Mixed brand MCBs and EICR coding

[stillp]

Life is full of risks, and only if they are quantified can one decide whether they warrant any 'avoiding action'.

Not true. Have you never come across ALARP?

I am being told of these 'known potential dangers', so I would like to see the nature of the evidence and, ideally, given some indication of the probability of these 'potential dangers' becoming realised dangers in practice.

What would be the benefit to the manufacturers of showing you the evidence? No manufacturer would release test results showing that a) their MCBs failed in another company's CU, or b) their CU failed when another manufacturer's MCBs were used.

I am being asked to have 'blind faith' in something without any of the evidence being available to me

No, you're being asked to adopt a precautionary principle.

I wonder, for example, how these "(small) changes in performance" due to the proximity of different-make device compare with, say, the changes in performance that can result from a 20°C change in ambient temperature.

What about a localised 60K change due to an adjacent device?

 

[stillp]

"Quite likely"? Do you seriously believe that??

Yes. Do you have a sound engineering reason to doubt it??

 

[JohnW2]

"Quite likely"? Do you seriously believe that??

Yes. Do you have a sound engineering reason to doubt it??

Yet again, this seems to be getting silly.

If the performance of a protective device were so very marginal that it failed to pass the required performance tests just because it had been housed in a different (but conceptually similar) enclosure, I would not want that protective device anywhere near my electrical installation!!!

Kind Regards, John

 

[stillp]

Yet you'd be happy to fit mixed manufacturer's MCBs in a CU in spite of professional instructions not to?

 

[stillp]

If the performance of a protective device were so very marginal that it failed to pass the required performance tests just because it had been housed in a different (but conceptually similar) enclosure

That is misuse, whatever you mean by "conceptually similar". Are there other misuses of protective devices that you think are acceptable?

 

[JohnW2]

Yet you'd be happy to fit mixed manufacturer's MCBs in a CU in spite of professional instructions not to?

On the basis of what little information I am allowed to have, I would be very much happier to have mixed make MCBs in a CU than to have a CU full of same-make MCBs whose design and performance was so marginal that they would be "quite likely" to fail the required performance tests if moved into a different enclosure.

Kind Regards, John

 

[JohnW2]

If the performance of a protective device were so very marginal that it failed to pass the required performance tests just because it had been housed in a different (but conceptually similar) enclosure

That is misuse, whatever you mean by "conceptually similar".

By "conceptually similar" I meant another 'CU' enclosure - and I only said that because some people around here are so determined to nit-pick my words that had I just written "enclosure" (as you had done), they might have asked me if I was talking about the "lions' enclosure" at a zoo or the "winners' enclosure" at a race course!

As for 'misuse', you need to tell yourself about that - it was you who wrote:

It's quite likely that they will pass the tests in one manufacturer's enclosure and not in another.

... and I merely responded by pointing out that it would be a frighteningly marginal product that would fail the required performance tests simply because it had been housed in a different enclosure - and nothing is going to change my mind about that!

Kind Regards, John

 

[kai]

What do panel builders do, whenever they want to mount an MCB onto a din-rail in a bespoke panel they have built?? Do they restrict themselves as to using breakers from the same firm as the enclosure??

 

[JohnW2]

Life is full of risks, and only if they are quantified can one decide whether they warrant any 'avoiding action'.

Not true. Have you never come across ALARP?

I have indeed (all the time, in fact!) and the whole point of the "RP" ("reasonably practicable") part is that it involves, to quote the HSE, "weighing a risk against the trouble, time and money needed to control it.". If one has no handle on level of the risk (probability of the event occurring), one cannot undertake the risk-cost assessment which is fundamental to ALARP. If a potential risk has been identified, it is therefore crucial to quantify the level of that risk if one wishes to apply ALARP.

If the level of risk is already extremely small, then ALARP would take the view that the "trouble, time and money needed to control it" could not be justified. We all face the risk every day that a meteorite could come through our roof, but the level of that risk is so low that we (per ALARP) don't take action to reinforce our roofs against that risk.

What would be the benefit to the manufacturers of showing you the evidence?

I don't care who generates, or 'shows me' the evidence. I am being asked to believe something, without any reason (other than 'blind faith') why I should. I don't care whether the evidence/facts comes from manufacturers, academia, independent organisations, government bdioes or wherever, but if you want me to believe it, you have to show me some evidence.

I wonder, for example, how these "(small) changes in performance" due to the proximity of different-make device compare with, say, the changes in performance that can result from a 20°C change in ambient temperature.

What about a localised 60K change due to an adjacent device?

In context? Are you seriously suggesting that, in the absence of faults, the temperature of a device could possibly differ by as much as 60° from the temperature of another device of the same rating, carrying the same current and compliant with the same relevant Standard(s), but from a different manufacturer??

Kind Regards, John

 

[Robin0577]

What do panel builders do, whenever they want to mount an MCB onto a din-rail in a bespoke panel they have built?? Do they restrict themselves as to using breakers from the same firm as the enclosure??

Interesting point. Likewise Rubber Box Co. and similar, who manufacture their own enclosures for portable distribution units and populate them with off the shelf devices*. To be fair, such products are not intended for "ordinary persons" and their use comes under BS7909 rather than BS7671.

*Admittedly, usually all matching devices within any given enclosure.

 

[ban-all-sheds]

The dangerous alternative is to buy second hand breakers off ebay, which may have interrupted a heavy short circuit just in excess of their breaking capacity (at 0.15 power factor!) a few times, so may be risky to buy.

So what should people do when they buy or lease a second hand property which has a second hand CU installed in it?

 

[SimonH2]

I don't care who generates, or 'shows me' the evidence. I am being asked to believe something, without any reason (other than 'blind faith') why I should. I don't care whether the evidence/facts comes from manufacturers, academia, independent organisations, government bdioes or wherever, but if you want me to believe it, you have to show me some evidence.

I agree, at present it's all "there might be a potential small problem" and "it might actually be measurable".

On thermal grounds, I have a hard time thinking that it could be an issue. If one breaker runs a bit warmer than another, then the worst case is that it very very slightly lowers the thermal tripping point of it's neighbours. But given that normal overload trips are at least 100% above rated capacity, and we all (or mostly, if I read the consensus right) don't design circuits to go into that zone between rating and actual trip point, I don't see that as being an issue. If it were an issue, then there's be a big problem with having some breakers under constant heavy load (and hence running warm) next to other breakers.

So we are down to magnetic effects. Now, a key design point of magnetic circuits is to contain the magnetic field so it goes where we want it - air gaps make for poor circuits. So I would be "somewhat surprised" if there were a significant leakage of field from the trip mechanism - and even more surprised if this, by the time it's jump the gap to the adjacent breaker, it is able to significantly influence the trip in that breaker. I certainly cannot see any way the field could be remotely strong enough to cause nuisance tripping, and to interfere with actual tripping then you'd need faults simultaneously in two circuits fed by adjacent breakers which in itself must be a very very very tiny probability. So a tiny effect, times a tiny effect, times a miniscule probability = what ?

I'd be, as John points out, more worried about failure of an individual device.

As as for the thought that a device may change it's characteristics in a meaningful way by being on a metal DIN rail in a tin can with "the wrong sticker" on the front is ... well I'll stay polite and ask for suggestions of realistic potential mechanisms for that.

 

[JohnW2]

I agree, at present it's all "there might be a potential small problem" and "it might actually be measurable".

Quite, at least in terms of what I am apparently allowed to know. Stillp keeps suggesting that there are 'facts' (which could include information relating to actually measurable problems), but will not or cannot tell me what they are.

On thermal grounds, I have a hard time thinking that it could be an issue.

So do I, but stillp would probably say that is due to my ignorance of 'the facts' and the engineering principles concerned. There could possible be some issues with adjacent MCBs 'running hot' (see below), but I find it very hard to believe that MCBs of same rating, compliant with the same Standard(s) and carrying the same current, but from different manufacturers, will differ appreciably in that respect.

...If it were an issue, then there's be a big problem with having some breakers under constant heavy load (and hence running warm) next to other breakers.

To be fair, manufacturers do acknowledge this potential issue. For example, Wylex say:

Adjacent thermal-magnetic MCBs should not be continuously loaded at or approaching their nominal rated currents when mounted in enclosures. It is good engineering practice to apply generous derating factors or make provision for adequate free air between devices. In these situations, and in common with other manufacturers, we recommend a 66% diversity factor is applied to the MCB nominal rated current where it is intended to load the MCBs continuously (in excess of 1 hour).

It does not indicate what might 'go wrong' if that 'good engineering practice is not exercised.

So we are down to magnetic effects. Now, a key design point of magnetic circuits is to contain the magnetic field so it goes where we want it - air gaps make for poor circuits. So I would be "somewhat surprised" if there were a significant leakage of field from the trip mechanism - and even more surprised if this, by the time it's jump the gap to the adjacent breaker, it is able to significantly influence the trip in that breaker. ... So a tiny effect, times a tiny effect, times a miniscule probability = what ?

Exactly my point - but, again, stillp will again presumably put my view down to my ('enforced') ignorance.

As as for the thought that a device may change it's characteristics in a meaningful way by being on a metal DIN rail in a tin can with "the wrong sticker" on the front is ... well I'll stay polite and ask for suggestions of realistic potential mechanisms for that.

Quite. Again the the name of remaining polite, I have avoided using words like "ridiculous" but, as I've said, if a protective device were so marginal/iffy that it was "quite likely" to not perform properly ('safely') if housed in the 'wrong' CU enclosure, I would be VERY unhappy about having such a device in my house, even if it were in the 'right' enclosure in the company of other 'correct' devices!

Kind Regards, John

 

[ban-all-sheds]

That was actually the second of two questions, the first being more relevant to a DIY forum. However, if the "commercial EICR" relates to a DB which is not "under the control of ordinary persons", how would you code it on an EICR and, in particular, which regulation would you cite as having been contravened?

... and, perhaps more to the point, if, as we are being told, this regulation exists because of a 'known potential danger' of mixing devices of different manufacturer in the same DB, why does this 'known potential danger' suddenly cease to be of concern if the DB is "under the control of" someone who is not and "ordinary person"?

Yes - you're right - sorry I missed half of the questions.

Although your second point again brings up the "you don't know" problem, so it seems to me that there is only one question, and it applies equally to domestic¹ and non-domestic.

And I would code it FI. The investigation required would probably be beyond the scope of the inspection, and quite possibly outside the competence of the inspector.

¹ In reality, of course, it would be cheaper to just replace the CU than pay a skilled person to investigate the pick'n'mix.

 

[ban-all-sheds]

If ban could have his way, we would go back to the Wylex Standard Range system (Plus RCD's), where everything is locked down. No pick-n-mic possible if that were the case, and secure terminals too, with those twin screw terminals. Do you bit Ban, as you have such an influence on those who write BS7671, you could make a safer Eighteenth edition come true.

Please show how that is a logical and intelligent conclusion to draw from what I have said.