| Measuring contact resistance of circuit breakers | Print this page  |
| The value of a primary circuit contact resistance (resistance between
two terminals of any pole of a breaker) is typically measured before a
circuit breaker is first put into service and during maintenance inspections.
The conditions, methods and procedure of the measurements should be compliant
to the regulations IEC 56 (now IEC62271-100) and ANSI C 37.09.
Four-wire (Kelvin) measurement method is used because it provides the best possible measurement. This ensures that the resistance of the connecting current cables is not included in the measurement. The test current is passed through a circuit breaker resistance using high current cables. The voltage drop across the circuit breaker is measured using sensing cables. The placing of cables is very important: the current cables should always be outside the sensing cables (see drawing). The resistance is calculated using Ohm's law: voltage drop divided by the test current (U=R*I i.e. R=U/I)
Procedure steps: 1. Make certain that the circuit breaker is de-energised on both
sides, closed and safety grounded and that local safety regulations
are followed. |
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