I have a substantial out building store cum workshop cum cold store cum damp room coming towards the end of comprehensive refurbishment and its time to look at the electrical install which i'd like to clarify afew matter upon.
Details are:
House is supplied by overhead lines and a PME system with local earth rod.
House has 30ma/30ms RCD's on sockets etc and 100ma Time delay RCD's on lighting etc as normal.
Shed is to have 10mm underground SWA supply from house (approx 10m) via
100amp DP isolator and a 63amp MCB. House earth will be exported to the shed via the SWA armour.
Shed will have SWA terminated via insulating box and its own earth via a rod and 10mm cabling back to 16way split load consumer unit.
The vague plan is:
a) 30ma/30ms RCD on one side of the CU, and either a 100ma time delay RCD or simple isolator on the other. Which??
b) put all the non critical sockets on the 30ma RCD side of the board including the damp room ones.
c) put the refrigeration and compressor supplies, lighting, smoke alarms and water heater on the other side.
d) Should the lighting in the damp room be on the 30ma RCD or the 100ma one bearing in mind the damp conditions, but there will be dangerous machine tools operating which wouldnt be good if the lights went out!
Sorry I'm not a sparks, but be aware that this will probably now come under the new regs that came into force on January 1st this year and you will have to get it certified either by a qualified electrician or the local Building Control Officer - have a look at this link to make sure
Tim wrote | House is supplied by overhead lines and a PME system with local earth rod.
Are you /sure/? Although there is an increasing tendency to fit earth rods as additional earthing to PME installations, most overhead line/earth rod installations will NOT be PME.
| House has 30ma/30ms RCD's on sockets etc and 100ma Time delay RCD's on | lighting etc as normal.
This further suggests TT earthing.
| a) 30ma/30ms RCD on one side of the CU, and either a 100ma time delay RCD | or simple isolator on the other. Which??
If it is TT earthing then you should use 100mA T/D RCD as main switch on the CU, unless you use RCBOs on all circuits.
| b) put all the non critical sockets on the 30ma RCD side of the board | including the damp room ones.
ALL sockets (and hard wired tools) should be on 30mA RCD as (a) they might be used for outside appliances (b) if it's TT earthing 30mA to socket circuits is required (c) you have dampness which will affect the insulation integrity of tools.
| c) put the refrigeration and compressor supplies, lighting, smoke alarms and | water heater on the other side. | d) Should the lighting in the damp room be on the 30ma RCD or the 100ma | one bearing in mind the damp conditions, but there will be dangerous machine | tools operating which wouldnt be good if the lights went out!
Lighting should be on the 100mA T/D RCD which should give discrimination. However, especially as you have dampness and rotating machinery, it would be
*much* better to use RCBOs on *all* individual circuits, and install battery-back-up emergency lighting units. With a 63A feed, you probably only need 3 or 4 circuits anyway - power / lights and smokes / refrigeration. If you only have one power circuit it is easier to incorporate a relay for no-volt-release and emergency-stop pushbuttons round the workshop.
Sounds like TT rather than PME, or do you actually have earth and Neutral bonded locally? Sounds unlikely!
Having just done these cacls for my own setup I realised that it isactually quite tricky to get discrimination between the outbuilding MCBs and a head end one like this. Found a HRC fuse at the feed end was simpler in the end.
Since both ends of this are TT you could get away with joining the earths IIUC.
Is the feed downstream of your existing time delay RCD at the house end? If so then you wont need another one here, and can use an ordinary isolator on the incomer. If you are not downstream of the house RCD then you will need another one here, or go for two CUs each with their own conventional (i.e. non delayed) RCDs (one 30mA, one 100mA)
Sounds ok.
As long as those circuits do not also have sockets that you could use to power hand held applicances that may be used outside then that is fine.
I assume you will be installing suitable switch gear to ensure the damp does not get into the electrics anyway ;-). The 100mA supply sounds best. If you don't want to do that, then consider adding a battery maintained emergency light (start at about 25 quit at TLC).
My two penneth....
I do hope this is not going a response regular... you need part P blaa blaa.... we know, we don't care (much) ;-)
No brick, you are quite correct since this is a "new circuit" and hence not a minor work... (unless the OP started this work prior to the begining of the year ;-)
Neither. I'd use 30mA for all circuits in what is essentially a barn. Does it really matter that much if anything loses power when it trips saving your life ?
Also I think you need proper local earthing but I'll leave comments on that to the real experts.
? "PME with local earth rod"? A contradiction in terms... PME, or in Reg-speak TN-C-S, means that your supplier is providing you with a nice low-impedance earth (combined with the Neutral in the case of TN-C-S, separate back to the substation in the case of TN-S). If you're providing the local earth rod, which you are, and that's what your installation relies on, as it seems to be, then you have a TT installation. To meet disconnection times, a TT installation needs a
100mA RCD, usually time-delayed to provide discrimination with any 30mA non-delayed RCDs for circuits which need/benefit from them.
Oh look - so it *is* TT. Fine so far...
When you say "exported", you don't really (I think) mean "exported", since the outbuilding won't be using the house earth. Rather, what I think you're telling us is that the SWA armour is to be connected to the house local earth rod, to provide a plausible way for usefully-more-than-63A to flow in the event of a phase-to-armour fault.
Your outbuilding will be a separate TT installation - "its own earth via a rod", and I'll twitter on about the protection of its final circuits below. For now, let's concentrate on fault protection of the SWA cable, in particular for a phase-to-(sortof-earthed)-armour fault. Is "earthing" the armour by connecting it only to the house earth rod going to be enough to assure disconnection by your 63A breaker so that (a) touch voltage on the armour doesn't go above 50V, (b) the cable won't smoke before the breaker trips? The proper calculations need Regs tables, but let's handwave for a moment. Your 63A breaker will keep supplying current more or less indefinitely - half an hour at least - at a current of 100A, which at 240V is as much as will flow with a total loop impedance of 2.4ohms (R = V/I). Earth rod could easily get up to or beyond that value, especially if it's dry the day your digger slices through the SWA connecting phase-to-armour (missing the N conductor by Sod's law), or some unlikely fault occurs in your outbuilding CU which has the same effect - and that's without taking into effect the SWA conductors' own resistance. So even without the Proper calculations, I'd say your SWA cable wouldn't be protected against a damaging overcurrent on a phase-to-armour fault if the armour's only relying on the earth rod as the current return path - and with the bulk of the resistance occurring at the earth rod, the bulk of the 240V supply voltage will be on the cable armour (and any spade you're holding!) for the duration of the fault.
So, you need a 100mA time-delay RCD for this cable. Can it be the whole-house 100mA time-delay RCD? Well... not ideally. If you're going to make the outbuilding a "separate" TT installation with its own earthrod, you'll want (at least conventionally) a 100mA time-delay RCD to cover the whole installation. Then you'd have two similar-characteristic RCDs in series (the house one and the shed one, both 100mA time-delay) and it's not at all clear that a phase-to-E fault in the shed would cause the shed RCD, rather'n the house one, to pop first. Just because the current flowing to earth happens to be going through the shed's earth rod rather than the house rod, doesn't affect that both RCDs see an imbalance between current-out and current-back, and it seems to me pot luck as to which one would pop first. Having only the one 100mA time-delay whole-installation RCD to cover both house and outbuilding seems poor practice, with a fault in either location disrupting supply to both.
So, one reasonable scheme is to give the shed supply its own 100mA RCD back at the house and take that RCD's L and N from your supplier's terminals (Henley block for the house supply and the shed supply). It's a little inconvenient to have the outbuilding's whole-installation RCD in the other building, but you said it was only 10 yards or so away, right? and part of the same "household"?
One other alternative which seems reasonable to me is for your shed to have all its final circuits protected by two (or more!) 30mA RCDs - for example using two Henley-blocked CUs each of which has a 30mA RCD as its main isolator; one for the final circuits which "must" have 30mA protection - sockets, particularly - and the other for the ones which "could" have only 100mA time-delay protection (lighting, &c) but which you want to have discriminated from the whole-house RCD. Then the whole-house RCD is protecting against phase-to-E faults in the SWA feed and the wiring up to the indvidual RCDs (or RCBOs on individual final circuits, if you're feeling flush ;-), while the local RCDs give you closer, discriminating protection against phase-to-E faults within the outbuilding. This avoids a separate 100mA RCD for the SWA cable, as it becomes reasonable to share the whole-house RCD which is now protecting only the outbuilding supply cable; though you're still vulnerable in this setup to a phase-to-E fault in the house on e.g. a lighting circuit cutting all the outbuilding power.
You could put a couple of "emergency" lights (rechargeable, non-maintained, i.e. turn on only when their mains disappears) on a non-30mA-protected circuit too, by using a split-load board as one of your two shed CUs. (Details of whether you choose two CUs, or a single split-load one where on the "non-RCD" side you put one or more RCBOs for socket circuit(s), depends as much on what pre-filled CU offers your local trade counter has in stock, and how much room you have for the incoming installation, as anything else!)
If you've got the 100mA RCD upstream, a simple isolator would be "OK", but a phase-to-E fault on the isolator-only side would cut all the shed power. I've suggested above that a 100mA RCD in series with the whole-house or dedicated-to-shed-supply-but-positioned-at-the-house-end one won't discriminate, i.e. either one might pop first (or both might!), which led to the (loony?) idea of separate 30mA trips for sockets and Other circuits in the outbuilding.
If you go for the two-or-more RCDs-in-the-outbuilding idea, you can reduce the likelihood of one fault taking out all the shed power. A bit of supplementary 'emergency' lighting could help you in the machine tools room (that's the damp one, right?) - and the more separate RCDs you have on the shed final circuits, the less "sensitive" the accumulated leakages make any one RCD.
I hadn't even thought of that possibility. And as there will be some impedance from the earth rod to the earth back at the transformer, surely all these bonded surfaces will still be live-ish ?
I have the same system (overhead PME plus local earth). Would it be a good idea to disconnect the neutral to earth connection and rely just on the earth rod ? Or convert to something else ? (suggestions as to best appreciated)
On Sun, 9 Jan 2005 00:12:33 -0000, "Mike" strung together this:
Nope, if you lose the neutral on a PME overhead and you have an earth rod the RCD will trip, providing you have one.
You need the 100mA RCD on the front end if you are going to add a rod so you may as well just go for fully fledged TT. If you were to add a rod and not have the front end 100mA RCD then non RCD circuits would still be live, unless there is a good flow that trips MCBs\blows fuses but you're chancing it.
On Sun, 9 Jan 2005 01:31:28 -0000, "Mike" strung together this:
Not if everything is already on a 30mA RCD. I was assuming, (not good I know but I got this idea stuck in my head), that the existing installation would be a normal split load board so the main switch would need to be changed for a 100mA RCD in this instance.
In your particular case you could just disconnect the earth from the PME terminal and connect it in to a seperate earth block then to an earth rod from there.
Are all your CU's insulated? If not there are precautions that need taking on a TT system that don't apply on PME. If you need more info post back as it's fairly involved.
Not nesser-celery. Bearing in mind that the M in PME stands for multiple, the local earth electrode may be one provided by the supply network operator as part of their multiple earthing arrangements, or may be added as part of your installation design to give a measure of additional protection against a broken supply neutral.
That's only one option for "properly" though, n'est ce pas? Using the available PME earth terminal in conjunction with proper equipotential bonding is just as, errm, proper. Better in fact, except for those cases where the equipotential zone concept can't easily be applied (e.g. a greenhouse), or where it's unlawful for the supplier to offer PME (i.e. caravans and boats).
Indeed- as is what happened at the neighboours house acouple of weeks ago- the N to E connection at the pole failed and all the metal work in the house rose to 240v! Someone had a far kick off the stainless sink (stood on quarry tiles) luckily it was dry at the time but incredibly dangerous.
On Sun, 09 Jan 2005 12:30:31 +0000, Stefek Zaba strung together this:
Yep, you can only be one or the other. If you just stick a rod in the PME terminal then a fault somewhere up the line could see all your neighbours trying to shove all their fault current through your rod.
The same could be said for PME on overheads with no rod added to the customers installation, if there's a fault upstream of you and your unearthed neighbours then their fault current will try to go through your equipotential bonding.
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