Earthing Generator TNCS

[Stuart Houghton]

My customer wants to have one ring main connected via a change over switch to a generator.

Earthing is TNCS

My idea was is move the ring main to a new consumer unit run a 6mm T&E to the change over switch from the 32 amp mcb in the main C/U where the ring main was. Then a 6mm T&E back from the change over switch to the new C/U.

I would install an earth rod which would be connected to the new C/U so the new C/U would have two earths one from the existing C/U and one from the earth rod.

The house is in a village with lots of overhead wires and I just thought what would happen if the neutral was brought down by a tractor then the return current that would normally go down the neutral of all of the houses after the break would be back fed through my customers C/U through the ring mains T&E and to ground via the earth rod. This could easily be a very high current which would surely melt the earth wires of the ring main.

Am I missing something ?

 

[Strima]

Why just one ring?

 

[davesparks]

How would the diverted neutral current flow via the cpc of the ring?

 

[Stuart Houghton]

How would the diverted neutral current flow via the cpc of the ring?

it would flow down the neutral after the break into my customers house at which point the neutral and earth are linked together, then to the existing C/U then to the CPC of the ring which is linked to the earth rod.

Hope that make sense !

 

[Stuart Houghton]

Why just one ring?

to keep it simple it would be the ring that supplies the fridge and central heating

 

[buzzlightyear]

The house is in a village with lots of overhead wires

try TT system.

 

[buzzlightyear]

to keep it simple it would be the ring that supplies the fridge and central heating

and what type of generator is this ?

 

[Stuart Houghton]

try TT system.

Not sure what you mean. I need to provide an earth which I am doing using an earth rod and the house is TNCS. I don't think I can switch the Earths as part of the change over switch so the TNCS will be connected to the earth rod as well. The generator will be a small 2kw portable type.

 

[buzzlightyear]

so you are using the earths separate ?

 

[Stuart Houghton]

so you are using the earth separate ?

Sorry still don't understand what you mean. The earthing will be be 6mm from existing consumer unit to new consumer unit then 6mm from new consumer unit to the earth rod. That way its earthed when there is a supply from the grid and earthed via an earth rod if the grid is completely lost. Grid connected it will be TNCS, generator connected it will be TT.

 

[buzzlightyear]

Sorry still don't understand what you mean. The earthing will be be 6mm from existing consumer unit to new consumer unit then 6mm from new consumer unit to the earth rod. That way its earthed when there is a supply from the grid and earthed via an earth rod if the grid is completely lost. Grid connected it will be TNCS, generator connected it will be TT.

now the I get the story.

 

[buzzlightyear]

My idea was is move the ring main to a new consumer unit run a 6mm T&E to the change over switch from the 32 amp mcb in the main C/U where the ring main was. Then a 6mm T&E back from the change over switch to the new C/U.

I would install an earth rod which would be connected to the new C/U so the new C/U would have two earths one from the existing C/U and one from the earth rod.

The house is in a village with lots of overhead wires and I just thought what would happen if the neutral was brought down by a tractor then the return current that would normally go down the neutral of all of the houses after the break would be back fed through my customers C/U through the ring mains T&E and to ground via the earth rod. This could easily be a very high current which would surely melt the earth wires of the ring main.

Sorry still don't understand what you mean. The earthing will be be 6mm from existing consumer unit to new consumer unit then 6mm from new consumer unit to the earth rod. That way its earthed when there is a supply from the grid and earthed via an earth rod if the grid is completely lost. Grid connected it will be TNCS, generator connected it will be TT.

Am I missing something ?

Sorry still don't understand what you mean. The earthing will be be 6mm from existing consumer unit to new consumer unit then 6mm from new consumer unit to the earth rod. That way its earthed when there is a supply from the grid and earthed via an earth rod if the grid is completely lost. Grid connected it will be TNCS, generator connected it will be TT.

 

[buzzlightyear]

The house is in a village with lots of overhead wires

I was meanly asking a question about the over head wires if it could be TT.

 

[davesparks]

it would flow down the neutral after the break into my customers house at which point the neutral and earth are linked together, then to the existing C/U then to the CPC of the ring which is linked to the earth rod.

Hope that make sense !

But that wouldn’t flow through the cpc of the ring circuit, it would flow via the earthing conductor and cpc to the changeover switch, then to the essential services CU, then to the earth electrode.

Whatever position the changeover switch is in, as you have described it, the route from the N-E link in the cutout to the earth electrode will always be via the 6mm T&E’s rather than via any final circuit.

 

[davesparks]

Grid connected it will be TNCS, generator connected it will be TT.

Why on earth would you make a generator connection TT? That’s just madness!
A generator at LV should be connected TNS or IT, TT would be ridiculous!

 

[buzzlightyear]

PAGE 153/10.5.5 in guidance note 8

 

[Risteard]

TT would be ridiculous!

And would require another earth electrode!

 

[Deleted member 26818]

Is the house you are working on fed via overhead wires?

 

[davesparks]

The generator will be a small 2kw portable type.

So what’s the earth electrode going to achieve with that? 2kW portable generators don’t generally have a N-E link.

 

[Deleted member 105166]

Not applicable here, but on a larger standby set, where there is an N-E link, am I correct in thinking the generator earth should be connected to the MET of the installation? (as there will be times when both genset and DNO supplies are energised concurrently, e.g. after the DNO supply is restored following an outage, prior to the AMF changeover switch reverting to the DNO supply)