Consumer unit

[EFLImpudence]

It's the old lights argument again and it's in at least one other active thread here too. Yes, the lights will go out if your RCD trips. How is the different from having the RCD in the garage? Yes there are fewer final circuits that could trigger a garage shutdown but it you'll still having a spinning down angle grinder in your hand. Would you put the lighting on a non-RCD final circuit/own RCBO? Consider this scenario:

It depends on how safe you want it to be.

House with a split load dual-RCD board. Upstairs lights on one side and downstairs on the other, because heaven forbid that all lights should go out if the RCD tripped. No complains about that setup, I take it, it's fairly standard after all. I'm standing in the kitchen, top side of beef on the bottom shelf of the oven and a baking tray full of goose fat heating on the shelf above. I take the fat out to add the par boiled potatoes and just as I'm half way across the kitchen, that old immersion heater trips the RCD and the kitchen plunges into darkness.

Then if you want, design it so that that doesn't happen.

With my one one remaining hand I mop the floor, decide to mash the spuds instead and replace the lighting with it's own RCBO. The spuds are now finishing off in boiling water, in a pan on the oven. Just as I'm carrying the pan to the sink to pour off the water, the YL comes home and turns the hall lights on. One of those GU10s reaches the end of its life and pops the B10 RCBO. Damn. Shall we have one more iteration? OK. Without any remaining hands, I get someone else to rework that lighting circuit to accommodate a C10 breaker. Just as I go to drain off the peas, I'm half way across the kitchen when a gas working in the street puts a drill through the service cable. From my hospital bed I call that mate back and get him to fit multiple, battery backed up lights to the kitchen and also get me a number for the local takeway.

Nothing is foolproof.

In summary, regardless of where the RCD should/shouldn't go, I'm not sure the lights going out in a given configuration is a very valid justification as there's plenty more configurations that can give rise to exactly the same result. I've also used the angle grinder in the house a few times...

People do all sorts of stupid things.

However, we are suppose to install systems with regard to what may happen and make them as safe as possible within the constraints of how much the customer wants to pay.

We do/should not install systems which are less safe than they could be with the same parts - i.e. with all the lights on the same RCD or the lights on the same RCD as the room sockets.

Are you calm enough, when the power goes off, to stand still while the angle grinder spins down or do you become startled and jump turning into someone stood next to you?

 

[paulc2]

I'd have thought the main reason for having an outbuilding's RCD located in the house, is to protect someone from electric shock if the cable between were accidentally damaged. Having the RCD in the outbuilding implies that either there's 2x RCDs in series (not good) or that the cable run between isn't protected (potentially unsafe).

I know the cable between should be SWA, dug in a trench, etc but then accidents happen which might still expose a danger, eg a part of the cable or its connections or fittings not within the SWA run is inadvertently damaged and not noticed over time.

 

[JohnW2]

I'd have thought the main reason for having an outbuilding's RCD located in the house, is to protect someone from electric shock if the cable between were accidentally damaged. Having the RCD in the outbuilding implies that either there's 2x RCDs in series (not good) or that the cable run between isn't protected (potentially unsafe). ... I know the cable between should be SWA, dug in a trench, etc but then accidents happen which might still expose a danger, eg a part of the cable or its connections or fittings not within the SWA run is inadvertently damaged and not noticed over time.

The situation is no different from most sub-mains - protected by a fuse or MCB but usually not an RCD. As you imply, cables obviously should be adequately protected (against mechanical damage) - and installing an RCD is a less-than-ideal solution for a situation of inadequate mechanical protection (although we accept that concept in terms of cables buried in walls).

Kind Regards, John

 

[EFLImpudence]

SWA doesn't have to be on an RCD because it is mechanically protected by the earthed metallic shield so that a short circuit occurs when damaged which will open the overcurrent protection.

The same as any other cable may be should you not want an RCD on a circuit.

I think it has been shown here recently that RCDs are not magic and even, if I may deduce, that ONE RCD protecting everything is possibly more hazardous than having none.

 

[ban-all-sheds]

I'd have thought the main reason for having an outbuilding's RCD located in the house, is to protect someone from electric shock if the cable between were accidentally damaged. Having the RCD in the outbuilding implies that either there's 2x RCDs in series (not good) or that the cable run between isn't protected (potentially unsafe).

I know the cable between should be SWA, dug in a trench, etc but then accidents happen which might still expose a danger, eg a part of the cable or its connections or fittings not within the SWA run is inadvertently damaged and not noticed over time.

The rules for concealed cables are the same whether the cable is part of a final circuit or a distribution one.

[EDIT]Quote added for clarity after intervening replies[/EDIT]

 

[JohnW2]

The rules for concealed cables are the same whether the cable is part of a final circuit or a distribution one.

Is that comment directed at me? If so, I agree with you, but what is your point?

Kind Regards, John

 

[paulc2]

I'd have thought the main reason for having an outbuilding's RCD located in the house, is to protect someone from electric shock if the cable between were accidentally damaged. Having the RCD in the outbuilding implies that either there's 2x RCDs in series (not good) or that the cable run between isn't protected (potentially unsafe). ... I know the cable between should be SWA, dug in a trench, etc but then accidents happen which might still expose a danger, eg a part of the cable or its connections or fittings not within the SWA run is inadvertently damaged and not noticed over time.

The situation is no different from most sub-mains - protected by a fuse or MCB but usually not an RCD. As you imply, cables obviously should be adequately protected (against mechanical damage) - and installing an RCD is a less-than-ideal solution for a situation of inadequate mechanical protection (although we accept that concept in terms of cables buried in walls).

Kind Regards, John

Ok let me think up a hypothetical scenario - a shed is supplied by a SWA mains cable, which is brought inside and connected to artic cable inside the shed feeding a garage consumer unit, some lighting, some power sockets etc. A trailer reverses into the shed at night and punches a hole in the side of the shed. It goes unnoticed. The next night, someone goes into the shed in the dark and fumbles for a light switch......

 

[JohnW2]

I think it has been shown here recently that RCDs are not magic and even, if I may deduce, that ONE RCD protecting everything is possibly more hazardous than having none.

Who knows - you're attempting to compare two minute risks, neither of which we have much clue about the magnitude of!

However, have you ever actually heard of a case of a serious injury (or even death) resulting from the theoretical 'lights all went out' consequence of a single RCD operating (itself a pretty rare event)?

Kind Regards, John

 

[EFLImpudence]

Do you mean a danger of splinters?

I suppose a trailer would also have artic cable wouldn't it?



One-off happenings can always be thought up that would be unprotected by whatever had been installed.
We do not have to install with road trafffic accidents in mind.

 

[JohnW2]

Ok let me think up a hypothetical scenario - a shed is supplied by a SWA mains cable, which is brought inside and connected to artic cable inside the shed feeding a garage consumer unit, some lighting, some power sockets etc. A trailer reverses into the shed at night and punches a hole in the side of the shed. It goes unnoticed. The next night, someone goes into the shed in the dark and fumbles for a light switch......

Sure - but that's no different from a tradesman (or householder) accidentally damaging the tails to your meter, or between meter and CU, or a submain cable protected only by a switch-fuse, and then someone touching the damaged cable without realising that it has been damaged.

Kind Regards, John

 

[EFLImpudence]

I think it has been shown here recently that RCDs are not magic and even, if I may deduce, that ONE RCD protecting everything is possibly more hazardous than having none.

Who knows - you're attempting to compare two minute risks, neither of which we have much clue about the magnitude of!

However, have you ever actually heard of a case of a serious injury (or even death) resulting from the theoretical 'lights all went out' consequence of a single RCD operating (itself a pretty rare event)?

I've just realised that is in reply to my post and not paulc2's.


No, I haven't but I thought that was the point.

That is - as no one seems to have heard of anyone being 'saved' by an RCD then logically the whole installation going off because one lamp has blown or another small fault presented a greater risk.

 

[JohnW2]

Who knows - you're attempting to compare two minute risks, neither of which we have much clue about the magnitude of! ... However, have you ever actually heard of a case of a serious injury (or even death) resulting from the theoretical 'lights all went out' consequence of a single RCD operating (itself a pretty rare event)?

No, I haven't but I thought that was the point. ... That is - as no one seems to have heard of anyone being 'saved' by an RCD then logically the whole installation going off because one lamp has blown or another small fault presented a greater risk.

No, that is (IMO) false logic IF no-one has heard of anyone being 'saved' by and RCD and no-one has heard of anyone suffering serious injury/death as the result of 'a whole installation going off', then how can you conclude that the latter represents a greater risk?

Kind Regards, John

 

[paulc2]

In my own home, in the past 10 years I've tripped the RCD twice. If it weren't there, I'd have only been dead once though.

The "lights going out" scenario also requires that it happens at night, eg no daylight available to see without the lights. Thus making it even less of an issue. But its a different argument, whether you need 1 or 2 RCDs (or equivalent eg RCBO on certain circuits). Mine is that the cable between a house and a shed will possibly either be more vulnerable than other non-RCD protected ones, or to make them as safe, is costly.

 

[EFLImpudence]

No, that is (IMO) false logic IF no-one has heard of anyone being 'saved' by and RCD and no-one has heard of anyone suffering serious injury/death as the result of 'a whole installation going off', then how can you conclude that the latter represents a greater risk?

Whilst I do not know of any injuries caused by either, I do know of several cases where people, particularly elderly, have been plunged into darkness causing inconvenience and worry, if not fear, but none of anyone who has caused this by 'nearly' being electrocuted - tripping the RCD.

So it would seem more likely to 'happen'.

 

[EFLImpudence]

In my own home, in the past 10 years I've tripped the RCD twice. If it weren't there, I'd have only been dead once though.

Doing what?

The "lights going out" scenario also requires that it happens at night, eg no daylight available to see without the lights. Thus making it even less of an issue.

It's dark here now.

But its a different argument, whether you need 1 or 2 RCDs (or equivalent eg RCBO on certain circuits).

It's up to you to spend as much as you like and design accordingly.

Electricity is dangerous. It's a compromise.
Remove it from your house and get knocked over by the proverbial bus.

Mine is that the cable between a house and a shed will possibly either be more vulnerable than other non-RCD protected ones, or to make them as safe, is costly.

But it isn't, is it?