Converting to a TT system... possibly

[chris day]

1.25ohm external earth fault loop impedance on TN-S supply to building

Supply authority stated our only option is a TT conversion as they will not covert to PME.

What would be my best course of action when converting to a TT as the building is quite old and i do not want a 100ma main switch for diversity reasons and nuisance tripping, so Would it be suitable to convert to TT and have all final circuits in a RCBO with a board charge?

 

[Strima]

What is this SDS adapter as i normally use a club hammer?

 

[Midwest]

View attachment 41179

I bought one of them ages ago, never used it in anger yet. Interestingly, mine hasn't got a hole at the end

 

[DPG]

There's a novelty.

 

[Strima]

I bought one of them ages ago, never used it in anger yet. Interestingly, mine hasn't got a hole at the end

I used it on my SDS max and it saves loads of effort.

 

[DPG]

I haven't used mine in a while. It does have a hole in the end though. Can get good depth with it.

 

[mhar]

I screw an earth rod driving bolt 5/8 into a rods coupler, then use an sds nut driver with the appropriate size socket, drill on hammer and rotostop. Don't expect to be able to use the nut driver and socket for anything else though

 

[The Ghost]

The DNO are obliged to maintain the earth they have there REG 25 ESQCR. Tell them you will take it up with HSE if they will not. They can put a steel band around the intake cable and braided earth from that. There is no excuse for them not to comply. Kick some arse!

 

[Nigel]

The DNO are obliged to maintain the earth they have there REG 25 ESQCR. Tell them you will take it up with HSE if they will not. They can put a steel band around the intake cable and braided earth from that. There is no excuse for them not to comply. Kick some arse!

The earth is in place and functioning.

 

[sparksburnout]

Surely they should come out and sort their earth out.

Correct. Here we go again, the fog is descending. If it's a TNS supply, then get the DNO out to sort it out, end of story. Next?

 

[sparksburnout]

The earth is in place and functioning.

1.25ohm external earth fault loop impedance on TN-S supply to building is functioning then is it?

 

[Nigel]

1.25ohm external earth fault loop impedance on TN-S supply to building is functioning then is it?

Yes it is. May not be optimal but most likely all OCPDs will not exceed their maximum permitted ao what is the issue?

 

[sparksburnout]

Yes it is. May not be optimal but most likely all OCPDs will not exceed their maximum permitted ao what is the issue?

Max value for TNS is 0.85 off top off my head. That's what DNO's have to work to. Either it is, or it isn't.

Yes it is. May not be optimal but most likely all OCPDs will not exceed their maximum permitted ao what is the issue?

Incorrect. It's well out of spec for a TNS system. Simple. Argue as much as you like, it's out of spec.

 

[Ian1981]

1.37 ohms max zs for a32 amp type B so unless your R1+R2 is ridiculously low then your max zs for this circuit will be exceeded.
I do not know what overcurrent protection is involved in this instance nore no of any rcd protection but no one in their right mind should except that ze measurement on that particular earthing arrangement.

 

[Ian1981]

I who hazard a guess that the DNO fuse would also not disconnect in the required time 5seconds if there was an live to earth fault on the tails.
Same statement could be said about a TT arrangement, however it’s not a TT arrangement at present.

 

[The Ghost]

The earth is in place and functioning.

So what is your Zs on say the RFC?

1.37 ohms max zs for a32 amp type B so unless

With the 80% rule of thumb? 1.06 so would not meet required disconnection time as already alluded to.