Short circuit

NEBITOMI · Jul 14, 2020

HELLO
i had an short circuit in my house!neutral to live short circuit!
the 30a old mcb is tripped but the 1361 13a fuse in the plug did not disconected!
why did not disconnect 13a 1361 fuse sooner then 30a mcb?
I tested the fuse in the plug still good, so the short circuit did not reach to 13a!but why 30a mcb tripped ?
The fuseboard has not RCD old wylex fuseboard!
please can someone sort explain ?

James · Jul 14, 2020

Modern circuit breakers are designed to trip faster than 13A fuses.
change the fuse as well as it is likely to have been weakened.

p.s. after you have found and fixed the short circuit.

telectrix · Jul 14, 2020

and the plug fuse is BS1362, not 1361, or should be.

Lucien Nunes · Jul 14, 2020

The way multiple OCPD in series will react is a complex subject, especially here where the time/current curves for fuses and MCBs are quite different. Far from being less than 13A, the fault current was likely thousands of amps but cut short by the MCB before the fuse pre-arc limit had been reached. With a different value of Zs and hence fault current, the fuse might have opened first.

The different characteristics can be exploited e.g. for protecting a submain with a fuse having an In closer to that of the MCBs in the downstream DB than would be possible with an MCB upstream.