UK Bonding of extraneous conductive frame

[Tequila]

TNC-S main supply with 16mm swa supplying garage consumer unit from main consumer unit in house, then 4mm swa supplying pond equipment through pond fuse box. (Pond cable run approx 40m)
Pond has a metal unistrutt frame and cover over pond equipment, this frame has its support legs concreted into the ground making then extraneous.
How is this best to be bonded? PME supply so has to have min 10mm bonding conductor, Can't use pond supply cable armour as not equivalent to 10mm, Can't use cpc of cable as only 4mm. Are these the correct assumptions?
Do I only have 2 options?
1) 10mm protected earth cable connected to extraneous pond frame all the way back to garage earth terminal giving me 10mm bonding conductor back to supply. OR
2) earth rod at pond, and turn pond supply into a TT system. So 4mm swa earth conductor disconnected and insulated, along with the armour at pond. And 10mm earth cable from new earth rod at pond to cpc connection of pond fuse box and pond frame?

 

[pc1966]

1) 10mm protected earth cable connected to extraneous pond frame all the way back to garage earth terminal giving me 10mm bonding conductor back to supply. OR

If your 16mm is 3C so has >= 10mm of copper CPC equivalent than that is an option.

2) earth rod at pond, and turn pond supply into a TT system. So 4mm swa earth conductor disconnected and insulated, along with the armour at pond. And 10mm earth cable from new earth rod at pond to cpc connection of pond fuse box and pond frame?

This might be simplest, and you don't need 10mm bonding in this case (as it is not carrying the open PEN fault current, only enough to clear the RCD and be physically strong enough). Also no risk of open-PEN fault voltages there.

Is the "pond fuse box" actually at the pond? Presumably it is just an RCD and the 4mm already has some form of MCB/RCBO protection?

 

[Tequila]

If your 16mm is 3C so has >= 10mm of copper CPC equivalent than that is an option.

This might be simplest, and you don't need 10mm bonding in this case (as it is not carrying the open PEN fault current, only enough to clear the RCD and be physically strong enough). Also no risk of open-PEN fault voltages there.

Is the "pond fuse box" actually at the pond? Presumably it is just an RCD and the 4mm already has some form of MCB/RCBO protection?

Thanks for the reply. Yes, so fuse box at the pond circa 35 to40m cable run from garage consumer unit. 4mm 3core swa is from 30ma rcd and 20amp mcb in garage unit. This terminates at 16amp mcb at pond fuse box which then feeds individual fused terminals to each pond device.
So, presumably, my easiest and probably most safe option is earth rod (less then 200ohms) at pond to make it a separate TT system.
I also have an earth rod connected at my main MET as I have solar/battery and supply failure backup for when grid fails when in island mode. Is there a danger of the two rods interference they are about 15m apart?
The pond is also one of the circuits which is backed up on DNO supply failure so is covered by the earth rod on grid failure.

 

[mainline]

Finding out how well the frame is earthed would be a good start.

 

[cliffed]

It’s not extraneous, it’s outside anyway, cpc for that circuit would suffice on the uni structure

 

[Julie.]

It’s not extraneous, it’s outside anyway, cpc for that circuit would suffice on the uni structure

Absolutely not!

If the metalwork has a good/reasonable connection to the earth then in the event of a broken PEN the local neutral connection will be connected to the substation star point by virtue of:
The neutral-earth bond in the head then
This earth/CPC connection to the earth mass then
The earth mass to any local earth bonding (if existing), and
The earth bonding at the substation where the star point is split to neutral/PEN/earth.

It could therefore be subject to exactly the same level of current that occurs with the main bonding under the same fault condition!

So yes it needs to be 10mm2 just the same as any bonding of ECP.

Unless it has a high resistance to earth that is - measure it, as @mainline says.

 

[cliffed]

Absolutely not!

If the metalwork has a good/reasonable connection to the earth then in the event of a broken PEN the local neutral connection will be connected to the substation star point by virtue of:
The neutral-earth bond in the head then
This earth/CPC connection to the earth mass then
The earth mass to any local earth bonding (if existing), and
The earth bonding at the substation where the star point is split to neutral/PEN/earth.

It could therefore be subject to exactly the same level of current that occurs with the main bonding under the same fault condition!

So yes it needs to be 10mm2 just the same as any bonding of ECP.

Unless it has a high resistance to earth that is - measure it, as @mainline says.

Ok… it’s definitely not extraneous, if there is a Broken PEN that would be a problem, it’s remote from the building so s TT System would be the answer.

 

[Julie.]

Ok… it’s definitely not extraneous, if there is a Broken PEN that would be a problem, it’s remote from the building so s TT System would be the answer.

It doesn't change what needs to be done - determine if it is actually electrically connected to the earth, if it is apply the correct sized bonding conductor or change this circuit to TT.

However, I suggest you check the BBB in part 2 and look at the definitions, ECP is a conductor which could introduce a voltage into the installation. It isn't restricted to that part of the installation inside of a house . The installation is the whole interconnected electrical system/equipment


EDIT

I forgot to mention, but the bonding is only required if it is class 1, if the equipment is class 2, then no talk of CPC/earthing needs to take place.

 

[Tequila]

It doesn't change what needs to be done - determine if it is actually electrically connected to the earth, if it is apply the correct sized bonding conductor or change this circuit to TT.

However, I suggest you check the BBB in part 2 and look at the definitions, ECP is a conductor which could introduce a voltage into the installation. It isn't restricted to that part of the installation inside of a house . The installation is the whole interconnected electrical system/equipment


EDIT

I forgot to mention, but the bonding is only required if it is class 1, if the equipment is class 2, then no talk of CPC/earthing needs to take place.

There is some class 1 equipment on the pond so any fault within the property or at the pond equipment along with a PEN fault on the supply, there would be risk of fatal shock if someone touched the class1 metallic equipment at the pond while touching the unistrutt frame correct? So I'm thinking TT is my only option. However with a low resistance rod already installed at main MET alongside the TNC-S would another rod be required at the pond? as the main rod should trip the 30ma rcd incase of supply PEN fault right?

 

[Julie.]

There is some class 1 equipment on the pond so any fault within the property or at the pond equipment along with a PEN fault on the supply, there would be risk of fatal shock if someone touched the class1 metallic equipment at the pond while touching the unistrutt frame correct? So I'm thinking TT is my only option. However with a low resistance rod already installed at main MET alongside the TNC-S would another rod be required at the pond? as the main rod should trip the 30ma rcd incase of supply PEN fault right?

TT requires an earth connection seperate from the PME connection.

 

[Tequila]

TT requires an earth connection seperate from the PME conconnection

Agreed, however the reason for the extraneous bonding at the pond is incase of a PEN fault!? If I have a PEN fault at supply I then have a TT system with the installed rod at the main MET!? So is there any need for separate TT at the pond?

 

[Julie.]

Agreed, however the reason for the extraneous bonding at the pond is incase of a PEN fault!? If I have a PEN fault at supply I then have a TT system with the installed rod at the main MET!? So is there any need for separate TT at the pond?

You could TT the whole installation, in which case the rod at the PME must be disconnected from the PME and used for the TT

But there could be other issues if the overall site is changed to TT, for example you may need to change to dp rcds/rcbos etc.

You could just change the one circuit to TT - leave everything else TN-C-S including armoured cable to the remote end, and fit the rcd and TT just that circuit at the load end.

 

[pc1966]

So, presumably, my easiest and probably most safe option is earth rod (less then 200ohms) at pond to make it a separate TT system.

Yes, and it seems Julie agrees (which is a big endorsement!).

I also have an earth rod connected at my main MET as I have solar/battery and supply failure backup for when grid fails when in island mode. Is there a danger of the two rods interference they are about 15m apart?

No issue there. Of course the rod connected to the MET should be on 10mm or above cable for the same open-PEN reasons.

Most rods in the several tens of ohms range won't sink enough current to need 10mm, but it is not impossible to have a bit under 10 ohms from a 2m rod in good conductive ground, and so tens of amps contentiously is feasible under worst-case circumstances and too thin a CPC/bonding conductor then becomes a fire hazard.

 

[Tequila]

You could TT the whole installation, in which case the rod at the PME must be disconnected from the PME and used for the TT

But there could be other issues if the overall site is changed to TT, for example you may need to change to dp rcds/rcbos etc.

You could just change the one circuit to TT - leave everything else TN-C-S including armoured cable to the remote end, and fit the rcd and TT just that circuit at the load end.

Thanks Julie could you expand a bit. I currently have TN-C-S supply to pond which on 1x rcd testing at pond disconnects within 300ms. With a PEN fault introduced at the supply, the 1x rcd test will still disconnect as fault current is taken by the parallel earth rod installed at the MET so wheres the danger of shock at the pond end via the extraneous frame with the supply disconnected?

 

[cliffed]

Agreed, however the reason for the extraneous bonding at the pond is incase of a PEN fault!? If I have a PEN fault at supply I then have a TT system with the installed rod at the main MET!? So is there any need for separate TT at the pond?

Agreed, however the reason for the extraneous bonding at the pond is incase of a PEN fault!? If I have a PEN fault at supply I then have a TT system with the installed rod at the main MET!? So is there any need for separate TT at the pond?

Sorry …is this equipment inside a building… are there any metallic pipework …except the uni strut …entering this.