Complex earthing situation - TT supply

[mwylde]

A while ago I moved into a house built in 1960. I'm not entirely happy about the wiring configuration which is quite complex. It's as follows.

Fused 60A TT supply (odd that as it's underground not o/hd and the substation is 50 metres away) comes into the detached garage. Two tails go to the meter and from the meter to a consumer unit. Earthing is by a wire which disappears through the garage floor, presumably to a rod, but I can't find the rod connection point outside. This wire is bare 7/.029 (corresponds to 2.8 sq mm for youngsters) from the earth bar in the CU.

The CU has a mains in switch that feeds a 63A MCB and a 30mA RCD, in turn feeding a 16A and a 6A MCB for garage sockets and lights (radial).

The 63A MCB supplies 2 core 3/16" (16 sq mm approx) PVC over copper sheathed Pyrotenax which goes about 30 metres to the house. The copper sheath is used as the earth.

At the house the Pyro goes into a split load CU with power on a 30mA RCD and lights not on RCD, and various MCBs. The pyro sheath goes to the earth bar in the CU. There is no other earthing i.e. not another rod.

The gas meter is external on the house wall, the gas pipe is bonded on entry to the house with 10 sq mm insulated back to the CU earth bar. The water pipe is bonded on entry, back to the CU earth bar, with bare 7/.036 (i.e. about 6 sq mm).

A 16A MCB, protected by the RCD, feeds an SWA cable going to a wooden stable outside. At the stable there is a small CU with switch and 2 MCBs for lights and sockets (1 radial). Earthing is by 1 core and armouring of the SWA, no rod.

I've toyed with the idea of asking if I can go onto PME but I can see complications! In particular I can't take continuous gas and water bonding back to the incoming supply. I think the best option I can take is to improve my TT earthing with new rod(s) and a 16mm earth cable at the garage to current standards. In effect then the house would be supplied TN-S from the garage and the stable TN-S from the house I think? Or should I rod the house and the stable as separate TT's with no links between them?

I'd appreaciate really expert comments please! I've been qualified in the past but I'm not fully up with current regs.

Thanks

Mike

 

[Adam_151]

So the MI is not RCD'd ?

I guess if you wanted TNC-S, it'd might be an idea to speak to someone at the DNO, at the DNO about earthing requirements and what they suggest, Whats the CSA of the sheath of the MI, if this meats the requirements of 54H, then its possible a TNCS earth could be 'exported' to a BEMT (building earth marshalling terminal) in the house


Or, looking at the familiar house/detached outbuilding situation backwards, get a TNCS terminal installed, use it for the garage switchgear, but once you're in the house, isolate the TNCS earth and have a plastic CU with the standard 100ma S type and 30ma split arrangement, with a local electrode, make sure no one can simutaniously come into contact with multiple earthing systems

Or you could leave the whole thing as TT, and sort it out better, with perhaps a time delay RCD at orgin, and boards with 30ma RCDS in other buildings


Personally, I think I'd be looking at a TNC-S supply (if available) with the house subboard made a TT system

One more point, might be best if supply to house came from a switchfuse, rather than a fuseway on a CU, I understand they arn't really supposed to supply big sustained loads, like heavy submains, or electric boilers, etc (but you could speak to the manufacturer)


Supply may have started off as TNS, but ended up getting re-designated by DNO due to corrosion of the lead sheath leading to bad Ze

 

[mwylde]

Thanks Adam, my comments below
Mike

So the MI is not RCD'd ?

## No it isn't. I can't see why it should be (other than that there should be an RCD at the supply point covering the whole installation, is that what you mean?) I think the urgent concern is that it means the house lighting circuits are not covered by anything.

I guess if you wanted TNC-S, it'd might be an idea to speak to someone at the DNO,

## I'm a Powergen customer on an EDF network (ex SEEBoard) - I've heard horror stories about trying to contact anyone technical!

at the DNO about earthing requirements and what they suggest, Whats the CSA of the sheath of the MI, if this meats the requirements of 54H,

## Not a problem, over 20sqmm.

then its possible a TNCS earth could be 'exported' to a BEMT (building earth marshalling terminal) in the house

## Haven't heard of that, was lead to believe TNCS earth never exportable. Will investigate.

Or, looking at the familiar house/detached outbuilding situation backwards, get a TNCS terminal installed, use it for the garage switchgear, but once you're in the house, isolate the TNCS earth and have a plastic CU with the standard 100ma S type and 30ma split arrangement, with a local electrode, make sure no one can simutaniously come into contact with multiple earthing systems

## That's one way I've thought about, think it would be TNS actually. The only thing I could bond would be a water pipe which comes through the floor up to a tap not within reach of anything electrical - does that need bonding?

Or you could leave the whole thing as TT, and sort it out better, with perhaps a time delay RCD at orgin, and boards with 30ma RCDS in other buildings

## Would the other buildings need their own spikes or rely on the MI sheath and the SWA?

## I used to run a mobile home park (permanent units not subject to special caravan wiring rules) and what a registered sparks did for me there and signed off for my compulsory safety inspection (under edition 16) was very similar. TN-S at main supply, exported (underground) to a CU in the shed at each pitch, MCBs to a light and an RCD socket in the shed, and onward through an underground link to a CU in the home. Thus all earthing dependent on the TN-S. All impedances came out OK on test. Is your view that that was not OK? I'm really proposing the same here.

Personally, I think I'd be looking at a TNC-S supply (if available) with the house subboard made a TT system

One more point, might be best if supply to house came from a switchfuse, rather than a fuseway on a CU, I understand they arn't really supposed to supply big sustained loads, like heavy submains, or electric boilers, etc (but you could speak to the manufacturer)

## Are you saying MCB ways in a CU are time limited not continuous rated? In fact there aren't any sustained loads such as heating anyway. Cooker is only high current item.

Supply may have started off as TNS, but ended up getting re-designated by DNO due to corrosion of the lead sheath leading to bad Ze

## Don't think so. The incomer is wrapped in old cloth tape from cutout down to floor and looks undisturbed. I assume it's lead sheath as it's quite stiff and steps down in size just before the cutout. That's why I'm assuming TNS is an option.

 

[Spark123]

So the MI is not RCD'd ?

No it isn't. I can't see why it should be (other than that there should be an RCD at the supply point covering the whole installation, is that what you mean?) I think the urgent concern is that it means the house lighting circuits are not covered by anything.

A fault on the MI cable as it is a TT supply may not cause enough current to flow to cause disconnection hence the reason it should protected by an RCD. This can mean all the exposed and extraneous conductive parts in your house becoming live in the event of a fault. Same problem with the lights, as it is a TT system they should be RCD protected.

 

[mwylde]

So the MI is not RCD'd ?

No it isn't. I can't see why it should be (other than that there should be an RCD at the supply point covering the whole installation, is that what you mean?) I think the urgent concern is that it means the house lighting circuits are not covered by anything.

A fault on the MI cable as it is a TT supply may not cause enough current to flow to cause disconnection hence the reason it should protected by an RCD. This can mean all the exposed and extraneous conductive parts in your house becoming live in the event of a fault. Same problem with the lights, as it is a TT system they should be RCD protected.

OK, yes, understood, it needs the whole installation RCD (100mA S) to cover it, not something specific, but if I can go to TN-S I don't need that. From a short term practical point of view the chance of a problem on this cable is negligible but I want to get protection on the lights quickly.

 

[Spark123]

I personally wouldn't class the risk on a piece of pyro outside as negligible, any sort of damage causing a breech and the IR can fall through the floor!! If it is converted to a TN system then the pyro won't need RCD protection. How about a 100mA (S) in the garage, then a double RCD with isolator board in the house (100mA and 30mA fed individually from the isolator)?

 

[mwylde]

I personally wouldn't class the risk on a piece of pyro outside as negligible, any sort of damage causing a breech and the IR can fall through the floor!! If it is converted to a TN system then the pyro won't need RCD protection. How about a 100mA (S) in the garage, then a double RCD with isolator board in the house (100mA and 30mA fed individually from the isolator)?

Sorry Spark 123 I didn't mention tht the Pyro is buried under the lawn which I'm not intending to dig up. In view of that, and the plastic sheath as corrosion proection, that's why i can't see it as a significant risk, and why I think it would provide an adequate earth to the house if it was TN-S connected at the garage. In fact the excellent earth return is - or used to be - promoted as a benefit of Pyro. What I'm not quite sure of now is whether that would meet the regs, but as explained above, I've had a directly equivalent system certificated in the past, on 50 permanent units on a mobile home park.

If I stayed TT and fitted a 100mA RCD at the garage, why would I want another 100mA one at the house please? What about discrimination - does making one of them an S satisfy that? And would I need another spike at the house?

Thanks for your time and thought!

Mike

 

[Spark123]

A cable without adequate protection against indirect contact is a risk. I don't think there is any denying that. It may be next week it takes a glancing blow with a spade, you don't know. A type S RCD is a time delay version, it should hold off long enough for a normal RCD to disconnect.

 

[mwylde]

Just realised I misinterpreted some of the wiring. The earth to the water pipe at the house attaches below the stopcock; it's not a bond, it is the earth, using the water pipe. This was accepted practice when house was built in 1960. I guess the wire in the garage goes to the water pipe too, not through the floor to an earth rod as I assumed! I'll have to peel back the pipe frost insulation and have a look.
I think i feel a new rod and a 100mA TD RCD coming on rather urgently. Talking to Powergen about changing the system can wait, although personally I think using TT on anything other than a long rural overhead should not be acceptable practise.

 

[Adam_151]

personally I think using TT on anything other than a long rural overhead should not be acceptable practise.

Getting away from the fact that TNC-S is not always available (it requires sufficient additional earth bonds in the local supply network, and the integritty of the CNE conductor to be assessed), it is in essence a poor compromise that works well most of the time given the standard house or small shop, but if you can work around the high impedance with RCDs, TT can be a nicer system, especially when you've got added complications