What would your method be?

[comms]

Morning all,
What would you do, decisions decisions.

Customer wants power out to a summer house.
He has purchased most of the materials already (2core swa 10mm sockets and conduit) and wants a CU supplied & installed in the outbuilding for lights + sso's.

Now the house is TT converted to PME (no pme label) no spareways on cu & he doesn't want the cu changed (place is minging so I'm chuffed with that) & the outbuilding is 12mtrs away.
I was planning on using henlys to split the 25mm tails, in to a hager switch fuse, run 10mm swa from switchfuse to outbuilding into 2 way cu with 30mA rcd as main switch & TT the outbuilding.

Now I'm doubting myself & thinking maybe RCD at the switch fuse ?
Or
Bin his 2 core put in 3 core and export the pme.

 

[bernardgreen]

For a summer house with lots of opportunity for contact with the ground my feeling would be to have the earth of the summer house as close to ground as possible. So my prefered would be TT with a good ground rod.

But as back up I would earth the armour of the SWA at both ends.

In effect exporting the PME earth to be parallel with the ground rod, this would tend to ensuring safety if the earth rod fails ( due to dry weather conditions )

 

[didthathurt]

I'd not put an RCD at the switchfuse, you're only protecting against the effects of cable damage and the Ze of a PME is so low that any L to E/N fault in the SWA will pop the breaker. I, personally, would earth through the SWA not a TT. Even the best TT Ze I've seen (sub 10 ohms) is far less effective than a PME. Using the SWA for the PME earth will still give the best EFLI (provided, of course, the min earth csa requirements are met which I think, off the top of my head, they will be with 10mm SWA).

 

[Spark123]

If you connect a PME to an earth electrode the size of the earth cable/armour needs to be taken into account. I think there is a bit in Guidance Note 5 regarding this.
I'd either use PME all the way through, or TT the summerhouse end but I wouldn't advise to try and mix the two together. Are there any extraneous conductive parts in the summerhouse?

 

[securespark]

I know that in theory at least, the maximum Ze is 0.35. However, TT conversions are often higher.

What is the Ze here?

 

[comms]

I know that in theory at least, the maximum Ze is 0.35. However, TT conversions are often higher.

What is the Ze here?

Ze of 0.12ohm

Didn't like the idea of mixing pme with rod.
Thought I'd use pme to swa armour from supply end & terminate armour into plastic adaptable box at summer house then TT the summer house cu.

Spark123 no extraneous conductive parts just an empty space mate

 

[scousespark]

Didn't like the idea of mixing pme with rod.
Thought I'd use pme to swa armour from supply end & terminate armour into plastic adaptable box at summer house then TT the summer house cu.

Use a metal adaptable box. SWA tends to break plastic, and you also need to earth the armour and you can't do this with plastic.

Also have a look at Wiring matters Autumn 2005 (IEE Site) which has an article describing their recommended method of doing what you want to do. Just in case mate, this is notifiable

 

[Pensdown]

Comms, as the Summer house is made of timber and is only 11 mtrs outside the PME zone what are you hoping to achieve by making it a stand alone TT installation?

 

[Spark123]

Spark123 no extraneous conductive parts just an empty space mate

In which case it is OK to export the PME earth without the issues of main equipotential bonding in the summerhouse.
Some useful info here: IET Wiring Matters

 

[bernardgreen]

Some useful info here: IET Wiring Matters

In the example of the garage with a water pipe : - If the PME earth is extended to the garage and bonded to the water pipe which is metal and goes into the ground is this not the same as an earth rod connected to the PME earth ?

 

[bernardgreen]

Spark123 no extraneous conductive parts just an empty space mate

But in damp weather a wooden floor on a concrete base can have a significant conduction to the ground. As can the single skin timber roof with an indoor ceiling rose fitted to it.

Especially where the screw fixing the rose to the ceiling has gone through timber and felt and as such is an extraneous conductive metal part.

An earthed bonded metal disk between rose and timber ensures the damp timber does not become live when an IP rated fitting is not acceptable.

 

[Spark123]

Some useful info here: IET Wiring Matters

In the example of the garage with a water pipe : - If the PME earth is extended to the garage and bonded to the water pipe which is metal and goes into the ground is this not the same as an earth rod connected to the PME earth ?

Similar yes, installing an earth rod would introduce un-necessary work and mean the supply earth size needs to be increased to suit PME conditions with main equipotential bonding.
With a metallic water pipe the reason for MEB is to ensure that all ECPs in the premises remain at or about the same potential. Current can also flow through these bonds from the PEN conductor to earth hence the minimum sizing specified by the DNO. Installing an electrode will have the same effect with the current from the PEN hence all the earthing/bonding conductors need to be sized accordingly.
If there are no extraneous conductive parts then you don't have the problem with the currents from the PEN conductor hence the earthing/bonding only really needs to be sized for automatic disconnection of supply.

 

[Spark123]

Spark123 no extraneous conductive parts just an empty space mate

But in damp weather a wooden floor on a concrete base can have a significant conduction to the ground. As can the single skin timber roof with an indoor ceiling rose fitted to it.

Especially where the screw fixing the rose to the ceiling has gone through timber and felt and as such is an extraneous conductive metal part.

An earthed bonded metal disk between rose and timber ensures the damp timber does not become live when an IP rated fitting is not acceptable.

The risks associated with PME only really come to light where there is a reduced body resistance or where the person is in good contact with earth.
Most of the items in GN5 re PME refer to special locations, a majority being where the body resistance is reduced.
The screw through a rose/felt doesn't strike me as an extraneous conductive part, you might get water coming in through your light though .