Replacing Main Earthing Conductor

[Clink]

This may be a newbies question but the older and wiser (or at least the more experienced) heads out there may be able to help....

Just inspected a clients house and amongst the, many, problems it only has a 6mm Main Earthing Conductor. I intend to upgrade it to 16mm but I can't see how to access the earth connector in the supplier's service head. Do I really need to contact the DNO to upgrade an earth? This seems excessive and possibly expensive.

 

[Richard Burns]

The earth cable from the DNO head to the main earth terminal is the DNOs property and should not be changed by you.
You could ring the DNO and say that their earthing conductor is not suitable for the load but I would check this carefully with the adiabatic equation before you ring other wise you would be charged .

 

[Clink]

Thanks Richard, that's confirmed what I thought, I'll go back and check the sums but I'm pretty sure that they are correct. It does make it potentially an expensive bit of cable though.

 

[Lew0125]

Just so you know. Underneath the main fuse, is a flat head screw undo that until you can slide the cover to the right. There will be the earth connections.

 

[Knobhead]

If it shows no sign of stress, leave it alone.

 

[Deleted member 26818]

What is the earthing system?

 

[Engineer54]

What is the earthing system?

If the earthing conductor is entering the Service head, it's pretty sure that it's a PME connection...

 

[Deleted member 26818]

Sorry Engineer54, but making the assumtion that it is TN-C-S just because the earth conductor enters the head is not really enough.
Many heads require that a link be connected inside the head to connect the neutral and earth.
It may be that this is a conversion that took place when 6mm² was the requirement, or may be that it is a TN-S system where 6mm² is still the correct CSA.

 

[dagrat]

As above, do the adiabatic. you will probably find that 6mm is adequate. If so leave it in

 

[Hiddo]

Have you measured the ZE? That's what the DNO will do and if they find the reading to be acceptable then they will say its OK.

 

[Engineer54]

Sorry Engineer54, but making the assumtion that it is TN-C-S just because the earth conductor enters the head is not really enough.
Many heads require that a link be connected inside the head to connect the neutral and earth.
It may be that this is a conversion that took place when 6mm² was the requirement, or may be that it is a TN-S system where 6mm² is still the correct CSA.

You have far more experience than i Spin, on the types of Service heads and connection methods used by the DNO'S in the UK. Just that i have never seen a TN-S connection back home, that wasn't taken from the cable sheath below the service head. Even on a conversion, the sheath connection should be taken off/discontinued and the bare area insulated with amalgamating tape or similar...

 

[Deleted member 26818]

They used to use (and perhaps still do in some cases) a split concentric cable, where one half of the outer sheath was the neutral and the other half the earth.
These cables were often made off in the head, and unless the link between the neutral and earth was made, they were TN-S, but without removing the cover, there was no way to tell.

 

[polo1]

^^^^Yep, we have lots of this arrangement in my area. Check the PSCC and PEFC, if they are virtually identical, it's most likely TN-C-S (but with a 6mm earth, it's probably TN-S!)

Regards

 

[HandySparks]

They used to use (and perhaps still do in some cases) a split concentric cable, where one half of the outer sheath was the neutral and the other half the earth.
These cables were often made off in the head, and unless the link between the neutral and earth was made, they were TN-S, but without removing the cover, there was no way to tell.

Like this:

 

[Deleted member 26818]

That is similar.
However this is the type I was thinking of:

 

[Engineer54]

They used to use (and perhaps still do in some cases) a split concentric cable, where one half of the outer sheath was the neutral and the other half the earth.
These cables were often made off in the head, and unless the link between the neutral and earth was made, they were TN-S, but without removing the cover, there was no way to tell.

Yep, i can see now what your saying!! Completely forgot about the split concentric type of cable... lol!!

 

[Clink]

Thanks guys, sorry for the slow response to questions, but yes it is TN-C-S and yes the Ze is in limits. My initial reaction was to replace it but having looked a bit further into the figures and there are no signs of deterioration I'm going to leave well alone.

 

[T&Earth]

Hi Dagrat

As above, do the adiabatic. you will probably find that 6mm is adequate. If so leave it in

When doing the adiadatic for say earthing & main bonding conductors where do you take the PEFC from? On a final circuit you would take it between Mcb Line & Earth.

Any advice would be appricaited

Many Thanks

 

[Richard Burns]

Hi Dagrat


When doing the adiabatic for say earthing & main bonding conductors where do you take the PEFC from? On a final circuit you would take it between Mcb Line & Earth.

Any advice would be appreciated

Many Thanks

The adiabatic equation does not apply to main bonding conductors, they are sized from the size of the incoming neutral for PME supplies or by the size required for the earthing conductor on non PME supplies.

For any installation the highest prospective fault current is found at the supply source. So PEFC is measured between the incoming line to the MET.
Anywhere else in the installation the fault current will drop because of the resistance of the wiring so work with the worst case (at the source).

 

[Deleted member 26818]

Hi Dagrat


When doing the adiadatic for say earthing & main bonding conductors where do you take the PEFC from? On a final circuit you would take it between Mcb Line & Earth.

Any advice would be appricaited

Many Thanks

The PEFC would be determined when measuring Ze/Zs at the origin of the installation.