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]

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?

 

[davesparks]

Yes. The MCB feeding the 2nd consumer unit cannot be left on an RCD due to selectivity issues.

Yes there is not selectivity but is that a problem, does it introduce danger? You've said it's a storage building, what is the actual problem here that you are trying to solve?
This is an EICR, it is not an assessment of absolute compliance with bs7671, it is an assessment of suitability for continued use.

This would not comply as a new installation, but that is not the point of an EICR. You have a building which is being used for storage on one RCD, under fault conditions the whole building would go off, but is that actually a problem?

 

[PS Electrics]

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

Storage not a problem but that is the second board, don't know what the first board does OP never said.

 

[Uplight99]

Yes there is not selectivity but is that a problem, does it introduce danger? You've said it's a storage building, what is the actual problem here that you are trying to solve?
This is an EICR, it is not an assessment of absolute compliance with bs7671, it is an assessment of suitability for continued use.

This would not comply as a new installation, but that is not the point of an EICR. You have a building which is being used for storage on one RCD, under fault conditions the whole building would go off, but is that actually a problem?

The board feeding all the circuits in the storage building is a sub-main off the board in the caretakers hut which has an office, toilet and a workshop used by the caretaker.

Both have front end 30mA RCDs hence the selectivity issue.

So yes currently all circuits on the TT are protected by a 30mA RCD. So at this point it becomes no different to having a front end RCD on a PME which would be a C3. Cheers for the advice. I am still going to recommend to swap it all out for RCBO and have a time delayed 100mA replacing the main switch in the first TT board to provide fault protection to the sub-main MCB.

 

[Pretty Mouth]

I think there is a little truth in what the guy in the video says, but he makes more out of it than he should. A N-E fault on a circuit protected with a single pole RCBO, combined with a high resistance fault on the N, upstream of the N bar, would leave some/all current trying to make its way back home via the earthing system. A TN system would be a low impedance path back, so wouldn't see a rise in voltage on the earthing system, but a TT earthing system, being typically high impedance, would likely see a rise in voltage. Thing is, it needs two faults present to become dangerous, so is acceptable.