RCBO sufficient for sub-circuits or is RCD also needed?

[daverk]

Moved here last year, adjacent to the main Consumer Unit there is a later Sub-board mini-CU which has a Protek RCBO 40/30B which gives protection to 2 feeds:

Feed 1 direct to internal study where there is another mini-CU with isolation switch and 32A mcb protecting computer sockets.
Feed 2 direct to substantial detached brick-built shed with CU with isolation switch + 32A mcb for shed sockets + 6A mcb for shed lighting + 20A mcb circuit for further detached garage (proper one in breezeblock, adjacent to shed).

The 20A garage circuit spur has an old-fashioned Wylex fusebox in the garage, with the usual DP switch and 3 fused circuits in total never drawing more than the 20A.

Question 1 is, given that the shed or garage may occasionally have the hedge trimmer or a drill plugged into them, is that main RCBO sufficient? (NB the circuit is stable, it has never tripped, & the RCBO trips when tested so presumably it is functional).

Question 2 is, would it be desirable to have additional RCD protection in the shed or garage (which would still be fed from the RCBO), and would having an RCD downstream from the RCBO affect its operation or that of the RCD?

Question 3 is, would it be better to separate the study circuit, use the RCBO only for that, and then put an RCD next to it to feed the shed/garage circuit?

Apologies for the diatribe, but am weighing up the options at this stage. Would obviously prefer to avoid spending the money if the circuits as described with RCBO are considered safe etc, but will not take risks with electricity... in particular need to be reassured that there is adequate shock protection.

 

[ColJack]

it would depend on the mA rating of the RCBO..
if it's 30mA then it's ok for the use of garden tools etc, if it's 100mA then you need a smaller 30mA for the sockets etc.

 

[Taylortwocities]

Also, if you put the RCBO/RCD at the start of the circuit then it will trip if ANY of the circuits it s feeding has a fault.

It is far better to have individual RCBOs for each circuit, then a single fault will not trip the lot - which is against BS7671 btw.

 

[daverk]

it would depend on the mA rating of the RCBO..
if it's 30mA then it's ok for the use of garden tools etc, if it's 100mA then you need a smaller 30mA for the sockets etc.

Thanks ColJack, the RCBO is 40A Current Rating with a 30mA Tripping Rating so it should be ok.

Also, if you put the RCBO/RCD at the start of the circuit then it will trip if ANY of the circuits it s feeding has a fault.

It is far better to have individual RCBOs for each circuit, then a single fault will not trip the lot - which is against BS7671 btw.

Thanks. I thought that might be the case - it didn't seem sensible to have both the study (with computer equipment) and the shed on the same tripping device, hence the question.

The sub-circuits & RCBO seems to have been installed by an electrician, probably in early 2000s by the look of it, but there's no paperwork. Interesting that you say that it is non-compliant with BS7671 - do you remember if it would have been compliant before 2008, i.e. before the 17th Edition/BS7671:2008?

Simplest option seems to be to leave the study feed on the existing RCBO, and separate the shed/garage feed and put it alongside off another RCBO (there's a 2 module-wide space there fortunately). In that way computer equipment would not get tripped by problems on outside circuits, although haven't had a problem so far. In this situation, is a 30mA RCBO or a 30mA RCD better for the shed/garage circuit?

 

[ban-all-sheds]

Both provide exactly the same level of RCD protection.

An RCBO also provides overcurrent protection.

There's no point in having an RCD and an RCBO, and if you've got a perfectly good MCB there's no point in tossing it away and replacing it with an RCBO rather than simply adding an RCD, unless you have space problems.

 

[daverk]

OK, thanks for all responses. I've decided that the 2 feeds should each have their own separate protection. The downstream sub-circuits are all well protected for overload by MCBs so in theory this just means an additional RCD for the shed circuit, and I'll leave the 30mA RCBO on the study circuit.

But in practice that means a power disconnection to move the tails and put on an extra pair from the connector block, and of course this sort of work is Part P now, so that means getting someone in.

I know that the electricity companies will put in a double pole isolating switch presumably after the meter to make life easier. Anyone know if they still do it for free? Unlikely, I bet!

 

[ban-all-sheds]

Many meters have isolators in them these days - are you sure yours doesn't?

Still notifiable though.

 

[daverk]

Many meters have isolators in them these days - are you sure yours doesn't?

Thanks, that hadn't occurred to me but I just had a look, and the meter dates from 1985, and there is no isolator that I can see.

 

[ban-all-sheds]

At 25 years old it might be worth trying to find out if it's due for replacement.

Also ask what their policy on installing isolators is.

 

[LoveRocket]

The cable run to your shed needs overcurrent proection - it's not acceptable to just have it on an RCD.

It's a good idea to split the outbuildings from the computer circuit though, and adding an additional RCBO for one of these is by far this simpliest solution to this (least amount of messing around rearranging your CU).

 

[daverk]

The cable run to your shed needs overcurrent proection - it's not acceptable to just have it on an RCD.

It's a good idea to split the outbuildings from the computer circuit though, and adding an additional RCBO for one of these is by far this simpliest solution to this (least amount of messing around rearranging your CU).

Thanks LoveRocket. If you look at my initial post, you'll see that the feed/cable run to the shed/garage already has overcurrent protection, with individual 6A, 20A and 32A MCBs on sub-CUs in those buildings. Ditto the study feed has a 32A MCB for the sockets (the study lighting is separately off the house lighting circuit) on a sub-CU in the study. At present both those feeds are connected to the same 40A RCBO with 30mA trippage in a sub-CU near the main CU.

Correct me if I'm wrong, but I think that if I just have the shed/garage feed moved from that 40A RCBO (leaving just the study feed on that RCBO), and have the shed/garage feed put on a new, separate, additional RCD then the individual 6A, 20A and 32A MCBs in the shed/garage will give the required overcurrent protection? OK? It seems pointless to put in another RCBO instead of an RCD given that there are plenty of MCBs already on that outbuilding circuit. But if I am missing something please feel free to berate me!

No way would I contemplate having a circuit only connected to an RCD without having fuses or MCBs as well in some form, if that's what you were thinking.

 

[LoveRocket]

I'm affraid I'm going to have to correct you as you're wrong!

All cables need overcurrent protection upstream. It's not acceptable to rely on the OPDs for the final circuits downstream.

For example, if there was a fault on your submain cable between L+N, or in the shed CU, the only thing protecting it from overcurrent is the supplier's cutout (probably 100a!).

So, an RCBO (sized below the current cappying capacity of the cable to your outbuilding) in your main CU will provide
1) Overcurrent protection for the submain
2) RCD protection for the final circuits in the garage and shed

It will also fit in a single way so there's no need to faff around in your CU.

Hope that makes sense.

 

[securespark]

Isn't that what he's got?

A 40A RCBO feeding the computer sockets then another CU in the shed?

 

[ban-all-sheds]

That's what he's got, but what he wants to do is

Correct me if I'm wrong, but I think that if I just have the shed/garage feed moved from that 40A RCBO (leaving just the study feed on that RCBO), and have the shed/garage feed put on a new, separate, additional RCD then the individual 6A, 20A and 32A MCBs in the shed/garage will give the required overcurrent protection?

 

[1john]

in short, you need overcurrent protection on the sub main cable from it's origin.