PME export to outbuilding

[dave_104]

Hi,
I’ve got a brick built outbuilding as an annex next to the main house (not physically attached) and I’d like to think about converting it from TT to PME. Main house is TN-C-S and the buildings are connected with 25M of 3-core 16mm SWA (one core currently unused). However there’s an external metal oil boiler outside the outbuilding (it's bonded to sub main CU). Water feed pipes/oil/etc from house are all plastic.
Reason for wanting to do this is excessive Zs, especially sockets which get about 16ohms. (Due to the earth rod Ze impedance).

I don't know if it's a snag, but the SWA armour is already cut back at the outbuilding CU (in a plastic stuffing gland) and would be fairly difficult to reconnect using a full brass SWA gland. There is obviously a spare 16mm core of the SWA though. The SWA armour is earthed at the house end.

Does anyone see reason why I can’t convert the outbuilding to PME?

Cheers,
Dave

 

[SUNRAY]

Hi,
I’ve got a brick built outbuilding as an annex next to the main house (not physically attached) and I’d like to think about converting it from TT to PME. Main house is TN-C-S and the buildings are connected with 25M of 3-core 16mm SWA (one core currently unused). However there’s an external metal oil boiler outside the outbuilding (it's bonded to sub main CU). Water feed pipes/oil/etc from house are all plastic.
Reason for wanting to do this is excessive Zs, especially sockets which get about 16ohms. (Due to the earth rod Ze impedance).

I don't know if it's a snag, but the SWA armour is already cut back at the outbuilding CU (in a plastic stuffing gland) and would be fairly difficult to reconnect using a full brass SWA gland. There is obviously a spare 16mm core of the SWA though. The SWA armour is earthed at the house end.

Does anyone see reason why I can’t convert the outbuilding to PME?

Cheers,
Dave

Well for a start it's rather good to hear the current installation has been done correctly without the brass gland.

Personally I'd improve the current earthing arrangement with more / longer earth rods.

 

[dave_104]

Hi Sunray - that's encouraging to start with about the glands
I thought about another rod, current one is about 4M from a river and knocked 8 feet down - so likely to be well in the water table. I'm guessing I'd need at least 6 of these the same to bring the value of Ze down to something more akin to PME though

 

[dave_104]

If it's not possible to convert to PME, would a Zs of 16 ohms on the sockets be acceptable though? They're on a 32A MCB and using a 30mA RCD. Regs book seems to suggest that 1667 ohms is the max Ze (although recommended not higher than 200 ohms). Earth faults would be taken care of by the RCD, and overcurrent by the MCB.

 

[SUNRAY]

Hi Sunray - that's encouraging to start with about the glands
I thought about another rod, current one is about 4M from a river and knocked 8 feet down - so likely to be well in the water table. I'm guessing I'd need at least 6 of these the same to bring the value of Ze down to something more akin to PME though

16Ω is OK for TT. Presumably you have suitable RCD.

 

[flameport]

would a Zs of 16 ohms on the sockets be acceptable though?

Yes, fault protection is provided by the RCD.

16 ohms is fine for a TT installation.
Under 200 is recommended by some, but under 100 would generally be a better target.

1667 is piffle and should be ignored. No real installation should be anywhere near that.

Is there some reason you want the Zs to be much lower?

 

[bernardgreen]

1667 is piffle and should be ignored.

Just to explain where the 1667 Ohm value comes from

It is considered that the CPC ( earth wiring ) can go as high as 50 Volts above Ground potential without creating a significant hazard. (*)

50 Volts applied to 1667 Ohms creates a current of 0.0299 Amps which is 29.9 milli Amps.

(*) I dispute that and would say 10 Volts should be the limit.

 

[ericmark]

The IET has a wiring matters about this the main consideration is you can't touch two different earthing systems at the same time.

 

[dave_104]

Thanks for all your help everyone and the clear explanations
I'm just about also to change the RCD to an A type rather than AC to be sure it can trip as there's a lot of electronic gear both in circuit and likely to be plugged in.
Cheers all !!!

 

[SUNRAY]

The IET has a wiring matters about this the main consideration is you can't touch two different earthing systems at the same time.

Hence my comment

Well for a start it's rather good to hear the current installation has been done correctly without the brass gland.

 

[ericmark]

Thanks for all your help everyone and the clear explanations
I'm just about also to change the RCD to an A type rather than AC to be sure it can trip as there's a lot of electronic gear both in circuit and likely to be plugged in.
Cheers all !!!

I have been looking at AFDD's, not to do what they are designed to do, but because many will also trip on over voltage, so will save equipment with loss of PEN. However it seems as you get better and better protection, it also costs more and more both to install and run.

Seems odd they don't publish how much power a type AC and type A RCBO uses within its self. May be they do? If I had no RCBO's I would consider fitting type A, but is it really worth swapping type AC less than 3 years old for type A, it actually said on the packet type B, what a con.

I was rather surprised with one job, to find the solid state contactors could be protected with a semi-conductor fuse, but not an MCB. I had thought a type B MCB would have been faster, but no. However fuses cost more, so not going to fit them in my consumer unit. However if I did I could have a go at @winston1 every time he tells people it's not a fuse box, it's a consumer unit.

 

[SUNRAY]

I have been looking at AFDD's, not to do what they are designed to do, but because many will also trip on over voltage, so will save equipment with loss of PEN. However it seems as you get better and better protection, it also costs more and more both to install and run.

Seems odd they don't publish how much power a type AC and type A RCBO uses within its self. May be they do? If I had no RCBO's I would consider fitting type A, but is it really worth swapping type AC less than 3 years old for type A, it actually said on the packet type B, what a con.

I was rather surprised with one job, to find the solid state contactors could be protected with a semi-conductor fuse, but not an MCB. I had thought a type B MCB would have been faster, but no. However fuses cost more, so not going to fit them in my consumer unit. However if I did I could have a go at @winston1 every time he tells people it's not a fuse box, it's a consumer unit.

Nothing wrong with fitting fuses in a consumer unit. Or for that matter fitting MCB's in a fuse box.

 

[ericmark]

Nothing wrong with fitting fuses in a consumer unit. Or for that matter fitting MCB's in a fuse box.

I agree, however to have a fuse and a RCD needs more room in the box to just a RCBO, so since my consumer unit is full, I will not be fitting fuses.

 

[SUNRAY]

I agree, however to have a fuse and a RCD needs more room in the box to just a RCBO, so since my consumer unit is full, I will not be fitting fuses.

Yep all understood, all fine with me whichever way.