TT system EICR questions

[Uplight99]

Wondering if anyone can advise as I’m struggling to understand the regs, I’ve never tested or even worked on a TT system.

I have tested an old farm that has been converted to a school so it is no longer an agricultural premises.

The main supply is a three phase PME which feeds all 6 consumer units. However 2 of the consumer units have been converted to TT off the same rod. The earth from the SWA has been left disconnected in the three phase panel so the PME has not been exported.

The SWA feeds one consumer unit with a front end 30mA RCD, which this consumer unit then feeds another consumer unit off in an old barn (now being used for storage) with another 30mA front end RCD. Obviously we have selectivity issues with this setup but that’s not my main concern.

The main issue is that this sub-main is just being fed from a 40A MCB in the three phase panel.

Would I be correct in saying I need a 100mA S type RCD to protect the sub-main?

My plan is to install a separate plastic enclosure next to the three phase panel which will house a 100mA S type RCD, using double insulated tails from the panel to the RCD which will then have the SWA on the outgoing to feed the 2x consumer units on the TT.

I will also replace both front end RCD consumer units with double pole RCBOs for all circuits (except the circuit that feeds the second consumer info to avoid selectivity issues).

Is my understanding correct?

 

[westward10]

The 40A submain should rely on the TN-C-S and this should earth the armour of the cable to provide ADS. At the TT end the cable armour and gland should be enclosed to prevent simultaneous contact between different earthing systems. If the existing protective device for the submain provides ADS then no RCD is required but the armour must be earthed to the earthing system at the origin.

 

[davesparks]

As above, the 40A submain must use the earthing system of the DB it is fed from, the armour of the cable and any cores not used as live conductors should be earthed at the supply end and insulated at the load end.

 

[PS Electrics]

Wondering if anyone can advise as I’m struggling to understand the regs, I’ve never tested or even worked on a TT system.

I have tested an old farm that has been converted to a school so it is no longer an agricultural premises.

The main supply is a three phase PME which feeds all 6 consumer units. However 2 of the consumer units have been converted to TT off the same rod. The earth from the SWA has been left disconnected in the three phase panel so the PME has not been exported.

The SWA feeds one consumer unit with a front end 30mA RCD, which this consumer unit then feeds another consumer unit off in an old barn (now being used for storage) with another 30mA front end RCD. Obviously we have selectivity issues with this setup but that’s not my main concern.

The main issue is that this sub-main is just being fed from a 40A MCB in the three phase panel.

Would I be correct in saying I need a 100mA S type RCD to protect the sub-main?

My plan is to install a separate plastic enclosure next to the three phase panel which will house a 100mA S type RCD, using double insulated tails from the panel to the RCD which will then have the SWA on the outgoing to feed the 2x consumer units on the TT.

I will also replace both front end RCD consumer units with double pole RCBOs for all circuits (except the circuit that feeds the second consumer info to avoid selectivity issues).

Is my understanding correct?

As Westward10 said but I would suggest you need an upfront time delay RCD for the first consumer unit if you have one way that is not RCD protected.

 

[Uplight99]

The 40A submain should rely on the TN-C-S and this should earth the armour of the cable to provide ADS. At the TT end the cable armour and gland should be enclosed to prevent simultaneous contact between different earthing systems. If the existing protective device for the submain provides ADS then no RCD is required but the armour must be earthed to the earthing system at the origin.

Ok thanks. At the moment the earth from the SWA has been left disconnected in the TN-C-S panel but the armour has been connected to TN-C-S and insulated at the other end. Changing this is going to be a ball-ache due to how they've terminated the SWA on the TT end (terminated and goes straight through the plastic enclosure, up the wall and up to the consumer unit).

Am I right in saying the use of double pole RCBOs means the upfront 100mA S type isn’t required and that it would be if I was using single pole RCBOs?

I will have an MCB in the first TT consumer unit as it feeds another board, so either way I am going to need a 100mA to provide protection to this circuit. Would using a 100mA S type RCD main switch for this first consumer unit suffice?

 

[westward10]

Your second sentence doesn't make sense. The earth from the SWA is disconnected but the armour is connected?
If you are writing EICRs it must be technically correct and understandable.

 

[PS Electrics]

Your second sentence doesn't make sense. The earth from the SWA is disconnected but the armour is connected?
If you are writing EICRs it must be technically correct and understandable.

I think I understand, there must be an earth cable in the swa that is disconnected?

 

[Uplight99]

Your second sentence doesn't make sense. The earth from the SWA is disconnected but the armour is connected?
If you are writing EICRs it must be technically correct and understandable.

Yes, that’s how it’s currently set up. The earth within the SWA has been left disconnected inside the TN-C-S three phase panel but the armouring has been glanded with a banjo and fly lead to the MET the left insulated at the TT side.

I’ve just never come across a TT system before. Sometimes you have to drop the ego and know when to seek advice. I have quite extensive testing experience, just none on TT.

 

[davesparks]

I think I understand, there must be an earth cable in the swa that is disconnected?

If it's disconnected then it's not an earth, an earth is only an earth when it is connected to earth, if it's not connected to anything then it's just an unconnected conductor .

 

[PS Electrics]

Yes, that’s how it’s currently set up. The earth within the SWA has been left disconnected inside the TN-C-S three phase panel but the armouring has been glanded with a banjo and fly lead to the MET the left insulated at the TT side.

I’ve just never come across a TT system before. Sometimes you have to drop the ego and know when to seek advice. I have quite extensive testing experience, just none on TT.

Have you checked with the DNO? Ask if they will allow the PME earth to be exported? Probably the DNO did not allow the earth to be exported because it was a farm? That way you can just convert the whole installation to PME and ditch the TT issues.

 

[davesparks]

Ok thanks. At the moment the earth from the SWA has been left disconnected in the TN-C-S panel but the armour has been connected to TN-C-S and insulated at the other end.

But the armour is the CPC for an SWA so if it has been connected to the TNCS earth then it is connected.

Changing this is going to be a ball-ache due to how they've terminated the SWA on the TT end (terminated and goes straight through the plastic enclosure, up the wall and up to the consumer unit).

There's no need to change it, as per the above posts this is how it should be connected.

Am I right in saying the use of double pole RCBOs means the upfront 100mA S type isn’t required and that it would be if I was using single pole RCBOs?

No.

I will have an MCB in the first TT consumer unit as it feeds another board, so either way I am going to need a 100mA to provide protection to this circuit. Would using a 100mA S type RCD main switch for this first consumer unit suffice?

What observations have you recorded on the EICR that you are now trying to fix?

I can't see, from what you've said so far, that there is anything that would require remedial action.

 

[Uplight99]

Have you checked with the DNO? Ask if they will allow the PME earth to be exported? Probably the DNO did not allow the earth to be exported because it was a farm? That way you can just convert the whole installation to PME and ditch the TT issues.

Yes, that thought has crossed my mind

There’s 6 consumer units in total on the installation, 4 of them are on PME including one of the two barns, and the other 2 are on a TT

I don’t see any reason why they be converted to PME

 

[davesparks]

I don’t see any reason why they be converted to PME

What is actually wrong with these outbuildings that needs fixing?

 

[Uplight99]

But the armour is the CPC for an SWA so if it has been connected to the TNCS earth then it is connected.

There's no need to change it, as per the above posts this is how it should be connected.

No.


What observations have you recorded on the EICR that you are now trying to fix?

I can't see, from what you've said so far, that there is anything that would require remedial action.

The earth core has been left disconnected, but the armouring has been connected to TN-C-S. I’m not sure how I can word that any better

On a TT all live conductors must be broken under earth fault conditions. Single pole RCBO would not break the neutral, but S type would, which is why I asked about using double pole RCBOs. There are currently no double pole RCBO/RCDs protecting these circuits on the TT consumer units.

 

[davesparks]

The earth core has been left disconnected, but the armouring has been connected to TN-C-S. I’m not sure how I can word that any better

If it's disconnected then it's not an earth, a conductor is only an earth when it is connected to earth, if it's not connected to anything then it's just an unconnected conductor.
Normal SWA doesn't have a designated earth core as it is designed for the armour to be used as the CPC.

Obviously this could be a problem in its own right due to capacitive coupling which may need to be recorded on an EICR.

On a TT all live conductors must be broken under fault conditions.

Which regulation requires this?

Single pole RCBO would not break the neutral,

No, but the main switch in a board does, this will satisfy the requirements of 462.2

 

[westward10]

For ADS there is only a requirement to disconnect the line conductor under fault conditions on a TT system.
When you say earth core (to be technically correct it is a cpc) is it identified by green/yellow insulation? If not then as previously stated it is just an unterminated conductor.

 

[PS Electrics]

The earth core has been left disconnected, but the armouring has been connected to TN-C-S. I’m not sure how I can word that any better

On a TT all live conductors must be broken under earth fault conditions. Single pole RCBO would not break the neutral, but S type would, which is why I asked about using double pole RCBOs. There are currently no double pole RCBO/RCDs protecting these circuits on the TT consumer units.

Double pole RCBO's are used if there is an upstream RCD that would be effected by an E-N fault.

 

[Uplight99]

For ADS there is only a requirement to disconnect the line conductor under fault conditions on a TT system.
When you say earth core (to be technically correct it is a cpc) is it identified by green/yellow insulation? If not then as previously stated it is just an unterminated conductor.

Yes it is pre-identified with green/yellow and just left disconnected in the panel

Depending on the current and impedance, the touch voltage could be horrific. Surely it’s safer using double pole RCBOs?

 

[davesparks]

Depending on the current and impedance, the touch voltage could be horrific. Surely it’s safer using double pole RCBOs?

How is a DP RCBO going to change the touch voltage?
Touch voltage will be limited to less than 50V by an SP RCBO just the same as a DP RCBO.

 

[westward10]

So you have a conductor identified by green/yellow but now disconnected suggesting it was once in use but now it is not. It is just a disconnected conductor nothing more.
Why could the touch voltage be horrific and how does a double pole RCBO make it safer?

 

[Uplight99]

So you have a conductor identified by green/yellow but now disconnected suggesting it was once in use but now it is not. It is just a disconnected conductor nothing more.
Why could the touch voltage be horrific and how does a double pole RCBO make it safer?

So you have a conductor identified by green/yellow but now disconnected suggesting it was once in use but now it is not. It is just a disconnected conductor nothing more.
Why could the touch voltage be horrific and how does a double pole RCBO make it safer?

I found a video that explains it a lot better than I ever could

 

[davesparks]

I found a video that explains it a lot better than I ever could

That video doesn't explain anything, it's just someone saying 'touch voltage will be horrific' with no explanation at all.

I think they may even have completely missed the point of what touch voltage actually is.

 

[PS Electrics]

I found a video that explains it a lot better than I ever could

This, but you cannot do without the upfront double pole RCD because you said you will have one way that is NOT rcd protected. So with the upfront double pole RCD you COULD have SP RCBOs in the consumer unit, except for the proviso that an E-N fault would take out the whole consumer unit. So DP RCBOs would avoid this.

 

[Uplight99]

That video doesn't explain anything, it's just someone saying 'touch voltage will be horrific' with no explanation at all.

I think they may even have completely missed the point of what touch voltage actually is.

This, but you cannot do without the upfront double pole RCD because you said you will have one way that is NOT rcd protected. So with the upfront double pole RCD you COULD have SP RCBOs in the consumer unit, except for the proviso that an E-N fault would take out the whole consumer unit. So DP RCBOs would avoid this.

Yes. The MCB feeding the 2nd consumer unit cannot be left on an RCD due to selectivity issues. This will have to be left on the MCB and not changed to an RCBO which is why I thought I needed to protect it with the 100mA S type to achieve selectivity

 

[davesparks]

This, but you cannot do without the upfront double pole RCD because you said you will have one way that is NOT rcd protected. So with the upfront double pole RCD you COULD have SP RCBOs in the consumer unit, except for the proviso that an E-N fault would take out the whole consumer unit. So DP RCBOs would avoid this.

It's a storage building, why would the whole building going off be a problem?