The message from Matt contains these words:
Oooh, couldn't be any of us - nothing but proper wiring with nice new colour codes here.
Anyway, I'm not sure whether you'd have to notify "Stuffing a live cable up the DPM's arse" to Building Control.
FWIIW 50m (150ft) is quite a long distance from your equipotential zone (ie your house). In fault conditions there could be quite a substantial difference between the local earth potential & that in the equi. zone. [This is covered in various books by Brian Scaddan]. Somewhat surprised not to see regulars on theis group not rise up en masse and say there should be a TT earth system in the shed.
IMHO opinion whilst your proposal for a 'loose' cable stretching to the shed might be OK for occasional use provided it is played out & taken in each time you use it, ISTM the cable should not be left out for prolonged spells (even if it is special cable sucj as your proposed Artic). Apart from deterioration due to weathering, there is the risk of it being physically damaged or causing an accident.
IEE (as was - now IET) has an article on putting elec power into an outbuilding in its 'Wiring Matters' series
IMHO if this supply is for anything more than incidental use (leaving it charging a battery for hours on end is not incidental use) then it should be connected into a properly protected supply - either the CU or other switchgear connected to the incoming 240V - in the house. At the shed there should be a proper switchboard with small CU, RCD & MCB protection. Standard practice is to run the cable in SWA. It isn't that expensive & is relatively easily sourced (eg TLC). It does need burying 500mm deep or so.
If you use 3 core SWA then you will obtain a much more substantial earth connection than you are proposing. You use the 3rd core for earth & also connect the metal sheath of the SWA. That should be much safer & would go a good way to extending (ie including) the equipotential zone to your shed. Even so I'd be sure there's some silent voices advocating TT somewhere....maybe they'll be along soon.
[a TT system gives you a local earth at the shed independant of your house earth system.]Either way you do need to make an assessment of the likely load in kW; & the voltage drop (max 4% allowed) at full load, so as to size the cable needed, & then a check that your MCB(s) & RCD protection is OK in the shed. Not hard when you've got this group to help ;-)).
If you do this job properly 1st time, you won't have to do it again.
HTH
Well, that's the advice I rather expected to be given.
[snip useful stuff]It does indeed. Many thanks. Douglas de Lacey
The issue really whether your charger is of Class 1 construction - i.e. does it have an earthed metal case? If it does _and_ you can touch the case while standing on the ground _and_ your house earthing is PME then TT earthing is indicated. The earth core of the extension cable or submain is earthed at the house end but insulated at the far end; the charger (and any other equipment at the far end) is then earthed to a local earth electrode (rod, usually). A 30 mA RCD provides protection for the TT installation; in principle this can be positioned at either end of the run, but in the case of the extension lead being used it should be at the house end. The earth resistance of the electrode should be under 800 ohms and preferably under 200 ohms.
I'm assuming that the charger includes a double-wound transformer which will provide some degree of electrical separation to the secondary side (and hence to the vehicle) - although this probably cannot be assumed to meet Class 2 or SELV requirements. If the charger provides no isolation at all (e.g. autotransformer) then the case for TT earthing is certainly strengthened.
This is a usefull referance.
But that refers to a permanent instalation wherereas OP is talking about a temporary extension lead which is not covered by the same regulations.
It doesn't say that at all, it's merely pointing out the need for bonding any extraneous-conductive-parts that do exist.
True, but both are governed by the same laws of physics. There's nothing to prevent use of the safer TT option, even with the extension lead. It's also pertinent to discuss the relevant the design issues, should the OP wish to make the installation permanent in the future.
Yup, and the OP has two concerns: to stay within the law[1] and to maintain reasonable safety. Since I don't equate the two I'm asking! I don't know much about TT earth systems: can I for instance just buy the appropriate spike from Screwfix? How do I test the installation? Do I bond it to the earth going back to the house as well? ...
Douglas de Lacey [1] Hence the use of the extension cable, though I know it's pretty daft.
That would require a building notice under Part P if you make this into a permanent installation (unless you use a registered contractor to do it).
Well I guess the extension lead could be used with reasonable safety, if the plug end is fused at the lowest practicable rating and RCD'd, and the lead is unplugged when not in use and you are around to supervise when it is in use.
Yes,
A very simple way would be to connect a low-current load such as a 15 W pygmy lamp between live and earth at the far end - this should trip the RCD almost instantly. This wouldn't necessarily ensure that your earthing is safe, but it will tell you if something is grossly wrong (i.e. the earth loop is open-circuit).
For a more thorough DIY way of testing the earth electrode resistance see the last two paragraphs of
No, that would defeat the object of the exercise. At the far end the earth coming from the house should be treated as potentially live and should be insulated or made inaccessible.
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