witing exterior sockets

[sm_jamieson]

A couple of questions about wiring an external socket fixed to the outside wall of the house.

1. What type of wiring is used to pass through the cavity wall to the socket (just TWE ?) Cavity is aircrete, fibreglass batt, brick.

1. Tilt hole in wall down toward outside to stop water getting in from the outside, or tilt hole down toward inside to prevent water running down the inside of the cavity (in theory !) and tracking into the socket box ?

2. If I use an exterior RCD socket (with the active feature that trips off with loss of power), are these usually 10mA RCDs, so you have discrimination from a 30mA RCD that may be protecting the wiring from the consumer unit to the socket ? Or do I have to use armoured cable from the consumer unit to avoid two RCDs ?

1. General question based on (3). Latest regs, wiring has to be protected by RCD or amoured (unless buried deep enough or in metal conduit etc). If you are wiring something remote with remote RCD (consumer unit in shed etc), does the TWE wiring through the house (before you go outside) have to be RCD protected ? Or do you really have to use armoured cable right from the consumber unit to avoid having RCDs in series and therefore lack discrimination ?

Thanks, Simon.

[sm_jamieson]

[The Natural Philosopher]

yes.

Neither. Use mastic.

pass.

I think you man armoured :-)

No.To the best of my knowledge. Howver an overall say 100mA RCD on the whole instalation is usual.

No.

Use big RCD on main incoming. Then RCBOs on spurs that need proper protection. Or remote RCD.Armour outside underground.

[Nightjar]

I used 20mm PVC conduit and passed T&E through that.

Not a problem if you use conduit and make sure it is well sealed to the box.

Colin Bignell

[Owain]

If you use a short length of plastic conduit to sleeve the hole and mastic it in, any water won't be going anywhere.

10mA vs 30mA will not necessarily give discrimination, you would have to check the time/current curves in the manufacturer's data.

Yes, or armoured etc.

You supply the circuit through an RCBO or MCB+RCD on the non-RCD side of the split load consumer unit. This might be as simple as using an RCD FCU adjacent to the consumer unit, if the circuit is

[js.b1]

Flat Twin & Earth of suitable rating through conduit through hole.

One way would be...

- 1G 20A DP Switch on inside in 1G 47mm backbox (wiring space)

- Female 20mm conduit fitting into wall

- 20mm conduit through wall

- Female 20mm conduit into rear of exterior box

Now, the conduit should ideally slope downwards to stop water tracking from the outer leaf to inner leaf. To do this with rigid conduit you slightly warm the conduit over a kettle to get a double bend to create an incline in the middle but horizontal to enter the conduit adapters. Alternatively you can buy flexible conduit which will slip inside female 20mm conduit fittings - look for 20mm *outside* diameter on Ebay, it is usually grey or white convoluted. Very useful to have some on the shelf for this kind of thing.

Exterior RCD socket are normally 30mA. You do not fit an external RCD socket to a 30mA RCD protected circuit, because there would be no discrimination - either or both would trip. A 30mA trip typically goes at 23mA and a particular number of milliseconds, another may trip at 22mA and slightly slower - but that does not matter because earth fault currents are usually much higher and thus trip both

If you want dual inline RCD protection for health reasons, such as a pacemaker & hedgetrimmer, I would fit a 10mA inline RCD to the hedge trimmer cable (CPC & Farnell sell these for about =A312) and not use an RCD protected socket. Not many external sockets have 10mA protection and would be pretty expensive (there is a 10mA non-outside metalclad Legrand on Ebay but the price has steadily risen as they have sold them).

So you want a remote RCD in a shed, thus no RCD at the head end?

1 - use BS8436 cable through the house buried

[sm_jamieson]

Is there any limit to how far the remote wiring (e.g. for the shed) can be away from the RCD in the consumer unit ? I will eventually wire up my shed, and it is about 25 metres from the house.

I wonder what the use of RCD sockets really is, since with a modern CU everything is likely to be protected in the CU.

Thanks, Simon.

[js.b1]

No.

An RCD works by detecting the imbalance between Live & Neutral. When current goes to earth, via a nail or person in contact with the ground, it creates an imbalance between Live & Neutral and the RCD will disconnect an Earth Fault so preventing electrocution or fire.

There is an issue re sizing the cable based on distance. The longer the cable the greater the voltage drop, which for fixed wiring needs to ensure that voltage at the end for lighting is within

3% of Uo (grid supply) which can be assumed to be 230V for calculation or actually measured (which can be higher). Sockets require the voltage at the end to be within 5%. In unusual circumstances you can ignore the figures where equipment is not likely to be substantially affected by lower voltage, that way you do not need to run hugely expensive 35mm cable to a tiny fountain pump some hundred metres away.

For 25m I would suggest 4mm 3-core, the 3rd-core & armour being for earth. If the shed is wet, in contact with ground, with metal tools in use you may need to make the shed a TT-supply rather than exporting any house earth (PME, TN-C-S) to it.

You need to pickup the Electrician's Guide to Domestic Wiring and Onsite Guide to 17th Regulations (about =A312 at Amazon UK).

In some instances you may have several sockets to which equipment may be connected which could trip an RCD, such as in commercial premises. In such instances it may be desireable to ensure only one socket shuts off rather than several. An example could be several freezers, you do not want one freezer fault taking out several (not that freezers are likely to trip an RCD).

There is an FAQ covering this, eg, cable depth , ground preparation, marking tape etc.

[ARWadsworth]

Yes T&E will do.

Tilt hole down towards the outside. Silicone the cable entry point on the outside box and put a drain hole in the outside box.

That will not provide discrimination in most cases.

That all depends upon how much you would expect the outside sockets to trip and the consequences of such a trip. On a rewire I would try to put the outside sockets on their own RCBO, but if adding an outside socket to a fully decorated house then I would just take the power from the nearest socket (usually via a DP switch)

If the T&E that you intend to use to power the outside sockets/sheds is "just behind plaster" etc then it would need to be RCD protected. And then it would only need the one RCD at the CU and not at both ends.

Cheers

Adam

[Dave Liquorice]

shed)

Up to a limit...

Or an imbalance due to the cable charging up. I have a plugin 10mA trip but it is a right begger to set with a long extension cable plugged into it.

[John Rumm]

T&E can be ok... a bit of conduit makes getting it through both leaves a bit easier.

yup

Different ratings of trip threshold does not give discrimination with RCDs - a 500mA leakage will trip one or both with no preference for the lower rated one going first.

or surface wire, or in (surface) trunking, or keep it protected via other means (i.e. >50mm from surface etc)

If not otherwise protected, then yes. This would be a good use for something like "earthshield" cable or one of the other metal clad (but not full on armoured) solutions.

All the gory detail: