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

[JohnW2]

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.

OK, given that I regard it as ambiguous, I'm happy to run with that - particularly since, as I've said, I do have a pair of 'adjacent' 16A immersion circuits.

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.

Exactly. So one can't have a pair of adjacent MCBs both 'continuously' (>1 hour) carrying more than 66% of their 'nominal rated current (~10.5A for a 16A MCB) - do you agree? ... which takes me back to my question - do you think that my adjacent pair of Wylex B16s is theoretically compliant or not?

Kind Regards, John

 

[EFLImpudence]

Firstly, I presume you mean that it is non-compliant because it does not follow the(se) manufacturer's instructions. Fair enough.

However, I would say I have never seen this instruction in any consumer unit or MCB etc. I have bought.

Also, the vast majority of consumer units I see are populated in the 'normal' manner.
That is, in descending order from the main switch so I therefore deduce that no consideration in relation to this subject or any other is given by anyone else.

Exactly. So one can't have a pair of adjacent MCBs both 'continuously' (>1 hour) carrying more than 66% of their 'nominal rated current (~10.5A for a 16A MCB) - do you agree? ...

That is what it says although that wouldn't be how it worked, would it? - unless you want to rewire.
Had you two adjacent 16A MCBs continuously running at 16A you would have to replace them with 25As.

which takes me back to my question - do you think that my adjacent pair of Wylex B16s is theoretically compliant or not?

I think they are compliant because I don't consider 13A to be 'continuously loaded at or approaching their nominal rated currents when mounted in enclosures' as can be attested by the fact that they haven't tripped or melted.

 

[JohnW2]

Firstly, I presume you mean that it is non-compliant because it does not follow the(se) manufacturer's instructions. Fair enough.

Indeed - that is what I mean.

However, I would say I have never seen this instruction in any consumer unit or MCB etc. I have bought.

I guess it depends upon what you regard as "manufactuer's instructions". Like you, I've never seen this mentioned in the 'inserts' which accompany CUs and MCBs. What I'm quoting is from Wylex's published Product Information - but I would imagine that also counts as 'MI' as far as the regs are concerned, wouldn't you?

Also, the vast majority of consumer units I see are populated in the 'normal' manner. That is, in descending order from the main switch so I therefore deduce that no consideration in relation to this subject or any other is given by anyone else.

Quite so - which is was the topic of my 'Saturday evening discussion' and the reason I started this thread.

Exactly. So one can't have a pair of adjacent MCBs both 'continuously' (>1 hour) carrying more than 66% of their 'nominal rated current (~10.5A for a 16A MCB) - do you agree? ...

That is what it says ....

** just make a note of the fact that you said that

which takes me back to my question - do you think that my adjacent pair of Wylex B16s is theoretically compliant or not?

I think they are compliant because I don't consider 13A to be 'continuously loaded at or approaching their nominal rated currents when mounted in enclosures' as can be attested by the fact that they haven't tripped or melted.

** yes but, as above, you've agreed that what Wylex appear to be saying is that one cannot have two adjacent MCBs both 'continuously' carrying more than 66% of their 'nominal rated current (~10.5A for a 16A MCB)!

Kind Regards, John

 

[EFLImpudence]

Exactly. So one can't have a pair of adjacent MCBs both 'continuously' (>1 hour) carrying more than 66% of their 'nominal rated current (~10.5A for a 16A MCB) - do you agree? ...

That is what it says ....

** just make a note of the fact that you said that

which takes me back to my question - do you think that my adjacent pair of Wylex B16s is theoretically compliant or not?

I think they are compliant because I don't consider 13A to be 'continuously loaded at or approaching their nominal rated currents when mounted in enclosures' as can be attested by the fact that they haven't tripped or melted.

** yes but, as above, you've agreed that what Wylex appear to be saying is that one cannot have two adjacent MCBs both 'continuously' carrying more than 66% of their 'nominal rated current (~10.5A for a 16A MCB)!

Yes, I see what you mean.

I was working on the assumption that had you two (or more) MCBs working at full load they would need to be increased by 50% because, presumably, the load couldn't be reduced. That would obviously satisfy the requirements. I.e. 25A MCB running 16A loads.

However if I do it your way and, indeed, the instructions' way it does introduce the anomaly that, to remove the effects of a continuous full load problem, it is not just sufficient to reduce the continuous full load but to reduce it to 66% loading.

Following that logic, all MCBs should have no more than 66% loading unless none of them is loaded at 100%.

I think I'm lost now.

 

[JohnW2]

Yes, I see what you mean. I was working on the assumption that had you two (or more) MCBs working at full load they would need to be increased by 50% because, presumably, the load couldn't be reduced. That would obviously satisfy the requirements. I.e. 25A MCB running 16A loads. However if I do it your way and, indeed, the instructions' way it does introduce the anomaly that, to remove the effects of a continuous full load problem, it is not just sufficient to reduce the continuous full load but to reduce it to 66% loading.

Exactly. I'm glad you understand 'the problem' now!

Following that logic, all MCBs should have no more than 66% loading unless none of them is loaded at 100%. I think I'm lost now.

I'm not surprised - that's why I started the thread!

Kind Regards, John