Rcd TT and SWA

[james king]

Hello all...

Quick q...

Ok, the tails will be split feed a small house and an annex.. The earthing system is TT. SWA will be ran to the Annex about 20m away. 25mm SWA 2 core will be used.

Am I correct in saying that a 30ma dtime delayed rcd should be installed befor the cu?

A earth rod will be fitted direct to the annex cu?

 

[tuckermot]

whats original board got ??

 

[james king]

No rcd front end. Tail run to cu in house about 15m non SWA.. Nothing to do with me.

 

[KFH]

I would be tempted to front end the lot with a 100mA time delayed RCD (House as well). You will need RCD protection on the SWA to the annex anyway, 100mA time delayed. Then put 30mA in the annex. Can't remember all the regs, it has been a hard day.

 

[james king]

Do u put a 100ma td on all TT systems?

 

[The Ranger]

Do u put a 100ma td on all TT systems?

not generally, only to create discrimination on duel RCD's on one circuit! to stop all the RCD's in one stream tripping!

 

[KFH]

OSG 3.6.3 b - For installations forming part of a TT system, all circuits must be RCD protected. Stuffing a 100mA S type up front solves a lot of problems as per Fig 3.6.3 (ii)

 

[Engineer54]

And..... earth the SWA at both ends, (or better still use a 3rd core) to link the 2 or more, earth electrodes (rods) together...

It is always better to have a front end S type 100mA RCD on a UK TT system, to back-up any failure of the split Cu's 30mA RCDs....

 

[SPARTYKUS]

And..... earth the SWA at both ends, (or better still use a 3rd core) to link the 2 or more, earth electrodes (rods) together...

It is always better to have a front end S type 100mA RCD on a UK TT system, to back-up any failure of the split Cu's 30mA RCDs....

... But only in a limited way will it back up the 30mA, yes? Not low enough to (theoretically) stop a death inducing shock.

 

[KFH]

The 100mA S type is to stop Line to Earth faults from causing a problem, it is not additional protection. If you have a Zs of 20 Ohm a line to earth fault current is going to be 11.5 A (Assuming the fault is not on a 30mA RCD circuit). This will only trip 6A beakers everything else will allow it to flow presumably causing some heating and high touch voltages? As Spartykus said it is not backing up the 30mA RCD but given the failure rate of RCDs it does provide protection where people are not the Line to Earth fault path.

 

[SPARTYKUS]

better than nowt.

 

[alarm man]

you dont need a 100matd if your fitting a dual rcd cu tho common practice every tt ive seen upgraded to a dual board has the 100matd left in as an isolator then tails to the cu

 

[SPARTYKUS]

you dont need a 100matd if your fitting a dual rcd cu tho common practice every tt ive seen upgraded to a dual board has the 100matd left in as an isolator then tails to the cu

hello alarm man

Yeah, we know, E54 was just saying its a good backup in case of a failure. Thats all. Or if you're really worried, fit 6 30mA RCCBS in series

 

[Engineer54]

I would still fit an S type 100mA front end device with an all RCBO CU be they DP or SP devices on a UK installed TT system.

Which by the way i have a similar set-up in my house in Cyprus, except the Front end RCD device is incorporated in a MCCB unit! and is 500mA (Standard for Cyprus) ....But then, i don't have a crazy, pointless, and totally ridiculous 200 ohm Ra to worry about either as my Ra is now approaching 1 ohm. My house isn't built on the best of soil types either, so it's taken a bit of getting too. I have Ufer grounding on both the house foundation and the swimming pool foundation tied together and a couple of 6m earth rods tied together all connecting back to my MET.... I'll be checking the Ra again the next time i'm home, hopefully after the rainy season, so i don't get any false readings