mcb failure

[nospam]

Hello,

The lights would not work this evening. I found the fault was the mcb. How common is mcb failure, what causes it, and what are the sympoms? I've never known it to happen before.

Thanks.

[John Rumm]

Generally it is fairly uncommon with most brands. However I have seen it happen with some Proteus MCBs made by Geyer.

The symptoms in that case were that sometimes when you switched them on mechanically they failed to make any electrical connection. Turning off then back on again would often fix the problem.

[nospam]

Hello again,

In this case we came home one night and the lights did not come on; there were no other symtpoms. We looked at the mcb and it was still in its "on" position, it had not tripped. I changed bulbs, checked wiring, etc and found no fault so tried another mcb and that worked. I put the old one back and it didn't, so I know it's the mcb.

It's a crabtree btw.

It's just I thought it would trip to "off" if it failed or take the rcd with it. No buzz, no smoke, no burning smell, nothing ;)

[John Rumm]

One possibility that can occur (although rarely) - especially if you are close to a substation - is the fault currents that can flow may exceed the breaking capacity of the MCBs. This can result in them failing terminally when attempting to clear a fault.

(most MCBs can break upto 6kA, although some designed as plug-ins for old wylex units can be as low as 3kA)

[ARWadsworth]

Sometimes they just fail. Infrequent but it happens.

Adam