Relying on loads not being able to overload

[JohnW2]

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.

Indeed - and, as we both have said, I don't think many people would 'bat an eyelid' about using a 50A MCB in that sitution....

... but, as I said, whether I would be equally 'comfortable' with hanging 1mm² lighting off that 50A circuit without any 'fusing down' is perhaps a different matter

Kind Regards, John

 

[JohnW2]

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.

Great minds I was wondering exactly the same earlier today but, as I've said, I was separated from my books for most of the day. However, you've now prompted me to have another look (albeit at my 'red' one). I'll let you know if I find anything!

Kind Regards, John

 

[THRIPSTER]

Yes, you are right John - you wouldn't 'bat an eyelid'. But you wouldn't contemplate hanging 1mm off a 50A MCB, as you know, because it would fall foul of other design criteria......so not entirely sure what you are saying Old Bean.


Regards

 

[JohnW2]

But you wouldn't contemplate hanging 1mm off a 50A MCB, as you know, because it would fall foul of other design criteria......so not entirely sure what you are saying Old Bean.

Well, I was suggesting that I probably "wouldn't be so comfortable", rather than that I "wouldn't contemplate"!

Whatever, what "other design criteria" did you have in mind?

Kind Regards, John

 

[THRIPSTER]

434.5.2 ?

Regards

 

[JohnW2]

434.5.2 ?

Maybe (it obviously 'depends' on the particular installation), particularly given the pretty extreme example I quoted (because we'd been talking about a 50A shower circuit). In the case of a far more likely scenario of 1mm² or 1.5mm² lighting hanging off a 32A circuit, I would imagine that it's far more likely that 434.5.2 would be satisfied.

That's for TN installations. With TT, where fault protection is reliant on an RCD and fault currents are (usually - although not in my house!) very much lower 434.5.2 would probably usually be satisfied, even in the 'extreme' scenario, wouldn't it?

Kind Regards, John

 

[JohnW2]

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.

I've now looked carefully at my (red) OSG, and agree that it appears to make no mention of omission of overload protection. In fact is says precious little about overload protection at all (although it has a lot to say about fault protection)!

You raise an interesting point. I wonder what proportion of practising electricians are even aware of the provision in the regs to omit overload protection in some situations?!

Kind Regards, John

 

[333rocky333]

I wonder what proportion of practising electricians are even aware of the provision in the regs to omit overload protection in some situations?!

Kind Regards, John

Im not aware, but I dont really do install work nowadays, what reg are we actually talking about john.

I only really recall the bits from my apprenticeship which allowed smaller cables feeding switchfuses and things from Busbars and henley blocks, though if i recall were limited by length and mechanical protection or am I diversing on to something different

 

[JohnW2]

I wonder what proportion of practising electricians are even aware of the provision in the regs to omit overload protection in some situations?!

Im not aware, but I dont really do install work nowadays, what reg are we actually talking about john.

Interesting. We're talking about 433.3.1(ii), the wording of which was posted by mfarrow a page or so back (Section 433.3 is entitled "Omission of devices for protection against overload")

I only really recall the bits from my apprenticeship which allowed smaller cables feeding switchfuses and things from Busbars and henley blocks, though if i recall were limited by length and mechanical protection or am I diversing on to something different

I'm not sure. 433.2.2 permits the OPD to be downstream of the conductors it is protecting in some circumstances, with some conditions (relating to length and/or mechanical protection, OR fault protection). Is that perhaps what you're thinking about?

Kind Regards, John

 

[333rocky333]

ok thanks will look tomorow.
I always recall the old henley blocks and busbars that had about 6mm coming off something like 200 amp busbars sometimes, usually feeding small 20 amp switchfuses for things like fire alarms, however these switchfuses were usually close and the cables proberly no more than a metre or so and was taught that the switchfuse limited the load and the chance of damage to the short supply run was minimal, thus making it okay.

But I cant recall anything about supplies as your discussing being acceptable so I will read up

 

[THRIPSTER]

RCD's would normally be expected to protect cable in respect of fault to earth but you would still need to protect cable for the time it takes the OPD to clear short cct's between line and neutral, line and earth - particularly in areas where there is high PSCC or high PEFC. Have a look at 543.1.1

Regards

 

[JohnW2]

RCD's would normally be expected to protect cable in respect of fault to earth but you would still need to protect cable for the time it takes the OPD to clear short cct's between line and neutral, line and earth - particularly in areas where there is high PSCC or high PEFC. Have a look at 543.1.1

Firstly, a 'short circuit between line and earth' is surely the same as a 'fault to earth', which should be cleared by an RCD - so that only leaves L-N shorts to consider. You have confused me a bit with the order in which you mentioned the regulations - 543.1.1, which you now mention relates only to protective conductors (hence not relevant to L-N shorts), whereas the very similar 434.5.2, which you mentioned last night, relates to any conductor (hence is relevant to L-N shorts).

However, you are clearly right in saying that the L and N conductors need adequate protection against over-currents due to L-N shorts, even in TT installations (since PSCC can be very high, even if PEFC is very low), and this is seemingly embraced by 434, which relates to negligible impedance L-E or L-N faults. In determining whether 434.5.2 is satisfied, one considers the PFC (i.e. the higher or PSCC and PEFC) - which, as above, can be high even in a TT installation.

However, even though the ‘fault protection’ (ADS) considerations are similar with both TN and TT, I still think that it will (for T+E cable >1mm²) usually be easier to satisfy them with TT than with TN - since with TN one has to satisfy 543.1.1 in relation to the CPC as well as satisfying 434.5.2 with respect to L & N conductors - and for T&E >1mm², the csa of the CPC is appreciably less than that of the L & N. When I wrote about TT last night, I forget that L, N and CPC all have the same csa with 1mm² T+E.

I don’t think that this significantly alters anything which anyone has said in this discussion, since comments about omitting overload protection have always (literally or by implication) been qualified by “provided fault protection requirements are satisfied’. However, it’s possible that satisfying those fault protection requirements may be rather more difficult than some people may think, particularly in relation to some of the more extreme scenarios we have considered (and particularly with TN).

Kind Regards, John

 

[THRIPSTER]

Seems like you have it all covered John.


Regards

 

[JohnW2]

Seems like you have it all covered John.

Maybe. In practice I suspect that those who rely heavily on the OSG may well usually/often end up concluding that fault protection requirements are not satisfied in relation to the 'undersized cable', leaving only those who are inclined and able to undertake adiabatic calculations in a position to (possibly) satisfy themselves that omission of overload protection (in appropriate circumstances) is acceptable.

Particularly after the one response last night, I'm actually starting to wonder whether a high proportion of practising electricians are even aware of the provision in the regs for potentially omitting overload protection!

Kind Regards, John

 

[THRIPSTER]

I try to avoid the OSG as it makes generalisations. I think it bases its assertions on standard circuits and that OPD are providing overload and fault current protection. As such, the circuits in the OSG should be inherently safe. With regards your postulate as to whether the subject is known abroad - I think not. I am no expert and am learning continually. Seems a lot more pleasant around here these days. Still contend that you would be better off posing questions of this type on the IET forum.

Regards