Exporting an earth xD

[Newtothis]

I know this is going to cause madness but how would this work exporting the earth is driving me nuts i feel like its a hard one to understand

for example a house has a tncs and it supplies a metal shed that is a TT system so the earth has not been exported.

the shed is 30m from the house

the house has a metal fence that runs from the house to the shed a class one light is on the fence at the house end supplied by the tncs

and at the shed end a light is also connected but supplied by the TT system

would that not link the two types of earthing systems?

 

[telectrix]

easy solution. isolate the light at the shed end from the fence , using non-conductive fixings.

 

[Pretty Mouth]

If you have continuous metal between the 2 lights, then the 2 earthing systems will indeed be connected. Class II fittings, or keep both lights on the same earthing system, perhaps supplied from the shed end.

 

[Julie.]

It depends, just because the luminares are class 1 it doesn't necessarily mean that contact of the earth/cpc with the fence is made.

Many fittings have insulated housings, whilst the main part is metal and connected to cpc.

You need to either physically check, or measure.

The one of great concern is the one connecting the fence to the house cpc, if it is connected, it in effect is bonding the met to an extraneous conductive part, which typically needs to be 10mm^2, not 1.5mm^2 or whatever it is currently.

This is assuming the fence goes into the ground, if it's isolated from ground then the cpc size is not so important

 

[telectrix]

This is assuming the fence goes into the ground,

jusrt trying to visualise a metal fence not going into the ground but hovering above it visions of Dynamo the magician.

 

[Julie.]

This is assuming the fence goes into the ground,

jusrt trying to visualise a metal fence not going into the ground but hovering above it visions of Dynamo the magician.

Metal trellis atop a wooden fence?

 

[telectrix]

or plastic fence posts. beats the corrosion issue.

 

[Wilko]

Hi - as per @Julie. ... at the house end do whatever is required (change the light fitting?) so that house CPC does not get connected to the metal fence.

 

[LastManOnline]

I know this is going to cause madness

Getting used to "madness" is unfortunately a requirement for electricians. The situation you describe is a perfect example. Many electricians on this forum can remember a, time when we did, nt did, nt have to cope with the type of problem you describe. However, cost won out over quality of service and so TNC-S was born. We are where we are.
Previous posts have mentioned some practical solutions. One other possibility is to put the fence light on with the shed supply and have it operated by a sensor.

 

[LastManOnline]

It depends, just because the luminares are class 1 it doesn't necessarily mean that contact of the earth/cpc with the fence is made.

Many fittings have insulated housings, whilst the main part is metal and connected to cpc.

You need to either physically check, or measure.

The one of great concern is the one connecting the fence to the house cpc, if it is connected, it in effect is bonding the met to an extraneous conductive part, which typically needs to be 10mm^2, not 1.5mm^2 or whatever it is currently.

This is assuming the fence goes into the ground, if it's isolated from ground then the cpc size is not so important

Even if it is connected to ground its worth measuring whether it has a high or low resistance to ground. An extraneous part with a high resistance won't require a 10mm strictly speaking as it can't import any currents that would have damaging thermal effects