Modern protection measures

[maltaron]

With the latest regulations specifying MCB & RCD protection I am wondering about the reliability of these units. With the old fuses, allthough slow in operation and protected only against excess current, they could be relied on to fail safe - the wire melted!!. Has anyone known of a MCB or RCD failing not safe? i.e. failing in the ON state. I do not advocate not using modern protection, I merely question its long term reliability.
Food for thought?

 

[RF Lighting]

Never known or heard of an MCB failing in the on position.

Some RCDs have been known to not operate, but this is rare, and less of an issue, as RCDs should only be relied upon as supplementary protection anyway.

If all else fails, you still have protection from the service cutout fuse.

 

[hairyben]

MCB's are fairly simple, granted not as simple & inherently reliable as a fuse, but simple and reliable all the same. I've never knowingly seen a failed one, but seen countless wrong fuses installed, and incorrectly sized fuse or plain copper wire in 3036 carriers, so it's all relative.

RCD's, replaced a couple operating "slightly" out of spec, ie still providing protection but a few millisecs too late, which in the real world amounts to nothing, and only recall 1 which refused to trip. And of course, it is recommended that these are tested regularly.

 

[Spark123]

Have had a failed MCB before - fault took the upstream 63A MCB instead of the local 15A MCB.
Have had a number of RCDs fail too, some out of spec, completely refusing to trip, and recently one with a faulty test button. Maybe it's just me that's unlucky

 

[hairyben]

Have had a failed MCB before - fault took the upstream 63A MCB instead of the local 15A MCB.

What else was in the board besides the 15A? I'm wondering in the 63A simply reached it's capacity through lack of discretion. B/C types? 15A could also mean it's pretty old too, maybe a type 3/4 vs. a B63?

 

[Spark123]

Nothing else on the board in use, 63A was a B type (MG) and the 16A was a type 2 (Wylex NN).

 

[hairyben]

Nothing else on the board in use, 63A was a B type (MG) and the 16A was a type 2 (Wylex NN).

I know modern circuit breakers work in two aspects- normally bimetallic strip (overload) and electromagnetic coil (short circuit)

I'm not so sure about older 3871's- anyone know more?

I'm not arguing for the sake , of course it's possible for anything mechanical to fail, I'm just curious whether the 63A was more sensitive to the type of fault or not.

Unfortunantly, we have no means of testing MCB tripping charicteristics, although I'm sure if you sent it back to the manufacturers they could bench it, I'm not so sure they'd be keen to admit it'd failed in service...

 

[Spark123]

The fault was a smouldering enclosure with an RCD in it which I was holding at the time!
Would have been a few hundred amps flowing at that point, not massive amounts but enough for a change of underpants.
The type 2 also has magnetic and thermal parts to the trip - x 4-7 iirc for the magnetic whereas the type B is a x 3-5

 

[hairyben]

Okay I believe you now

Whats the design life expectancy of MCB's and RCD's generally?

I tend to flog people new consumer units, evan when they have older MCB-only units on the basis that they normally require RCD protection (always now) plus older MCB units like old crabtree and MK (siemens type) are simply old and who knows how reliable. I've never really known for sure just how reliable they are, there are in all likelyhood hundreds of faulty MCB's for everyone that makes it's status known in the manner you describe.

Has anyone ever done bench testing on a significant sample of secondhand MCB's to find out?

 

[industryspark]

Read somewhere that 3% of rcd's are believed to be faulty, i.e outside their tripping times,ect

Quite a large percentage if you think about it (if the statistic is true of course)

Ive heard people mention before that they think mcb's should be tested but the problem is how to test them, cant imagine an effective way of testing them safelly and guaranteeing not to damage them.

Would think there will be faulty ones out there but problem is they might not show until needed. Percentage wise tho i cant imagine faulty mcb's would be anything like the 3% qouted for rcd's (probably below 0.10% at a guess or lower.)

 

[Click-Sure]

I install a lot of Crabtree starbreaker units, around one in 50 MCBs fail, IE will not stay in the closed position.

Around 1 in 70 of their rccb's fail too, test button will not operate and rcd tester will not operate it either.

Had an EATON (MEM) RCBO fail out of the box recently too.