introducing an shed earth rod and part p

[bertoli]

From the advice gathered from this site, my final set up for my timber shed electrics is as follows. I will only be running low drain devices, but on the occasions of using mitre saws, heaters etc, these will only be used exclusively.


PME house supply

32mcb in house CU ->

4mm t&e ->

junction box with 30A choc block ->

22m of 4mm SWA 3 core cable underground (with CW glands, though junction boxes will be inside house and shed)->

junction box with 30A choc block ->

4mm t&e ->

shed CU (with 63A RCD, 6 and 16 MCB)


From the shed CU, I will be running 2 double sockets, and 4 x 50W mains voltage downlighters.

The SWA cable armour is earthed at the house and garage end, and also the 3rd core of SWA is connected right through from house CU to shed CU as an earth.

I have read that it is not advisedable to use a PME earth for a shed because of the broken neutral scenario, and to use an earth rod instead.



My questions are;

If all this work was done last summer, does Part P still apply if I introduce an earth rod at the shed to this setup, then disconnect the house earth at the the shed? The shed is approx 12m from house.

Secondly, if the broken neutral scenario arose, would testing the shed RCD not show this up? Does the shed RCD not require a working earth to function?

Many thanks for any advice given.

 

[yai46]

part p is from jan 2005 which means your be fine,

As for the earth rod if you have tested the consumers unit in your shed and it tests out ok your be fine, normally earth rods are used when a tt system is supplying a propertry which is purely a live and nuetral supply with a pit outside with a earth rods inserted.
If your supply was a TT then it's required that earth rods are fitted at every addition consumer units are req.

Personaly I would say this is fine how you have done it.

Kind regards
Rob

 

[mattux]

hi m8

as far as i can remember the earth on a pme system should not leave the building.
you need to put an earth spike in for the shed and just supply live and neutral, so 2 core swa would be fine.
use 16mm from shed cu to earth spike.
disconnect earth from house cu end.

hth

matt

 

[Spark123]

disconnect earth from house cu end.

Does this not leave the SWA cable protected by a 32A MCB and earth electrode?

 

[yai46]

I would leave well alone and i would rely on the armored earth at the consumers unit. as it is now

 

[Spark123]

My point being it is highly unlikely there will be a low enough efli through the earth rod for a 32A mcb (assuming type B) for a 5s disconnection, i.e. Zs = 1.2ohms max measured @ 10 deg C.

 

[mattux]

disconnect earth from house cu end.

Does this not leave the SWA cable protected by a 32A MCB and earth electrode?

sorry i meant to say "disconnect earth from house cu end aswell"

matt

 

[Spark123]

You did, so where is the earth for the armour from?

 

[mattux]

You did

if its been glanded and terminated properly it should.
im sure i learnt this in 2391, i will try and find the info.

matt

 

[Spark123]

What I am asking is it safe to have an armoured cable with a live and neutral fed from the house CU from a 32A MCB (no mention of an RCD)? The earth to the armour if separated from the house CU earth can then only be bonded to the earth electrode. 32A MCB requires efli 1.2ohms max @10degC for 5s disconnect. Taking into account this is now effectively a TT system for the shed, touch voltage RAIa<50v also has to be fulfilled. This means an efli of 0.31ohms is now required where protection is provided by a 32A type B MCB (160A for a 0.4-5s disconnection.)

 

[mattux]

What I am asking is it safe to have an armoured cable with a live and neutral fed from the house CU from a 32A MCB (no mention of an RCD)? The earth to the armour if separated from the house CU earth can then only be bonded to the earth electrode. 32A MCB requires efli 1.2ohms max @10degC for 5s disconnect. Taking touch voltage into account, RAIa<50v also has to be fulfilled (TT system). This means an efli of 0.31ohms is now required.

im not sure what you mean m8
you do not need the 3rd core, it gets disconnected at both ends.
the armoured is protected by the armour sheath which should be earthed at the house cu but isolated from the shed cu.
basically you are feeding the shed as an tt arrangement.
why is every opne editing their posts

matt

 

[Spark123]

The bombshell is when you said "disconnect earth from house cu end."
This can be read as disconnect the earth core and armour earth, as I did.

 

[mattux]

The bombshell is when you said "disconnect earth from house cu end."
This can be read as disconnect the earth core and armour earth, as I did.

sorry for not making myself clear in the first instance,
none the less a good discussion for people to learn from

matt

 

[bertoli]

What I am asking is it safe to have an armoured cable with a live and neutral fed from the house CU from a 32A MCB (no mention of an RCD)?

The 32MCB will not be on the RCD side of the house CU, only a 63 RCD at the shed CU.

So my current setup is not acceptable? I still need a TT arrangement at my shed, or will I get away with leaving my setup as-is?

Also my other point, does my shed RCD not require a working earth so I can test the shed RCD properly? If no earth is present, will the RCD fail to trip?

Again. thanks for all advice given.

 

[Steve]

Just dont connect the house earth electrically to anything in the shed. Keep the rod earth and the PME earth seperate. Earth the sheath on the SWA with the house earth as it is likely to be a better earth, being PME. Don't connect the sheath to anything in the shed though, but still be sure to terminate the SWA correctly. You only really need 2 core SWA.