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Protection Riddle No.114 - Discrimination
In one of our MCC we have encountered a problem tat whenever there is flashover or any motor burnt, the fuse blows out but it also trips the MCC incomer. the MCC incomer has release devices with earth-fault and overcurrent protection. kindly suggest a solution.
Author : Kailash - From: India
Wed, October 15th, 2014 - 11:30
It seems the discrimination between upstream circuit breaker and downstream fuse should be checked again. For more information and practical experience you can refer to Switching, Protection and Distribution in Low-Voltage Networks (Siemens handbook) as illustrated below:

Discrimination between a circuit-breaker and a downstream fuse in the overload range and up to the tripping current IAn of the instantaneous overcurrent release in the circuit-breaker, conditions for discrimination are met if the time-current characteristic curve of the fuse (including its tolerance band) does not touch the tripping curve of the current-dependent delayed overload release in the breaker. This must also hold true if the circuit has been under load and the fuses/overeurrent release are at operating temperature (Fig. below).
For prospective short-circuit currents which would equal or exceed the tripping value of the instantaneous
Over current  release, discrimination is only possible if the fuse limits the actual short-circuit current during rupturing to a let-through value which is lower than the tripping current of the release. This can only be expected if the fuse rating is much lower than the rated current of the circuit breaker.
Practical experience has shown, that short-circuit discrimination is ensured if the delay time t, of the short time delayed over current release shifts the tripping curve to at least 100 ms above the time-current characteristic curve of the fuse.

Author : Hamid - From: Iran
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