At the handover acceptance and maintenance site of the switchgear, circuit resistance testing is an essential item that cannot be missed every time. Many people know that it needs to be tested, but they cannot explain why it needs to be tested or what it is - today I will explain this question from the beginning.

What is circuit resistance

The conductive circuit of switchgear such as circuit breakers and isolating switches is composed of moving contacts, stationary contacts, and their connecting conductors. When the switch is in the closed state, current enters from one side, passes through the contact surface of the moving and stationary contacts, and flows out from the other side. The resistance of this complete current path is called "loop resistance" or "contact resistance".

The magnitude of circuit resistance is extremely small, usually ranging from tens of micro ohms to hundreds of micro ohms. But it is precisely these hundreds of micro ohms that generate significant thermal losses - I ² R - when high currents pass through. Once the current reaches the rated value, the thermal power at the contact point may be considerable.

What will happen if the contact resistance exceeds the standard

Oxidation, carbon deposition, insufficient contact pressure, or contact wear on the contact surface can all lead to an increase in contact resistance. There are two consequences of high contact resistance: one is that during normal operation, the contact point continues to heat up, accelerating insulation aging and contact erosion, forming a vicious cycle; Secondly, in the event of a short circuit fault, the contact point may overheat locally, causing the contact to melt and the switch to fail to open normally, resulting in protection failure.

That's why national standards and regulations in the power industry clearly require that the conductive circuit resistance of switchgear must be measured after leaving the factory, handover, and major repairs, and the test current must not be less than 100A.

Why is it necessary to use 100A testing

When measuring low current (such as milliampere level), it is impossible to penetrate the oxide film on the surface of the contact, and the measured contact resistance is low, which masks the real problem. Only by applying a sufficiently large current (above 100A) can the conduction state of the contact be truly reflected under actual high current conditions.