Exporting earth because of concrete floor?

[James Carmichael]

Hi,

I'm having a shed/home office built in my garden, about 20 metres from the house. My house has a TNC-S supply and the electrician has told me the outhouse needs a local TT earth. Fine by me, but ...

The outhouse will have no incoming gas or water supplies, and has no other classic "extraneous conductive parts". The reason she told me it needs a TT supply is because it has a concrete floor. Fine by me, but ...

As she was telling me this, she was stood on the ground floor of my house. It's a Victorian house with a terrazo (polished concrete) hall - with no evidence of an electrical insulating layer beneath it. Indeed, as she was speaking she was stood where my electrical supply comes into the house, in the original integral coal shed with brick laid directly on the earth. It was raining at the time and the floor was noticeably wet.


She rebuffed my suggestions (but without a reason) that her reasoning suggests either my house floors needed adding to the house's equipotential bonding or my house needed to be moved to a TT supply.

Why the difference?

(If it matters: Both my house and the outhouse are of stone and brick wall construction. I also won't be doing anything 'challenging' electrically in the outhouse; no welding or anything like that. Like the main house, I'll have lights and plug sockets for powering the laptop etc.)

 

[flameport]

Earth electrode(s) installed into the concrete base of the new building, TN-C-S supply to the new building, earth electrode connected to the TN-C-S earth.
Use 10mm² or larger for the supply from the existing building to the new one.

TT may be an option, but only if you have a specific reason for having it.

 

[James Carmichael]

Thanks @flameport - so essentially, you're recommending that the concrete floor in the ouhouse is bonded?

Why is this not a requirement in the main house?

 

[flameport]

Earth electrodes for TN supplies have been permitted for ever but no one bothered.
Since the most recent amendment of BS7671 they are recommended.
In the near future it's inevitable that they will be required.

In theory you could just install the TN-C-S and no electrode, but as you have the opportunity, it should be installed.

The electricians reasoning about concrete floors and TT supplies is incorrect.

 

[FrodoOne]

I'm having a shed/home office built in my garden, about 20 metres from the house. My house has a TNC-S supply and the electrician has told me the outhouse needs a local TT earth. Fine by me, but ...

It seems that there may be a "nomenclature" problem here.
If one "looks -up" https://en.wikipedia.org/wiki/Earthing_system one will find that a TN-C-S system is illustrated/specified without a "local" "Earth" electrode.
However, in countries where TN-C-S systems are (virtually) universal (North America, Australia, New Zealand etc), one, or more, local "Earth Electrodes" are also required.

The reason for this is actually for "System Protection", which is not the same as "Equipment Protection" !!!!

I will not here go into the differences but simply refer you to

This is a rather long video (and couched in American terms and voltages - with "Ground" used in place of "Earth").
However, the part about "System Protection" is from 21:15 to 26:35

Sadly, many "electricians" do not understand the different "needs" of "System Protection" and "Equipment Protection"

If you follow the discussion of Mike Holt, you will see that the installation pictured in https://commons.wikimedia.org/wiki/File:HomeEarthRodAustralia1.jpg is definitely not the way in which to install a "System Earth" electrode connection for a TN-C-S System.

If your "Outhouse/Shed" should happen to have a "Ranforced Concrete" floor, you have an excellent opportunity to provide a "Uffer" Ground/Earth connection (in lieu of "Ground Electrodes".)

I invite you to "Look-Up" "Uffer Ground". (https://en.wikipedia.org/wiki/Ufer_ground )
In this it is stated: - "When homes are built on concrete slabs, it is common practice to bring one end of the rebar up out of the concrete at a convenient location to make an easy connection point for the grounding electrode.
Ufer grounds, when present, are preferred over the use of grounding rods."

Such "Uffer" interconnection and Earthing is now required in all Australian "concrete slab" home constructions

 

[SUNRAY]

If you follow the discussion of Mike Holt, you will see that the installation pictured in https://commons.wikimedia.org/wiki/File:HomeEarthRodAustralia1.jpg is definitely not the way in which to install a "System Earth" electrode connection for a TN-C-S System.

One of the companies I work for did a full rewire on a rental property for the landlord between tenants and before decoration but (of course) did not provide a document labeled EICR.
So the new tenant discovered the lack of EICR and called an electrician with the instruction to invoice the rental agent. Upon receiving the EICR pointing out the 'lack of earthing' they appointed one of their regular electricians to correct and invoiced the landlord for it and the EICR.

Upon receiving the invoice the landlord tried to charge it back to the original installer. They of course didn't understand the claim and insisted on inspecting the repairs.

We found;
the MET block adjacent to the CU had been removed with the wires simply chopped off,
in the gas meter box the earth clip still in place but the wire chopped,
in the electric meter box the earth bond chopped from the C/O earth terminal,
under the sink the hot and cold water and the CH pipes bonds removed but clips left in place.

In their place;
the original MET block had been fitted elsewhere,
a big hole chopped into the plaster and brick to fit a clip to the galvanised gas pipe (which we knew from supplied pictures was after 2 T pieces) and a coil of wire,
a very rusty earth spike very close to the electric supply cable - with a very long coil of wire,
a new clip on the cold water pipe with a coil of wire.
All of the wires were clipped direct either on the front of the skirting or into the dampcourse outside and all looked like it was done by a chimpanzee. Additionally there was the regular spiders webb of bonding under the boiler.

It ended up in small claims court, my boss got more out of expenses claim than out of the original rewire but I don't know who lost out. Digital photos prior to rewire and of original installation were revealed only to landlord prior to court hearing.

Among the things listed by the independant inspector being;
the incorrect coils of wire,
the excessively long runs of cable and
the lack of bonding to the pipes emerging from the concrete floor.
Additionally pointing out the way cables had been cut leaving sharp hazards.

All the tenant had to do was ask about an EICR and all would have been made clear to them, they had a copy of the DEIC along with the Landlords gas cert and full set of appliance instructions in a package. There was no excuse for any of the additional work as the sticker on the CU had shown the company details and date of installation and clearly visible in the agents photos dated the day the tenants moved in but by the time we got back it had been stripped leaving only the sticky residue and torn paper. Also showing in their photos dated a month or so before were the earth wires under the sink prior to being cut (taken to show the water meter and washing machine plumbing.)

 

[ericmark]

Earth systems are not really a DIY subject, personally I feel TN-C-S without some auto disconnect device with loss of PEN is wrong, but I am it seems in the minority, the problem arises when two earthing systems are used one can be a significant voltage difference to the other, and there needs to be enough distance between the two systems, a caravan may need to have a TT earth, but fire regulations ensure it is not too close to any buildings, TN-C-S was called PME and the M stands for multiple, but these rods are sized so excessive current can't flow, a 6 mm² cable is likely good for around 45 amp, depending on the route, so at 230 volt so an earth rod would need to be less than 5 Ω before there is any real problem, but using the re-bar of a concrete slab it could well be less than 5 Ω and you don't want earth wires heating up and melting, if the DNO fits earth rods then the load is shared between them, and if rules say everyone must fit earth rods again load is shared, but one odd house with a really good earth mat connected to the PEN is not a very good idea, I have seen the melted wires as a result.

Commercially we fit huge resistors to take the load with loss of PEN but in a domestic it is really an either or, either TT or TN-C-S not good to mix them. To fit a 60 Ω earth rod is not going to be a problem, but to bond to the re bar in the slab is another thing.

However distance between buildings does make a difference, I am sure with my garden I could have a garage with a TT supply and an EV charging point without a problem, but most gardens there is likely a problem, and to say yes or no without being on site and knowing the loop impedance or earth rod readings is wrong.

The @FrodoOne video repeatedly talks about turning it off. This is the only way to stop the current flow. But there is no way to turn off the current flow between two earth electrodes.

 

[SUNRAY]

Earth systems are not really a DIY subject,..

I Very much agree with that. I once found a TT earth rod in a wooden planter standing on wooden decking.

but one odd house with a really good earth mat connected to the PEN is not a very good idea, I have seen the melted wires as a result.

hence: https://www.diynot.com/diy/threads/a-real-moan-tonight.552183/ no sign of melting but that was with the outbuilding L & N isolated

The @FrodoOne video repeatedly talks about turning it off. This is the only way to stop the current flow. But there is no way to turn off the current flow between two earth electrodes.

In fact it would mostly be contravining regs.

 

[ericmark]

In fact it would mostly be contravining regs.

But is how we do it with EV charging points.

 

[SUNRAY]

But is how we do it with EV charging points.

hence:

In fact it would mostly be contravining regs.

 

[flameport]

I feel TN-C-S without some auto disconnect device with loss of PEN is wrong,

Problems with those devices
- there is no standard for them, so each manufacturer does whatever they want
- they do not work for all instances of broken CNE conductors
- there are other network problems not covered by such devices even when they do work

Or to put it another way - these are sold as critical safety devices which by design will only work some of the time.

But is how we do it with EV charging points

A sticking plaster solution to the broken LV distribution networks that have been installed in the UK since the 1970s.

 

[ericmark]

A sticking plaster solution to the broken LV distribution networks that have been installed in the UK since the 1970s.

Glad to see some one else feels the same.

 

[flameport]

The reason why CNE cables were introduced - simpler, faster, cheaper.