoring of this minimum current.

2. Additional Determination of Insulation Resistance under High Voltage

If the insulation resistance is determined beyond mere excess current and breakdown detection, additional time-dependent effects must be considered. After applying the DC high voltage, phenomena such as dielectric absorption, polarization processes, and effects often occur, as known, for example, in the determination of DAR (Dielectric Absorption Ratio) or PI (Polarization Index).

These effects cause the measured current or the calculated insulation resistance to be time-dependent and only stabilize after a certain waiting period. To determine the insulation resistance as accurately as possible, these transient effects must have completely or largely decayed. Only then does the measured current essentially represent the actual ohmic leakage current through the insulation.

In this extended test approach, the high voltage test thus combines two aspects: on the one hand, the safety-relevant assessment regarding excess current and breakdown, and on the other hand – with sufficient measurement duration and suitable evaluation – the qualitative and quantitative assessment of the insulation condition via the determined insulation resistance.

High voltage test probes: manual testing at multiple test points

During the high voltage test with DC voltage, the Capacitance present in the device under test is charged. After switching off the test voltage, this charge initially remains, meaning a dangerous Residual voltage can still be present at the device under test. This poses an acute risk of electric shock for personnel and a risk for subsequent work steps.

For this reason, it is imperative to discharge the device under test in a controlled manner after completing the DC high voltage test. Modern high voltage test devices are equipped with built-in discharge facilities that automatically and safely discharge the device under test to a non-hazardous voltage level. Only after complete discharge may the device under test be touched or further processed.

Reliable and monitored discharge is therefore an essential component of DC high voltage testing and indispensable for the safety of operating personnel and the test environment.