TT Earthing shed

Toby250

Hi all,We have an old shed at the top of the garden around 50m from main fuse board our supply seem to be TT earthing but shed is wired like diagram A should it not be like B or C

2011

Are you sure your installation is TT ?
This is quite unusual.

Regardless of this I would be installing an earth rod local to the shed and connecting this earth electrode to the earth bar in the shed sub distribution board. I would also connect the shed earth bar to the earth bar in the main distribution board.

Toby250

2011 wrote: »

Are you sure your installation is TT ?
This is quite unusual.

Regardless of this I would be installing an earth rod local to the shed and connecting this earth electrode to the earth bar in the shed sub distribution board. I would also connect the shed earth bar to the earth bar in the main distribution board.

As far as i can make out it is,it has an earth from esb meter to distribution board but not from main fuse to esb meter, if it was tn-c-s would you not have an earth before or from main fuse going to distribution.. but then what is the earth from esb meter to distribution board about?

Toby250

2011 wrote: »

Are you sure your installation is TT ?
This is quite unusual.

Regardless of this I would be installing an earth rod local to the shed and connecting this earth electrode to the earth bar in the shed sub distribution board. I would also connect the shed earth bar to the earth bar in the main distribution board.

Also if it was TN-C-S should you even have an earth rod conected?

2011

Toby250 wrote: »

Also if it was TN-C-S should you even have an earth rod conected?

An earth electrode ties the earth bar it is connected to to the local ground potential. This is desirable as extraneous conductive parts should be at the same potential as the ground they are local to. So from that perspective it is best to connect an earth rod at the shed regardless of whether it is a TT or TNCS installation. If this was not done then earths connected to the earth bar would be at the same potential as the MET which of course may be slightly different to the potential at the shed.

2011

Toby250 wrote: »

if it was tn-c-s would you not have an earth before or from main fuse going to distribution.. but then what is the earth from esb meter to distribution board about?

With TNCS the neutralising point is at the cut out. As such you would have L, N & E from the meter cabinet to the main distribution board.

See page 4 of this link.

Toby250

2011 wrote: »

With TNCS the neutralising point is at the cut out. As such you would have L, N & E from the meter cabinet to the main distribution board.

See page 4 of this link.

Thanks for the info, I don’t see any at the cutout just one going into the meter from distribution board so this is TT then? Regardless I need to get an earth sorted in the shed.

Risteard81

It sounds like you are describing an old installation where it is neutralised at the ESB meter.

This makes it a TN-C-S installation. The neutral tail and main protective conductor (neutralising link) both connect to the ESB's PEN (Protective Earthed Neutral) conductor. So you have a phase conductor and PEN conductor from the DSO (ESB).

I agree with the advice that a local electrode is desireable at the shed in either case to keep the MET firmly tied to Earth potential.

Toby250

Risteard81 wrote: »

It sounds like you are describing an old installation where it is neutralised at the ESB meter.

This makes it a TN-C-S installation. The neutral tail and main protective conductor (neutralising link) both connect to the ESB's PEN (Protective Earthed Neutral) conductor. So you have a phase conductor and PEN conductor from the DSO (ESB).



I agree with the advice that a local electrode is desireable at the shed in either case to keep the MET firmly tied to Earth potential.

Would that not be a TT the fact I have a consumer earth? Just trying to understand the difference.

Risteard81

Toby250 wrote: »

Would that not be a TT the fact I have a consumer earth? Just trying to understand the difference.

No.

A TT (Direct Earthed) system is Terra-Terra, i.e. Earth to Earth.

Therefore the Earth fault loop goes from your electrode through the general mass of Earth to the ESB's electrode and back through the transformer windings.

Your electrode is simply an Earth reference for the ESB's PEN conductor.

TN-C-S is Terra/Neutral Combined and then Separate, i.e. Neutral and Earth are combined (in the PEN conductor) and then separate within your installation. The electrode merely ties the PEN to Earth again at your installation to try to prevent a dangerous voltage developing between the MET and true Earth.

The overwhelming majority of the Earth fault current will flow through the PEN conductor back to the star point of the transformer. A tiny amount will flow through the electrode as this will be of a much higher impedance.

I would advise against looking at diagrams from the north or Britain as it isn't usual to install an electrode in a TN-C-S installation under BS7671 - although most of the world (rightly) requires it.

2011

Toby250 wrote: »

Would that not be a TT the fact I have a consumer earth? Just trying to understand the difference.

TT systems have no neutralizing link.

Toby250

Thanks for all the info, I’ll get an electrode fitted for the shed and get the old shed installation checked out to be sure everything is safe, thanks again

Bruthal

My own opinion would be, in the case of a domestic tncs, that an earth rod at the neutralising point is adequate.

2011

Bruthal wrote: »

My own opinion would be, in the case of a domestic tncs, that an earth rod at the neutralising point is adequate.

It probably wouldn't make much difference to TN-C-S alright.
I got the impression that the earth rod was already installed or it was possibly a TT installation, in which case I would connect it as described. Either way wouldn't hurt.

It would be important to determine whether it is TT or T-C-NS.

Toby250

Risteard81 has determined its TNCS just an old way of doing it, so should i bother with earth rod in shed? if it improves the safety of the installation any bit i will get one fitted or would i be just wasting money

2011

Toby250 wrote: »

Risteard81 has determined its TNCS just an old way of doing it, so should i bother with earth rod in shed? if it improves the safety of the installation any bit i will get one fitted or would i be just wasting money

If you can confirm definitely that it is TNCS then I wouldn’t bother.

Toby250

2011 wrote: »

If you can confirm definitely that it is TNCS then I wouldn’t bother.

Would i have to contact esb to confirm? would an earth wire from meter to distribution board be there for any other reason than a TNCS system?

Zarco

Toby250 wrote: »

Would i have to contact esb to confirm? would an earth wire from meter to distribution board be there for any other reason than a TNCS system?

Normally you just confirm it visually

The connection looks to be hidden with yours

Zarco

It's better to neutralise at the fused cutout than the meter but ESBN used to neutralise at the meter too on older iinstallations like yours

There's no other reason for a earth connection at the meter

Zarco

To prove the connection you would do what they call an external fault loop impedance test in the UK(Ze)

A continuity test with the main conductors disconnected also proves it

2011

The resistance between earth and neutral is low in a neutralized installation as they are deliberately connected together. Where as the resistance is far higher when not neutralized as the only connection is though the general mass of earth.

Toby250

2011 wrote: »

The resistance between earth and neutral is low in a neutralized installation as they are deliberately connected together. Where as the resistance is far higher when not neutralized as the only connection is though the general mass of earth.

So continuity between earth and neutral at a socket should confirm this for me should it?

Zarco

Toby250 wrote: »

So continuity between earth and neutral at a socket should confirm this for me should it?

No

You get erroneous connections on the fixed wiring so you wouldn't prove it that way

2011

Zarco wrote: »

No

You get erroneous connections on the fixed wiring so you wouldn't prove it that way

What do you mean?

2011

Toby250 wrote: »

So continuity between earth and neutral at a socket should confirm this for me should it?

If you measured this with a suitable test instrument I would expect the resistance between earth and neutral to be very low (like under a couple of Ohms) for TNCS. For a TT installation it would be in the hundreds of Ohms. That is assuming everything is wired properly.

Risteard81

2011 wrote: »

If you measured this with a suitable test instrument I would expect the resistance between earth and neutral to be very low (like under a couple of Ohms) for TNCS. For a TT installation it would be in the hundreds of Ohms. That is assuming everything is wired properly.

It would read through any neutral/Earth fault in the installation though so there's no guarantee that everything's wired properly or not damaged.

Toby250

Risteard81 wrote: »

It would read through any neutral/Earth fault in the installation though so there's no guarantee that everything's wired properly or not damaged.

if you disconnected main earth at board and you still got continuity/low ohms at a socket would this then tell you have neutral/Earth fault somewhere?

2011

Risteard81 wrote: »

It would read through any neutral/Earth fault in the installation though so there's no guarantee that everything's wired properly or not damaged.

I agree 100%, this is why I said “ That is assuming everything is wired properly.”

2011

Toby250 wrote: »

if you disconnected main earth at board and you still got continuity/low ohms at a socket would this then tell you have neutral/Earth fault somewhere?

If you mean the earth from the meter cabinet, then yes as the installation should only have one neutralizing point in circuit and in this case it would be disconnected.

It would be best to get an experienced electrician to confirm the earthing arrangement IMHO.