Wooden Shed and how to proceed TT or not

[GD16]

I thought the key issue is that what ever earthing system is used if there is not a separate earth connection wire run all the way back by the DNO to their substation transformer you run the risk of lost neutral causing extraneous parts to rise to line voltage or close to it. Any bonded water/gas pipes may end up transferring any earth fault currents to earth in this situation but obviously this must not be relied upon.

Its just a case of the current taking path of least resistance 

Thanks for all your help.

 

[steptoe]

Yes it relies on a PME derived earth you don't export it stops at the service head.

You do NOT export PME , you extend the zone

You cannot export TN-C-S and remain within the confines of it still being a compliant installation without numerous other expensive alterations. 

 

[steptoe]

I thought the key issue is that what ever earthing system is used if there is not a separate earth connection wire run all the way back by the DNO to their substation transformer you run the risk of lost neutral causing extraneous parts to rise to line voltage or close to it. Any bonded water/gas pipes may end up transferring any earth fault currents to earth in this situation but obviously this must not be relied upon.

Its just a case of the current taking path of least resistance 

Thanks for all your help.

The big problem with "exporting" a TN earth is the fact that the general mass of earth may not be at the same potential as the earth that you electrical system presents. 

PME negates this by giving you a general mass of earth at the same potential as your electrical system presents. 

The easiest (IMO) way if doing this is to ensure your TN-C-S system is a pure PME , with localised rods if necessary

 

[binky]

Great thanks to be honest the selectivity issue wont matter practically as the RCBO is in the Main CU only supports the radial circuit to the outside socket on the house currently If it triggers its not going to affect the rest of the house.

So wether the RCBO triggers in the Main CU or the RCD protected circuits in the shed go first it's not going to make any difference to the Shed.

How do you think an inspector will interpret this from the perspective of selectivity and  the requirements of 18th ed?

if you are leaving the RCBO in situ, there's no point in further RCDs in the shed.

 

[GD16]

Hi guys just a quick question take a look at the diagram am I right in thinking the 2.5mm T/E in the house leading to the external wiska box must be protected by the RCBO as its just Twin and Earth.

 

[binky]

if the house cable has been plastered into a wall, then you should leave it. If it's run through a cavity, then you probably don't need it. You don't need to RCD a submain

 

[GD16]

Great thanks Binky, I need it to be right as its going to be inspected by building control later on.

I will use the current 20A RCBO to protect the whole sub-main and just use MCB's in the garage CU that way everything should be protected and no selectivity issues.

Buy for now

 

[Murdoch]

I have to confess but a TT rod in such a set up seems rather pointless .........

And do you have a tester to properly test a TT earth?

 

[GD16]

Hi Yeah I have a tester . I'm using a KT66 with the earth spike test pack.

The other option I suppose I would have is to change the diameter of the SWA to one that meets the regs for CPC CSA size.

 

[steptoe]

Hi Yeah I have a tester . I'm using a KT66 with the earth spike test pack.

The other option I suppose I would have is to change the diameter of the SWA to one that meets the regs for CPC CSA size.

I'm not familiar with that tester, 

Can it test Ra as well as Ze/Zs ? 

Will you need to change the SWA,? Have you calculated the armour csa? 

 

[GD16]

Ignore my last comment I don't need to worry about increasing the the CSA of the earth conductor, (its 4mm 3 core SWA currently) as there are no extraneous parts in the shed  so no need to worry about the combined bonding and CPC size requirement. 

Perhaps your right TT system is a bit of an over kill in this situation but I wasn't sure if its better to try to protect from a possible loss of neutral especially as there are metal light switches currently installed in the shed.

I see some have suggested just bringing the CPC to the shed and also using an earth rod to give additional protection in that eventuality however rare.

It seems there is a lot of different opinions on this.