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?

 

[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.