Discrimination issue

[Spaced]

Hi everyone

I hope that someone can shed some light on a small probem I have inherited.

I have a submain suppled directly from a Henley block - 25mm tails to a 63A 100mA RCD main switch and a 40A MCB. This supplies a small inhabited outbuilding via a 16mm SWA cable to a C.U fitted with a
80A 30mA RCD main switch. The issue I have is one of discrimination as since the 100mA RCD is within the main house, should this trip whilst the tennants are out then the attached property cannot be re energised. I ran a complete RCD test from the outbuilding and the RCD in the house only tripped on the 5x 180 degree test but obviously if I can create this scenario it can occur during normal use. Any ides as to how best to resolve this issue.


Thanks for your help.

Mac

 

[Guest123]

Exchange the 100mA RCD for a time delayed 'S' type one.


Edit.....just re-read the post and noticed it's wired in SWA......what type of earthing arrangement is there??? maybe RCD is not required to the sub-main as it's SWA and you have additional 30mA RCD protection at the sub DB. Could also exchange the RCD for a simple main switch.

 

[pennywise]

Is it a TT supply at the house end. Surely if it isn't then the SWA doesn't need an RCD, your discrimination problem is at the outbuilding end as it only has one RCD, I'm probably reading this wrong a couple of RCBO's at the outbuilding and replace the RCD being used as the main switch with a standard double pole variety

 

[Spaced]

Hi Lenny

The main supply is via a 60A PME - TNCS. This feeds Meter to Henley to Submain RCD currently.
Thanks for the advice. I'll change the RCD for a D.P main switch.

Am I correct in thinking that the outbuilding should have it's own TT rod - from memory I think it's currently using the main house earth.

Thanks again.
Mac

 

[Guest123]

Ok so you've got a TN-C-S system....makes things slightly different.

I take it the outbuilding is remote from the house i.e, not joined on to it???

Also I assume there is water and possibly gas/oil in there???

Is the 16mm SWA a 3 core??? if so then the 3rd core can be used as both the CPC and an MEBC, so no need to TT it, just make sure that any services in that outbuilding are bonded in 10mm back to the sub-board in the outbuilding. Just make sure that the armourings are correctly terminated also.

The RCD in the house can be done away with in favour of a standard DP main switch, therby eliminating the discrimination issue.

 

[pennywise]

wouldn't he need to replace both the RCD's with double pole switches at the house and the outbuilding, as he has only one RCD at the outbuilding which is inhabited, so this wouldn't give discrimination between lighting and power, that's why I was thinking on the lines of RCBO's in the outbuilding.

 

[Guest123]

True enough....depends how far he intends to go and any cost issues.

 

[pennywise]

If only every job had an unlimited budget

 

[Griffin]

Hi all,

I am pricing a similar senario at the moment,

TNCS property with a large wooden shed approx 10m away in the garden.

I was going to run in a 2 core 10mm SWA out then TT the shed.

If I want to use the earth provided by the DNO will I have to increase the sub to 16mm???

10mm SWA is adequate for loading and VD

regards

G