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.

 

[Deleted member 9648]

An unprotected buried cable otherwise properly installed to a previous edition of the regulations without additional protection would be a code 3. As a code 3 would not result in an unsatisfactory assessment it need not be rectified if you are just re-using it and not altering it in any other way.

 

[telectrix]

i concur with wp. however, it could be argued that as you are changing the OCPD for the 16mm T/E, you are altering the circuit and should therefore ensure it complies with current edition.

 

[Deleted member 9648]

i concur with wp. however, it could be argued that as you are changing the OCPD for the 16mm T/E, you are altering the circuit and should therefore ensure it complies with current edition.

....Don't think he is changing the OCPD is he?

 

[telectrix]

he didn't say specifically. if not, then 100% can be left as is.

 

[jedisparks]

he didn't say specifically. if not, then 100% can be left as is.

Hi and thank you both for your replies. Currently this 16mm doesn't have an OCPD other than the suppliers fuse. Only a defunct 500 mA Rcd. So am planning to install an OCPD for it. Way I see it, I can't satisfy the regs unless I fit a 30mA. Either way it's a departure. Just wondering what others might do in such a case.

Thanks again.

 

[Plonker 3]

Run a new cable on a route or re-route the existing cables so that RCD/RCBO protection is not needed.

 

[jedisparks]

Hi Dillb

Yes,I think that is the only way I can tick all the boxes so to speak. It's a horrible run though, grade 2 listed house, meter downstairs with CU upstairs. It's going to mean some trunking or conduit LOL. Not impossible though.

Thanks for your input

 

[Deleted member 9648]

Hi Dillb

Yes,I think that is the only way I can tick all the boxes so to speak. It's a horrible run though, grade 2 listed house, meter downstairs with CU upstairs. It's going to mean some trunking or conduit LOL. Not impossible though.

Thanks for your input

Might be a tricky conversation with the client suggesting they spend hundreds of pounds rewiring a circuit which just a few years ago was considered perfectly safe.

 

[jedisparks]

Might be a tricky conversation with the client suggesting they spend hundreds of pounds rewiring a circuit which just a few years ago was considered perfectly safe.

I know and it's going to be a serious pain to do as well. It's either that or have a departure noted on the cert which might not look good if they ever have to claim on their house insurance - which may also come back on me, however unlikely. I'm slightly helped by the fact that they've just bought and moved in to the place and knowing that it is need of some modernising. I will go back round sit down with them and try to explain it.

Don't know what else to do tbh. They want the installation to be safe and to carry out alterations with additional sockets etc, so the installation is in need of modernising. The wiring is in good nick but the protective devices and non existent bonding needs sorting! 522.6.202 has me over a barrel.

If it was always easy it would be boring

Thanks again

 

[Octopus]

If he slaps in a front end 30mA RCD then he'll foul of inconveniencing the client (sorry can't remember the reg no!) and then have discrimination issues with the other 30mA RCDs

I like the code 3 for the circuit!

 

[jedisparks]

If he slaps in a front end 30mA RCD then he'll foul of inconveniencing the client (sorry can't remember the reg no!) and then have discrimination issues with the other 30mA RCDs

Yep, that's the problem. I think it's 531.2.9 might be wrong

I like the code 3 for the circuit!

Can't really as the circuit is currently only protected by the supply company fuse. It's a 20 meter run so should have its own protective device on the consumer side of the meter. Then If I'm going to install an OCPD for it I'm kinda responsible for the circuit.

Thank you for replying

 

[eddiecme]

Had exactly the same thing myself last year.

Archaic dangerous Main D/B in a secure locked plant room with 8 x 16mm T&E feeds going to single c/u's within a sheltered accommodation block. Each c/u was running dual 30mA RCDS.
Old horrible building and you could see the 16mm T&E snaking up the wall on a diagonal before disappearing into the loft/floor, both of which were inaccessible. We we're asked to change the Main D/B by the client off the back of the EICR, hitting the same problem as you describe above (which as it goes is probably pretty rare). the task of pulling in new submains would have been thousands of pounds worth of work so they didn't want to do it.

Main D/B was changed to one incorporating MCBs with no RCD protection and detailed risk assessment was put in which included the site supervisor being fully notified of the potential hazardous areas, this also involved us marking (where possible) the route of the cables. NICEIC were happy with this.

 

[Octopus]

Can't really as the circuit is currently only protected by the supply company fuse. It's a 20 meter run so should have its own protective device on the consumer side of the meter. Then If I'm going to install an OCPD for it I'm kinda responsible for the circuit.

Thank you for replying

have ve you asked for guidance from your scheme? I would interested to hear what they suggest! Failing that the IET as they write the rules - yet seem not to understand the impact of their decisions.

 

[showerman]

No mention of timedelay rcd ? just a thought !