Connecting a garage to house with TT using buried SWA

Hi,

I hope someone can help me out here. I have tried to find the answers to this in the wiki and in past threads, but can't find the answers for when the house is wired TT.

I have a garage with RCD and MCBs. It is currently linked to the house with an overhead cable. The house is a TT system with a single RCD and wired fuses.

The overhead cable is a very poor lash up that was there when I bought the property 16 years ago and could have easily been there for 10 years before that. (Yep, I should have changed it years ago... ).
It is a length (10ft?) of 4mm T & E wrapped around a length of what looks like washing line. It runs about 7feet off the floor - a foot above a gate and fence. The wire has no mechanical protection and has been exposed to UV for decades.

It has become an issue because the fence and gate is to be replaced and it is rather 'in the way'.

Obviously I would like to replace it. It is just at the thinking stage at the moment.

I have two questions:

Firstly, if I use SWA cable under flag stones does this need to be buried? I have read elsewhere that cables should be 18" deep or more, but if covered by a flag stone patio is this neccesary? I was thinking it might actually be safer to run the cable just under the flags, perhaps with yellow warning tape over it, as then anyone who takes the flags up would see it straight away?

Secondly, the distance from the house to the furthest socket point is quite long, maybe 40ft. The soil outside the rear of the garage is very deep and moist, the water table being quite high. It looks like an excellent site for an earth spike and I am wondering if fitting the garage with it's own earth electrode and creating a local TT system might be a better option?

The aricle that is linked to from this forum whenever garages are mentioned, the IET one by John Ware, says that an outbuilding on a TT system must not be connected to the earth of the incomming SWA cable. This article assumes the main dwelling is PME.

"Where the installation in the garage is supplied by an armoured cable, the armour or any protective conductor in the cable must not be
connected to and must not be simultaneously-accessible
with any exposed-conductive-parts in the outbuilding."

Question: Does this also apply to installations where the house is TT?

Is is verbotten to link two TT systems together? The earth spikes would be about 40ft apart. If so, just out of academic interest, why so?

Thanks.

See Issue 16 Autumn 2005 Wiring Matters Electrical installations outdoors: a supply to a detached outbuilding at http://www.theiet.org/publishing/wiring-regulations/mag/2005/index.cfm it covers most of what you need.
Also Issue 21 Winter 2006 Multiple earth electrodes in a TT system for a dwelling may help.

Steve2008 said:

Firstly, if I use SWA cable under flag stones does this need to be buried?

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Yes.

I was thinking it might actually be safer to run the cable just under the flags, perhaps with yellow warning tape over it, as then anyone who takes the flags up would see it straight away?

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Unless they drag them out with a digger.

Or whatever they drive between the flags to lift the first one cuts into the cable.

ban-all-sheds said:

Steve2008 said:

Firstly, if I use SWA cable under flag stones does this need to be buried?

Click to expand...

Yes.

I was thinking it might actually be safer to run the cable just under the flags, perhaps with yellow warning tape over it, as then anyone who takes the flags up would see it straight away?

Click to expand...

Unless they drag them out with a digger.

Or whatever they drive between the flags to lift the first one cuts into the cable.

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ok, fair point.

I think I read somewhere a 450mm trench with a tape at 150mm, then...

any ideas on the question of linking two TT systems together?

The multiple earth electrode article already mentioned uses multiple electrodes to decrease Ze, but assumes a single earth point on the installation.

That's not quite the same as having two seperate electrodes linked together - but with two seperate points for the circuits to be earthed to.

Would this cause a problem?

Issue 16 Autumn 2005 Wiring Matters Electrical installations outdoors: a supply to a detached outbuilding link already given does refer to using TT and as to is completely independent or linked will depend on how close the shed is and what is between the two house and shed.
Because a lawn mower or similar could be plugged into either supply and both house and shed may have earthed metalwork outside I would link the two earth rods.
But not a DIY job anyway as the meters used to measure the resistance on earth rods are quite complex and you really need to involve who every is signing the paperwork in the design as some people have very fixed ideas as to what is allowed and too much work to do twice because he does not like it.
I have fitted 100's of earth rods and I still have to revise a little before testing and sometimes it is hard to get far enough away from the rod to test.
I was assuming you were Part P registered or a Scottish Electrician hence giving web site links.
It is all too easy to miss out a vital instruction including protecting people from the rod of course.

Even if you create two seperate TT systems, the sub-main cable to the outbuilding needs RCD protecting from the house. What type of board do you have at the house - is it split load with 100mA time-delayed RCD main switch and then 30ma RCD for those circuits requiring such?
If that is the case you could feed the sub-main from the 100mA side and have the local 30mA RCD at the out-building.
If the whole of your installation is protected by a front end 30mA RCD then there is not much point in having a secondary 30mA at the out-building as there will be no discrimination. But this set-up is a recipe for nuisance trips.

ricicle said:

Even if you create two seperate TT systems, the sub-main cable to the outbuilding needs RCD protecting from the house. What type of board do you have at the house - is it split load with 100mA time-delayed RCD main switch and then 30ma RCD for those circuits requiring such?
If that is the case you could feed the sub-main from the 100mA side and have the local 30mA RCD at the out-building.
If the whole of your installation is protected by a front end 30mA RCD then there is not much point in having a secondary 30mA at the out-building as there will be no discrimination. But this set-up is a recipe for nuisance trips.

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Hi,

The main board at the house is an ancient fuse box with a single 30Ma RCD. It is mounted in the meter box which is mounted on the outside of the house.

I would love to see it replaced but alas this has caused some ... difficulty . I have asked two electricians about this and they will not replace the unit in situ - it must be moved indoors. This creates severe cabling problems as it must be moved some distance from it's present location - effectively requiring the house to be rewired.

There are good and a bad times to rewire a house, and for a lot of non-technical reasons this is a really bad time. I intend to get the whole house rewired in the hopefully not too distant future, but right now have a dodgy overhead cable that I think needs addressing asap as it is runs across the top of a fence which needs to be replaced.

You could call your supplier and ask if they can convert your supply from TT to TNC-S - it will make things much easier.

Pensdown advice on getting TN-C-S although good will not necessary solve the problem.
But you need right patter between you and supplier. The supplier does not have to give you a TN-C-S supply.
All supplies from a transformer should be the same so in theory he can't just change your house but neither can he change all houses together so he can claim the other houses from same transformer are work in progress.
However it is up to the supplier to tell you what supply you have and on a few occasions in the past I have phoned a supplier on customers behalf and asked them what supply I should have which they are duty bound to tell me. Each time they were unable to tell me over the phone and had to send someone out to check and each time they arrived they tested supply and provided me with a TN-C-S supply FOC.
It's not what you ask but how you ask!
If there are no extraneous-conductive-parts on you garage then just taking the house supply into garage is likely OK. However metal doors can mean there are earthed metal parts which can be touched both inside and outside garage and consideration has to be made as to if the earth can be exported to the garage.
In a housing estate where the garage is between houses all with TN-C-S supplies then exporting the earth is normally OK.
But if beyond the garage you have fields an earth gradient may exist and you may need a TT supply even if your house is TN-C-S.
Because it is not straight forward I would suggest you need the advice from someone who has been to the site and fully understands the area and it is not something to be done remote as we are on the forum.
If in England or Wales it will come under Part P and in many cases it is cheaper to get a registered electrician to do the work than to DIY with the £115 charge in this area to register the work with the council.

The houses in our street are all supplied with a pair of overhead wires on wooden poles. I think a TN-C-S supply is unlikely in the near future, it would require a substantial amout of work and investement by the supplier.

I am happy to call in a pro, but the two electricians I have approached so far will not wire into the existing installation. The first guy refused to touch anything unless the whole house was rewired - he wasn't prepared to wire into a 1970s fuse box. I asked him if he would put a few of the problems right and come back in the future and do a complete rewire and he said "NO". A second guy I approached, following advice from members on this board to get a PIR, was more sympathetic but still wants to move the consumer unit location which effectively requires a rewire. I really don't want the house rewired right now.

So, I am a bit stuck with a potentially dangerous hanging cable to the garage and also a 6mm feed to a shower which should be 10mm. The earth electrode has been concreted over and the consumer unit is an ancient fuse box (not in the best of condition). I would gladly pay to have all these put right tomorrow, but can't get a pro to touch any of them without installing a new consumer unit in the kitchen, 10 feet from where it is now, and rewiring.

I don't want the consumer unit moving or the house rewiring, yet.

So we're stuck for the time being.

At the moment I'm just thinking of possibilities. If I can DIY the cable and when the house is rewired in two or three years the work is ripped out and replaced, well, no problem if it is £20 worth of cable and a few hours work. Trouble is, it is notifiable. Hassle with the local authority and £150 fees makes it different. The temptation is just to leave it all as it is and not bother.

Steve2008 said:

The first guy refused to touch anything unless the whole house was rewired - he wasn't prepared to wire into a 1970s fuse box.

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Charlatan.

If all you want sorting is the garage circuit then that is nothing to do with what's already there, and your existing fusebox is still a recognised method of circuit protection.

Apart from earthing and bonding there is no requirement to update anything not directly worked on as part of the garage supply.

A second guy I approached, following advice from members on this board to get a PIR, was more sympathetic but still wants to move the consumer unit location which effectively requires a rewire. I really don't want the house rewired right now.

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Personally I wouldn't want my CU outside, as it's so ****** inconvenient. But unless your existing wiring is in such a poor state that you should have it replaced for your own safety I can't see what's wrong with replacing it with an IP rated box with DIN rail terminals and simply extending the cables to the new indoor CU location.

Nor can I see what would be wrong, if you want to leave the existing CU completely alone, with splitting the tails and taking a new supply via a switchfuse to the garage CU.

Try again to find an electrician - maybe third time lucky and you'll get one who's actually got a brain....

All supplies from a transformer should be the same so in theory he can't just change your house but neither can he change all houses together so he can claim the other houses from same transformer are work in progress.

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Certainally aint true on my patch (old EMEB area) as all the supply poles are marked 'PME System, do not disconnect neutral' and most of the houses have PME incomers with the exception of a few here and there, which are TT. And I know thery're all on the same transformer, I know which phase most are on too!