Cascading Overload


Thread Starter

Power System

What happens in case of putting cascading overload, i.e. the main circuit breaker has the thermal magnatic protection and this circuit breaker connect to more contactor with overload "as one combination". Contactor with overload is connected to machine. This is right system or wrong?
This is the normal setup where motor protection is involved. The reason is this. The thermal/magnetic setup in the circuit breaker will provide overcurrent protection similar to a fuse. However when this setup is applied to motor protection, the overcurrent settings have to cater for the motor starting currents which normally lie somewhere around 6 times the normal running current. If this overcurrent protection is used to protect the motor when it is running normally, rest assured that the motor will burn up before the overcurent protection detects that there is something wrong. To overcome this, a thermal overload unit is fitted on the contactor. This replicates the thermal characteristics of the motor, in that when the motor is cold (and so is the overload unit) the motor can withstand the high starting currents without damage and so overload unit does not operate. However when the motor has been running for some time and the motor windings are at their working temperature, the overload unit becomes 'hot' similar to the motor. So in this case when the motor goes in an overload condition (eg. 1.5 times normal current), the overload unit also goes in an overload condition (same 1.5 times normal current) and therefore operates tripping the motor. As can be seen from the example mentioned, the 1.5 times current passing through the motor is far below the 6 times tripping setting of the circuit breaker. This means that while the overload unit will detect the fault, the circuit breaker will not.