Some advice ahead of running wiring to shed

[riveralt]

You cannot export a TN-C-S supply.

You will need to isolate the armouring at the shed end, and fit a plastic CU with RCD main switch, and earth electrode.

Care to quote some regulation numbers or other references for this little gem??

I was under the impression that permisiion of the DNO was required to export a PME supply - and that was rarely given especially if the shed was more than two meters from the supply.

So I assume that londonboy's proposal was to TT the shed hence the plastic CU and RCD.

Don't know whether Westie has a view on this?

 

[Electrifying]

DNO aren't interested in anything our side of the cutout - I know, I've asked at least two of them and that was the reply I got.
Their only stipulation is that the installation complies with BS7671 for them to supply it........and there's nothing in the 'regs' that says you can't run an earth to an outbuilding - regardless of earthing system.

I've also asked the IET Technical Helpline and they actually encouraged extension of the equipotential zone to outbuildings.

What you have to take into account (as designer and installer), when dealing with a PME supplied TN-C-S system is:
1/ The construction of the outbuilding - is it brick, steel, wood??

2/ Are there any 'Extraneous Conductive Parts' in the outbuilding,

3/ Flooring - insulated??

You would then make an informed decision as to whether you should utilise the 'Supply' earth - or seperate the outbuilding from the 'supply' earth and install a 'Rod'.

You're the designer/installer - it's your call........nothing to do with the DNO.

 

[bernardgreen]

DNO aren't interested in anything our side of the cutout -~~~~~~.nothing to do with the DNO.

My local DNO are insisting that the CPC of the new PME supply to a cottage is isolated from the CPC of the adjoining shop's TT supply. This will require the insertion of plastic sections in the water pipe that links the two properties. Until now the TT supply to the shop also fed the cottage.

The reason seems to be that if there was a network fault and the PME derived CPC went high voltage then the CPC from the ground rod in the shop ( in linked via water pipework ) would also try to go high with heavy currents flowing in CPCs and water pipes.

In that that case the ends of the plastic isolating section in the water pipe would be at different potentials.

A similar problem would seem to be possible with an out-house with CPC to ground rod supplied from a house with PME derived CPC.

Confusing isn't it. When touching two "earths" at the same time could result in a minor electric shock.

 

[westie101]

Don't know whether Westie has a view on this?

This is within our "rules" that will comply with the regulations

Building Complexes with Interconnecting Services

Where there is one customer occupying several separate buildings on the same site, there will normally be only a single service for the whole site. The subservices from one building to another will be afforded by private cables. The ESQCR Regulation 8(4) prohibits customers from using CNE within their installations and ENA ER G12/3 section

4.9 gives guidance on the sizes of bonding conductor to be used. The required bonding sizes (copied as Table 2 in this CP) are related to the size of the incoming cable’s neutral conductor.
This is important in cases where a cable is laid between two separate buildings belonging to a customer on his own premises.
If there are incoming metallic services in a remote building which are separate from those in the building containing the Network PME connection, the bonding between the two buildings is related to the size of the incoming service, and not to the size of the cable linking the two buildings.
The reason for this is that the bonding might have to carry diverted neutral current following a fault on the Network.
It shall be pointed out that whilst the wire armouring of a PVC insulated cable may provide adequate capacity for short duration faults within the installation it will almost certainly be too small for equipotential bonding purposes.
In these cases then either a separate protective conductor sized in accordance with Table 2 shall be run, or, as a non-preferred option the cable shall be converted to CNE, and an ad-hoc approval sought

You're the designer/installer - it's your call........nothing to do with the DNO.

In the strictest terms that is correct, but if we feel an installation is unsafe we can issue an improvement notice or in xtreme circumstances disconnect the supply without notice.
Installtions that comply with the Wiring Regs should comply with our requirements but as you can see above that may not be the case. For a TNC-S supply to an outbuilding the above must be complied with. The alternative is to completely seperate the earths and use a TT system for the outbuilding - this I posted on another thread particularly relating to agricultural buildings (

 

[westie101]

In that that case the ends of the plastic isolating section in the water pipe would be at different potentials

Just thinking, if this insulated section was on a part of the pipe where it was buried the risk would be reduced, the length of the insulated section could also be longer to remove the possibility of touching both parts.
An alternative would be to convert from a metal pipe to plastic just outside the building and bring the plastic pipe to inside, again there would be little or no possibility of touching both parts

 

[ban-all-sheds]

Installtions that comply with the Wiring Regs should comply with our requirements but as you can see above that may not be the case.

Might it not?

The regs call for e-c-ps to be bonded, so.....

For a TNC-S supply to an outbuilding the above must be complied with. The alternative is to completely seperate the earths and use a TT system for the outbuilding

Out of interest I re-read the Wiring Matters article on supplying outbuildings, and it seems to be using a PME supplied one just as an example. If you have a TN-S supply, and you extend the equipotential zone to the outbuilding, you'd need to bond an e-c-ps there just as you do in the main house - the only difference would be the size of bonding conductor required.

I don't think the article means to imply that you only need to bond e-c-ps in an outbuilding when the supply is TN-C-S.

 

[westie101]

Might it not?

Amongst all the advice given on here regarding supplies to garages, outbuildings, etc. I have not seen any mention of sizing the cable such that the copper equiveilant c.s.a of the earth complies with the copy of our COP I just posted to afford the use of a TBC-S (PME supply)
Given that the minimum size of this cable is 10mm2 copper equivalent.

So if the wiring regs are not specifying this the answer is yes!

 

[ban-all-sheds]

The Wiring Regulations specify that e-c-ps should be bonded, and they specify that where PME conditions apply the minimum size for the main protective bonding conductor is 10mm² copper, or its equivalent.

So any connection to an outbuilding that genuinely complies with the regs should be OK from your POV.

Unless I'm missing something.

 

[westie101]

So any connection to an outbuilding that genuinely complies with the regs should be OK from your POV

Absolutely (reality is that once a supply is connected we don't go roundchecking for continued compliance anyway. It's usually only when things go wrong or we spot something that we look closer)

 

[JohnW2]

The Wiring Regulations specify that e-c-ps should be bonded, and they specify that where PME conditions apply the minimum size for the main protective bonding conductor is 10mm² copper, or its equivalent.
So any connection to an outbuilding that genuinely complies with the regs should be OK from your POV.
Unless I'm missing something.

What about the requirement (I think!) I recently mentioned, which I still feel sure I didn't dream (but still can't find) of a maximum impedance of 0.05Ω for main bonding conductors? If that's actually true, the maximum length of 10mm² (copper) would be around 27m (at 20 degrees C, about 22.7m at 70 degrees C). Some of the recent discussions have involved much longer cable runs than that (particularly the 350m one!) and would require extremely large cables to achieve the 0.05Ω. - it looks to me like at least 150mm² (copper) for 350m, and probably 50mm² (copper) for 100m.

...or did I really dream this 0.05Ω thing?

Kind Regards, John

 

[ban-all-sheds]

 

[JohnW2]

.

Did you have something to say?

Kind Regards, John.

 

[ban-all-sheds]

I did, but it was b****cks, so I unsaid it.

 

[riveralt]

What about the requirement (I think!) I recently mentioned, which I still feel sure I didn't dream (but still can't find) of a maximum impedance of 0.05Ω for main bonding conductors? If that's actually true, the maximum length of 10mm² (copper) would be around 27m (at 20 degrees C, about 22.7m at 70 degrees C). Some of the recent discussions have involved much longer cable runs than that (particularly the 350m one!) and would require extremely large cables to achieve the 0.05Ω. - it looks to me like at least 150mm² (copper) for 350m, and probably 50mm² (copper) for 100m.

...or did I really dream this 0.05Ω thing?

Kind Regards, John

The value 0.05Ω appears in Guidance Note 3 under Continuity of protective conductors including main and supplementary bonding - Test Method 2.

This method can also be used to confirm a bonding connection between extraneous-conductive parts where it is not possible to see a bonding connection e.g. where bonding clamps have been 'built in'. The test would be done by connecting the leads of the instrument between any two points such as metallic pipes between which a bonding connection was required and looking for a low (minimal deflection) reading of the order of 0.05Ω or less.

 

[JohnW2]

...or did I really dream this 0.05Ω thing?

The value 0.05Ω appears in Guidance Note 3 under Continuity of protective conductors including main and supplementary bonding - Test Method 2.
This method can also be used to confirm a bonding connection between extraneous-conductive parts where it is not possible to see a bonding connection e.g. where bonding clamps have been 'built in'. The test would be done by connecting the leads of the instrument between any two points such as metallic pipes between which a bonding connection was required and looking for a low (minimal deflection) reading of the order of 0.05Ω or less.

Many thanks; at least I didn't dream it! I must say that my recollection was that it was more than an explicit 'requirement' than that seems to imply, but maybe that recollectiuon is clouded! However, I still have a distinct feeling that I've seen it somewhere other than in GN3. Dreaming again, perhaps!

How do you (and others) interpret all this? Do you feel that satisfying the minimum CSA requirements for bonding conductors per the regs (which I presume are based primarily on the CCC of the cable) is always adequate, regardless of the length of the cable (hence even if the resistance of the conductor is far more than 0.05&#937, or what?

Kind Regards, John.