Shed Electrics

[Paul]

Hi,

I've been reading many of the threads regarding shed or greenhouse electrics, but I'm still not 100% on the way to go, so I thought it best to clarify some things before I proceed.

I have a split-load consumer unit. From the CU to the outside of the house it approx 8m. From the outside of the house to the brick built shed it is approx another 8m. I am planning to have a double socket and a light. The double socket will be used for the occassional use of a power tool such as a jigsaw or a lawnmower (about 2000w max). The light will be a standard 100w. Also I plan to power a water feature (2 metres from shed, 25w), a pond pump (15m from shed, 25w), a security light (500w) attached to the outside of the shed and also have a single waterproof socket (15m from shed, by pond) just incase I need to plug something in at the top of my garden.

My questions are (sorry there are a lot of them) :

1. I'm going to use a spare 20Amp 'B' type MCB on the non-RCD protected side of the CU using 4mm T+E cable. Can I use normal T+E cable for the first 8m (up to the outside of the house) and then connect it using a junction box, to 8m of SWA 4mm cable to the shed?

1. I was planning to place the RCD spur unit inside the shed at the end of the SWA cable. Can I then continue the radial circuit using normal 4mm T+E or could I use a Fused Connection Unit after the RCD and continue with 2.5mm T+E?

2. Can I use a plastic back box for the double socket or are there special 'shed' style sockets?

1. I have estimated a total of 10m of cable being used inside the shed. Does this have any impact of whether I should use a separate earth rod or can I just connect the power to the house earth?

Just so you know, I was going to spur to use a 5amp FCU for the shed light and the security light. Also, all the outside cabling is SWA and will be in that 20mm blue plastic piping that you can buy from Wickes, buried deep deep underground.

Thank you in advance!

Paul

[Lurch]

On 21 May 2004 08:09:02 -0700, in uk.d-i-y [e-mail] (Paul) strung together this:

Yep, all looking good so far.

Why not use a 'garage unit'. A 25A RCD with a 16A and 6A MCB. Run the lighting in 1mm from the 6A and the power in 2.5mm from the 16A. (screwfix.com Cat No. 63138).

You can use plastic sockets, if they're not likely to get battered. I usually fit metalclad accessories in out buildings and the like.

As it's only over a relatively short distance I would export the house earth to the shed.

Yep, that or the way I suggested above.

You don't need to enclose SWA in anything, if you do make sure it isn't in a water pipe! It only confuses people in the future. I would bury the SWA as it is.

[BigWallop]

Any external wiring should be protected as much as possible, so I'd personally advise you to use the RCD side of the CU.

It's also best and safest if you can make the SWA in a single run all the way between the two points. Joints in anything, by nature, make them less efficient. And I'd also go for 6 mm csa' SWA on that full length to make sure that supply is adequate enough to take any fault currents.

The circuits you intend to install from the shed supply are not going to be huge loads, so 2.5 mm csa' should be enough for radial connection to them.

You might also find that you need to install quite a large, not huge, junction box inside the shed to take all the seperate circuit connections you want to make. Not just now, but in the future.

Try to go for industrial socket plates and light fittings because they are slightly safer for outdoor use. They also take more of a bashing in extreme weather conditions. Weather proof is even better for building that are not going to be at constant temperature and humidities.

Relying on the Wire Armour is never a good thing really. So I'd advise that a seperate local spike be driven. You should then be using a separate CU in the shed and make your individual circuits from that.

[Lurch]

On Fri, 21 May 2004 15:46:37 GMT, in uk.d-i-y "BigWallop" strung together this:

Ignore this bit, it is unneccesary and adds to the nuisance tripping.

Somewhat excessive that is. 4mm is plenty adequate, and as for the joints, 1 mid point connection isn't going to have any influence on the current carrying capacity of the circuit or its reliability.

He never asked if he could use the SWA as an earth, just whether he can use an external earth rod. If he uses 3 core SWA cable then the third core is the earth.

[BigWallop]

But you know what like these DIYers are. They say they'll only put a certain load on things, and then, before you know it, they're running disco's in the garden for the kids. :-))

[Lurch]

On Fri, 21 May 2004 19:52:09 GMT, in uk.d-i-y "BigWallop" strung together this:

Well, quite! I've just re-read my post and it sounded a bit angry, sorry about that! But, I've run complete DJ rigs, including most if not all of the lighting, from 13A extension leads before now. I don't think a kids disco is going to pull more than a couple of amps.

[steve]

With ELECTRIC CABLE BELOW tape

over the top of the cable not directly on top a good few inches above it for obv reasons and might be a good idea to put an accurate map of garden with the cable run marked on it near the CU for future referance

Steve

[John Rumm]

Not only that - if the OP selects the correct MCB rating at the house end then one of its jobs will be to protect the SWA from overloading.

[Lurch]

On 21 May 2004 15:35:24 -0700, steve_ [e-mail] (steve) strung together this:

Erm, yes. I thought that at the time but the thought never travelled as far as my fingers! Good advice.

[Joe Lee]

No. It must be RCD protected.

Joe Lee

[Joe Lee]

Wrong. also please explain how it adds to nuisance tripping.

Joe Lee

[Paul]

SNIP

I like the idea of using a garage unit - so much tidier!

Could I still use the 4mm SWA (3 core) to this consumer unit? Also, taking into account that I need to run another 15m of cable from the shed to the top of my garden for the pond pump and weatheproof socket, do I need to provide the shed CU with it's own earth rod? If so what is the correct proceedure to install one?

Paul

[Lurch]

On 22 May 2004 02:29:03 -0700, [e-mail] (Paul) strung together this:

That's the idea!

Yes, that's fine.

You could do, I personally wouldn't for what it's running. If you did want to the correct procedure is,

Hammer 4' rod into ground without going through any pipes, it's easier if you fit the bottom half of the PVC box and the clamp to the rod first.

Test the rod with an earth loop impedance tester. If the reading is too low install anothe rod somewhere near the first, although generally 1 rod does it.

When you're making the armoured of into the CU in the shed you must ensure that the SWA armouring isn't connected to anything earth related in the shed, nor should the third core be used for an earth. The earth for the outbuilding end should be solely provided by the rod.

Now, using a miniimum of 4mm earth cable connect the rod to the earth bar on the shed CU.

In basic terms, that's it. In practice it's not as simple as it sounds, but not overly taxing!

[Lurch]

On Sat, 22 May 2004 03:03:48 +0100, "Joe Lee" strung together this:

Correct, sort of.

Well, I didn't mean it simply trips the RCD all the time, what I meant was as it's power equipment in the shed and garden it's more likely to be the cause of tripping the RCD. That's why it's better with it's own RCD at the shed end, you don't lose power to half the house when the pond pump goes u\s.

[John Rumm]

Good advice I thought. "Wrong" seems rather too strong even if the argument were balanced with pro's and con's

If you add a circuit (any circuit for that matter) to the RCD protected side of your house CU, then you are by definition adding a new source of earth leakage that can eat into your total combined "budget" of 30mA allowed by the RCD. Even perfectly wired circuits may have some small leakage due to inductive or capacitive coupling in either the circuit itself, or more likely, the appliances used on them. Electrical instalations sited in outbuildings are also more likely to experience dampness from time to time which will also raise the likelihood of them generating a leakage related trip. Every trip of this type is going to take out all the RCD protected circuits on your split load CU - sounds like a nuisance to me.

Since there is very little likelihood that you will be accidentally chopping through your SWA cable with the hedge trimmers, there is no great need for it to be RCD protected.

[N. Thornton]

From: Andy Wade ( [e-mail])

hence the

Remember that

example)

earth

I think this is something of a misunderstanding, and a very common one. An RCD doesnt limit fault voltage at all, just as it does not limit fault current either. What it does is ensure that fault voltage/current is short lived. With a TT system a leakage fault is liable to give you more or less the full 240v on your hand drill, RCD or no RCD. The RCD simply cuts the shock time down.

Given that appliance leakage faults are far more common than PME earths going live, the advantage is clearly not with the TT option.

There is that too: RCDs do fail, and earths are normally not maintained. I've seen whole house earths disconnected at the rod before now.

In short TT should not be used unless there is no safer method available. TT allows fault voltages to rise to 240v, and relies entirely on an RCD to disconnect such faults. Neither of these potential risks occur with supplier-earthed systems.

If this is going to go into an FAQ this should really be cleared up first - unless I'm mistook somewhere along the line.

Regards, NT

[Andy Wade]

Well, I think you missed my point a bit - although I agree that the last sentence should have ended something like "then with a 100 mA RCD any sustained lift in the voltage of the installation earths will be limited to

20 V."

Yes, perfectly true, but the requirements for RCD tripping times are based on coordination with the IEC touch voltage curve, such that the shock should not cause death or serious injury.

There's a lot of information on shock protection in Paul Cook's Commentary on BS 7671 [ISBN 0852962371]. See for example Table 4C: in dry conditions a touch voltage of 220 V is not considered dangerous (in the sense mentioned above) if its duration is less than 180 ms. Under such conditions the residual current will be at least 4.4 times the RCD tripping current (220 V/50 V) and the tripping time will be very short.

Advantage from what point of view? Both systems will give the indirect contact protection required by BS 7671 - for what you rather ambiguously call "leakage faults" ('fault' usually means a dead-short). The TT system has the additional advantage in limiting the sustained touch voltage which can occur between the installation's protective conductors and the local 'true' ground to a safe value, something which the TN systems don't do at all. The TN system circumvents this problem by using equipotential bonding, but that concept fails when Class I equpment is used outdoors, for example.

Moreover, the touch voltage in a TN system in the presence of an appliance earth fault might be higher than you've realised. This comes about because of the use of reduced-size circuit protective conductors (CPCs) in (for example) twin & earth cables. The worst case is with the 4mm^2 cable, which has a 1.5mm^2 CPC. Thus the resistance of the CPC is nearly 2.7 times that of the phase conductor, which makes the touch voltage about 73% of the supply voltage - i.e. about 170V - and it will increase during the fault as the CPC heats up more than the phase conductor. Also the fault can take up to 400 ms to clear, or 5 s in the case of fixed equipment. This combination is outside the safe area of the IEC touch voltage curve (which 'allows' 100V for 400 ms) but is permitted - by tradition, I guess - except in those wet areas where local supplementary bonding is reguired. Supplementary bonding is simply a measure to reduce the touch voltage that can occur between the accessible exposed-conductive-parts and extraneous-conductive-parts in the locality.

Quite true - although (IME) disconnected earths are more likely to be found in old house installations with an ELCB and an earthing conductor of puny size, 1.5 or 2.5 mm^2. With modern practice with a 16mm^2 (or conduit protected) earthing conductor and a proper joint to the earth electrode in an earth pit, there should be much less to worry about and 'maintenance' is really limited to checking the resistance during periodic inspections. The greater weakness is probably that most people will not bother to do the

3-monthly RCD test.

Yes, but only outside the equipotential zone, and then only very briefly, whereas a TN system can produce a touch voltage as high as ~170 V for 5 s within the equipotential zone (except where supplementary bonding is used).

But different risks obtain in the TN case, as I hope I've manged to show.

I wasn't aware of any FAQ proposal - although I have been meaning for ages to write a piece on "Electricity Supplies to Outbuildings" for ages. Now where's that round tuit gone?

Anyway, thanks for an interesting post.

[N. Thornton]

Good points, but 3 points tilt the table a bit:

1. Appliance leakage faults are many many times more frequent than a supplier earth going live. This rather tilts what is more favourable. The TT system performs worse on such faults.
2. The fact that with most domestic TT installs, the earthing and RCD functions are never tested after the installer leaves, not even once.

It is these 2 primarily that lead me to conclude that the odds of a nasty will be higher with a TT system.

Note also the 170v scenario you present with TN only occurs when the live to earth fault resistance introduced is zero, and that is something rarely achieved by real world faults. So the situation with TN is perhaps not as grim as it may have appeared.

Ultimately both systems have their imperfections, but with the death rate from fixed wiring being at probably less than 1 per year, considering all the other hazards in life it begins to become academic in the end.

Regards, NT

[Christian McArdle]

If it isn't zero, then the calculations for earth fault tripping via an MCB are false, and the MCB may well take more than 10 seconds to trip, which may be a fire hazard, depending on the nature of the fault. Remember that fire protection is actually vastly more important than shock protection. Hardly anyone gets killed by electrocution in the home, whilst many are killed by electrical fires from appliance earth faults.

Christian.

[Andy Wade]

Worse on touch voltage, but probably better on fault clearance time. But does it matter, both systems comply with BS 7671 if properly designed and built?

From posts here it seems to be coming quite common for a periodic inspection to be done when a property changes hands.

Bear in mind the thread's subject. I'm certainly not advocating use of TT earthing for house installations in preference to use of an available distributor's earth terminal. That would be counter to the trend in the supply industry, which is definitely towards universal PME (which principle is ensconced in the ESQC regulations.) What I continue to maintain though is the value of the TT system in outbuilding installations where the equipotential zone principle is difficult or impossible to apply - e.g. garages, sheds and workshops with damp floors, greenhouses or outdoor equipment (Class I) with exposed-conductive-parts.

See Christian's post. For design purposes you only need to consider s/c faults. Any 'leakage' situation which produces enough voltage drop across the CPC to give a dangerous touch voltage will generate an awful lot of localised heat, and in practice will quickly turn into a negligible-impedance fault as insulators track and carbonise, or whatever. Certainly an RCD will give earlier disconnection in the event of this type of failure (and might even prevent a fire) and in the TT installation you get such protection 'for free' (the RCD has to be there anyway). But you can fit RCDs in TN installations to give similar protection if you choose - and you then get into the arguments about nuisance tripping (always a sign of a definite problem, IME) and fire safety, premature loss of lighting from exit route [see Peter Parry, /passim/].

Tell that to the advocates of Part P. (Though I think it's a lot more than one a year if you include deaths in fires started by wiring problems.)