Part P assessment question

[markadams]

One of the questions on a recent Part P assessment was how would I wire an external outbuilding if the earthing arrangement to the property was PME.

I said I would run a three core SWA from the property to the outbuilding and use the cable armour and the third core as the exported CPC.

I was highly criticised and was told that I should install a rod at the property and at the outbuilding.

I can find no mention of this in 17th ed AMD3 book, can anyone tell me where i would find this if it is the case.

I have had a couple of other Assessments previously where they have checked out building installs and never mentioned this so I am not happy.

 

[Wilko]

On a 30ma RCD ?

Hi Dave - the fault I'm thinking of is loss of N on the DNO side of the cut out. So with TNCS the local N and E are still connected, but floating now. Initially the kettle will not carry any current, as N is dis. But when it's earthed metal casing is touched by me, I provide a path for current flow. This current travels via L and N and the element (so the RCD is still happy) and back to cutout, onto installation E and circuit cpc back to the kettle and me ... Ugh.

 

[Deleted member 26818]

Hi Dave - the fault I'm thinking of is loss of N on the DNO side of the cut out. So with TNCS the local N and E are still connected, but floating now. Initially the kettle will not carry any current, as N is dis. But when it's earthed metal casing is touched by me, I provide a path for current flow. This current travels via L and N and the element (so the RCD is still happy) and back to cutout, onto installation E and circuit cpc back to the kettle and me ... Ugh.

If there is bonding in place to metallic services (not plastic) current will still flow through the kettle.
Reason for the proposed requirement for consumer earth electrodes, is because of the increase in plastic services.

 

[Ian1981]

If there is bonding in place to metallic services (not plastic) current will still flow through the kettle.
Reason for the proposed requirement for consumer earth electrodes, is because of the increase in plastic services.

Is it?
Not because of open circuit PEN conductors?

 

[Deleted member 26818]

Of course it’s because of open PEN conductors.
Not been a real problem though because of bonding and metallic services.
Now though, as more and more of our services are plastic, bonding is pretty much pointless.

 

[Toneyz]

How often does this open circuit happen?

 

[Deleted member 26818]

How often does this open circuit happen?

Probably more than they’d like to admit.
There are supposed to be systems in place to cut power in the event of a lost neutral.
Of course with everything being put out to tender, there’s no guarantee the people maintaining the Network know what they’re doing.

 

[telectrix]

All the streetlighting in Cornwall is on PME.

well bugger me. i thought it was all on gas.

 

[Dave OCD]

well bugger me. i thought it was all on gas.

Oh very droll, it was converted from gas at least a fortnight ago.

 

[telectrix]

Oh very droll, it was converted from gas at least a fortnight ago.

another lamp lighter on the dole then.

 

[ruston]

another lamp lighter on the dole then.

Na , they will have given him a new post.