best way to do it?

[GPack]

Hello. A quicky that hopefully someone can clear up for me.
Consumer unit in house to supply cabin at bottom of garden. I have designed the circuit as follows.
25A RCBO in main c/u -37 meters of 4mmSq SWA to 4 way/2module c/u in cabin - Main switch & 20A MCB for power and 3A for lighting. Does this sound ok? Any way to improve it.
1. This would mean terminating SWA onto an insulated board. Is there a better way?
2. I have been told it would be better to put a Switched fuse isolator just outside the main consumer unit so the circuit can be isolted on 2 poles to work on it if necessary. Is this how you would do it?
3. What fused d/p isolator would you use?

Thankyou

 

[davelerave]

not familiar with uk rules so ill be interested in the answer

the obvious issue i see is that a socket fault may knock out the light -creating a potential hazard

 

[Deleted member 9648]

Sounds fine to me.....if you have an RCBO at the origin of the circuit that will be double pole .
Terminate the SWA into a galv through or adaptable box outside the CU and connect an earth wire to the box.

 

[GPack]

RCBO is single pole.

 

[Spudnik]

Indeed most RCBO's are only single pole.

If double pole isolation is required to work on the submain, then the main switch will do.

Must admit, i would run in a 6mm SWA with 32A upfront.

Plenty of headroom then.

 

[davelerave]

so whats the rcbo on the swa for then? someone will have to tell me

even if it's tt does the main rcd not cover automatic disconnection on the swa itself

 

[Deleted member 9648]

My mistake....always assumed Rcbo's disconnected both poles.

 

[GPack]

The RCBO is in an unprotected way so it is independent of the other circuits.

 

[davelerave]

so whats the rcbo on the distribution circuit for?
excuse my ignorance

 

[GPack]

No problem Dave. You will see the way I have designed the circuit from the first post. The circuit starts with an RCBO. The RCBO provides both overcurrent protection and protection against any Earth leakage currents for that circuit as required by the 17th edition. The RCBO works independent of the RCDs that supply the rest of the circuits in the house. This means if there is a fault in the cabin, the household circuits won't be affected. The distribution circuit, SWA is then connected from the RCBO to the small consumer unit in the cabin. Hope that clears it up for you. Please say if it doesn't.

 

[davelerave]

hey i work in the ROI hoping to move to UK-hence my confusion on UK rules


so here for tn-c-s we would have overcurrent /short-circuit protection on swa at main DB

and then separate 30ma protection for each final ciircuit if required at the sub-board in shed


the rcbo at main board would be considered a potential nuisance as lights and sockets trip together

and you have to go back to main board to reset/troubleshoot if a problem arises in shed


just curious-is it 'TT'? what exactly is the logic ?

 

[GPack]

You raise a very fair point. Looking at that I would say that your circuit is designed better than mine. Thank you very much. So the DB at the cabin will have a main d/p switch with a 20A RCBO and a 3A RCBO.

The installation is TNCS not TT.

If it were TT I take it you would be happy with my original design?

What do you think about having to isolate the submain circuit using the main switch on the house consumer unit? You could isolate it from the MCB but it would only be single pole isolation.

Would you come from the main consumer unit, from the submain MCB into a d/p isolator and then into the SWA and down to the Cabin?

Apologies for my sucking eggs post previously. I understand where you are coming from now.

 

[davelerave]

i would be thinking of 6sq supply if possible and prob a 6amp rcbo for the lighing circuit

whatever rules you have for isolation as well(generally DP for TT and SP for T-N-CS but rules may vary)

do rules require rcd/rcbo for submain? certainly wouldnt be using it if not required for tn-c-s as mcb will ensure 'automatic disconnection' if cable is cut

good practice is usually to avoid potential nuisance when installing earth-leakage protection

 

[johnboy6083]

Could'nt you just run the SWA from an MCB in the main CU. At the shed/garage CU you then provide RCD protection. Localises any faults to nearest CU.

Isn't there some rule about earthing in outbuildings also? Something about an earth rod? Id be happy to be educated?

John

 

[GPack]

I don't think I need 6A for lighting. I don't think we need 6mmSq SWA. I know it is sometimes a good idea to allow for future loads but in this instance it isn't going to happen. Because of this i have tried to be as efficient as possible without over engineering the installation.

I wouldn't say we need to use a rod for the cabin. No need. There are no extraneous parts that need bonding, no structural steel etc.

 

[GPack]

A quick question.

If I used an MCB at the main board, SWA to the cabin board which was RCD protected, is it true that I would need a double pole switch (2modules), and RCD (2 modules) and 2 MCBs for lighting and power (2modules)? I don't think you can do away with the d/p switch can you? reg no. if possible would be
helpful when answering this one.
Or just a D/P switch and 2 RCBOs (4 modules)

 

[davelerave]

sure ya i would use the 6sq for Zs and Vd as well as capacity

as price difference is small

3amp is small for any final circuit-you would certainly be risking nuisance with incandescents anyhow

The final circuit ocpd protects the conductor- design current may be small but mcb can be bigger than 3amp as cable is still protected

If light fitting needs protection it should have an internal fuse fitted