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?

 

[sparksburnout]

He would usually make a derogatory comment about landlords
No offence intended Burnsy!

Naughty, naughty.

 

[Ian1981]

My point is that you stated that not meeting maz permitted Zs values was grounds enough to go back to the DNO.

I am just pointing out that that is rubbish.

never said anything of the sort!
Your design would be a non starter.
We are talking about a hypothetical ze of 0.85 you have brought to a scenario not in relation with the OP ze problem.
Stop looking for confrontation, if you want that go join Twitter and spout aload of nonsense to complete strangers.

 

[Ian1981]

Duplicate post

 

[Nigel]

never said anything of the sort!
Your design would be a non starter.
We are talking about a hypothetical ze of 0.85 you have brought to a scenario not in relation with the OP ze problem.
Stop looking for confrontation, if you want that go join Twitter and spout aload of nonsense to complete strangers.

Ok - you don't seem to understand. That is fine. We shall move on.

 

[Wilko]

... temperature correction ... will it ever reach 70 degrees? In most cases probably not.

I normally wouldn't apply the temperature correction to the suppliers network for this reason.

 

[Wilko]

I would be constantly checking the Ze on the existing system to see if it falters and check any neighbouring premises for comparison.

I agree. Ze being outside their own target and DNO not fixing means it's only going to get worse. The installation should not be left in this state.
On the plus, SSE recently improved my home TNS Ze from 0.6 to 0.5 - I noticed a new (bigger) transformer has been installed locally and no doubt they've cleaned everything up. Proper planned preventative maintenance .

 

[mhar]

Getting back to basics, is it a DNO supplied earth or does it look home made?
If it is DNO supplied, how did you report the issue? As an earthing enquiry or as an earthing fault? Their regulator has targets for both but but they are dealt with differently.
If DNO supplied I would maybe re-report it as a fault under ESQCR.

 

[Midwest]

Think the OP is waiting for the dust to settle.

 

[The Ghost]

So when you say building, what kind of building it? Domestic or ...?
I think what I am trying to say and has been alluded to, if you have a Ze of 1.25 ohms and there is say an RFC which will require a Zs of 1.37 that will give you a wapping 0.12 ohms for compliance with disconnection times via an MCB for OCP on your Zs.

 

[westward10]

I agree. Ze being outside their own target and DNO not fixing means it's only going to get worse. The installation should not be left in this state.
On the plus, SSE recently improved my home TNS Ze from 0.6 to 0.5 - I noticed a new (bigger) transformer has been installed locally and no doubt they've cleaned everything up. Proper planned preventative maintenance .

That Ze maybe perfectly stable and as I said before those max values are not carved in stone.

 

[Nigel]

So when you say building, what kind of building it? Domestic or ...?
I think what I am trying to say and has been alluded to, if you have a Ze of 1.25 ohms and there is say an RFC which will require a Zs of 1.37 that will give you a wapping 0.12 ohms for compliance with disconnection times via an MCB for OCP on your Zs.

This is when you need to incorporate further protection into your design to provide fault protection. We have no control over the earth provided and if the DNO state that is what you are getting then you need to factor in additional protection.

In this instance I would move all the circuits that could not achieve the maximum permitted Zs value onto a new DB and protect this DB with a 500mA RCD with 30mA RCDs DP protection for any circuits that need it additionally.

This would be a hell of lot more economical than making the system a TT and doing away with a stable earth in favour of an unstable earth.

 

[Ian1981]

I normally wouldn't apply the temperature correction to the suppliers network for this reason.

No but the regulations always work to the worst case scenario so I always stick to the maximum zs stated in appendix 3 but when testing at site temperature, it is taken into account.

 

[chris day]

Hello Dave I suppose i would then need to ensure all final circuits have 30ma protection as my only concern is the entire building turning off with an earth fault.

 

[chris day]

This would be a hell of lot more economical than making the system a TT and doing away with a stable earth in favour of an unstable earth.[/QUOTE]

hmm yes Essex i think this would be the best way to go, as long as all final circuits will meet the required disconnection time then we have no issues.
I think converting to a TT is money for old rope, would the 500ma downstream RCD just to for backup and to protect the tails?

 

[chris day]

That Ze maybe perfectly stable and as I said before those max values are not carved in stone.

The building is a small hut in the middle of a park which has a few ring final circuits for heaters and lighting etc, the Ze is stable, give or take 0.05 over 3 months.
I think from what everyone has said i may stick with the DNO 1.25Ze and RCBO everything?