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Topic Title: 433.3.1 (ii) - open to interpretation?
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Created On: 09 May 2013 08:38 AM
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 09 May 2013 08:38 AM
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alanblaby

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After an argument in another place, I'd like some opinions on 433.3.1 (ii).

My understanding of it is that for something with a fixed resistance load, such as an electric shower, there does not have to be overload protection for the cable (fault protection is required).
Clearly, this is not a good design practice, but it came to light recently, where we could not get a 45A CB for a design load of 42A, yet the cable could only hold 46A. I said because of 433.3.1, we could use the 50A CB which was available.

I spoke to my Tech Helpline yesterday, and got an inconclusive answer, mainly that a shower can overload, I said most unlikely, it'll either be pulling 42A, or it'll have a fault which will draw 100's of amps, and the 50A breaker will cover the fault (so long as Zs etc comply).He didnt agree, but said that yes, that Reg. is open to interpretation, but I should not use a CB that is rated higher than the cable capacity. Well, no, it isnt good practice, but allowable as I read it.

Any thoughts on this?

Thanks
Alan.
 09 May 2013 08:55 AM
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bartonp

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I'd have used a 40A MCB. You'd be able to shower for a while before it tripped.
50A - within the letter if not the spirit. It's you that'll have to defend the design if owt goes wrong.

Phil.
 09 May 2013 08:57 AM
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KFH

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Alan, I am with you on this. While perhaps not good practice, 433.3.1. is quite clear. Our interpretation is confirmed in the IET Electrical Installation Design Guide (4.2.2).
 09 May 2013 09:05 AM
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Thripster

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Perhaps it is not good design to have an MCB carrying a higher current than its rating - however small the over current is?
 09 May 2013 09:27 AM
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Legh

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Running a B40A circuit hot at 2A over its rating, as far as i can see, will not trip the protective device although bad practice, according to 433.1.1 and may go someway in affecting its operation over a period of time.

The circuit appears to be a 9.6kW load with a cable of either 6.0mm2 clipped clipped direct or a 10mm2 buried in thermal insulation.
So there would be no allowance for future adjustments.

The shower should have a thermal overload cut out within the design (I know its not a washing machine or a bathroom fan - lol, but then would you rely solely on the safety features in the manufacturer's design?)

Personally, I wouldn't have thought overload would be an issue and the protective device would provide S?C protection. However, you might as well say goodbye to any warranty if you did underrate the protective device.

Legh

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 09 May 2013 09:31 AM
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AJJewsbury

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I think the principle is clear enough - and commonly done - not only in the likes of industrial motor supply circuits, but even the humble domestic unfused spur. So there shouldn't be any arguments about it being 'not good practice' or anything like that.

The only doubt in my mind about this particular situation is whether the shower can "mis-behave" in such a way that it presents something akin to an overload to the supply. Clearly there's no possibility mechanical jamming causing an overload as might be the case with a motor, but what if internal faults weren't of negligible impedance as we usually assume for wiring faults. I think that shower elements are rather like the ones in cookers - i.e. thin wire surrounded by (mineral?) insulation, within an overall earthed metal sheath. If the insulation did break down part-way along the element for whatever reason (as sometimes happens with cooker elements) the fault would be in series with part of the element - shorter element and same supply voltage would suggest to me an significantly increased current - but significantly lower than would be expected from a simple short - i.e. very much like an overload.

Is that a "credible scenario"? (as OMS might say) - I don't know, I can only guess. I'd suggest it's a possibility that's worth considering though.

Perhaps another tack is that you're not entirely omitting overload protection - it's there at 50A in this case - which narrows the field somewhat. Also if the only forseeable "overload" situation also involves an earth fault and there's RCD protection, then disconnection will happen anyway.

I reckon a good argument could be built to show that there's no actual danger in this case.

- Andy.
 09 May 2013 09:51 AM
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Parsley

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You don't need overload protection as an electric shower is a fixed resistive load. It's not bad design practise to omit what's not required.

433.1.1 (i) requires the PD In to be not less than Ib.

A PD should not be used as a load limiting device, small overloads are not likely to cause operation of the PD, but may reduce the life expectancy of the conductors.

Fault protection is still required.

Regards
 09 May 2013 10:04 AM
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Legh

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Originally posted by: Parsley

You don't need overload protection as an electric shower is a fixed resistive load. It's not bad design practise to omit what's not required.

433.1.1 (i) requires the PD In to be not less than Ib.

A PD should not be used as a load limiting device, small overloads are not likely to cause operation of the PD, but may reduce the life expectancy of the conductors.

Fault protection is still required.

Regards


I'm not sure that i understand this as the conductors (Iz - in this case) are rated at 47A, the load is 42A and the solution of whether or not to use a B40A as opposed to a B50A where a B45A is unavailable ......

Legh

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"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 09 May 2013 10:53 AM
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Parsley

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Originally posted by: Legh

Originally posted by: Parsley



You don't need overload protection as an electric shower is a fixed resistive load. It's not bad design practise to omit what's not required.



433.1.1 (i) requires the PD In to be not less than Ib

A PD should not be used as a load limiting device, small overloads are not likely to cause operation of the PD, but may reduce the life expectancy of the conductors.



Fault protection is still required.



Regards




I'm not sure that i understand this as the conductors (Iz - in this case) are rated at 47A, the load is 42A and the solution of whether or not to use a B40A as opposed to a B50A where a B45A is unavailable ......



Legh


Sorry Legh, I should have added that if you include overload protection a 40 MCB won't comply as per 433.1.1(i) if you select a 50A MCB to comply with 433.1.1(ii) a larger CSA will be required or you could omit the overload protection as per 433.3.1(ii) ensuring you comply with section 434 which will normally allow a smaller CSA to be used.

Regards
 09 May 2013 11:04 AM
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rovermaestro

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As a customer rather than an installer I argued this point on an EICR with regard to a shower installed some years ago. 8.5kW drawing 37A on 6mm ref B in conduit on a wall. Cable therefore rated at 38A only 32A or 40A rcbo available. Circuit has 40A told I had to use 10mm cable or clip direct. Glad to see the experts teasing this apart for us.
 09 May 2013 12:59 PM
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geoffsd

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Is the correct voltage being used for the calculation?

If nominal voltage is being used when the manufacturer has stated 240V then you will have too high a result.

There are a lot of threads where this is the case.

E.g. 9.6 @ 240 = 40A but 9.6 @ 230 (wrongly used) = 41.74
 09 May 2013 01:53 PM
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Parsley

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Originally posted by: geoffsd

Is the correct voltage being used for the calculation?



If nominal voltage is being used when the manufacturer has stated 240V then you will have too high a result.



There are a lot of threads where this is the case.


E.g. 9.6 @ 240 = 40A but 9.6 @ 230 (wrongly used) = 41.74


Most shower manufacturers state the highest KW that is only possible at the highest voltage. If the voltage is lower the current drawn will also be lower the elements resistance doesn't change because the voltage has.

Regards
 09 May 2013 04:53 PM
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geoffsd

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Yes, I realise that but was pointing out that because nominal voltage is 230 that is what a lot of people use instead of the 240 which the manufacturer has stated.and so making it look as if a larger CPD is required.

Hence "wrongly used".

Plus, of course, it won't be the 'highest KW' as the voltage can rise above 240.
 10 May 2013 06:50 AM
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leckie

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Originally posted by: AJJewsbury

I think the principle is clear enough - and commonly done - not only in the likes of industrial motor supply circuits, but even the humble domestic unfused spur. So there shouldn't be any arguments about it being 'not good practice' or anything like that.



The only doubt in my mind about this particular situation is whether the shower can "mis-behave" in such a way that it presents something akin to an overload to the supply. Clearly there's no possibility mechanical jamming causing an overload as might be the case with a motor, but what if internal faults weren't of negligible impedance as we usually assume for wiring faults. I think that shower elements are rather like the ones in cookers - i.e. thin wire surrounded by (mineral?) insulation, within an overall earthed metal sheath. If the insulation did break down part-way along the element for whatever reason (as sometimes happens with cooker elements) the fault would be in series with part of the element - shorter element and same supply voltage would suggest to me an significantly increased current - but significantly lower than would be expected from a simple short - i.e. very much like an overload.



Is that a "credible scenario"? (as OMS might say) - I don't know, I can only guess. I'd suggest it's a possibility that's worth considering though.



Perhaps another tack is that you're not entirely omitting overload protection - it's there at 50A in this case - which narrows the field somewhat. Also if the only forseeable "overload" situation also involves an earth fault and there's RCD protection, then disconnection will happen anyway.



I reckon a good argument could be built to show that there's no actual danger in this case.



- Andy.


Although I can also think of scenarios where an overload could occur, say a cable being crushed and a low IR resulting in an overload, the regulation does not ask us to consider that. Only a fault of negligible impedance. So I agree with Parsley and think for a fixed resistive load, overload protection can be omitted. If you we're to fit a 50A PD you could argue that the device was for fault protection only. However as already pointed out, the warranty for a shower install would probably be void.

This reg does not actually mention fixed resistive loads, I haven't got the BGB to hand but I think it says something like loads that are unlikely to cause an overload. What other loads could these be other than fixed resistive loads?
 10 May 2013 07:55 AM
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ebee

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we always assume a fault to be of negligible impedance when calculating for short circuit or earth fault protection although we do realise that in real life that might not always be the case.

In the same way we must decide if overload protection is required or not and clearly fora fixed load such as a heating element it is (deemed) very unlikely that such an overload could occur.

Andy Jewsbury has pointed out a set of circumstances that, although very unlikely, could possibly occur , however as he states other protection (earth fault/rcd) should negate this anyway.

To the OP,
your "design load of 42A" was this a shower based on 240V or was it something else/based on 230v ?

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Ebee (M I S P N)

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 10 May 2013 06:39 PM
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alanblaby

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The 42A load / 47A cable and 50A CB were just an example, I was interested in the views of 'people in the know' of this Reg, which mostly correspond with my reading of it, yet on another forum, I was roundly told I was wrong, and such a thing would never be allowed.
Thanks for the replies.
 10 May 2013 07:43 PM
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dg66

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A good example is, spurring from a ring with a 32A CPD using 2.5mm cable to a twin socket. 2.5mm isnt rated at 32A , but we dont bat an eyelid at this arrangement do we?

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Dave(not Cockburn)
 10 May 2013 09:13 PM
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leckie

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Hi Dave, no we don't bat an eyelid but isn't his because a twin socket can only take two 13A loads, i.e. 26A?
 10 May 2013 10:00 PM
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rovermaestro

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Good point. Another question from me is whether the design of radial final circuits would allow a higher rated CPD, say 32A protecting 4mm t&e with two seperate13A sockets if the cable were derated by installation method to 30A. I realise the load might not be simple resistive. I have this situation in my utility room where the circuit serves a washer and a tumble drier, combined connected load approx 22A, so can't use a 20A CPD. It seems to me the maximum demand would be 26A. If additions were made to the circuit these parameters would be taken into account and lower CPD substituted if the new design current was greater than the cable capacity. Intuitively and using common sense this seems fine but is it compliant?
 11 May 2013 09:56 PM
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leckie

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Rover, some manufacturers make 25A rated circuit breakers, etc, Hager for one. That opens up a few options. Although I haven't got any 25A fuse wire.
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