Can any one advise the regulations concerning Over rated protection devices, I am well aware that a circuit breaker must be adequate for its application, but what are the regulations regarding a circuit breaker that is over rated for the load, I.E. a 10A load with a 32A circuit breaker, what is the rule regarding the maximum above the load rating that a circuit breaker should and can be and is there a calculation for this.
any help would be greatly appreciated.
There are two aspects for the overcurrent rating, the first is for fault protection - in this case ALL circuits must have protection which will disconnect in the given times, and must prevent damage to the cable - it doesn't have to be the nearest breaker that provides this, but it makes more sense that it is rather than one further upstream.
The breaker must be sized low enough to provide this.
The second aspect is overload, and this is only required if there is a possibility of over load, so a 32A MCB feeding a 2.5mm2 radial having more than one socket would not be suitable because one could plug 2 x 13A loads in which case the cable would be overloaded and unprotected.
If this same 32A MCB now supplies a single fixed load of say 18A, then this would be OK - because the circuit cannot be overloaded - typical examples are showers, or cookers etc, once the shower is on full at say 10kW, you can't turn it up beyond this so overload protection is not required (not that this would be a 32A MCB or 2.5mm2 cable of course).
Another example might be a motor, this can be overloaded, but if the motor has a proper starter with overload protection at the motor end, the breaker supplying the cable to the motor doesn't need to prevent overload in itself , but just like every other case it needs overcurrent for fault protection.
