Relying on loads not being able to overload

[stillp]

John, you've rather missed my point, which was to wonder why you, as someone who is AFAIK neither a qualified electrical engineer nor an electrician (although probably a very capable DIYer), should be questioning the collective wisdom of the 'great and good'.

 

[JohnW2]

John, you've rather missed my point, which was to wonder why you, as someone who is AFAIK neither a qualified electrical engineer nor an electrician (although probably a very capable DIYer), should be questioning the collective wisdom of the 'great and good'.

In turn, I would suggest that you have missed my point in starting this thread.

I did not start this thread to 'question' anything - but, rather, to learn (as, as you say, a non-professional in the field) the views of practising electricians about the matter I raised. I did, admittedly, express my 'discomfort', but only really as a means of ascertaining whether (m)any electricians felt similarly.

I'm quite probably totally wrong, but I still have a suspicion that an appreciable proportion of electricians might feel at least a little like me - which is why I'd love to hear more views from electricians.

Kind Regards, John

 

[JohnW2]

The case to which I referred was intended to point out that cable damage could result in a L - E fault or a L - N fault of almost any impedance.

When I read this (which, as I've said, echoes something I've been saying in this thread), I thought you were sympathising with my view, but that doesn't seem to be the case in terms of what you've written subsequently....

... regardless of what the load is, and whether or not it can result in an overload, we appear to be agreed that cable damage can, even if very rarely, result in an L-E or L-N fault of 'almost any impedance' - including impedances which would result in fault currents which theoretically would threaten the cable if the cable were not protected by an OPD with an In appropriate to the CCC of the cable.

I imagine you must agree with that, but if you don't regard it as a reason for not having an 'over-rated' OPD, is that because (although it was you who wrote of the possibility), you regard such a risk as being "too low to worry about"?

Kind Regards, John

 

[mfarrow]

The regs say that 'adequate' overload protection for a cable is not required IF the load is deemed to be unable to result in an overload situation. AFAIAA, the IET/regs offer no opinion as to what loads are, and are not, capable (under some circumstances) of resulting an an 'overload' situation - that decision is left to the designer.

The term is not 'unable':

for a conductor which, because of the characteristics of the load or the supply, is not likely to carry overload
current, provided that the conductor is protected against fault current in accordance with the requirements of
Section 434.

Secondly...

Overload current. An overcurrent occurring in a circuit which is electrically sound.

Whilst I agree they have passed the buck to the designer, one must realise also that the regulations are there to regulate against the designer who would, whether by incompetence, misunderstanding, human error, or otherwise, design a circuit which is not safe. Therefore I stand by my original statement.

BS 7671 has been derived to regulate, as far as is deemed practicable or necessary, poor design, misuse, and poor installation which is likely to cause danger. Your idea that a designer must decide to sit and mull over the points not chosen to be regulated in BS 7671, or seek advice elsewhere, is all well and good. Someone sat in an air-conditioned office who's rarely been on site may decide he's got the time and expertise to set up a working group and do this, but the average electrician will not. They would do what they consider is allowed within the regulations, where required.

 

[JohnW2]

The term is not 'unable':

Fair enough - I haven't had the regs to hand whilst writing in this thread (but I do now). As you say, the regs talk in terms of conductors "not likely" to carry overload current.

Secondly...

Overload current. An overcurrent occurring in a circuit which is electrically sound.

Indeed. However, as I've repeatedly tried to say, the regs consider protection only in relation to two over-current situations ['overload' (as defined above) and 'fault' (current due to an abnormal or unintended path of negligible impedance)], whereas there are actually three. The third, which the regs do not really address, arises when an excessive current flows through an abnormal or unintended path which is not of negligible impedance.

Whilst I agree they have passed the buck to the designer, one must realise also that the regulations are there to regulate against the designer who would, whether by incompetence, misunderstanding, human error, or otherwise, design a circuit which is not safe. Therefore I stand by my original statement.

I'm not sure what 'original statement' you refer to. If the regs wanted to reduce the risk of an unsafe design due to incompetence, misunderstanding or human error etc., the safest course would surely be to not include any provisions for 'conventional' overload protection to be omitted in some circumstances, wouldn't it?? If 'conventional' overload protection of a cable was always required, without exception, there would surely be less scope for possible errors?

Your idea that a designer must decide to sit and mull over the points not chosen to be regulated in BS 7671, or seek advice elsewhere, is all well and good. Someone sat in an air-conditioned office who's rarely been on site may decide he's got the time and expertise to set up a working group and do this, but the average electrician will not. They would do what they consider is allowed within the regulations, where required.

You've really lost me here, since that sounds much more like an argument I might present 'for the other side'! The regs surely force electricians/designers to do the very thing which you say "is all well and good". The regs include provision for omission of overload protection in some situations, but give no explicit guidance as to what those situations are, instead requiring the designer to 'mull over' the situation and decide whether or not the characteristics of the supply and/or load render it unlikely that the cable could ever carry an overload current.

Kind Regards, John

 

[THRIPSTER]

I don't think anybody would out of choice. However, what about a new 9500W shower as a replacement for, say, 8.5Kw shower fed with 6mm² T/E ?

45A MCB not available for the make of CU that you have. Ref method C stipulates max of 47A. What do you do, tell customer he needs 10mm² OR fit 50A MCB (after first having made sure that doing this satisfies all other criteria)?

I don't know, you tell me.


Regards

 

[JohnW2]

I don't think anybody would out of choice. However, what about a new 9500W shower as a replacement for, say, 8.5Kw shower fed with 6mm² T/E ? ... 45A MCB not available for the make of CU that you have. Ref method C stipulates max of 47A. What do you do, tell customer he needs 10mm² OR fit 50A MCB (after first having made sure that doing this satisfies all other criteria)? ... I don't know, you tell me.

I'm not so sure about your actual example. A 9,500W shower (presumably, as usual, 9,500W at 240V), would take only about 39.6A at 240V, around 38A at 230V - so a 40A MCB would be fine - for both shower and 6mm² (method C) cable.

However, if you had said 10,500W, the dilemma you were attempting to present would then exist - about 43.75A at 240V, about 41.9A at 230V.

I suppose this one is almost a no-brainer. A 50A MCB is so close to the CCC of the cable that only an ultra-paranoid person would be worried about invoking the option to 'omit' overload protection. Probably a preferable option to using a 40A MCB and slightly overloading it.

However, I think this is very different from, say, not fusing down when running a light, fan or whatever in 1mm² cable off a 32A sockets circuit (or even, I suppose, a 50A shower circuit!) - which just does not 'feel right' to me!

Kind Regards, John

 

[EFLImpudence]

9500 / 240 = 39.6A

With a 10.5kW (43.75A) shower your example is apt and I would have no problem with the 50A MCB.

 

[JohnW2]

9500 / 240 = 39.6A
With a 10.5kW (43.75A) shower your example is apt and I would have no problem with the 50A MCB.

Quite so - but you real need to try to type faster ...

I'm not so sure about your actual example. A 9,500W shower (presumably, as usual, 9,500W at 240V), would take only about 39.6A at 240V, around 38A at 230V - so a 40A MCB would be fine - for both shower and 6mm² (method C) cable.
However, if you had said 10,500W, the dilemma you were attempting to present would then exist - about 43.75A at 240V, about 41.9A at 230V.

Kind Regards, John

 

[stillp]

The regs include provision for omission of overload protection in some situations, but give no explicit guidance as to what those situations are, instead requiring the designer to 'mull over' the situation and decide whether or not the characteristics of the supply and/or load render it unlikely that the cable could ever carry an overload current.

A better way to look at it is that BS7671 allows the designer some flexibility in situations such as Thripster describes, provided that he (the designer) can justify using that flexibility by assessing that overload current is unlikely.

 

[stillp]

I did not start this thread to 'question' anything - but, rather, to learn (as, as you say, a non-professional in the field) the views of practising electricians about the matter I raised. I did, admittedly, express my 'discomfort', but only really as a means of ascertaining whether (m)any electricians felt similarly.

I'm quite probably totally wrong, but I still have a suspicion that an appreciable proportion of electricians might feel at least a little like me - which is why I'd love to hear more views from electricians.

May I ask why John? What are you going to do with the opinions of the few electricians who post on this DIY forum? If you want a cross-section of electricians' opinions you might be better posting on the IET forum, or even asking ESF (as the ESC have, laughably, rebranded themselves).

 

[JohnW2]

The regs include provision for omission of overload protection in some situations, but give no explicit guidance as to what those situations are, instead requiring the designer to 'mull over' the situation and decide whether or not the characteristics of the supply and/or load render it unlikely that the cable could ever carry an overload current.

A better way to look at it is that BS7671 allows the designer some flexibility in situations such as Thripster describes, provided that he (the designer) can justify using that flexibility by assessing that overload current is unlikely.

I was obviously referring to, and quoting, mfarrow's language - but I think that the two above statements are saying essentially the same thing.

As I said in responding to Thripster, his example was so 'borderline' that I suspect most people would have little problem in exercising discretion and accepting the 50A MCB even if they felt that the shower was likely to create an overload. However, as I wrote to him, I see that as rather different from, say, running 1mm² cable to lighting from, say, a 50A shower circuit without any 'fusing down' - but maybe that's just me!

Kind Regards, John

 

[JohnW2]

May I ask why John? What are you going to do with the opinions of the few electricians who post on this DIY forum? If you want a cross-section of electricians' opinions you might be better posting on the IET forum, or even asking ESF (as the ESC have, laughably, rebranded themselves).

A good point, I suppose. However, I'm not going to 'do anything' with the opinions, whether from here, the IET forum or elsewhere. It's really just a matter of interest - I know what I am, and am not, comfortable with as far as my own electrical installation is concerned, and (dare I say!) the extent to which I am comfortable to 'twist', 'adapt' and/or deviate from, the regs in my own installation!

The interesting thing in terms of this forum is, of course (as EFLI pointed out early on), despite all this discussion about omitting ('coventional') overload protection is only 'for the electricians' (and maybe a few others ) - since (one would hope), for example, you would probably never see any DIYer here being told other than to 'fuse down' when dropping to a cable size whose CCC was less than the In of the circuit's primary OPD (since very few DIYers will have the ability to confirm that fault protection is satisfactory). On the contrary, they will virtually always be told that they must 'fuse down'. That sort-of makes sense, although the reality is, of course, that, even if they do 'fuse down', they would still actually face a requirement (which few could fulfill) to confirm that fault protection is satisfactory!

Kind Regards, John

 

[THRIPSTER]

Yes, you are right guys - I had used a nominal voltage of 230V instead of the 240V that the shower is rated at - so, as you say, the point I am trying to make is made by substituting a >10Kw shower.


Regards

 

[EFLImpudence]

One more point, John:

Is reference made to this (omission of overload protection) in the OSG?
I've had a quick look, but I don't like reading it, and can't see one so perhaps it is only apparent to those who know of it already.

Just a thought typed slowly.