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

[daverk]

I'm interested in all your thoughts, honest. But I think that there are too many words on here (me included) so the essence of this is getting too complicated.

Bear with me, at the weekend (power outage Friday because some trees are getting lopped by the electricity board!) I'll knock up the circuit schematics and try to post them here, to make what exists, and what could be, as clear as possible, and then you can tell me definitively whether it's good or bad!

 

[daverk]

Firstly, thanks for the responses on Thursday. In particular LoveRocket and 1john.

The penny dropped and I'll just say it again in my way to show that I've understood : on a cable run where the MCBs are distant and down the run, if there is only an RCD at the start of the circuit, then the cable between the start of the run and the MCBs would not be protected for overcurrent other than by the main 100A fuse. But an RCBO provides overcurrent protection as well as acting as a residual current device so it gives overcurrent protection to all the circuit including the leg between the start of the run and the downstream MCBs. I hope that's right now...

So in essence I already have a safe system because the study and shed/garage feeds are both on an RCBO. However it would be better to separate them to avoid computer and garden equipment being on the same RCBO, and it should be a relatively simple job to add a 2nd RCBO, move Shed/Garage feed to the 2nd RCBO, and add an extra Supply feed from the Connector Block to 2nd RCBO.

Now, I stress that because this change would be notifiable that I would have to get in a Part P registered electrician to do the work, but I do want to understand what is a safe and a sensible configuration (and ensure that his spec is an appropriate one).

So for clarity I have done some circuit schematics (either will open full size in a new window if you click on them) of what
exists already: and what is proposed:

The changes are minimal but in answer to the point you made, LoveRocket, there is no space on the main split load CU to accept an RCBO so it will have to go in the enclosure (sub-board #1) alongside the other RCBO. There is a free double module space large enough.

Would be grateful for any comments regarding the layout. Having checked the voltage drops of the existing cable runs on http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html they seem to be OK/adequate:

• 4mm2 SWA to RCBO to Study at 23 metres run would be good for a max 9.6kW with a max current load of 41.7A in a 42A max cable load.
  
  10mm2 SWA to RCBO Shed at 39 metres run would be good for a max 16kW with a max current load of 69A in a 73A max cable load.
  
  2.5mm2 SWA Shed to Garage at 24 metres run would be good for a max 6kW with a current load of 26A in a 31A max cable load.

So some questions:
1. would the existing 40A current rating/30mA trip rating RCBO be OK for the feed to Study, given the 42A-ish maximum load (which is unlikely ever to be reached, and there's a 32A MCB in the Study anyway - presumably there's a need for some difference between the two)?
2. if the 10mm2 run to the Shed can handle 69A and still be within the max cable load then presumably another 40A current rating/30mA trip rating RCBO would be suitable for that too (40A should be more than enough)?

As always, any other general observations would be welcomed.

Thanks all.

 

[ban-all-sheds]

IMO this would be better, as it would move the RCD protection to each location so that you don't have to schlep back to the house if it trips, and means that if you are in the shed or garage doing something you don't get plunged into darkness with a slowly spinning down finger amputator in your hand because of a socket circuit trip.

 

[daverk]

Thanks ban-all-sheds, for taking the time to consider this. I take your point about having the trip resets local to where the trip might happen but this would be quite an expensive solution - about £140 for the MCB, the 4 RCBOs, & Garage CU.

The other thing - that 50A MCB at the top of the shed/garage circuit would give overcurrent protection to that whole circuit, but surely, without an RCBO or RCD at the top of that circuit there would be no residual current detection on the shed/garage cable runs (mainly in cleats on external walls, with a lesser section in conduit and underground), neither on the 10mm2 cable run from the 50A MCB to the shed, nor on the 2.5mm2 cable run from the shed to garage, so if they were sliced there would be a risk of shock?

 

[ban-all-sheds]

They're armoured cables - whatever slices through them will be earthed when it hits the live.