Consumer unit

[Taylortwocities]

no rdc protection on your lighting circuits. So, if the shed develops a earth fault your lighting in the house will remain on, so no hazard. You can isolate the shed at the 32a mcb if you suspect the fault is in the shed wiring.

Please check the definition of isolation in the BGB.
It says …separate the installation or section from EVERY source of electrical energy.
Even with the MCB off, a neutral fault in the shed will cause the house RCD to trip with the ensuing mayhem (freezer food melting, no central heating, granny stuck on the Stannah stairlift, etc).

 

[knowall]

I totally agree, however any circuit on the RCD side of the board with an earth/neutral fault would cause the RCD to trip no doublt causing you mass hysteria !

KA[/b]

 

[SimonH2]

I'm also with those who say "don't".
It could be used provided the circuit is properly designed and installed, but from practical experience I wouldn't advise it. As others have said, it's a right PITA if you are in the garage and trip the RCD taking out most of the house in the process. It's especially fun at night !
Some time ago we had the CU moved at home (to move it out of what was to become a shower room) and the sparky put the garage supply on the RCD side without asking. We had to move it to the non-RCD side for the reasons given - it's still compliant as the cable routing does allow the cable to be non-protected. There's an RCD in the garage where it's easier to get to when it (infrequently) needs resetting.
As a bonus, the outside lighting runs from the garage circuit, so a fault on that also doesn't take out the house RCD.

As to welders, we haven't had a problem tripping a B32. That's with either a stick welder or MIG welder - but then we generally aren't running particularly high powers.

 

[ban-all-sheds]

Incomer

Service connector block(s) to

house CU

switchfuse => SWA => Outbuilding

Sorted.

 

[Iggifer]

BAS is right, nevermind other considerations, the balancing on that CU is poor already, adding another large load to the RCD side is a poor idea

It seems the RCD side has 6x32a and 1 x 40 and the non-RCD side just has 6a and 16s. Could be wrong about the non-RCD side as there's no close up.

Cooker and Shower on the same split is a poor design, one needs to go on an RCBO on the other side really.

 

[aesmith]

Incomer

Service connector block(s) to

house CU

switchfuse => SWA => Outbuilding
Sorted.

What you've described is exactly how the electrician did our supply to the outbuildings. Out of interest why are you preferring that over taking the supply from a 40A MCB on the non-RCD side in this case, an option that wasn't available for us as our CU is all RCD?

 

[flyingsparks]

Incomer

Service connector block(s) to

house CU

switchfuse => SWA => Outbuilding

Sorted.

Shouldn't that be...


Incomer

DP Isolation Switch

Service connector block(s) to

house CU

switchfuse => SWA => Outbuilding

Sorted.

 

[SimonH2]

Out of interest why are you preferring that over taking the supply from a 40A MCB on the non-RCD side in this case, an option that wasn't available for us as our CU is all RCD?

Personally, if the CU had space and capacity, then I'd just take it from the CU. But there are several reasons you might not want to :

1) If you can't get a suitable size breaker - eg if you wanted say 50 or 63A but the manufacturer only does up to 40A

2) You've not got space/spare non-RCD ways

3) It might potentially take over a load limit on some component - though I can't see that being an issue in a domestic environment

4) With a separate switch-fuse, you can a) choose a unit that is easy to terminate SWA into, and b) you can leave the power on to the shed/garage while turning off the house and vice versa

5) A fuse may well be preferable to an MCB - it gives better discrimination. Eg, if you have a "hard" fault in teh garage, then it will trip the MCB feeding that circuit - but the magnitude of the fault may also trigger the MCB in the house. In theory the one ion the garage should trip slightly faster as it's a higher overload %, but by the time it clears the fault, the upstream one is likely to have reached the point of no return in operating it's mechanism. A cartridge fuse gives coarser protection and is less likely to blow in those cases - thus leaving the garage lights on when you trip the sockets.

 

[JohnW2]

Please check the definition of isolation in the BGB. ... It says …separate the installation or section from EVERY source of electrical energy.

The definition in the BGB does,indeed, say that. However, as you know, the second sentence in the section on isolataion also says "In a TN-S or TN-C-S system, it is not necessary to isolate or switch the neutral conductor ....".

Even with the MCB off, a neutral fault in the shed will cause the house RCD to trip with the ensuing mayhem (freezer food melting, no central heating, granny stuck on the Stannah stairlift, etc).

I agree totally, which is why I think it would be very unwise (even in a TN system) to not have DP switching/isolation in this situation. However, as above, in a TN installation, it's not necessary for compliance with the regs.

Kind Regards, John

 

[Taylortwocities]

As always, on this DIY forum, my aim to advise 'best practice'.

 

[JohnW2]

As always, on this DIY forum, my aim to advise 'best practice'.

Agreed, and (in this context) both you and I have done this. However, it was you who directed folk to the BGB definition of isolation - and, as I've said, if one merely only strove for BGB-compliance, in this context one would (not fore the first time!) not end up with what you and I would call 'best practice'

Kind Regards, John

 

[ekmdgrf]

You mention in your other post you plan on using a welder in your garage.

A welder will take out that breaker every now and again, and will take out your 20A RCBO 7 times out of every 10 times you turn the welder on.

Also, you'll end up with your RCD protection at the house, so a single fault in the garage could take out the whole installation. Imagine being plunged into darkness whils holding a grinder or similar.

Why not let someone who actually knows what they are doing design this installation for you?

I wouldn't wish to get drawn into the on going debate on the suitability of a house based RCD to serve an outbuilding but some of a logic behind the reasoning doesn't really make sense to me.

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:

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.

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.

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...

 

[JohnW2]

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...

Partially true - but the point is that some configurations make it less likely that the lights in any particular place will go out as a result of a fault. The lights going out in the garage/workshop when you are using a power tool is a real issue - ideally you should not have 'workshop' lights protected by the same RCD (or MCB/fuse) as the sockets and, in truth, you really should have battery-powered emergency standby lighting, so you don't find yourself in the dark with a 'running down' power tool if, say, there is a power cut.

Kind Regards, John

 

[aesmith]

That just prompted me to check, and right enough there's a lot of nearby stuff on the same RCB in our installation. Within the kitchen for example we have sockets, lights and water heater on RCB 1, and cooker, central heating and water pump on the other RCB. I suppose there's no ideal way of splitting services between the two, but I would have thought it might be better with lights not sharing with any other service in the kitchen.

On the other hand, to put it in context we were 11 years at our last but one house, with a single RCB covering the whole house and also the outbuildings (stupid layout with outbuilding RCD inside the house RCD). I can't remember a single trip caused by the outbuildings, and only one caused in the house (appliance fault).

How frequently do people experience RCB trips?

 

[JohnW2]

On the other hand, to put it in context we were 11 years at our last but one house, with a single RCB covering the whole house and also the outbuildings (stupid layout with outbuilding RCD inside the house RCD). I can't remember a single trip caused by the outbuildings, and only one caused in the house (appliance fault). How frequently do people experience RCB trips?

Quite so. I lived for about 15 years with single RCDs covering each phase of a 3-phase installation (roughly one phase per floor), and experienced virtually no trips. In the subsequent 10 years or so, I've had circuits split between mutiple RCDs and, again, have experienced hardly any trips (except those cause by me!). ... and, yes, the RCDs have been regularly checked, 'properly' as well as by use of the test buttons.

Particularly given that even if 'the lights do go out', 99+ times out of 100 that is not going to result in problems/injury, the rarity of RCD trips is such that, although the hazard we are discussing theoretically exists, it is an incredibly low risk. Crossing a road is probably represents more of a risk!

Kind Regards, John