A contactor is another form of switch operated automatically. Its rating should be matched to the size and type of load it is controlling. The use of a contactor allows the reduction in size of the contacts in a control device (eg time switch).

AC1 and AC3 are utilisation categories or switching capabilities of load types, taken from this standard:

• AC1 – non inductive or slightly inductive loads, resistance furnaces.
• AC3 – inductive loads, squirrel cage motors: starting, switching off motors during running

First check the RCCB by disconnecting the load cables and operate the test button. If it works, the RCCB is OK. Ensure neutral cables correspond to active cables. There may a crossed neutral, ie the current flow in active and neutral is not equal, thus tripping occurs.

All circuit breakers are subjected to short circuit testing at their marked rating and are capable of successfully interrupting the fault current without undue damage to the circuit breaker. The circuit breaker should not be installed into an area where the prospective fault level is higher than the rating of the circuit breaker (except where cascading is used as a design feature under Wiring Rules). Commercial installations and installations close to distribution transformers will have relatively higher fault levels. Consult your energy distributor for the fault level at a given installation.

There are 3 curve characteristics for magnetic operation,

• B 3-5 times line current used on generators and long runs, not widely available in Australia
• C 5-10 times, light and power circuits, most common, widely available
• D 10- 20 times, highly inductive loads, motors, transformers, some discharge lighting, welders