Hi
I need to wire in my garden electrics with armour cable etc... Can anyone advise on what MCB's and what size armour cable to use?
I have done a small diagram if it helps?
You need to tell us the cable lengths before an accurate answer can be told :-)
House to Garage = 8m Garage to Shed = 6m Shed to Summerhouse = 5m
cheers ;-)
There are lots of ways you could do this. I'd run 6mm 2 core SWA from the house to the garage, with a 32A Type B MCB in the house.
At the garage, I'd have:
30mA RCD for the entire lot. Isolate the earth from the house. Do not connect to the garage. Use an earth rod. 6A MCB for the lighting 16/20A MCB for the sockets (you can go up to 32A if you wish) 16A MCB for the shed/summer houseThen 3 core 2.5mm SWA, exporting the garage earth down one core, from there to the shed and summer house. Run shed/summer house lighting from local 3A FCUs if the fittings do not indicate compatibility with a 16A circuit.
Don't forget that you will need to notify the council and have the installation tested.
Note that your proposed spotlights for your garage might not be a good idea. Spotlights are appalling inefficient for space lighting and contribute significantly to global warming.
Christian.
Nice one Christian cheers
Why the earth rod for shed? would you like to quote a reg to justify it please. My preferred way would be 5 way consumer unit in garage rcd sockets where nessary, no rcd protection to server (to prevent nusicence tripping) agree with all mcb/cable sizes but would put small 2way "garage consumer units" with rcd main switch in shed and summer house
Not an engineer etc...
I doubt there is a specific reg that says "you must do TT etc". It's going to be a matter of making the design compliant with several relevant regs.
If you are going to export the house earth then it must be safe to do so.
See here first for some excellent background info:
PME supplies are dodgey as it's not impossible for the earth/neutral to fall off upstream (esp. in rural locations) then your neutral AND your local earth is now being pulled up towards phase by attached loads (yours or neighbours'). In the case of a metal greenhouse with supply and earth bonding, your frame is now at 240V and the ground is still earthed - so you get the idea. And you don't bank entirely on the main RCD to get you out of trouble...
TT - you have your own electrode with a relatively high impedance. Phase to earth fault in the house will float this up (and thus your greenhouse frame in the previous example) to potentially dangerous voltages relative to the real earth. Wet grass, bare feet leaning on the greenhouse you would not need full mains voltages to potentially kill you.
Pause for thought indeed, been an electrician for nighon 30 years and would not consider earth rod for outbuildings unless their was an existing tt system, just as likely to loose your earth rod connection as your main supply earth surly, how often should your earth rod be tested
3mts, 6mths a year? That just will not happen in a domestic house, As long as rcd protection is used,imo using house earth is the safest way.
The NICEIC did a crackng article a few months ago in Professional Electrician and Installer magazine (the free one at the wholesalers) on how the export an earth to outbuildings with a PME supply.
The main criteria was that the combined CPC and main bonding conductor for the exported earth meets regs 547-02-01, 542-01-09, and 543-01-01.
Unfortunately the article was covering out buildings with extraneous conducting parts and made no mention buildings without.
Adam
..and of course think about what happens to the earth rod impedance when we get a long hot spell & the ground dries out...
Yes indeed - I did say it was a minefield...
I've heard differing opinions from engineers too, each with a perfectly goog sounding argument to back his theory up. I think this is one of those areas where getting expert advice in a specific situation is most definitely a good idea, even if it costs... Then I did say I wasn't an engineer.
If it were me, and I'd concluded I ought to TT each out-building,
I'd probably sink two of the longest rods I could physically bash into the ground (at least 4 feet, preferably more) at diagonally opposite corners of the building and bond the lot solid.
Then that's just me - "if in doubt, double it, if still not sure double it again!" :)
Cheers
Tim
I won't argue with experience (especially as I have little, but just happen to have read around the issue a bit)...
The only thing I would add is that at least a local rod (or rods) can be maintained under reasonably controlled conditions and inspected/tested regularly IMHO.
As to the likelihood of a PME supply failing wrongside - I don't know how likely that is in reality, but it is outside of the householders control.
Perfectly valid point on (not) testing local TT.
Just out of interest, how far do you consider it reasonable to export a TN-S earth - I'd be interested for when I have to do it?
Cheers
Tim
Deep enough rods hopefully and re-test if there's an element of doubt.
Cheers
Tim
Hi don't think distance is relevant, a decent earth loop reading is the thing i would look for. and another thing...... if you decided to put in a new pme supply from local electricity supplier to said garage there would be no need for an earth rod. My point is all customers really on electricity supplier for an earth point unless their a tt system. I have had issues with tn-s supplies, first thing to check is earth loop had quite a few with high readings, they were simply converted to tnc-s by local electricity supplier, but the house had been without an effective earth for who knows how long.
.
Calculate the cable size needed needed using
It is more likely that you will see a TT supply converted to a TN-C-S without the main earth connection being made as the bonding on the installation was not up to TN-C-S standards and so the installation is left as a TT.
Adam
get a long hot spell &
Well it only needs to muster a feeble 8K ohms to be able to trip a 30mA RCD.
The thing to bear in mind is that with a PME system and an exported earth, there is the possibility that you will pass no leakage current in the case of a disconnected CNE conductor to the property. That can leave phase voltage on your exposed metalwork indefinately. Hence why maintaining the equipotential zone in the outbuilding is important if you are exporting the earth.
Although the limit of 1667 ohms to comply with BS 7671 [413-02-20] is a bit more relevant and ensures that the touch voltage won't exceed 50 V. If the supply feeds a greenhouse then section 605 applies and the limits become 25 V and 833 ohms [605-06-01].
If you achieve the OSG recommendation of The thing to bear in mind is that with a PME system and an exported
Absolutely. And where that's just not practical - the aluminium greenhouse being the archetypal example - go for TT.
I suggest the following are relevant:
130-02-02 131-01-01 (i) 541-01-01 541-01-02 542-01-09
would agree regs quoted are relevent, to every installation, just don't see the relevance to an earth rod being required on an outbuilding installation
131-01-01 (i) requires you to /design/ an installation to be safe. A PME earth connection is potentially unsafe if it's exported outside the equipotential zone and there is risk of simultaneous contact between exposed-conductive-parts of the electrical installation and the general mass of earth. These risks will arise if the DNO's combined neutral and earth (CNE) supply conductor is broken, or if a phase-neutral or phase-earth fault occurs elsewhere on the distribution network, temporarily raising the PME 'earth' potential above ground, due to the voltage drop along the CNE conductor. Such faults can take more than 0.4 s, and even more than 5 s to clear.
There's usually no difficulty if the outbuilding is dry. Any incoming service pipes that constitute extraneous-conductive-parts can be main-bonded to the incoming submain's earth in the usual way, creating an equipotential zone. The earth conductor in the submain (usually the SWA's armour) then functions as both a CPC and a main bonding conductor. Because of the latter function the min. size for this 'earth' is 10 mm^2 copper, or 22.6 mm^2 steel armouring. This is met by 2-core SWA (BS 5467) of size 6 mm^2 or larger.
So far, so good. But if the outbuilding is a garage with a damp floor, or a greenhouse, the only way you could create a dependable equipotential zone would be with a buried earth mat in the floor structure, or in the soil - not very practical for existing structures. Worse still is when the user uses an extension lead to use Class 1 equipment outdoors. The problem could be avoided by using only Class 2 equipment in the outbuilding, but that would preclude installing any
3-pin socket outlets, using metalclad accessories, which are usually desirable on grounds of robustness.The problem can be avoided by using TT earthing for the outbuilding - a much safer arrangement all round, provided that the earth system and the RCD(s) can be maintained. This is fairly standard practice, and has been since use of PME became well-established during the 1970s.
The ESQC regulations legally forbid suppliers from providing PME earth connections on supplies to caravans or boats. (Regulation 9 (4) "The distributor shall not connect his combined neutral and protective conductor to any metalwork in a caravan or boat.")
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