Discrimination

[mark28118]

Does anybody have any detailed information on discrimination, or could explain in abit of depth for me. Ive read BS7671 536.1 and know the basics but would be grateful of any more information that could be provided.

Thanks in advance

 

[johnboy6083]

Discrimination is the term used to describe slectivity of protective devices. For protective devices to discriminate, it means that only the device closest to the fault operates.

In basic terms, an upstream device can have a rating 1.6x the device below it and it should discriminate.

In more advanced terms, the energy let through of the downstream device should be less than the pre-arcing value of the upstream device.
With MCB's it is possible to provide overload discrimination by using them in series, but not short circuit discrimination. Its always best to feed an MCB from a fuse.

RCD's are different, and only by using S types can you acheive discrimination.

 

[mark28118]

So say for instance a 32A mcb in a cu, was feeding cu in a garage with a 40A main switch would this produce overload discrimination and be faulted in a EICR for example ? Whats the benefit of using a fuse upsteam ?

 

[telectrix]

the 40A switch is not a protective device. the 40A is it's max. safe load. the 32A MCB upstream protects the cable supplying the 40Aswitch.

 

[gutterball]

if you have a 32a mcb feeding another board and that board has another 32a mcb it is a 50/50 which one trips. (usually the most inconvenient). but if the downstream (nearest the load) mcb was a 16a mcb it would trip 1st.

 

[gutterball]

forgot to add fuses usual except a higher loading before they fail.

 

[Bosi]

So say for instance a 32A mcb in a cu, was feeding cu in a garage with a 40A main switch would this produce overload discrimination and be faulted in a EICR for example ? Whats the benefit of using a fuse upsteam ?

I would say, what's the benefit with MCB's apart from being more safe for grandma to "replace" :wink5:
Regarding overcurrent protection, fuses are outstanding.
They can handle short-circuit currents of 50-100 kA while an MCB typically can handle Icn of 6 or 10 kA.
They are current-limiting (as well as the energy) while MCBs are not, meaning that fuses will lower the short-circuit current which will allow a 3 kA MCB after it.
They are selective while MCBs in series, with different In, will trip randomly at high short-circuit currents.