discrimination conundrum

[jedisparks]

Hello to all,

Have just completed an EICR for a client of mine who would now like me to do the remedial work. I've hit a problem with the design of the job.

Part of the work will involve a new CU with RCDs and possibly a couple of RCBOs

The CU is remote from the meter position and is fed via a 16mm Twin & earth sub main that comes from the meter cabinet via an old 500mA RCD (which does not operate by the way). Now, it is likely that this 16mm T & E runs in partitions within 50mm of the surface and as such should have a 30mA RCD(522.6.202).

Trouble is if I protect the sub main cable with 30mA and have further RCDs in the CU I'm not going to have any discrimination between RCDs, resulting in inconvenience in event of a fault.

Have been thinking of protecting the sub main with a 100mA S type RCD to provide discrimination and some protection too but this is still a departure. Have asked NIC tech help who say I need to ensure reg 120.3 when claiming any departures. Which I don't think I will be with a 100mAmp RCD on the front end.

Any thoughts or help on this will be gratefully received.
Thanks for reading.

 

[Richard Burns]

No mention of timedelay rcd ? just a thought !

That was part of the Op's question!

 

[jedisparks]

Thanks to everyone who replied yesterday.

Quick Update: Spoke to NIC tech help today. The chap I spoke to suggested that I protect the sub main cable with a time delay 100mA RCD in order to maintain discrimination with the downstream RCDs. But that this is a departure and must be noted as such. Also, that it would be a good idea to provide a risk assessment to identify the risk of the potentially buried cable without a 30mAmp (additional protection).

Also to note within the risk assessment that I am in fact improving the overall safety of the installation and not making it less safe than it already is.

Will update again once I've spoken to client

Cheers again

 

[showerman]

That was part of the Op's question!

Sorry missed the s type ref. when scanning through ! , however my " thought" seems
to have been a possible solution to the problem

 

[Richard Burns]

It is an interesting choice for the NICEEIC to make, the regulations specifically state that any departure from the regulations, that may be noted on the electrical installation certificate, may not provide any less safety than that provided by compliance with the regulations.
This is not the case in this instance and should not be considered to be complying with the regulations or as an acceptable departure.

However in practical terms increasing the safety of the installation is of benefit and it could be considered that increasing the safety could be undertaken even in contravention of the regulations as otherwise you are retaining a more dangerous state. But that is a decision for the electrician on site to make as a call on a case by case basis.

 

[Plonker 3]

If the NICEIC are happy for that, I would request it in writing from them so that should the brown stuff hit the fan they will be stood along side you in court.