Electrical Earth in Observatory

[mbhyde]

Ok just to recap.  What you are saying is (?)

1. The SWA metallic sheath is Earthed at the house end.

2. The SWA metallic sheath at the obs end is NOT earthed there - can be terminated on the plastic of the PVC consumer unit for neatness

3. The Earth for the plugs etc INSIDE the Obs does not use the Earth fed from the house but uses a "local" Earth inside the Obs which is provided by means of a Earth Rod.

Assuming [3] is true why is 3 core needed to be run from the house then?  Why not just run Postive and neutral?

1 yes that is right

2 yes that is right

3 earth rod out side the obs in ground about 1m down and 2 core is will do if you use 3 core the earth is terminated with sheath at obs end

[JamesF]

Just for my own understanding, if you have equipotential bonding that means that any exposed conductor should hopefully be at the "house earth" potential as I understand it.  Does it mean though that it need not necessarily be at the "local earth" potential -- let's say if you live in a long narrow house with the earth at one end and you're at the other?  And therefore there's still a risk if the house earth through some fault becomes significantly higher potential than the local earth?

James

[Paul M]

Just for my own understanding, if you have equipotential bonding that means that any exposed conductor should hopefully be at the "house earth" potential as I understand it.  Does it mean though that it need not necessarily be at the "local earth" potential -- let's say if you live in a long narrow house with the earth at one end and you're at the other?  And therefore there's still a risk if the house earth through some fault becomes significantly higher potential than the local earth?

James

That's right. So your copper water pipes are at the same potential as the chassis of the washing machine. Still need to be careful on the ground floor of an old stone floor property. There is a good chance that the kitchen floor is at true earth potential whilst your equipotential zone might be at some other potential.

It's a huge subject. But like I say, RCD's are now seen as a capture-all solution to many potential electrical defects ans faults!

[JamesF]

That makes sense to me then.

I dread to think what our house is like.  It was completely re-wired thirty years ago, but the regs have changed a huge amount since then.  I don't know if the CUs in the outbuildings would be considered "safe" these days, but even with my limited understanding I'd imagine not.

James

[kirkster501]

1 yes that is right

2 yes that is right

3 earth rod out side the obs in ground about 1m down and 2 core is will do if you use 3 core the earth is terminated with sheath at obs end

Thanks mbhyde.  I don't quite get what you said in point three.  Sure about the Earth Rod bit. But you're saying that if I were to use three core then the Earth of the 3-core is terminated with the sheath at the obs end so does not actually "connect" to anything in the obs because the Earth of the Obs (inside the obs consumer unit) is provided locally by the Earth rod? I think that is what you are saying?  So the only true Earth anywhere near the obs is local to the obs via the Earth rod?  The sheath and the Earth of the 3-wire is joined together at the obs end but is actually truly earthed back at the house end?  I think that is what you've said?  In other words, at the Obs end the Earth rod Earth supplying the Earth to the obs' sockets does not come into contact with the sheath and internal Earth of the SWA (those two being bonded at the obs end but earthed at the house end?).

Thanks for your advice.

Steve