Three Phase PME

[stevesey]

Hi

I've just looked at a three phase PME (labelled) system where there is no earth connection to the neutral at the meter or 100A fuses.

Am I right that this is incorrect?? (and should be fixed ASAP).

The supply used to be overhead but was converted to underground about 5 years ago (and I assume was PME'd at this point). There seems to be an earth wire going outside to and earthing rod, but I can't trace it and anyway it's no where beefy enough for current regs (and I think should be disconnected for PME?)

Steve

N.B There are no genuine three phase loads connected, just (approx) balanced single phase circuits.

 

[andy]

Hi

I've just looked at a three phase PME (labelled) system where there is no earth connection to the neutral at the meter or 100A fuses.

Am I right that this is incorrect?? (and should be fixed ASAP).

The supply used to be overhead but was converted to underground about 5 years ago (and I assume was PME'd at this point). There seems to be an earth wire going outside to and earthing rod, but I can't trace it and anyway it's no where beefy enough for current regs (and I think should be disconnected for PME?)

Steve

N.B There are no genuine three phase loads connected, just (approx) balanced single phase circuits.

if it is PME there there should be a MET next to the cutout with an earth going into the neutral side. contact you DNO and they should be able to tell you if your susposed to have TNCS or TT

 

[stevesey]

Most definately no earth going anywhere near the cutout (just R,Y,B and Netural).

Thought TT was usually only overhead.

 

[andy]

Most definately no earth going anywhere near the cutout (just R,Y,B and Netural).

Thought TT was usually only overhead.

TT usually is overhead, but it can also be underground

 

[plugwash]

you need to talk to the rec/dno they may just tell you to put your earth in a spare terminal on the neutral block of the cutout but you MUST check with them before doing anything like this.

it appears to me that they did a PME capable feed and labeled it as such but the system was never actually converted from TT to PME.

 

[stevesey]

So it could still be TT especially given it was overhead originally. My concern is that there was no obvious central earthing point on the supply board when the "contractors" converted the street to underground, so they simply missed it out. I'll check with power company.

If it is TT the earth wire/rod will need replacing. Which leads to another question - the kitchen/toilet etc is the other end of the building, running a suitable earth from the pipework back to the fuseboard will be a real pain. In a TT system is it valid to have another earth rod and and connect to this?

 

[plugwash]

So it could still be TT especially given it was overhead originally. My concern is that there was no obvious central earthing point on the supply board when the "contractors" converted the street to underground, so they simply missed it out. I'll check with power company.

right now it *IS* TT whether it is TT for a reason or whether its TT because noone bothered hooking up an earth to the neutral is something that you will have to clarify with the power company.

If it is TT the earth wire/rod will need replacing. Which leads to another question - the kitchen/toilet etc is the other end of the building, running a suitable earth from the pipework back to the fuseboard will be a real pain. In a TT system is it valid to have another earth rod and and connect to this?

NO NO NO the whole point of those bonding runs is to keep stuff at the same potential. it can't do that if its not connected together.

are you planning to do this work yourself? because if so you certainly don't seem qualified to do so given that you are even suggesting that.

 

[stevesey]

Qualified? How about a member of the IEE and a degree in Electrical Engineering Just a bit short of real world practice - hence the questions (and reading of regs) before deciding what the final configuration will be.

Yes agreed the system is now TT, whether is is supposed to be or not is a different question, I did say I would check with the power company what it was supposed to be.

NO NO NO the whole point of those bonding runs is to keep stuff at the same potential. it can't do that if its not connected together.

Equal potential within the same room (or arms reach) is the idea behind the regulations. Hence no need to run a cable back to the fuse board from kitchen/toliet (nor any need for a second earth rod - apologies for that red herring).

Steve

 

[ban-all-sheds]

Equal potential within the same room (or arms reach) is the idea behind the regulations. Hence no need to run a cable back to the fuse board from kitchen/toliet (nor any need for a second earth rod - apologies for that red herring).

Nor any need for supplementary equipotential bonding - that's only in bathrooms.

I take it your main equipotential bonding is in place, and OK, and doesn't need to run from the kitchen?

 

[plugwash]

NO NO NO the whole point of those bonding runs is to keep stuff at the same potential. it can't do that if its not connected together.

Equal potential within the same room (or arms reach) is the idea behind the regulations. Hence no need to run a cable back to the fuse board from kitchen/toliet (nor any need for a second earth rod - apologies for that red herring).

Steve

in normal uk installations we do two types of bonding

main equipotential bonding which serves to keep stuff at more or less the same level throughout a building by bonding anything that has an independent earth path (ie pipes) to the earthing system

and supplementry equipotential bonding which serves to keep things at even closer to the same potential in high risk locations such as bathrooms and swimming pools

 

[Big_Spark]

Steve, what is the installation this supply feeds as this may give a clue as to why the installation is thus connected.

(Reread original post, please feel free to ignore this question )

 

[stevesey]

in normal uk installations we do two types of bonding

main equipotential bonding which serves to keep stuff at more or less the same level throughout a building by bonding anything that has an independent earth path (ie pipes) to the earthing system

and supplementry equipotential bonding which serves to keep things at even closer to the same potential in high risk locations such as bathrooms and swimming pools

Quite correct - and of course main bonding may be negated by plastic pipes or the effectiveness reduced by compression joints stuffed with PTFE etc. Where as supplementary bonding should ensure that all the surfaces within one locale are at the same potential so reduce the risk of shock (by touching two surfaces at a different potential) under a fault condition.

 

[stevesey]

I take it your main equipotential bonding is in place, and OK, and doesn't need to run from the kitchen?

The main bonding in place - yes. OK, almost certainly not - I haven't traced it yet but the choices I have are all uninsulated and lacking in mm2. The same reason why the earth wire/rod will need replacing.

 

[Big_Spark]

Steve, I am assuming that the main tails of the installation are either 25mm or 35mm, as such the connection to the Earth Rod need only be 16mm to comply, and the Earth you have identified as exiting the building is likely connected to the original Earth Rod/stake/plate that is buried in the ground. It could be it was never in a true pit, as with modern installations, and so it's exact location is not visible.

I would suggest that firstly you undertake, or arrange for, a full periodic test to be done, including the measuring of the Resistance to ground of the Earth you have. This can be done with a Rod meter or basically using a multimeter or Meggar.

 

[stevesey]

I would suggest that firstly you undertake, or arrange for, a full periodic test to be done

Actually a full inspection is where I am heading. The installation is a small church building where I have taken responsibility for the building etc. Generally the more visible side of the electrics are in pretty good shape, metal clad sockets, MCB/RCD fuse boxes etc. At the moment I'm checking things out, fixing minor problems and labelling/documenting what we have. The main issue seems to be that whilst the obvious stuff has been upgraded over the years no inspections have been done and no one has picked up on the things like the antiquated earthing.

Once I understand whats needed I plan to get an electrician in, agree/do remedial work and inspection.