Saturday evening discussion - Part 1 (of 1 or 2!)

[Spark123]

I'm hungover

 

[JohnW2]

the breaking capacity of the MCB is increased to 16,000A". I assume that we're just seeing badly/inorrectly written Product Information - quite possibly written by a non-technical person!

Do they mean that when the stated circumstance apply/arise then the MCB breaking capacity IS increased to 16kA? ... I.e. not miraculously changed but must be replaced by an MCB with that capacity.

Or to word it better - the REQUIRED breaking capacity of the MCB is increased to 16,000A".

If that's what they mean, then I would say that it's incredibly (and potentially dangerously) misleading.

However, I doubt that such is their intention. What I quoted was from the "Technical Specification" of their products, not general advice on what MCB breaking capacity is required in the presence of a 100A BS 1361 fuse (and, indeed, advice that would mean that the product whose 'Technical Specification' they were presenting would not be suitable in that situation!)

Kind Regards, John

 

[seasickstevie]

Would be interesting to hear manufacturer's answer to the query.

 

[JohnW2]

Would be interesting to hear manufacturer's answer to the query.

Well, I think they probably mean what they have written (not what EFLI has suggested) - and, if that's the case the question to which I would be very interested in hearing their answer is "Why". Maybe I'll ask them.

However, folks, interesting though this lengthy tangential discussion has been, I've still only really had one response to the question I posed in my OP - about the interpretation of Wylex (not MK) product information - would anyone like to offer an opinion?

Kind Regards, John.

 

[EFLImpudence]

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).

I would say that means that two (or more) such loaded mcbs should not be situated adjacent each other.

One such loaded mcb can be adjacent a lowly loaded mcb (or two).

That is, it is not necessary to have an empty space between all mcbs, merely that the fully loaded ones should be separated in the group of mcbs.


As for the use of the word 'diversity', (as electricians understand it) it would seem to be the opposite of the normal usage - where higher loads can be connected.
'Derating' the mcb is more correct.

 

[JohnW2]

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).

I would say that means that two (or more) such loaded mcbs should not be situated adjacent each other. One such loaded mcb can be adjacent a lowly loaded mcb (or two). That is, it is not necessary to have an empty space between all mcbs, merely that the fully loaded ones should be separated in the group of mcbs.

Yes, that is one possible interpretation. Do you I take it that you think it means that both of two adjacent highly-loaded MCBs have to be de-rated to 66% of their In ratings (e.g. 32A derated to about 21A, 16A derated to about 10.5A)? What if there are three adjacent high-loaded ones (the one in the middle would then seem to be in a worse position than one which has another high-loaded one only on one side)?

As for the use of the word 'diversity', (as electricians understand it) it would seem to be the opposite of the normal usage - where higher loads can be connected.
'Derating' the mcb is more correct.

Quite. As I said, it's odd that they have used 'diversity' when they spoke far more conventionally of 'derating factor' in the previous sentence - which is why I wondered if they perhaps had some different meaning in mind!

Kind Regards, John

 

[EFLImpudence]

Do you I take it that you think it means that both of two adjacent highly-loaded MCBs have to be de-rated to 66% of their In ratings (e.g. 32A derated to about 21A, 16A derated to about 10.5A)?

I suppose so.

What if there are three adjacent high-loaded ones (the one in the middle would then seem to be in a worse position than one which has another high-loaded one only on one side)?

I suppose that would mean leaving a space between then no derating would be necessary.

As I said, it's odd that they have used 'diversity' when they spoke far more conventionally of 'derating factor' in the previous sentence - which is why I wondered if they perhaps had some different meaning in mind!

I don't know what that would be.
The dictionary definition has nothing to do with this .

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).

I am, of course, thinking in domestic circumstances where this is unlikely to occur.
Are they considering the MCB melting or does the extra heat lead to the MCB tripping prematurely and so the 66% acts as a balancing effect.

Also, are there cases of MCBs which are not in enclosures?


If taken as written, we have to accept the statement.
However, several pages of discussion could follow should there be unknown mistakes.

It could be akin to taking everything as Gospel while reading the local newspaper and discussing at length things which did not happen.

 

[JohnW2]

What if there are three adjacent high-loaded ones (the one in the middle would then seem to be in a worse position than one which has another high-loaded one only on one side)?

I suppose that would mean leaving a space between then no derating would be necessary.

As I understand what they're trying to say, one can always avoid derating by having space(s), but that would be true of two just as much as three. My point was that, in the absence of spaces, they seem to be requiring the same derating factor whether there are one or two adjacent high-loaded MCBs.

Having said that, there is an alternative interpretation of what they say (which I suggested in the OP) - namely that there has to be either a space or derating for a highly loaded MCB even if the adjacent ones are not highly loaded. In other words, the first sentence could be saying that an MCB should not be run 'continuously' at high load if there were any 'adjacent MCBs' (regardless of how much loading they had).

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).

I am, of course, thinking in domestic circumstances where this is unlikely to occur.

Well, that obviously depends upon which of the interpretations one makes. If a lone highly-loaded MCB (surrounded by other MCBs) 'qualifies', then that would be more common in the domestic situation ... ** see below.

Are they considering the MCB melting or does the extra heat lead to the MCB tripping prematurely and so the 66% acts as a balancing effect.

I had assumed (yes, I know!) the latter - or, at least, some malfunction of intended MCB functionality

Also, are there cases of MCBs which are not in enclosures?

Yes, I wondered about that, too. I suppose it would be technically possible to manufacture a standalone MCB which did not have a meaningfully distinct 'enclosure', but I've certainly never seen or heard of one. Even then, the absence of a 'meaningfully distinct enclosure' would be almost a semantic issue, since there would have to be some sort of 'cover'.

If taken as written, we have to accept the statement.

As above (and, indeed, per the whole reason I started this thread), we cannot really 'accept' it until we've decided how to interpret it!

** What was going to be 'Part 2' of this thread, if/when we had reached a consensus about interpretation, exists in my (domestic!) house. One of my CUs has two adjacent Wylex B16s for two dedicated (3kW) immersion heater circuits. Admittedly, we often don't use both, but sometimes we do. When we do, each of those MCBs is carrying appreciably more than 66% of 16A (about 10.5A), and could well be on for well over an hour if the tanks were pretty cold at the start. So, given what the Wylex MI appears to say, one could argue that my arrangement is not compliant with BS7671 - what do you think?

Of course, if one uses my initial interpretation (see OP) of what Wylex are saying, then the same situation (and question about compliance) would arise in almost any home which had a 16A MCB supplying an immersian heater - unless it happened that there were no adjacent MCBs!

Kind Regards, John

 

[bernardgreen]

As a MCB being heated by an adjacent MCB will trip sooner than it would at normal temperature then I see it as being safer for that circuit. That is of course ignoring the question of why the other MCB is getting hot which may be a result of a less than safe overload on its circuit.

 

[JohnW2]

As a MCB being heated by an adjacent MCB will trip sooner than it would at normal temperature then I see it as being safer for that circuit.

That is certainly what one would expect of the thermal tripping (and is reflected in the limited Wylex data on the effect of ambient temperature on In which I quoted). However, how certain are you that excessive temperatures may not alter the functionality of the magnetic tripping, maybe in the 'unsafe' direction? The magnetic tripping is presumably a pretty finely balanced bit of engineering, so it's not beyond belief that marked changes in ambient temperature could have an appreciable effect.

Kind Regards, John

 

[EFLImpudence]

As I understand what they're trying to say, one can always avoid derating by having space(s),but that would be true of two just as much as three.

Yes but -

My point was that, in the absence of spaces, they seem to be requiring the same derating factor whether there are one or two adjacent high-loaded MCBs.

I do not agree in the 'one' case because - note the plural -
"Adjacent thermal-magnetic MCBs should not be continuously loaded at or approaching their nominal rated currents".

Having said that, there is an alternative interpretation of what they say (which I suggested in the OP) - namely that there has to be either a space or derating for a highly loaded MCB even if the adjacent ones are not highly loaded. In other words, the first sentence could be saying that an MCB should not be run 'continuously' at high load if there were any 'adjacent MCBs' (regardless of how much loading they had).

But it doesn't say 'an MCB', it says 'MCBs'.

If taken as written, we have to accept the statement.

As above (and, indeed, per the whole reason I started this thread), we cannot really 'accept' it until we've decided how to interpret it!

True.

** What was going to be 'Part 2' of this thread, if/when we had reached a consensus about interpretation, exists in my (domestic!) house. One of my CUs has two adjacent Wylex B16s for two dedicated (3kW) immersion heater circuits. Admittedly, we often don't use both, but sometimes we do. When we do, each of those MCBs is carrying appreciably more than 66% of 16A (about 10.5A), and could well be on for well over an hour if the tanks were pretty cold at the start. So, given what the Wylex MI appears to say, one could argue that my arrangement is not compliant with BS7671 - what do you think?

I would just say that neither of them has yet melted or tripped, so...what has happened to cause the statement from the manufacturer(s)?

Also, 13A on a 16A MCB is not "continuously loaded at or approaching their nominal rated currents".

Of course, if one uses my initial interpretation (see OP) of what Wylex are saying, then the same situation (and question about compliance) would arise in almost any home which had a 16A MCB supplying an immersian heater - unless it happened that there were no adjacent MCBs!

I stand by my reference to the plural and as above (13A).

In a domestic situation the only case of such a situation arising would/could be a Socket Ring or Radial with very heavy loads on at the same time - even then an hour would be unlikely - or possibly a very large cooker.

 

[JohnW2]

My point was that, in the absence of spaces, they seem to be requiring the same derating factor whether there are one or two adjacent high-loaded MCBs.

I do not agree in the 'one' case because - note the plural -
"Adjacent thermal-magnetic MCBs should not be continuously loaded at or approaching their nominal rated currents".

In that specific case, you have misunderstood me, What I meant was the situation in which one or two high loaded MCBs were adjacent to another high-loaded MCB (i.e either two or three high-loaded ones next to one another). However, in terms of the more general issue about 'plurals'....

Having said that, there is an alternative interpretation of what they say (which I suggested in the OP) - namely that there has to be either a space or derating for a highly loaded MCB even if the adjacent ones are not highly loaded. In other words, the first sentence could be saying that an MCB should not be run 'continuously' at high load if there were any 'adjacent MCBs' (regardless of how much loading they had).

But it doesn't say 'an MCB', it says 'MCBs'.

Of course, if one uses my initial interpretation (see OP) of what Wylex are saying, then the same situation (and question about compliance) would arise in almost any home which had a 16A MCB supplying an immersian heater - unless it happened that there were no adjacent MCBs!

I stand by my reference to the plural and as above (13A).

Isn't that just how English grammar works? Consider the following examples:
"Sockets should not be sited where they could be affected by water"
"MCBs must have an In no greater that Iz of the cable they protect"
"FCUs on spurs from ring final circuits must be within 3m cable run from the origin of the spur" ... etc. etc.
The 'plural' grammatical constructions obviously are not meant to imply that those statements do not apply if one is only talking one one socket, MCB or FCU. I think the same could be said of the statement about 'adjacent MCBs'. In other words, that first sentence from Wylex could be intended to mean 'when MCBs are adjacent, none of them should be continuously loaded at or approaching their nominal rated current'.

So, given what the Wylex MI appears to say, one could argue that my arrangement is not compliant with BS7671 - what do you think?

I would just say that neither of them has yet melted or tripped, so...what has happened to cause the statement from the manufacturer(s)?

I obvioulsy can't answer that question - and maybe, as someone has sugegsted, I will ask them. However, I don't have to tell you that their reason is essentially irrelevant to the matter of compliance. As we've often discussed, there are things required by MIs which some electricians do not really feel should be required (e.g. 3-pole isolators for fans) - but that doesn't alter the fact that, if one wishes to be compliant with BS7671, one has to follow those MIs - regardless of why they say what they do.

Also, 13A on a 16A MCB is not "continuously loaded at or approaching their nominal rated currents".

This is where Wylex's 'instructions' get very confusing - almost a circular argument. One can certainly argue that 13A is not 'at or approaching' 16A. However, they seem to actually be saying that, for 'continuous' (>1 hour) loading, the limit should be 66% of 16A (i.e. about 10.5A). In other words, their instructions do not allow a 'continuous' load to be 'at or approaching 16A'! Hence my question about compliance.

Kind Regards, John

 

[EFLImpudence]

"Adjacent thermal-magnetic MCBs should not be continuously loaded at or approaching their nominal rated currents".

namely that there has to be either a space or derating for a highly loaded MCB even if the adjacent ones are not highly loaded.

I disagree.
It says "Adjacent thermal-magnetic MCBs should not be continuously loaded at or approaching their nominal rated currents".

If only one is at that state then adjacent thermal-magnetic MCBs are not "continuously loaded at or approaching their nominal rated currents".

This is where Wylex's 'instructions' get very confusing - almost a circular argument. One can certainly argue that 13A is not 'at or approaching' 16A. However, they seem to actually be saying that, for 'continuous' (>1 hour) loading, the limit should be 66% of 16A (i.e. about 10.5A). In other words, their instructions do not allow a 'continuous' load to be 'at or approaching 16A'! Hence my question about compliance.

I disagree again.
As the MCBs are not "continuously loaded at or approaching their nominal rated currents" then the instructions do not apply.

 

[JohnW2]

"Adjacent thermal-magnetic MCBs should not be continuously loaded at or approaching their nominal rated currents".

namely that there has to be either a space or derating for a highly loaded MCB even if the adjacent ones are not highly loaded.

I disagree. It says "Adjacent thermal-magnetic MCBs should not be continuously loaded at or approaching their nominal rated currents". If only one is at that state then adjacent thermal-magnetic MCBs are not "continuously loaded at or approaching their nominal rated currents".

I can't argue, because I regard it as ambiguous. You may therefore be correct.

This is where Wylex's 'instructions' get very confusing - almost a circular argument. One can certainly argue that 13A is not 'at or approaching' 16A. However, they seem to actually be saying that, for 'continuous' (>1 hour) loading, the limit should be 66% of 16A (i.e. about 10.5A). In other words, their instructions do not allow a 'continuous' load to be 'at or approaching 16A'! Hence my question about compliance.

I disagree again. As the MCBs are not "continuously loaded at or approaching their nominal rated currents" then the instructions do not apply.

Again, as I said, I find this very confusing. What exactly do you understand by this 66% 'diversity (presumably 'derating') factor'? As I see it, they are saying that if one were 'thinking' of creating a situation in which an MCB would be 'continuously loaded at or approaching its nominal rated current', then one can't. Instead (one assumes) one has to install a higher-rated MCB such that it's 'continuous' load is no greater than their specified '66% of its nominal rated current' (and one is not 'allowed' to load it 'continuously' to greater than that 66%). If so, how does that differ from saying in the first place that one can't have a 'continuous' load >66% of In? Do you really think this is straightforward?

Kind Regards, John

 

[EFLImpudence]

Again, as I said, I find this very confusing.

Presumably it is only confusing if you read it in more than one way.
I have explained what the words mean to me.

What exactly do you understand by this 66% 'diversity (presumably 'derating') factor'? As I see it, they are saying that if one were 'thinking' of creating a situation in which an MCB would be 'continuously loaded at or approaching its nominal rated current', then one can't.

In my world that only applies if two (or more) adjacent MCBs are continuously loaded at or approaching their nominal rated currents when mounted in enclosures.

Instead (one assumes) one has to install a higher-rated MCB such that it's 'continuous' load is no greater than their specified '66% of its nominal rated current'

Or move or separate them but if not possible then - yes.

(and one is not 'allowed' to load it 'continuously' to greater than that 66%).

You are saying 'it'.
I think it only applies if 'them' so, as above.

If so, how does that differ from saying in the first place that one can't have a 'continuous' load >66% of In? Do you really think this is straightforward?

Because it does not apply unless you place such MCBs adjacent each other.

In design terms would it be considered good (design) to have two or several MCBs in a board which are knowingly going to be run at this state for long periods and, if so, place them all adjacent each other?

As I see it there is nothing wrong with running an MCB at its rated current but if that is the case then two or several adjacent each other may overheated (with whichever consequence is anticipated).