TT system

[big-steve]

hi all. I am after some advice and i cannot find an answer in the regs.
I have undertaken a job around a family members where the garage is being converted into a guest house. Its a three phase supply too the house and a TT system. I have run an armoured too the garage too a single phase consumer unit. and balanced the phases at the consumer units in the house. the consumer unit in the house has a 100A 30mA RCD main isolator. The consumer unit i have installed has a 100A 30mA RCD main isolator. I have took the earth from the Consumer unit in the house too the garage but do i have to put in another earth rod for this new consumer unit even though it is fed off the house?
thanks
Steve

 

[spark 68]

No you don't need to add an additional rod at the remote end, however it would be better if you did, as this would lower your overall Ra, and also provide some redundency.

My only other concern is that you have two 30mA RCD's in series, this will not provide any discrimination, and either or both could trip in a fault condition, knocking the supply for the whole house out. It may be better to split the supply at the house for the garage via Henly's and provide a 100ma S type RCD and MCB for the garage distribution cable.

 

[Rockingit]

^^^ what he said.

 

[Engineer54]

You haven't told us what the present Ra value of the existing TT system is??

With all TT systems (especially UK systems) it is always advisable to add rod positions and link them, ...in your case via the SWA using the armour or a separate core!!

Your RCD protection is in series so will probably both trip at the same time.

 

[big-steve]

call me stupid, but what is an Ra value?

 

[spark 68]

Ra is the Earth rod resistance, which on a TT install is also your Ze.

 

[big-steve]

ohh the ze is 52.1 ohms

 

[spark 68]

So if the far end is rodded (Highly recommended, but not a reg), it will lower your Zdb at the garage end, lower is better, and contribute to a lower overall Zs throughout the whole install. :shades_smile:

 

[big-steve]

thanks spark, thats 1 question out the way then lol . I dont work in domestic, this is a favour (wished id said no lol), am i okay too break the meter tails via a henley then into a 63A 100mA rcd? too incorperate discrimination from the house C/U and the Garage C/U beins as they are both 30mA ? your advice is much appreciated lol!

 

[spark 68]

am i okay too break the meter tails via a henley then into a 63A 100mA rcd?

This depends on the size of the DNO fuse feeding the tails, if the supply fuse is 100A then you will need a 100A RCD.

What you could do though, is feed a 100A DP isolator then feed your MCB before the RCD, this will reduce the rating of your RCD to match the MCB =>.

 

[big-steve]

yeah but a 100A RCD wont protect the 16mm armoured for the garage, if i take 25mm tails to the incomer of the 100A 100mA RCD that will be sufficient protection for the circuits

- - - Updated - - -

sorry the incomer of the 63A 100mA RCD***

 

[spark 68]

An RCD does not provide overcurrent protection, the 63A rating is how much current it can safely handle, as presumably your garage is more than three metres away, you cannot use the forward fusing regs. It will need to be suitable for the DNO's fuse.

For example, If say you feed a 40A MCB first, then you could fit the 63A RCD after.