Earth spike for outbuilding?

23 Apr 2009
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United Kingdom
Sorry to ask another question about outbuildings, but I can't find an answer.

Is it mandatory to have an earth spike to earth a detached garage that is supplied from the house CU?

Reason I ask is because I am planning to have a small CU installed in my garage to replace an older fusebox in the garage. Electrician says that I must have a spike driven into the ground by the garage to earth it and not rely on the earth supplied from the cable from the house - as it is at present.

A few other details:

Garage is detached and only 3 metres from the house at closest point.

Supply to garage is underground cable in conduit ("toughened" I believe, but not SWA)

Garage circuit is RCD protected at house CU, supplied by 20A mcb.

Garage does NOT have any water supply.

House supply is bog standard - TNCS I believe?

Garage will run just standard stuff - Fridge, lights, a few power tools when needed.

Any advice would be appreciated. Thanks.
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Your electrician is responsible for the design, installation and testing.There are several ways of providing a safe earthing system to an outbuilding. If your electrician has decided that to make the building a TT unit and not export the earth then s/he must have had good reason to use that method. This may be due to he limitations of the earthing in the house or other factors. The only way to find an answer is to ask him.
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Forgetting about rule books to start with the problem with outbuildings is the voltage gradient too far away from the other buildings and the earth may be ineffective and two close and you may be able to touch parts earthed by the two systems at the same time. The electrician designing the system has to decide which to use.

On to the rule book and with the ruling that caravans, marinas, petrol stations, and electric vehicle charging systems should not have a TN-C-S earth system means that it could be said a garage which may be used for charging a car should not be TN-C-S although personally I would not consider this as a valid reason.

Fitting an earth rod is not a big job and I can't really see why you would not want one fitted. If the earth being imported from the house is not to the required cross sectional area then this approach would be a cheaper option than installing a new cable.

So why don't you want a TT system?
If your sparky is up to the mark he'll bang in an earth rod, take a reading, and if needed use a joint and knock the second one down. Maybe it was the way I was taught but always TT'd an obviously separate garage or shed.
My opinion, based on experience, is TT always for out buildings. And for main house as well in some circunstances.

The "earth" in a PME system is the incoming neutral which is invariably a few volts different to ground and in exceptional circumstances, ( network problems with high impedance or broken neutrals etc ) can become live relative to ground. For people inside the equipotential zone of house this is not a hazard as they ( in theory ) cannot make contact with ground. The hazard occurs if they are using an electrical item with an earthed metal case in the garden when the neutral goes faulty. If ( when) this happens they will almost certainly feel the shock and it could be fatal. ( as it is delivered via the "earth" wire no RCD will see it as they only monitor Live and Neutral ) Hence the seemingly un-safe requirement that electrical tools in the garden must ( should ) be double insulated and should not have an "earth" connection.

Consider the outbuilding where simultaneous contact with "earth" and ground is possible and using TT becomes the more sensible option.
For people inside the equipotential zone of house this is not a hazard as they ( in theory ) cannot make contact with ground.

If that were true then yes. But to read the meters in my house one stands outside in a narrow ally close to next doors meters and the pipework within the box is connected to the house earth system. There are many other points where the house earth system connects to extraneous-conductive-parts including boiler overflow and the like which is at a level where people outside the house can touch it. Be it a tap for watering garden or a soil pipe there are many items external to house which may be connected to house earth.

I have had this argument with caravans parked next to house the regulations say the caravan should be TT yet it's within arms length of metalwork on the house connected to a TN-C-S system. The house is also joined to next door house again with metal work which is within an arms length from the two houses so to change one house to TT would mean all houses in the block would also need to be TT. This is why the DNO decides on earthing method not the visiting electrician.

To my mind we should have TN-S rather than TN-C-S but that is not going to happen. So we need to do a risk assessment on connecting a supply to an out building and have to consider which method is most appropriate in that situation. There is no one size fits all.

We all hope TN-C-S is PME and the DNO have multi-earth rods around the estate or some other earth connection. I have normally assumed TN-C-S is PME. Your reply will be interesting on this.

The question is there a mandatory requirement and I would say no. It is up to the electrician after a risk assessment to decide. The statement that there is only 3 meters between house and garage means this would not be a cut and dried answer but would need some consideration as to best method.

However the question is why is the owner worried about it being on a TT system? Although another electrician may question it, for the owner it seems odd.

I have noted the IET has said that charging systems for electric cars should be from a TT earth system and I question the idea of a car parked inside the house in my case having a different earth to rest of house? The same applies when outside very close to the house. I am sure the one electric car in our street is not on a TT earth system.

This could go on and on. However as far as the guy posting is concerned I would say if his electrician says he needs an earth rod then he needs an earth rod. No one on this forum could really say the electrician is wrong. But should some one else could read the post and say "Well my garage is on the house earth which is TN-C-S" neither could we say that is wrong. We as electricians do a risk assessment and we do what we think is right for that situation and sign on the dotted line to say we made the decision.
I think sometimes we get too caught up in inisting on TTing all outbuildings when there is a TNCS supply in the main building.

Consider a home shed/workshop three foot away from the house, if it wasn't there and there was an extension / conservatory on the house there instead no one would suggest TTing that. A workshop is likely to be used by persons wearing suitable footwear, not slippers/sandels etc. So therefore risk of TNCS is low. However if it were a greenhouse in the middle of the garden then you have got a metallic structure which is likely to be touched by folk with muddy hands which standing on wet ground in sandels, so far higher risk,

Sometimes I think the risk of relaying on an RCD installed in what is often an unheated environment to provide sole indirect contact protection due to a high impedance TT earth is greater (due to the possible failure of the RCD) than the risk of a few volts rise of accepting TNCS and having a low impedance earth that allows faults to clear the OCPD.

There must be tens of thousands of private lighting columns in car parks up and down the country connected to TNCS earthing and while the idea doesn't quite site right with me, it doesn't actually seem to generate any problems in practice
But to read the meters in my house one stands outside in a narrow ally close to next doors meters
I know a gas meter man who always wears gloves after getting a "noticable" tingle from a meter he was wiping clean to read the dials. And a local plumber who always puts an isolating plastic section when fitting outside water taps.
Well, thank you all very much indeed for your comprehensive and informative replies. I've got enough here to at least engage with the electrician and explore the options a bit more. My reasons for asking the question in the first place. are as follows:

1. I haven't had much luck with electricians in the past and so i am rather wary. The calibre of people that contribute to this forum never seem to turn up at my house! (The last one I had at my house did a legal and certified job, but the quality and attention to detail and customer satisfaction was so bad, even his horse was laughing).

2. Your posts actually hit on my main concern: I don't know a vast amount about electrics, but I instinctively felt that there was indeed a trade-off to consider. ie. swapping a tried and trusted earth from the house for a spike in the ground, which I understand would be higher impedance. I know it's probably the right thing to do in the circumstances, but it just doesn't feel right, if you know what I mean.

Thanks all
Restrictions on exporting TNCS earthing to an outbuilding!

Assuming you are talking from a DNO point of view, what restrictions?
Have seen TN-CS exported all over the place, one of them was 350m+ on a 70mm 4c SWA. From BS7671 pov as long as the CPC is sufficient, in a copper equivalent capacity, for use as a main protective bond where it is required to provide that function then I don't see a problem. (exceptions for some special locations of course - caravans/marinas etc)

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