Separating PME & TT earthing for outbuildings

[Msitekkie]

I would appreciate peoples thoughts on fixing this earthing setup to correctly separate PME & TT:

Setup: Workshop at the bottom of a long garden connected to supply via 3 core armoured cable. Earth rod fitted at workshop, but the earth has also been connected to the PME from the supply & the armouring is earthed throughout.
Just to complicate matters further, a No2 Pratley box (Metal) has been used to T off the armoured cable in a garage located between supply & workshop.

More detail: Both outbuildings have individual consumer units & have concrete floors & block walls. Floor in the garage tends to be damp. SWA is partly buried, partly clipped & connected to a consumer unit in the meter box with 40A RCD (not S type unfortunately, but that doesn't affect the earthing & DNO seem happy with CU in the meter box).

Questions/thoughts:
I am aware TT & PME should not be joined, so would begin by disconnecting the SWA earth core at the supply end. Probably also in the Pratley box on the supply side. Leaving the TT earthing connected between workshop & garage.
I am not so sure about the armouring... If it was just the workshop I would just isolate the armouring from the consumer unit there. Would you just do the same with the garage consumer unit and leave it at that?

Additional thoughts:
Would you expect the garage to need it's own earth rod?
Pratley have told me they can supply "non metallic glands" for their box so maybe for the garage consumer unit I could isolate the armouring where it comes out of the Pratley box rather than at the consumer unit end of that short leg (on the wall). Otherwise at the CU end like the workshop.

Summary: So hopefully I would end up with the circuits in both outbuildings TT earthed, but the armouring PME earthed (& isolated from TT) except the short leg on the wall from Pratley box to CU which would be TT earthed.

 

[marcuswareham]

In theory both buildings could use the PME earth instead of TT, if you remove the rod at the workshop then there is no extraneous conductive parts at either building so no need for 10mm bonding conductor back to the house.

Although a lot of people would favour the option I just mentioned, I myself due to the fact of the wet concrete floor having good conductivity to true earth I would ditch the PME earth and make both outbuilding TT as you are suggesting,

What I have done in my own outbuildings is use a plastic 2 module enclosure and terminated the SWA onto the bottom of this enclosure using an insulated gland so this isolates the PME earth connected to the amour, within this enclosure I have a 100mA delay RCD for fault protection and then T&E coming out of the top of the enclosure with the CPC connected to the TT earth going to a small garage-style consumer unit which contains a 30mA RCD as the main switch for additional protection.

I opted for doing it this way rather than using a plastic wiska box for isolation because: it gives me a plastic enclosure to fit an RCD for fault protection and means that the supply cables going into the metal garage consumer unit are RCD protect, if I used a wiska box for isolation in the unlikely event that one of the supply cables feeding the garage consumer unit shorted out to the metal case of the consumer unit, due to the high resistance of the earth electrode the MCB feed the SWA would not disconnect, make sense?

 

[Rockingit]

In theory both buildings could use the PME earth instead of TT, if you remove the rod at the workshop then there is no extraneous conductive parts at either building so no need for 10mm bonding conductor back to the house.

Be interesting to actually do this by calculation rather than assume the tabular values?

 

[marcuswareham]

Be interesting to actually do this by calculation rather than assume the tabular values?

There could be other extraneous conductive parts we don't know about, do you mean do the calculation for the earth rod and cable coming into the building or for the conductivity of the floor

A cable coming to a building connected to an earth rod in the ground surely would obviously bring an earth potential into the building

 

[telectrix]

I'd use the PME and dry th floor. ever heard of Trench Foot. That's worse than the occasional electric shock.

 

[Msitekkie]

I'd use the PME and dry th floor. ever heard of Trench Foot. That's worse than the occasional electric shock.

I have tried hard to get the damp problem fixed without success unfortunately, although with this type of outbuilding where you are in contact with the ground, my understanding is TT is the correct choice anyway even without the damp because of the risks from a broken PEN conductor. Different scenario if you have a wooden building. There are some metal accessories too.

 

[Msitekkie]

Be interesting to actually do this by calculation rather than assume the tabular values?

? Sorry, I am not sure what calculation you have in mind? I have a mix of TT & PME which needs to be separated, are you suggesting there is some calculation that could in theory suggest having both is OK?

 

[Rockingit]

? Sorry, I am not sure what calculation you have in mind? I have a mix of TT & PME which needs to be separated, are you suggesting there is some calculation that could in theory suggest having both is OK?

It's (theoretically) possible that by applying the calculations for minimum supplementary bonding csa that if it comes back in small enough (min 4mm) that your SWA armoured might be up to the job, in which case just extend the PME. Have a read of GN8.

 

[marcuswareham]

I have tried hard to get the damp problem fixed without success unfortunately, although with this type of outbuilding where you are in contact with the ground, my understanding is TT is the correct choice anyway even without the damp because of the risks from a broken PEN conductor. Different scenario if you have a wooden building. There are some metal accessories too.

It is up to the designer to assess the risks associated with a broken PEN conductor, and then put mitigation in place to reduce the risk, if the risks are unacceptable.

One way of mitigating against a broken PEN conductor risk is by making the outbuilding TT, although this is not as simple as a fix-all solution sometimes using TT can increase the risk and careful consideration should be taken into account when setting up TT systems very close to TN systems

 

[Msitekkie]

It's (theoretically) possible that by applying the calculations for minimum supplementary bonding csa that if it comes back in small enough (min 4mm) that your SWA armoured might be up to the job, in which case just extend the PME. Have a read of GN8.

There is no supplementary bonding requirement. This is mainly about mitigating the risks of a broken PEN conductor.

 

[marcuswareham]

It's (theoretically) possible that by applying the calculations for minimum supplementary bonding csa that if it comes back in small enough (min 4mm) that your SWA armoured might be up to the job, in which case just extend the PME. Have a read of GN8.

The minimum size for a bonding conductors with a PME supply is 10mm

see Reg 544.1.1 & table 54.8

I believe this is todo with high current under PEN fault conditions and the potential for diverted neutral currents

 

[Msitekkie]

It is up to the designer to assess the risks associated with a broken PEN conductor, and then put mitigation in place to reduce the risk, if the risks are unacceptable.

One way of mitigating against a broken PEN conductor risk is by making the outbuilding TT, although this is not as simple as a fix-all solution sometimes using TT can increase the risk and careful consideration should be taken into account when setting up TT systems very close to TN systems

Yes this is the scenario I am working with.

 

[marcuswareham]

There is no supplementary bonding requirement. This is mainly about mitigating the risks of a broken PEN conductor.

Although you say currently you have an earth rod connected to the PME earth at the workshop, this earth rod could be counted as an extraneous conductive part, if you were to leave it this way with that earth there would need to be a minimum of a 10mm conductor connecting that earth rod to the PME MET

This is also the case for any other extraneous conductive part in either of the buildings, like water pipes etc

 

[Msitekkie]

The minimum size for a bonding conductors with a PME supply is 10mm

see Reg 544.1.1 & table 54.8

I believe this is todo with high current under PEN fault conditions and the potential for diverted neutral currents

There is no bonding requirement. No water pipes or anything like that in the outbuildings.

 

[Rockingit]

The minimum size for a bonding conductors with a PME supply is 10mm

see Reg 544.1.1 & table 54.8

I believe this is todo with high current under PEN fault conditions and the potential for diverted neutral currents

But are we discussing main bonding or supplementary??