electrics to shed/workshop

[glav]

ok, all taken on board i will go with running a 16mm swa and stick with the 40amp mcb at the house CU.
thanks for the advice.

 

[JohnW2]

Does that set-up achieve discrimination?

Clearly not - but what would you suggest as a practical alternative? The cable needs protection as do the final circuits. You could omit the 32A MCB in the shed (but run the lighting off a 6A local MCB or FCU), hence relying only on the 40A MCB in the main CU - but then you would then not be able to use a 2.5mm² for a ring final or 4mm² for a radial final in the shed.

Personally, I'd prefer to live with the lack of discrimination than have to wire the shed's sockets in 'shower cable'

Kind REgards, John.

 

[glav]

ok all taken on board, i think i will stick with the 16mm swa and 40 amp mcb at the house CU though incase i ever want too add on too the shed supply CU. i.e pond pump etc.
thanks for the advice

 

[Monkeh]

ok all taken on board, i think i will stick with the 16mm swa and 40 amp mcb at the house CU though incase i ever want too add on too the shed supply CU. i.e pond pump etc.
thanks for the advice

A pond pump will use less than your lights.

 

[JohnW2]

ok all taken on board, i think i will stick with the 16mm swa and 40 amp mcb at the house CU though incase i ever want too add on too the shed supply CU. i.e pond pump etc.

A pond pump will use less than your lights.

Indeed it would, but the "etc." might possibly take a lot more

Seriously, though, maybe I'm too allergic to digging (or approaching an age when I couldn't do digging!) - but, with a task like this, I would usually always 'go up one size' relative to size of SWA I currently needed, or could envisage needing - simply because there are often unexpected surprises in the future (and, as I said, it's only about a 23% increase in cable price)!

Kind Regards, John.

 

[timbim]

Seriously, though, maybe I'm too allergic to digging (or approaching an age when I couldn't do digging!) - but, with a task like this, I would usually always 'go up one size' relative to size of SWA I currently needed, or could envisage needing - simply because there are often unexpected surprises in the future (and, as I said, it's only about a 23% increase in cable price)!

What digging?

 

[JohnW2]

What digging?

The digging I'd feel obliged to do in order, as a matter of principle, to avoid paying £215.05 + VAT (aka about £2582) to hire a toy for a day!

Kind Regards, John.

 

[securespark]

Does that set-up achieve discrimination?

Clearly not - but what would you suggest as a practical alternative? The cable needs protection as do the final circuits. You could omit the 32A MCB in the shed (but run the lighting off a 6A local MCB or FCU), hence relying only on the 40A MCB in the main CU - but then you would then not be able to use a 2.5mm² for a ring final or 4mm² for a radial final in the shed.

Personally, I'd prefer to live with the lack of discrimination than have to wire the shed's sockets in 'shower cable'

Kind REgards, John.

Just pointing out a non-compliance that could be a PITA if the house MCB trips in place of the local CPD's.

 

[JohnW2]

...Personally, I'd prefer to live with the lack of discrimination than have to wire the shed's sockets in 'shower cable'

Just pointing out a non-compliance that could be a PITA if the house MCB trips in place of the local CPD's.

It might be a (very occasional) PITA but, as I was implying, the only realistic alternative I can think of would be more-or-less as big a PITA - namely to have the house MCB as the one and only protection (with either a 32A MCB, or a 40A one with oversized socket cabling in the shed), which therefore would obviously trip (in the house) in the event of a fault in the shed.

One obviously cannot have protection only in the shed - the cable between house and shed obviously has to have some protection. If one wants a 32A+6A supply in the shed and wants that to have local protection, but also wants to retain discrimination from the device protecting the cable, that upstream MCB would have to be 100A+, and the SWA also capable of carrying 100A+ ... unless you can think of some other way of doing it that I've missed.

BTW, I don't think it's particularly clear that a lack of discrimination (which we're now meant to call 'Selectivity') between MCBs is actually a 'non-compliance' - 536.1/536.2 are far from explicit (at least, to me!).

Kind Regards,John.

 

[securespark]

My apprenticeship was with a guy who trained under the first copy of the 14th Ed. He was a TV engineer prior to that. So I learnt most of the 14th Ed. regs from him, while simultaneously attending college and doing the 15th....

Therefore, I am guilty as charged of not familiarising myself with the new regs in that respect.

Can't believe I've missed that change.

Slap on wrists!

I was remembering reg. A7. in the 14th, which is quite clear:


A7. The characteristics and settings of excess-current protective gear, including fuses, and the arrangement of the installation, shall be such that discrimination in the operation of the excess current protective gear is ensured.


The 15th gets a bit wishy-washy:


533-6. Where necessary to prevent danger, the characteristics and settings of devices for overcurrent protection shall be such that any intended discrimination in their operation is achieved.


Wishy-washy bit in italics.

The 16th reg. is little changed from the 15th.

So it seems this reg. is being watered down over time.

To my mind, Where necessary to prevent danger makes the reg. all but redundant.

Failing to achieve disc, I mean selectivity does not really give rise to danger, only inconvenience.

 

[JohnW2]

So it seems this reg. is being watered down over time. To my mind, Where necessary to prevent danger makes the reg. all but redundant. Failing to achieve disc, I mean selectivity does not really give rise to danger, only inconvenience.

Indeed so - that was what I was implying.

I'm not sure I understand why those earlier regs you cite were so much tighter - as you say, with a very small number of pretty obsure/contrived exceptions, failure of discrimination/selectivity is only going to be an inconvenience, not a danger. It therefore seems to me that, for once, the current regs are more sensible.

... particular in cases such as we've been discussing. As I've been trying to point out, if we were obliged to 'ensure discrimination' in this scenario, we would (unless I've missed something) be stuck with a choice between two or three equally ridiculous ways of achieving compliance.

Also, given that the bit of the 14th ed you cite says 'including fuses', use of a BS1363 plug with a 13A fuse on a 15A/16A/20A radial final would presumably have been non-compliant, since that would be a case of very poor discrimination, I would have thought I would also imagine that a good few sub-mains with high rating downstream MCBs might also fail a 'discrimination test'.

Kind Regards, John

Kind Regards, John.

 

[securespark]

Agreed.

Think about your average 13A 1362 fuse protecting an appliance connected to a 32A MCB.

I have come across 100's of instances over the years where the breaker has popped before the 1362.

 

[JohnW2]

Agreed. Think about your average 13A 1362 fuse protecting an appliance connected to a 32A MCB. I have come across 100's of instances over the years where the breaker has popped before the 1362.

Exactly. In fact, when it's a matter of fuse vs. MCB, in the presence of a very high fault current, I suspect that the MCB is very likely to operate first, almost regardless (within reason) of the relative In's of the two devices.

What I think many people mayoverlook is that once the fault current exceeds 5*In for a Type B MCB (or 10*In for type C or 20*In for Type D), the time/current curve is essentially vertical - so that the disconnection times for very high currents may actually be incredibly short - so short that a fuse probably doesn't have enough time to even start thinking about heating up and popping

Kind Regards, John.

 

[ban-all-sheds]

the "etc." might possibly take a lot more :-

And the OP might possily not have a clue what he's doing.

glav - nobody is born knowing anything about any of this, everybody has to learn it all.

The question is have you learned enough to be competent to do this work?

If, when you read through this topic, including all of the information linked to, you find one single thing which you don't understand, or even makes you think "I never knew that" then please take that as a sign that you should be using an electrician for this work rather than DIYing it.

What will you do about testing, for example? Do you know what tests you should carry out on the installation - what sequence you'd do them in and at what point you would energise the installation, and for each test do you know what is being measured, why it is important, how you would carry out the test, and with what equipment, and what sort of results you would expect to get if everything was OK?

What do you plan to do to ensure that the work complies with P1, and how do you plan to convince your LABC that it will comply?

 

[JohnW2]

If, when you read through this topic, including all of the information linked to, you find one single thing which you don't understand, or even makes you think "I never knew that" then please take that as a sign that you should be using an electrician for this work rather than DIYing it.

Whilst I agree with the spirit of what you're saying, that statement is surely way OTT? If the finding of "one single thing" that one doesn't understand or which makes one think "I never knew that" indicated that one should turn to some other (presumably omniscient) person to undertake electrical work, then I would suggest that there would not be any 'competent' electricians to turn to! .... Hands up any electricians who have never said "I never knew that" and who have never found 'one single thing' that they don't understand!

I fear that you undermine your well-intentioned comments by making them too extreme.

Kind Regards, John.