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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

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  • 0

    My concern is that the earthing of all of these lights relies upon exported TNCS from the CU (s). This is via a 1.5mm core plus armour in the SWA. Is this sufficient?. The lights are fairly old and I believe designed to be earthed ( I do need to check this). Each one is bolted down to concrete in the ground. 



    I practice it's probably absolutely fine.


    In theory the lighting columns should really be treated as extraneous-conductive-parts and so be main bonded - in that way you can ensure that the small c.p.c.s aren't obliged to carry large diverted N currents (or large earth fault currents from outside the installation) on their own. In practice the resistance to Earth of the lighting columns in likely to be significant - several tens if not hundreds of Ohms - and that alone will limit currents flowing through the c.p.c.s to Earth via the columns to something quite modest and within the capabilities of small c.p.c.s. even in the worst case conditions.


    BS 7671 doesn't really recognise the situation of extraneous-conductive-parts having a large resistance to Earth - the only concession (intended really for street lighting columns connected direct to the mains) is that the bonding size may be reduced from 10mm2 to 6mm2 - but even that's likely to be over-engineered for your situation.


    Generally it's acceptable for PME Earth to be used on outdoor lighting - even steel street lighting columns are normally PME'd - despite the recognised risks of a broken PEN.


       - Andy.
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  • 0

    My concern is that the earthing of all of these lights relies upon exported TNCS from the CU (s). This is via a 1.5mm core plus armour in the SWA. Is this sufficient?. The lights are fairly old and I believe designed to be earthed ( I do need to check this). Each one is bolted down to concrete in the ground. 



    I practice it's probably absolutely fine.


    In theory the lighting columns should really be treated as extraneous-conductive-parts and so be main bonded - in that way you can ensure that the small c.p.c.s aren't obliged to carry large diverted N currents (or large earth fault currents from outside the installation) on their own. In practice the resistance to Earth of the lighting columns in likely to be significant - several tens if not hundreds of Ohms - and that alone will limit currents flowing through the c.p.c.s to Earth via the columns to something quite modest and within the capabilities of small c.p.c.s. even in the worst case conditions.


    BS 7671 doesn't really recognise the situation of extraneous-conductive-parts having a large resistance to Earth - the only concession (intended really for street lighting columns connected direct to the mains) is that the bonding size may be reduced from 10mm2 to 6mm2 - but even that's likely to be over-engineered for your situation.


    Generally it's acceptable for PME Earth to be used on outdoor lighting - even steel street lighting columns are normally PME'd - despite the recognised risks of a broken PEN.


       - Andy.
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