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shed supply

Guys,

Just some advice please!

my son has installed a new 6m/m 3C SWA supply to his new wooden shed,( no extranous parts) its going to be terminated into a 4 way consumer unit (RCD main swt) and used to power & lighting in the shed and some lighting in the garden,

question is:

The source end in the house is a twin RCD board, so using a spare way on either side will give the supply cable RCD protection, but what about the RCD in the shed board? is it advisable to keep both?

0 Alan Capon over 5 years ago

Unless the supply is TT, I am sure the local DNO would recommend that the shed has TT earthing, separated from the house. The shed will need its own RCD and rod for this, and ideally a time-delay RCD at the house end to protect the cable (unless it can be done with over current only), otherwise you have no discrimination.

Regards,

Alan.
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0 Eclipse over 5 years ago

Alan,

Thanks for that,

the incomng supply is tnc-s, so initially i was thinking of spliting the incoming tails through blocks and installing a single enclosed MCB which the SWA would terminate to, this then leaving the RCD at the shed end, correct or not?

if not, then as you suggest i'd have to try and retro fit an S type RCD into one side of the existing board.

Thanks for your advice

Alan
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0 Chris Pearson over 5 years ago

Eclipse:

Alan,

Thanks for that,

the incomng supply is tnc-s, so initially i was thinking of spliting the incoming tails through blocks and installing a single enclosed MCB which the SWA would terminate to, this then leaving the RCD at the shed end, correct or not?

if not, then as you suggest i'd have to try and retro fit an S type RCD into one side of the existing board.

If you fit an S type RCD in place of one of the RCDs in your main board, you will lose your additional protection in that part of the house.
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0 John Peckham over 5 years ago

If your shed is all wooden with a wooden floor or a concrete floor with a DPC and no extraneous parts then then the risk of shock in the event of the loss of the PEN is low. So you could supply the shed with an SWA cable from a non-RCD circuit relying on the over current protection to protect the SWA cable.Then a garage type consumer unit with a 30mA RCD main switch. You still have the risk of the outside lighting but this could be minimized with the use of Class 2 fittings.

If you want to reduce the risk further for PEN loss run the SWA cable as before but insulate the armour wire at the shed end and terminate the SWA in a Wiska gland in to the CU. Over sleeve the line and neutral and take the conductors straight to the main switch keeping them as short as possible.Connect an earth electrode to the earth bar and make sure the earth resistance is as low as possible ideally under 200 ohms.
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0 Alan Capon over 5 years ago

Eclipse:

. . . the incomng supply is tnc-s, so initially i was thinking of spliting the incoming tails through blocks and installing a single enclosed MCB which the SWA would terminate to, this then leaving the RCD at the shed end, correct or not?

Correct. You don’t really need to protect the SWA with an RCD as long as you can operate the protective device from the far end in the event of a short to the armour. As John said, you need to insulate the armour and take just the phase and neutral into the consumer unit in the shed. The earth would then come from a local earth rod, ideally less than 200 ohms.

Regards,

Alan.
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0 Eclipse over 5 years ago

Thanks John and Alan,

it confirms what I was thinking, just got to put it all together now?

cheers guys

Alan
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0 Former Community Member over 5 years ago

Alan Capon:

Unless the supply is TT, I am sure the local DNO would recommend that the shed has TT earthing, separated from the house.

Hi Alan, do you have any links available relating to info on this recommendation?
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0 gkenyon over 5 years ago

Alan Capon:

Eclipse:

. . . the incomng supply is tnc-s, so initially i was thinking of spliting the incoming tails through blocks and installing a single enclosed MCB which the SWA would terminate to, this then leaving the RCD at the shed end, correct or not?

Correct. You don’t really need to protect the SWA with an RCD as long as you can operate the protective device from the far end in the event of a short to the armour. As John said, you need to insulate the armour and take just the phase and neutral into the consumer unit in the shed. The earth would then come from a local earth rod, ideally less than 200 ohms.

Regards,

Alan.

Although, the consumer's earth electrode to form a TT system must also be separated (below ground) from buried metalwork connected to the PME earthing system:

(a) To ensure it really isn't connected to the PME earthing system

(b) To avoid HV fault transfer voltages if the PME earthing system is connected to a Global Earthing System

(c) To help reduce the risk of touch/step potentials between the two earthing systems.

Some guidance recommends this separation is as much as 10 m; this is based on Figure 16 in BS 7430; some DNO's may require a lesser separation distance, but it would be of the order of metres, often at least 2 m.

And then there's BS 7671 - there must be no possibility of exposed-conductive-parts (this time above ground) of anything connected to the TT earthing system being simultaneously-accessible with (within 2.5 m of) any exposed-conductive-parts or exposed-conductive-parts connected to the PME earthing system.

Hence, in a small curtilage property, especially where there are no extraneous-conductive-parts associated with the outbuilding (which would require minimum 10 sq mm main bonding to the MET in the house), it could well be safer to do as John originally said, and simply connect the shed to the property's PME earth.
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shed supply

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