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DIY domestic wiring to outbuildings

Former Community Member
Former Community Member
Hello, I am new to the forums and would like to introduce myself. In the past I have undertaken a lot of domestic wiring as a DIYer. Most of this predated the Part P requirements. However, in 2013 I undertook a fairly major wiring project at my home that included two new consumer units and half a dozen or so circuits. I did this under building control and they arranged inspection and testing. I secured a full set of certificates to underpin my work. I am about to go through the same process to add in two outbuildings to my set up. I have submitted a building notice and am awaiting to hear from Building Control re appointment of a firm to undertake inspection and testing.

Whilst I gather most DIY work scares many of you you, worth pointing out that whilst I am not any form of electrical Engineer, I do have (from many years ago) a HND in Mechanical and Production Engineering and a B Eng (Hons) in Engineering Technology. Both of these included a significant element of electrical technology and electronics. I have also worked in engineering (aerospace) for much of of my early career and still work in a STEM environment. Further,  intend to do a short course on the latest regulations and may also undertake a C&G course on testing and inspection to possibly get to a point where I can register with one of the governing bodies.

I have come to this forum as I am designing my changes and preparing for the first fix. i have been seeking guidance on the internet and from the regulations themselves. I find the regulations open to many interpretations and the advice on many forums to be wildly different. I really want to get this installation right and will have many questions, most requiring practical experience that I simply do not have. I believe this forum will be able to guide me in the right direction when I need it.


My first question relates to burial of SWA. I had a digger in last week to put in a series of trenches for cables. I intend to do all of the external wiring in SWA and in an ideal world would have buried it at 600mm. However, I have places in the garden where it is not possible to get that deep. For an example, under one lawn I have an underground structure that limits the trench depth to 300mm. What would be a appropriate method for installing SWA at this depth? Do I need to add something to prevent a spade/fork penetrating the cable, if so, what would you recommend? I also need to watch my budget here so looking for cost effective views if possible.


There will be further questions. Thank you.


regards

  • 0
    522.8.10 SWA sheath needs to be earthed; location should be marked (usually by tape); "Buried cables ... shall be at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground."


    HTH.
  • 0
    Strictly speaking that is the armour Chris, not the sheath (which is plastic). The yellow warning tape is a good idea, and the depth beneeth the lawn at 300 mm and the tape at say 100mm should warn anyone digging adequately. It is quite difficult to cut SWA with a spade, and if you do you will certainly trip the CPD (circuit protective device) or an RCD if used. You don't say if this is a distribution circuit (to a remote consumer unit) or a final circuit, but if the latter you should protect the whole lot with an RCD. There will now be discussion about the earthing method and PME from some people, please ask for further advice although you will need to provide details, and use information. Any joints or ends to this cable will need special glands, which clamp the armour, and have specific sizes for various sizes of cable. Waterproof types are available where you need to prevent water ingress to the cable itself. The armour may be used as the earthing conductor, although many people like to use 3 core cable, with one core for earth, but the armour must be earthed anyway in case of spade attack etc. It is not a lot more expensive than 2 core, or sometimes even cheaper so shop around. 3 core cable has brown, black and grey cores for 3 phase use, but you can relabel as you wish, although it is probably best to use the brown core for live, and many like the grey for neutral with a blue identification added. The black core should be labelled with green and yellow as normal for earth. Fitting the glands (particularly CW waterproof ones) needs some practice but you should see instructions on youtube. Come back for more information as required.
  • 0
    single phase SWA is available that has the right colours like this  stuff but it is a few pence more, so the contractors do not seem to like it !


    Any safe depth is OK  - arguably under a path the depth could be near zero, as it will never be dug up,  a vegetable garden quite the reverse.

    No need to do anything very special, if you are really bothered then bricks or bits of paving slab. But ask yourself, will it really be dug ? (and if yes, re-route it may be preferrable !)
  • 0
    Plastic duct (often twinwall - e.g. https://www.tlc-direct.co.uk/Products/FXKVR50.html) can add some extra protection - especially if only hand digging is expected. (Also makes drawing in extra or replacement cables at a later date somewhat easier).


    In the old days you'd have brick-like cable tiles - but I think they're not fashionable any more. As Mike says, bit of paving slab work almost as well.


       - Andy.
  • Former Community Member
    0 Former Community Member
    Thanks all for replies so far. I like the idea of a hard slab like material above the cable, do you think my BC employed inspector will? I have seen that you can still buy cable tiles, just quite expensive. I do believe 300mm to be within spade/fork depth but agree that a piece of robust slab with warning tape above should reduce risk to almost insignificant. The option of going around the obstruction to avoid areas that may be dug in the future is extremely difficult if not impossible. The cable will be a  distribution circuit and be in 16mm SWA. The armour and the third core will be connected to the house earth to protect the cable, this will not be continued beyond the cable end. The house is TNCS, the outbuilding will be a TT installation with a (is one enough?) earth spike. 


    Which brings me onto another question, is a single earth spike in chalk a good enough earth? How would I test it when I only have access to a multimeter? If when tested by the BC inspector, it fails, it will cost me for a full retest. I am trying to avoid that. It cost me an additional £400 last time down to silly errors that I could have easily avoided.
  • 0
    IF the buildings are not yet up, put some steel re-bar grid into the footings and bring a bar out to connect to that as well - cement below DPC is not such a bad conductor, and a foundation electrode assists the electrode quite a bit.


    Testing an electrode - well, if you  hire a tester, it will inject a known voltage and read the current. . The simple tester flashes mains live onto the electrode for a short time and reads the current. The tester limits the duration and cuts off if the resistance  is too high - I do not recommend doing that without the right instrument.

    The complex testerfor when there is no supply  puts in additional electrodes and injects knowm currents and measures the voltage drop and does some cancellation to eliminate the effects of the test electrodes. "Identical" readings often vary by 25% or so -  that is just luck of the mud or weather.


    If you have access to  12V or 24V AC transformer and your meter does AC amps, you can safely get quite a long way without the official meter - put one side of the 12V to mains supply earth and the other to the electrode under test, measure the current. R = 12V/ RMS current (or 24V or whatever voltage) Do not use DC (or the meter 'ohms' ranges), electrolysis from DC causes bubbles and other polarization effects that party insulate  the surface of the electrode & confuse the results.


    Reverse the sense of the transformer (i.e. swap the output) and verify that it is more or less the same to eliminate mis readings due to any pre-extant voltage gradient in the soil from AC leakage from other users.

    if need be put in more than one rod, one rod length or more apart, and aim for a combined result of less than 100 ohms, and be smug, as the upper limit is rather higher.

  • 0
    You might want to consider where in 10 years time say, the next house owner might decide to sledge-hammer in some fence posts. Warning tape won't help there.
  • Former Community Member
    0 Former Community Member
    Keep a look out for one of these:

    https://www.ebay.co.uk/itm/Robin-Digital-Earth-Tester-Loop-/114130565623?hash=item1a92b639f7%3Ag%3AGxUAAOSwal9eVkO6&nma=true&si=%252Bfp1R4l3mFwWGuxsogXiAdOBLoI%253D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557


    Regards


    BOD
  • Former Community Member
    0 Former Community Member
    Thank you again. 


    The building is already up, so no chance of using the footings. Although I do need to build a couple of small walls in the proximity of one of them. I may well embed some re-bar in the footings for them. Would there be some advantage in embedding a copper rod? I can always revert to a spike type if results are poor. I will still need a spike type electrode for the other building. I assume that the best ground will be the wettest area, or is that a poor assumption. 


    Bod - that is a useful tip, thank you. I was surprised how cheap it was. I will keep an eye out. 


    Mike- If I can't source  a test meter, will try your 12/24V AC transformer route. Thanks for the detailed response.


    Regards
  • 0
    With regard to depth and gardening, and speaking as a gardener for half a century or so ...


    Only vegetable patches get dug over regularly. Even then, it is unlikely that you will go below a foot. I have never seen the need for double digging. Some years ago, I found a straight black "root" which turned out to be the service cable. Ordinary hand digging isn't going to trash anything and in any event, you would find the warning tape well before reaching any cable.

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