Shed Installation Disconnection time?

[Just]

Hi I have a socket Radial off a 20A MCB and a lighting Radial off 6A MCB in a shed the board is fed by a 32amp MCB protected by an RCD on a TNCS syatem in the house. If I want the first point of fault ie the shed to trip first should I be using an RCBO for a TT system that trips under .2s? This would avoid both RCDs or the one in the house tripping first if I use an RCD 0.4s disconnection. I could change the type of RCD in the house but that protects a number of circuits. I have a 4 way CU so could either have a RCBO main switch, protect two MCBs with a different type RCD or fit two RCBOs. Too many options!
Thanks for your advice,

 

[Octopus]

Is this for you or a customer?

 

[ezzzekiel]

but it starts getting expensive or messy.

not really, split tails and fit board at shed

 

[Deleted member 26818]

That's what I was refering to as messy.
Don't forget you'd need overload and fault protection.

 

[ezzzekiel]

how is that messy? and protection goes without saying but its no different to any other setup

 

[Deleted member 26818]

First off you would have to place an enclosure for the circuit protective device, in such a position that the SWA which has already been terminated into the CU will reach.
Then you will have to split the tails, I assume you were intending using henly blocks?
These have to be placed somewhere and then two pairs of tails will have to go from these, one pair to the CU and the other to the enclosure.
Finally there's the earth from the SWA to be installed, either to the earth bar in the CU, or to the MET if the earth bar in the CU is not being used as one.
To my mind, a lot messier than using a non-RCD protected way in the CU.

 

[ezzzekiel]

thanks your explanation is very clear and now i have a better understanding of how it is done and obviously it would end up quite messy, unless of course you happen to be an electrician and have the skill and capability of adding to an installation in a tidy, safe manner in a location which is (in most cases out of sight) away from view.

 

[ian.settle1]

No not the circuit, just the cables.

Are cables not part of a circuit?

 

[Deleted member 26818]

thanks your explanation is very clear and now i have a better understanding of how it is done and obviously it would end up quite messy, unless of course you happen to be an electrician and have the skill and capability of adding to an installation in a tidy, safe manner in a location which is (in most cases out of sight) away from view.

Get a grip.

 

[Deleted member 26818]

Are cables not part of a circuit?

I believe so.

 

[ezzzekiel]

Get a grip.

whatever you say say, youre the boss

 

[malcolmsanford]

The cable supplying the shed is a distribution circuit. As such there is no requirment in BS7671 for this to be RCD protected. Depending on the earthing system used for the shed, it may be that only the socket-outlets require RCD protection.
You mention that the house is TN-C-S, have you exported this earth to the shed, or isolated the shed from this earth and TT'd the shed?
If the shed is TT'd then both circuits in the shed will require RCD protection.
What cable have you used to supply the shed?

If the distribution cable falls under regulations 522.6.5 or 522.6.6 then it will have to be RCD protected. Though in a later post the OP did inform us that the cable was a SWA and therefore RCD protection my be omitted. But I think a blanket statement that a distribution circuit not needing RCD protection is not correct.

Regardless of what earthing system is in use for the shed be it TT TNC-S or TN-S as this is a domestic situation regulation 411.3.3 regarding sockets under 20amp by the use of ordinary persons will require RCD protection

 

[Deleted member 26818]

Have I made a blanket statement that distribution circuits do not require RCD protection?
BS 7671 only requires circuits of special locations to have RCD protection, as this circuit supplies a shed not a bathroom, swimming pool or sauna; and the installation is not agricultural, there is no requirement for the circuit to have RCD protection.
No, if the cable fell under Regulations 522.6.5 of 522.6.6 then the cable might require RCD protection. However it doesn't and wouldn't as it is SWA.
Yes Regulation 4113.3 is applicable, not just because of use by ordinary persons, but also because they are likely to be used for mobile equipment outdoors.
I have stated that the socket-outlets require RCD protection.
My reference to the earthing system was in relation to the lights requiring RCD protection.

 

[malcolmsanford]

"The cable supplying the shed is a distribution circuit. As such there is no requirment in BS7671 for this to be RCD protected"

I have to say that I read that as it was worded, that a distribution cable to a shed as no requirement for RCD protection as per BS 7671-2008, and so if I didn't know better I would think that even if regulation 522.6.6 and 522.6.7 affected the cable RCD protection was still not required.

If I read this wrong then I do applogise.

 

[Deleted member 26818]

It is not worded distribution cable, it is worded distribution circuit.
I reiterate, there is no requirement in BS7671 for any circuit other than of a special location to be RCD protected.
Regulation 522.6.6 and 522.6.7 do not neccessarily entail that RCD protection is required for circuits. RCD protection is just one option.
The others being as I am sure you are aware: earthed metallic sheath, earthed conduit or trunking or mechanical protection.
When designing or constructing an installation or addition or alteration to an existing installation, consideration of all the Regulations in BS7671 should be made, not just those relating to RCD protection.
In this instance compliance with Regulation 314.1 parts (i), (iii) and (iv) has not been achieved.

 

[ezzzekiel]

As stated earlier, as you wish rcd protection at the shed end, split main tails to suitably rated enclosure run swa to shed and fit a two way rcd protected db at shed end for sockets and lighting, also preferable to tt the shed end (not necc required just a prefered option) other alternatives may be available.

You can choose to listen to "spin" irrational argument but his point is neither here nor there, you will require rcd protecting cables/ outlets or both at one end or the other.