Rewire.

[ebee]

Agreed, it would be very rare to Test MCBs except for type testing an fuses we never check them either, we go with the flow with them.

Yes that RCD data is quite old and it was collected abroad I think so it may not have been truly representative for the UK even back then. what a reasonable guess is nowadays I have no idea and yes an RCD failure includes those outside their given parameters by how ever much,

I have come across RCDs that do fail to trip at all and a few that are wildly out too.
Most RCDs I have actually tested are well within to to the extent it narrows down the difference between the old BS standard and the newer BS(en) standard to virtually nothing at all in real terms, so any perception that our old BS spec was better are not really reflections of what is!
It`s all theoretical at best.
But they do sometimes fail though, so what can we reasonably do?

on an aircraft try to get a back seat, they tend to not very often "back into a mountain"!

 

[SUNRAY]

Exactly. So long as the status quo remains unchanged, I thing of it as "TT+"

Indeed. The greatest 'risk' is not that something would go wrong with the bonding within either of the houses but, rather, that the (presumed)) metal water supply pipe which is currently (presumably) joining our electrical installations (via bonding) could e changed to plastic 'one dark night'. That's why I wrote ..

As above, that's exactly my position and thinking.

I have that sort-of 'covered', at least in terms of clearing faults before anyone gets a shock, since there are up-front 100 mA TD RCDs upstream of the RCDs (RCCBs and RCBOs) in CUs.

I'll never knock anyone for being ultra-cautious, whether in terms of wanting redundancy of protective devices or anything else, even when I personally regard it as a bit 'OTT' (but even I would be very nervous stepping onto an aircraft if I wasn't aware of all the built-in redundancy!)

As for this "7% failure rate", people have been quoting that figure for very many years (probably 20+ years, maybe a lot more), hence based on some very old, maybe very 'early' (in terms of RCDs), data. I also seem to recall that the 'failure' in question related to 'testing', so many of that "7%" may have been only fractionally 'out-of-spec', which could be of little consequence given that the 'spec' itself is essentially arbitrary ... not to mention the fact that being 'early' the data may well have related to generations of RCDs less reliable than current ones. There surely must be a lot more, more recent data relating to recent devices?


However, as always remind folk, it's all very well being concerned about the reliability of RCDs for providing fault protection, but the alternative is to rely on MCBs or the over-current functionality of RCBOs - and since there is no practical way of routinely testing them, we really have no idea about their in-service reliability - maybe their 'failure rate' is 70%?

Kind Regards, John

"no practical way of routinely testing them" - Why not?
One company I worked for had an MCB tester, rated for Type 1 & 2 to 60A. Are MCBs now so reliable that such testers do not exist anymore?

 

[JohnW2]

"no practical way of routinely testing them" - Why not? ... One company I worked for had an MCB tester, rated for Type 1 & 2 to 60A. Are MCBs now so reliable that such testers do not exist anymore?

I don't doubt that 'MCB testers' exist, not the least because it would be the easiest thing in the world to design and manufacture. However, do you think that any electrician has ever possessed one and used it to test MCBs in a domestic installation (or, come to that, any installation)?

In reality, what 'testing' is done is essentially dictated by BS7671 and GN3, and I think you would struggle to find any mention of testing MCBs in either of them As for ...

Are MCBs now so reliable that such testers do not exist anymore?

.... as I recently observed, I haven't got a clue about in-service reliability of MCBs - not the least because, as above, they are never, or virtually never, tested whilst in service (at least, not in domestic/commercial installations). As I said, for all I know they could be a lot less reliable than RCDs. In fact, it wouldn't actually surprise me all that much if they were less reliable - MCBs very rarely operate 'in anger' (many never operate through their in-service life) and, unlike what hap[pens (is meant to happen) with RCDs, they are not regularly 'exercised' in the name of testing. It therefore could well be that MCBs are, in practice, more likely to fail because of 'sticktion' than RCDs.

There's another possible issue with testing MCBs. I don't know whether this thought process is still around but there was a time when some people believed that (analogous with crash helmets, child car seats, seat belts etc.) if an MCB had 'operated in anger' (i.e. as a result of over-current) it should be immediately replaced - and it would be a bit ironic (aka plain daft) if the act of testing resulted in the tested device being scrapped and replaced

 

[SUNRAY]

I don't doubt that 'MCB testers' exist, not the least because it would be the easiest thing in the world to design and manufacture. However, do you think that any electrician has ever possessed one and used it to test MCBs in a domestic installation (or, come to that, any installation)?

How else do you think I know about one?

In reality, what 'testing' is done is essentially dictated by BS7671 and GN3, and I think you would struggle to find any mention of testing MCBs in either of them As for ...

.... as I recently observed, I haven't got a clue about in-service reliability of MCBs - not the least because, as above, they are never, or virtually never, tested whilst in service (at least, not in domestic/commercial installations). As I said, for all I know they could be a lot less reliable than RCDs. In fact, it wouldn't actually surprise me all that much if they were less reliable - MCBs very rarely operate 'in anger' (many never operate through their in-service life) and, unlike what hap[pens (is meant to happen) with RCDs, they are not regularly 'exercised' in the name of testing. It therefore could well be that MCBs are, in practice, more likely to fail because of 'sticktion' than RCDs.

There's another possible issue with testing MCBs. I don't know whether this thought process is still around but there was a time when some people believed that (analogous with crash helmets, child car seats, seat belts etc.) if an MCB had 'operated in anger' (i.e. as a result of over-current) it should be immediately replaced - and it would be a bit ironic (aka plain daft) if the act of testing resulted in the tested device being scrapped and replaced

Yes I've heard of those stories and it became a reason to use the tester

 

[JohnW2]

How else do you think I know about one?

As I said, I've never doubted that they exist, not the least because testing is clearly a necessary part opf the development and manufacture of MCBs. However, as I said, I've never seen (or heard of) one being used as part of the I&T of an electrical installation. I'm pretty sure that you are the first person I've ever heard saying that they had personal experience of such a bit of kit.

Yes I've heard of those stories and it became a reason to use the tester

 

[Collectors]

Nice to see a double red on the switch drop, rare to see these days.

 

[ebee]

that is very interest

"no practical way of routinely testing them" - Why not?
One company I worked for had an MCB tester, rated for Type 1 & 2 to 60A. Are MCBs now so reliable that such testers do not exist

That is very interesting Sunray.
If a manufacturer made one I am sure that some would be bought. But enough to make sales worthwhile? not quite sure.
Of course some of us could make our own, not many would bother but some would do I think.
I did think about it a couple of times it it remained as purely a thought experiment really. Rely on MCB makers to Type Test and hope for the best.
As for timing, it takes a bit more, a simple stopwatch type thingy is one thing but a reasonably accurate timer and monitoring is something else for nearly all.
But hey yes, if enough folk had them then all for the good I thinks.
Anyway Sunray, thanks for that comment. I have learned something.

 

[ebee]

are not really

It therefore could well be that MCBs are, in practice, more likely to fail because of 'sticktion' than RCDs.

Yup I am with you on that one.

 

[pete01]

Nice to see a double red on the switch drop, rare to see these days.

Its 2 single reds, well it was.

 

[plugwash]

If a manufacturer made one I am sure that some would be bought. But enough to make sales worthwhile? not quite sure.

I did some searching and it seems that omicron advertise their "compano 100" for MCB testing, but it only has an output current of 110A, so seems only suitable for B20 and smaller MCBs.

 

[ebee]

yes double reds were rare but some installed nonetheless. were more expensive so red and black with often but not always a red sleeve or tape flag. I always sleeved Red where required. I found it actually made it easier to look at to check connections funnily enough (I did realise that the majority favoured true Red as COM on switching two ways in 3 core and E though, unlike me)

 

[ebee]

Thnks Plugwash, interesting

 

[SUNRAY]

As I said, I've never doubted that they exist, not the least because testing is clearly a necessary part opf the development and manufacture of MCBs. However, as I said, I've never seen (or heard of) one being used as part of the I&T of an electrical installation. I'm pretty sure that you are the first person I've ever heard saying that they had personal experience of such a bit of kit.

OK yes I understand that and to be honest it isn't the sort of thing to lug around willy nilly, it is big and substantial to dissipate the energy from a 60A MCB on test (the reason it was limited to 60A and and only included type3 to something like 15A) with a noisy tangenital fan and huge single pole contactor, I'm guessing 12"x12"x18".
For a long time it didn't occur to me it was a rare tester (and why not upgraded) until I became aware of them being hired. But one has to consider the environment I've mostly worked in is commercial and some very significant kit, pump and fan motors being in 10's of KW, boilers of 100's of KW for example.

 

[SUNRAY]

As I said, I've never doubted that they exist, not the least because testing is clearly a necessary part opf the development and manufacture of MCBs. However, as I said, I've never seen (or heard of) one being used as part of the I&T of an electrical installation. I'm pretty sure that you are the first person I've ever heard saying that they had personal experience of such a bit of kit.

OK yes I understand that and to be honest it isn't the sort of thing to lug around willy nilly, it is big and substantial to dissipate the energy from a 60A MCB on test (the reason it was limited to 60A and and only included type3 to something like 15A) with a noisy tangenital fan and huge single pole contactor, I'm guessing 12"x12"x18".
For a long time it didn't occur to me it was a rare tester (and why not upgraded) until I became aware of them being hired. But one has to consider the environment I've mostly worked in is commercial and some very significant kit, pump and fan motors being in 10's of KW, boilers of 100's of KW for example


I have no idea why I have a double post 5 minutes later, my PC was closed and I was eating my dinner.