Outbuilding Earthing

[EFLImpudence]

That don't make it right.

 

[JohnW2]

That don't make it right.

Indeed - it doesn't necessarily make it right.

More to the point, I don't think that the implication of the suggestion is even correct. Although, assuming it sticks to draft, the 18th will require all installations to have a local earth electrode (even if TN), I presume that would normally be close to the origin of the installation and I am not aware of any suggestion that there would be a requirement for a further earth electrode for an outbuilding which was part of the installation.

If that is correct, then the issues and various views about the earthing of outbuildings will presumably remain essentially the same as at present. In other words, even if there is an additional earth electrode close to the origin of the house's installation, I imagine that one will still have to think of a TN-C-S system as being TN-C-S when considering issues of outbuilding earthing.

Kind Regards, John

 

[ericmark]

If the earth rod is on the short side so around the 100Ω mark with 10 mm² cable i.e. smaller than the supply cable neutral and close to the head it will simply enhance the DNO multiply earths and be no real harm.

The problem I saw was when 4 earth rods around 3 meters in length were sunk in each corner of the garden, and in a 750 mm deep trench there was bare copper strip joining them so the total resistance was likely less than the DNO earth pit, less than one ohm. Other than radio hams I think a earth like this is very rare.

 

[JohnW2]

If the earth rod is on the short side so around the 100Ω mark with 10 mm² cable i.e. smaller than the supply cable neutral and close to the head it will simply enhance the DNO multiply earths and be no real harm.

Quite.

The problem I saw was when 4 earth rods around 3 meters in length were sunk in each corner of the garden, and in a 750 mm deep trench there was bare copper strip joining them so the total resistance was likely less than the DNO earth pit, less than one ohm. Other than radio hams I think a earth like this is very rare.

Indeed - and I would suggest so rare as to be of virtually no relevance to domestic (or even most commercial) electrical installations.

I don't even see why this necessarily needs to be an issue in the case of radio hams, since the 'enhanced earths' they may sometimes wish to have can usually be restricted to the RF circuitry and isolated from the electrical installation's 'earth'. Indeed, in the distant past I have used 'ground planes' (constructed using insulated conductors), and they have not even been in electrical continuity with 'the earth'.

Kind Regards, John

 

[mwatsonxx]

I've seen steel-armoured distribution cables where the steel has rusted so much it has broken apart and hangs in limp segments around the core.

Tested by time, and failed.

My tenner says that was steel tape armouring, not SWA

 

[mwatsonxx]

In this situation there is no need for an earth rod, unless you deliberately want to connect to dangerous stray currents and introduce them into the shed and the house.

 

[bernardgreen]

My tenner says that was steel tape armouring, not SWA

I can think of at least two cables where the SWA ( not tape ) had rusted away.....

In this situation there is no need for an earth rod, unless you deliberately want to connect to dangerous stray currents and intruduce them into the shed and the house.

If the shed is wired as a TT then the Earthing of the shed would not introduce "dangerous stray currents" into the house.

Did you really mean stray currents ? Stray currents are only dangerous if they are large enough to heat cable beyond a safe operating temperature.

Stray voltages can be dangerous if they are able to drive currents up to dangerous levels through cables or people

 

[ericmark]

My tenner says that was steel tape armouring, not SWA

I have seen SWA fail, and I know one of the jobs done with street lighting was the so called pressure test, where the resistance of the outer cables to earth was measured and logged so after road works it could be retested and contractors fined if they damaged the outer covering of the cable. Be it rust or electrolysis cables with the outer covering damaged can fail, so they need testing on a regular basis, with commercial where the results are logged all well and good. However with domestic measuring is not so common and logging the readings even less common, having a means to disconnect the earth to test is also a problem.

So down to the old risk assessment, the risk of SWA failing is far higher than an inner copper core, so using a copper core even in 25 years time it is unlikely to have failed, where using SWA even in 10 years time which is the recommended time between EICR there is too great of a risk that SWA will have failed.