Dual grounding vs. One-side grounding

When working on a high-voltage circuit breaker (HV CB) at an operating substation, it must be grounded on both sides to ensure safety. This is implemented to protect personnel working on the equipment from induced voltage and the possible occurrence of high voltage.

To measure the contact resistance of a HV CB with a micro-ohmmeter, the HV CB must be closed. When the CB is closed with two-side grounding, a pair of two parallel resistances is created, which in theory will affect the CB contact resistance measurement reading. To avoid creating two resistances in parallel (the resistance of the CB and the resistance of the grounding circuit), the HV CB must be ungrounded on one side. This will eliminate one of the parallel resistances, and only one resistance will be involved in the measurement – the contact resistance of the CB.

This method is the easiest and most accurate for measuring contact resistance. However, removing one of the two ground connections reduces the safety of personnel working on the HV CB.

To ensure personnel safety, grounding must remain enabled on both sides. The question is how this will affect the results of the contact resistance measurement and whether the difference will be significant. To this end, a series of tests were carried out at various substations to determine the effect of the grounding circuit, as a parallel resistance, on the HV CB contact resistance results.

Several tests and their results:

When measuring on a two-side grounded CB, the actual resistance is underestimated by the values given in the table above.

The measurement error depends on the resistance of the grounding circuit. This resistance may vary depending on the condition of the disconnector grounding blades and the quality of the contact of the installed portable grounding device.

In general, for the initial determination of the HV CB condition, the method of measuring the contact resistance with two-side grounding is more efficient. In this case, personnel safety is fully maintained and no additional manipulations are required to unground one side of the HV CB.