Earthing a metal container

[bernardgreen]

The CPC in the Twin and earth cable is the CPC from the house's wiring. It is NOT the CPC of the container's wiring.

I thought that was clear enough to make the point.

 

[JohnW2]

I thought that was clear enough to make the point.

I must say that I did not find what you wrote to be in any way ambiguous. However, having said that, I knew what you meant (since I had written essentially the same) - so I suppose it's possible that it would not have been as totally clear to someone who didn't know?

Kind Regards, John

 

[EFLImpudence]

 

[Lectrician]

If the feed from the house some how comes into contact with the container or the CU housing the resistance of the earth rod will be too great to allow automatic disconnection to occur without an RCD in the supply.

I concur. It would need the RCD protection, or provide some reinforced protection to limit damage to the cable. With a metal CU, the glands for tails and tail clamps achieve this. As for the container, you would need to protect the cable somehow.

RCD would be the best method.

 

[Spark123]

The supply shouldn't be T&E, even on a catenary. It is not suitable for installation in direct sunlight, nor where there may be movement.
The cable may degrade and is attached to a supposedly metal catenary wire, personally if they refuse to change it I'd run a mile!!

 

[JohnW2]

I concur. It would need the RCD protection, if provide some reinforced protection to limit damage to the cable. With a metal CU, the glands for tails and tail clamps achieve this. As for the container, you would need to protect the cable somehow.

I think I may need some education .... Do I take it that it's just the semantics which are confusing me, since I don't see that the feed cable would, per se, need any protection against the consequences of it "somehow coming in contact with the container" - since the greatest current one could realistically expect to be able to get down a domestic earth rod ('single figure' amps) would do no harm to 10 mm² T+E.

The concern presumably is that contact between L of the feed cable and the container (assuming it was bonded to the TT MET, or even just incidentally earthed) would result in the container (and TT electrode) rising to a high potential relative to true earth, without any protective device operating. Whilst, given adequate bonding, that should not be a hazard to those within the container, it could, of course, be a potential hazard to those touching the container from outside. The greatest worry is probably that such a fault to could persist undetected for a long time - and, as you say, an RCD is probably the simplest way to prevent that.

Am I talking/thinking sense?

Kind Regards, John

 

[Adam_151]

I would TT the supply at the container but this presents an issue with the non-RCD protected feed entering the metal container / metal consumer unit, so the supply would need an upfront 500mA type S RCD at the supply end.

Having attended to a cabin that had become live due to questionable installation practices which allowed exactly that to happen, its an important consideration, especially as the environment in which a site cabin is likely to be situated means it is at perhaps greater risk of damage.

 

[Spark123]

I concur. It would need the RCD protection, if provide some reinforced protection to limit damage to the cable. With a metal CU, the glands for tails and tail clamps achieve this. As for the container, you would need to protect the cable somehow.

I think I may need some education .... Do I take it that it's just the semantics which are confusing me, since I don't see that the feed cable would, per se, need any protection against the consequences of it "somehow coming in contact with the container" - since the greatest current one could realistically expect to be able to get down a domestic earth rod ('single figure' amps) would do no harm to 10 mm² T+E.

The concern presumably is that contact between L of the feed cable and the container (assuming it was bonded to the TT MET, or even just incidentally earthed) would result in the container (and TT electrode) rising to a high potential relative to true earth, without any protective device operating. Whilst, given adequate bonding, that should not be a hazard to those within the container, it could, of course, be a potential hazard to those touching the container from outside. The greatest worry is probably that such a fault to could persist undetected for a long time - and, as you say, an RCD is probably the simplest way to prevent that.

Am I talking/thinking sense?

Kind Regards, John

In an upshot, yes!
You wouldn't want to be trying to enter the container with it at 230v wrt mother earth!!

 

[Lectrician]

I concur. It would need the RCD protection, if provide some reinforced protection to limit damage to the cable. With a metal CU, the glands for tails and tail clamps achieve this. As for the container, you would need to protect the cable somehow.

I think I may need some education .... Do I take it that it's just the semantics which are confusing me, since I don't see that the feed cable would, per se, need any protection against the consequences of it "somehow coming in contact with the container" - since the greatest current one could realistically expect to be able to get down a domestic earth rod ('single figure' amps) would do no harm to 10 mm² T+E.

The concern presumably is that contact between L of the feed cable and the container (assuming it was bonded to the TT MET, or even just incidentally earthed) would result in the container (and TT electrode) rising to a high potential relative to true earth, without any protective device operating. Whilst, given adequate bonding, that should not be a hazard to those within the container, it could, of course, be a potential hazard to those touching the container from outside. The greatest worry is probably that such a fault to could persist undetected for a long time - and, as you say, an RCD is probably the simplest way to prevent that.

Am I talking/thinking sense?

Kind Regards, John

Exactly as your last paragraph. It could sit at or near to 230v indefinitely.

 

[Lectrician]

My if was meant to be or.

 

[JohnW2]

Exactly as your last paragraph. It could sit at or near to 230v indefinitely.

Thanks - so am I right that it was just the semantics of "the need to protect the cable somehow" which was confusing me?

Kind Regards, John

 

[JohnW2]

My if was meant to be or.

It seems that it was "or" by the time I saw it

Kind Regards, John

 

[seasickstevie]

It seems that it was "or" by the time I saw it

It still is

It would need the RCD protection, if provide some reinforced protection ...

There is always the 'Edit' button

 

[JohnW2]

It seems that it was "or" by the time I saw it

It still is

Oh, now that is confusing . I assumed that his 'corrective comments' referred to his post about 2 minutes previously (which did contain an "or" when I saw it - probably always did!), not an earlier post!

Kind Regards, John

 

[Lectrician]

It seems that it was "or" by the time I saw it

It still is

It would need the RCD protection, if provide some reinforced protection ...

There is always the 'Edit' button

Must admit, I thought the edit button was time locked. It won't edit quotes anyway, which gets even more confusing.