electrics to shed/workshop

[Stoday]

Please sir, please sir. My hand's up. I didn't know that the regs on discrimination between protective devices had been watered down.

Indeed even if "selectivity" is necessary to prevent danger you only need to consider selectivity under the rule.

It's fortunate you don't need to do anything about it since such situation regularly occurs in garage/workshop supplies. Often a 32A breaker in the house feeds a 32A breaker protecting a ring circuit in the workshop. Tripping of the house breaker will shut down the lights where machines may still create a cutting hazard in the dark whilst they run down after loss of power.

 

[JohnW2]

Please sir, please sir. My hand's up. I didn't know that the regs on discrimination between protective devices had been watered down.

I'm sure that if they were being honest and forthcoming, the hands of all the electricians here would be up (in relation to something they didn't know/understand, not necessarily discrimination) - so you're in very good company!

Please Indeed even if "selectivity" is necessary to prevent danger you only need to consider selectivity under the rule.

Quite - that's part of the 'watering down' which has apparently happened - and, as I said last night, for once I think the current regs are probably being sensible.

It's fortunate you don't need to do anything about it since such situation regularly occurs in garage/workshop supplies. Often a 32A breaker in the house feeds a 32A breaker protecting a ring circuit in the workshop. Tripping of the house breaker will shut down the lights where machines may still create a cutting hazard in the dark whilst they run down after loss of power.

That's certainly a potential danger, but it's not anything to do with discrimination/selectivity (per 536 of the regs) - which is about discrimination between protective devices (and your scenario relates to the operation of only one). However, although the discrimination regs have been watered down, others have been tightened up. 314.1 now says:

"Every installation shall be devided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault

Note that this says 'shall', rather than 'shall consider'. The potential danger you mention will exist whenever a shed/garage where power tools etc. may be used has an electricity supply which shares any protective devices (MCBs and/or RCDs) with any circuits in the house - which is quite common (mainly in relation to RCDs), but seemingly probably non-complaint with 314.1(i). In terms of '17th-ed installs' I think compliance in that respect could only be achieved by the shed/garage having its own RCBO (since it could not share an RCD with any other circuits). In the case of a TT installation which had an upstream Type S RCD, the shed/garage's RCBO would presumably have to be supplied upstream of that.

The situation you describe, with 32A MCBs in both house and garage/workshop probably would not be non-compliant with 314.1(i), provided that the house MCB served nothing other than the garage feed - since it is incredilby unlikely that the house MCB would operate due to anything other than things happening in the garage (in which case the garage MCB would operate, and create hazards, even if there wasn't one in the house).

Kind Regards, John.

 

[Stoday]

I intended that the garage had a garage consumer unit such that a 32A mcb/rcbo protected the ring and a 6A or 10A mcb the lights.

I really don't see much need for a ring in a domestic workshop. A 20A radial would be better because under overload it would provide some discrimination.

The biggest motor in my workshop is 2kW, for a table saw. Runs off a 13A plug and has only fused once. Again, in a domestic workshop, only one machine (plus dust extractor) is used at a time

 

[JohnW2]

I intended that the garage had a garage consumer unit such that a 32A mcb/rcbo protected the ring and a 6A or 10A mcb the lights.

That's what I thought you meant.

I really don't see much need for a ring in a domestic workshop. A 20A radial would be better because under overload it would provide some discrimination.

and I agree with that.

However, getting back on topic, if you're suggesting that the CU in the garage would enable its 32A MCB to trip without taking out the garage lights, then what is going to be protecting the cable between house and garage? As I said before, unless you had an MCB (or other protective device) in the house (and corresponding SWA) rated at 100A+, I don't see how you would hope to get reliable discrimination from the 32A MCB in the garage - and without that discrimination, the garage lights still get taken out if there is a fault on the garage socket circuit. Or am I missing something?

Kind Regards, John.

 

[securespark]

But he's now considering a 20A radial, so there would not be a 32A MCB in the garage.

Wasn't the idea to have a bit of SWA protected by a 40A MCB, feeding a sub-board with (now) a 20A & a 6A MCB?

 

[JohnW2]

But he's now considering a 20A radial, so there would not be a 32A MCB in the garage.

Well, he may now be 'considering' a 20A circuit, because Stoday recently suggested it, but I haven't seen the OP talk about anything other than a 40A MCB feeding a shed CU with 32A and 6A MCBs.

Wasn't the idea to have a bit of SWA protected by a 40A MCB, feeding a sub-board with (now) a 20A & a 6A MCB?

See above - I don't think this has (at least, not yet) been the OP's idea. In any event, I don't think you'd get any reliable discrimination between a 20A and 40A MCB.

Kind Regards, John.

 

[ban-all-sheds]

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

Well - I'll plead guilty to imprecision/incompleteness giving the impression of extremity.


Understanding - I stand by that - you do need to understand it. I took it as read that understanding might not be instantaneous, and might need some studying and thinking, so sorry if that wasn't clear - maybe I should have said "if you're unable to understand it...."

Where I really wasn't clear was with the "didn't know that". I meant, and should have said, that that's an indication not to DIY yet, as clearly there are still things you don't know.

 

[JohnW2]

Understanding - I stand by that - you do need to understand it. I took it as read that understanding might not be instantaneous, and might need some studying and thinking, so sorry if that wasn't clear - maybe I should have said "if you're unable to understand it...."

Yes, that would have been more reasonable - and I agree that is some cases we do have good reason to believe that a poster is probably unable to understand things which they need to be able to understand. However, in the context of this thread, I would suggest that the OP has given us no reason to think that there is an 'inability to understand' issue.

Where I really wasn't clear was with the "didn't know that". I meant, and should have said, that that's an indication not to DIY yet, as clearly there are still things you don't know.

I still think that you are being too 'sweeping'. It's not at all uncommon to see experienced electricians, in this forum and elsewhere, saying that they "didn't know X" until someone points it out - but I wouldn't suggest that they should not yet undertake electrical work, would you? The mere fact that people come here and ask questions indicates that they recognise that they need to know things which they don't yet know, and are attempting to remedy that.

Kind Regards, John.

 

[ban-all-sheds]

In terms of the information in this topic, and that in the links in my first post, until you know it all you shouldn't be DIYing a task of this nature.

 

[securespark]

Talking of experienced electricians not knowing things...

Not only is Selectivity a little known principle, but there are other areas too.

I have encountered many NIC sparks recently (not the boy-wonders, the fully-fledged older generation sparks) who know little about IP breaches in CU's. Not only that, their inspectors don't seem clued-up about them, either!

 

[JohnW2]

In terms of the information in this topic, and that in the links in my first post, until you know it all you shouldn't be DIYing a task of this nature.

A fair proportion of those links are to reference material, and I don't think it's realistic to expect anyone, even experienced electricians, to 'know it all' - i.e. carry around tabulated data etc. in their heads

Talking of your list of links, the first is, as you know, to a wiki article which you wrote. ISTR we've been through this befoe, but I can't help but wonder whether it is not, in one respect, unnecessarily confusing for the DIYers for whom it is intended. Having initially explained:

I2: its fusing/tripping current. For a Type B MCB this is 1.45In, and it must trip within 1 hour

... you then subsequentyly write:

Finally, 1.45 times the current carrying capacity Iz of the cable must be no less than I2, the current causing effective operation of the device:
1.45Iz ≥ I2

... taking those two statements together, would it not be less potentially confusing to simply say "...Iz of the cable must be less than In of the device" - which, as well as being simpler, makes intuitive sense?

Kind Regards, John.

 

[JohnW2]

I have encountered many NIC sparks recently (not the boy-wonders, the fully-fledged older generation sparks) who know little about IP breaches in CU's. Not only that, their inspectors don't seem clued-up about them, either!

I can believe it. I fairly recently heard about a (very experienced) electrician who complained about some sealant (caulk or somesuch) which had been used to seal around cables entering the top of a CU (so as to achieve currently-required IP level) on the grounds that 'it impedes ventilation and therefore could lead to overheating'!

Only a fool believes that everyone in any trade or profession knows, or should know, everything!

Kind Regards, John.

 

[ban-all-sheds]

... taking those two statements together, would it not be less potentially confusing to simply say "...Iz of the cable must be less than In of the device" - which, as well as being simpler, makes intuitive sense?

The current carrying capacity of the cable must be less than the rating of the device?

So on a B50, for example, we must use a cable which cannot carry 50A?

That doesn't make intuitive sense to me....

I'll assume that you meant to write ""...Iz of the cable must be no less than In of the device"?

The article does say that:

"So Ib ≤ In ≤ Iz"

Having initially explained:

I2: its fusing/tripping current. For a Type B MCB this is 1.45In, and it must trip within 1 hour

... you then subsequentyly write:

Finally, 1.45 times the current carrying capacity Iz of the cable must be no less than I2, the current causing effective operation of the device:
1.45Iz ≥ I2

... taking those two statements together, would it not be less potentially confusing to simply say "...Iz of the cable must be less than In of the device" - which, as well as being simpler, makes intuitive sense?

Have you forgotten that the 1.45 in I2 = 1.45In is not the same 1.45 in 1.45Iz ≥ I2 - it's just coincidence?

If I2 = 1.8In the requirement for Iz would remains 1.45Iz ≥ I2 (or I2 ≤ 1.45Iz if you prefer).

It would not become 1.8Iz ≥ I2.