Using MCBs instead of switch fuses on tail extensions????

[dichroic]

Note. For overcurrent protective devices incorporated in a consumer unit complying with BS EN 60439-3: 1991 including Annex ZA, dated 2006 and having a rated current (In) not exceeding 50 A, the rated short-circuit breaking capacity may be taken as 16 000 A. This rating is however conditional upon the consumer unit being protected by a type II fuse to BS 1361 rated at not more than 100 A, or any other short-circuit protective device having the same let-through energy (I2t) and cut-off current.

 

[trebor]

So if there was a fault on the tails at say 16KA and the breaker is rated at 10KA are you now saying that we let the breaker go into meltdown why the 1361 fuse is rupturing ?

The BSEN60898 mcb's in a consumer unit are usually rated at 6KA. These are also on the load side of the meter so are we relying on the main fuse to stop these committing suicide in the event of a fault of negligible impedance ? If so why do we have to test PSCC and PFC to ensure the mcb's are rated correctly?

Regards

Dichroic

I admit that the regs as usual are a bit of a mish mash with lots of ifs buts and maybe`s but

In the extreme case of very high pfc where breakers are used at origin both b and c curve mccb`s are available at 10ka and above, this is normally only the case with industrial services of 200A and above where the service cables are larger and could be the case where a large service is brought into a block of flats say.

The normal domestic service is unlikely to exceed 6ka

in the case of 60898 cb`s in consumer units these can be taken to withstand 16ka where they are max 50A rated, enclosed in a BSEN 60439-3 or BS5486-13 enclosure, the main fuse is Type 2 BS1361 rated at no more than 100A

Trebor

 

[trebor]

Note. For overcurrent protective devices incorporated in a consumer unit complying with BS EN 60439-3: 1991 including Annex ZA, dated 2006 and having a rated current (In) not exceeding 50 A, the rated short-circuit breaking capacity may be taken as 16 000 A. This rating is however conditional upon the consumer unit being protected by a type II fuse to BS 1361 rated at not more than 100 A, or any other short-circuit protective device having the same let-through energy (I2t) and cut-off current.

Sorry Dichroic

I posted the reply below before I saw yours above, and ive just really repeated what you had already correctly said, i hope your post was not copyrighted

 

[Spudnik]

Sorry but i havent read the whole thread yet, but, MCB's generally have a 6or 10kA rating which could be blown to pieces in the event of a fault.