The contacts in the circuit breaker need to checked periodically to ensure that the breaker is healthy and functinal. Poorly maintained or damaged contacts can cause arcing, single phasing, and even fire.
The two common checks conducted on the contacts of a circuit breaker are the visual inspection check and the contact measurement check.
The Visual inspection check involves examining the contacts of the circuit breaker for any pitting marks due to arcing and worn or deformed contacts.
The second check is the contact resistance measurement. This involves injecting a fixed current, usually around 300 A through the contacts and measuring the voltage drop across it. This test is done with a special contact resistance measuring instrument.
Then, using Ohm's law, the resistance value is calculated. The resistance value needs to be compared with the value given by the manufacturer. The value should also be compared with previous records.
Both these tests need to be done together. As there are cases of contacts having good contact resistance yet being in a damaged conditions.
Thus, for a contact to be certified healthy, it needs to have a good contact resistance and should clear the visual inspection test.