Basement CU

[JohnW2]

...discrimination has been offered to the circuit, so in that way it is compliant.

I guess it must just be me - but what does the above mean?

WTF do you think it means, stop being an A.R.S.E

It was a genuine question - I truly don't understand what you mean. As I said, maybe it's just me, but I really don't understand what "offering discrimination to the circuit" means.

Kind Regards, John

 

[JohnW2]

Well it the case of the garage, the situation described was a B40 in the house and a B32 in the garage. It was posed that a fault significant enough to trip the B32 in the garage, would also likely trip the B40 in the house. The danger then being the likelyhood of being plunged into darkness when the power-tool in your hands spinning at high speed has just gone faulty and tripped both the B32 and the B40.

All true. In my previous post, I'd overlooked the fact that there was lighting involved.

I'd imagine the same situation would arise in the above configuration? Good design is supposed to minimise these effects?

Yes, the same would be true. As you say, good design should seek to minimise such risks - but, as I said before, there's not really any design solution if one wants to run the secondary CU from another CU - the only real solution would be for the second CU to have it's own separate supply - which would make it a significantly more major job.

Kind Regards, John

 

[PrenticeBoyofDerry]

Well it the case of the garage, the situation described was a B40 in the house and a B32 in the garage. It was posed that a fault significant enough to trip the B32 in the garage, would also likely trip the B40 in the house. The danger then being the likelyhood of being plunged into darkness when the power-tool in your hands spinning at high speed has just gone faulty and tripped both the B32 and the B40. Or as john said, an earth fault taking out not only the whole garage supply, but half the house (or all of the house) as well.
I'd imagine the same situation would arise in the above configuration? Good design is supposed to minimise these effects?

This could be said of a lot of scenarios, it could happen whilst doing work within the main house. You try to minimise the likelihood of it, but to what extremes can you go, to be absolutely positive that this event will never take place.

 

[JohnW2]

This could be said of a lot of scenarios, it could happen whilst doing work within the main house. You try to minimise the likelihood of it, but to what extremes can you go, to be absolutely positive that this event will never take place.

I think that you and I have been saying essentially the same - that there is a limit to how far one can reasonably go to avoid these problems.

As I have implied, one obviously can virtually eliminate the risk to which Aragorn referred (whether in the 'main house', a garage or wherever) by having no protective devices common to both lighting and sockets circuits. If one does that, then nothing short of the cutout fuse (or the fuse protecting a submain) operating will result in loss of lighting because of a fault on the sockets circuit. However, battery backed-up emergency lighting would probably be a much easier/cheaper solution in a cellar or garage.

Kind Regards, John

 

[mfarrow]

I personally don't think that this discrimination business is really much of a issue... MCB trips are... rare events, and possibly having to reset two of them... 'once in a blue moon' is not really much of a hardship.

John, I'm sorry but what planet are you on when you say that? Discrimination isn't just about convenience, it's about safety for the user, especially if the basement sockets and lights are going to be supplied by the same upstream MCB, as in this instance, and especially in a basement which may be devoid of natural light, which we do not know.

 

[JohnW2]

I personally don't think that this discrimination business is really much of a issue... MCB trips are... rare events, and possibly having to reset two of them... 'once in a blue moon' is not really much of a hardship.

John, I'm sorry but what planet are you on when you say that? Discrimination isn't just about convenience, it's about safety for the user, especially if the basement sockets and lights are going to be supplied by the same upstream MCB, as in this instance, and especially in a basement which may be devoid of natural light, which we do not know.

I agree with what you say - you presumably missed my subsequent post in which I wrote:

All true. In my previous post, I'd overlooked the fact that there was lighting involved.

However, I still essentially stick by what I said about discrimination between OPDs. Discrimination between two OPDs in series is IMO essentially a matter of convenience. The problem to which you refer is not diretly related to that - it's a consequence of having lighting and sockets sharing a protective device, regardless of whether or not there were two non-discriminating OPDs involved (see ** below). As I subsequently wrote (and as you presumably also will have seen):

As I have implied, one obviously can virtually eliminate the risk to which Aragorn referred (whether in the 'main house', a garage or wherever) by having no protective devices common to both lighting and sockets circuits. If one does that, then nothing short of the cutout fuse (or the fuse protecting a submain) operating will result in loss of lighting because of a fault on the sockets circuit. However, battery backed-up emergency lighting would probably be a much easier/cheaper solution in a cellar or garage.

In fact, in terms of this thread, I'm not at all sure why there is a perceived need for this 'basement CU' to supply 'two lights and two sockets'. Why not run the two basement lights from an existing lighting circuit and used a spare way (ideally on a different RCD) in the house CU (e.g. the one which was going to provide the supply to the basement CU) to supply the two basement sockets? If there were no common RCD, taht would eliminate the hazard which concerns you.

** Even people who are on the same planet as you commonly have, or advise, a situation in which the lighting in a garage, shed, or other outhouse is derived from a single feed to the building (for sockets + lighting) via an FCU. That creates the hazard to which you refer, even when there are no non-discriminating OPDs.

Kind Regards, John

 

[westie101]

Sometimes I wonder about this fear of lighting going off for faults on other circuits on a common RCD.

If it was felt, by the appropriate authorities, to be that serious an issue they should have, don't you think, been suggesting properties have two separate supplies from the DNO "just in case". (obviously fed from 2 different HV & LV systems in case of an HV fault).

Just how far does this aversion to any risk at all actually go?

 

[JohnW2]

Sometimes I wonder about this fear of lighting going off for faults on other circuits on a common RCD. If it was felt, by the appropriate authorities, to be that serious an issue they should have, don't you think, been suggesting properties have two separate supplies from the DNO "just in case". (obviously fed from 2 different HV & LV systems in case of an HV fault). Just how far does this aversion to any risk at all actually go?

I have to largely agree with you. As you imply, loss of lighting whilst one is using a 'dangerous' tool is a risk we have to live with in in terms of 'power cuts'. There is undeniably some (tiny) risk, of course, but I presume that most of those who worry about it are the same people who regularly cross roads and drive cars and, heaven forbid, some may even smoke, drink alcohol or eat unhealthily

If I were, say, holding an operating circular saw in a basement when the power failed and the lights went out, I think I'd have the sense to wait until the blade stopped spinning before I put it down or tried to extract myself from the dark basement ... and I think that the great majority of others would do the same. So, even when this pretty rare situation arises, the risk of it resulting in harm is going to be small.

Kind Regards, John

 

[westie101]

the risk of it resulting in harm is going to be small.

40 years and I've never known it happen, as I'm sure if it did there would have been a huge amount of publicity!

 

[DetlefSchmitz]

We should probably decide what discrimination means.

I expect everyone knows this, but it may be worth saying in case someone doesn't.

I used a cartridge fuse of 30A in my garage protecting the SWA to a shed, and an MCB of 6A to protect the wiring for the lighting in that shed.

I caused a fault, and both the MCB tripped and the fuse blew.

For a fault, discrimination doesn't occur (in my experience).

For an overload it probably will, but I can't see how my two luminaires will suddenly cause an overload.

So I'm less impressed with the concept of discrimination than I used to be.

 

[JohnW2]

the risk of it resulting in harm is going to be small.

40 years and I've never known it happen, as I'm sure if it did there would have been a huge amount of publicity!

That may be going a bit far! Some people will panic when the lights suddenly go out, and I'd be surprised if at least one hasn't ever put their hand into a bit of revolving machinery or suchlike as a result.

Talking of which, the main 'hazard' we're talking about is that which arises if 'the lights go out' whilst one is in proximity to dangerous tools/machinery - but I have not heard the 'worriers' campaigning too loudly for dual lighting circuits, or emergency lighting, in every domestic garden shed, basement or garage? A 'lighting only' failure is theoretically even more dangerous than a lighting failure secondary to 'power' circuit failure (e.g. due to a common protective device) - since the tools/machinery will carry on operating indefinitely.

Kind Regards, John

 

[westie101]

not heard the 'worriers' campaigning too loudly for dual lighting circuits .....

The question I find myself asking more and more is: - "Are they worried about safety or their liability?" if the latter then they should say so.

A read of the HSE website shows they hold the same views on a number of subjects

 

[JohnW2]

We should probably decide what discrimination means.

Indeed. That's the point I was trying to make to mfarrow. Having lighting fail as a result of a fault on a sockets circuit because both share a common protective device is nothing to do with 'discrimination' in the normal electrical sense.

I used a cartridge fuse of 30A in my garage protecting the SWA to a shed, and an MCB of 6A to protect the wiring for the lighting in that shed. I caused a fault, and both the MCB tripped and the fuse blew. For a fault, discrimination doesn't occur (in my experience). ... So I'm less impressed with the concept of discrimination than I used to be

I think the main issue is that, in the face of high fault currents, it's always going to be rather hit and miss, particularly if the two protective devices are of different types. Even in my limited experience, it's not uncommon for a, say, 32A MCB to operate when a 13A, or even smaller, fuse in a plug doesn't - whilst sometimes it happens 'how one might expect', with the lower rated fuse operating but the MCB not operating. Having said that, I presume the chances of the 'expected discrimination' occuring increases as the ratio of the two devices' Ins increases.

Kind Regards, John

 

[JohnW2]

not heard the 'worriers' campaigning too loudly for dual lighting circuits .....

The question I find myself asking more and more is: - "Are they worried about safety or their liability?" if the latter then they should say so.

Indeed, but I think that 'conscience' probably sometimes comes into it a bit as well. If someone's knowledge enables them to envisage some risk, even if increadibly small, they would 'never forgive themselves' if some tragedy resulted from their not acting, or at least speaking, on the basis of that 'knowledge'. ... but if that's how their mind/psychology works, they probably should never carry passengers in their cars they are driving.

For others, it's probably just 'academic'. If their knowledge enables them to envisage some extremely small risk, they feel the need to tell the world about it.

Kind Regards, John

 

[ban-all-sheds]

Just how far does this aversion to any risk at all actually go?

To what can be expected to be reasonably possible, and what is reasonable provision to make to make it less possible.

Dual DNO supplies?

Not really. I have a customer with 4 supplies to separate rooms and a 5th local standby to each, but they have good reason to be exceedingly averse to loss of too much capacity.

But taking account of hazards that may arise from the failure of a single circuit such as a lighting circuit? That doesn't need dual DNO supplies, it just needs not having a t**t design something where lack of discrimination between a sub-main OPD and downstream devices could cause a fault on one circuit to take out the lights on another.