Discrimination

[Blacktail]

Hi, my first time on this or any forum, be gentle. I install/erect agricultural barns and more recently domestic sheds. I have recently taken my 17[SUP]th[/SUP] regs and part P and before any one says anything, I am NOT now an electrician. I have in the past installed the electrics in the agricultural barns but more and more feel I am in a gray area regarding agricultural and say a garden summer house, which is obviously Part P. I am planning on becoming a competent person in the next six months or so once a get a domestic project that’s big enough. Hope that give you an idea where I sit.

I guess this is a typical lack of experience question. Have been asked to install an outside light. Domestic property. Yes i know about building control, we all have to start somewhere. TP & N supply into a Hager board with a single 100 A 4pole 100mA RCCB. No RCBO on any of the circuits. Should I replace the 4 pole with a 30mA or install RCBO on light circuit I am coming off. My main question is about discrimination. There is none at the moment as everything comes through the 4 pole. If I swap the 100mA with a 30mA still none but at least the rest of the house is now closer to the mark. Both final rings are rated at 32A (so OK?) and there is a fire alarm circuit. What to do?

 

[Rockingit]

It's probably a 100mA RCCB main switch because it's a TT system - in which case it has to remain where it is. That's why the rest of the house is protected by default. It doesn't need 30mA RCD's as it was probably all done to a previous edition of the regs.

You have two choices, either swap the 100mA RCCB for a 's' type (time delayed (and pricey)) or put the outside light on it's own circuit.

If you wire the light back to the board in conduit or trunking thoughout the entire run then you don't need a 30mA RCD for mechanical protection.

 

[stuart569]

Hope you are familiar with safe isolation working on 3 phase installations!

 

[Knobhead]

What’s the difference between single and three phase isolation?

Non as far as I know!

 

[stuart569]

What i meant to say was make sure you know how to safely isolate before working!Wouldn't like a shock between phases now would we

 

[telectrix]

the isolators have more bits inside.

 

[Blacktail]

Rockingit, thanks for that, sorry yes it is a TT system should have said. And yes it was a new build installed just pre 17th. Owner was not happy about my suggestion of surface mounting. When you say swap with s type this is if I put the preferred circuit on a rcbo yes? Outside light on own circuit meaning running from its own rcd board consumer unit?

Stuart569 Safe isolation. I check each phase against each other then each phase against N, then each against earth and finally E against N (then proving unit!!) Is that correct.

 

[stuart569]

Prove your tester against a known live source/proving unit first.Then as you have said and remember to lock off and put up signs.Not everyone does it but better safe than sorry

 

[Blacktail]

Thanks Stuart. Do all that as well. Always reassuring for a (supposedly) trained newbie like me to be told i'm doing it correctly.