External earth fault loop tt system

[brightspark2010]

Evening all could anyone please tell me the the maximum ze for tt systems the jobs entail Rec cut out fuse bs88 16a to feed 1x 30 ma rcd fuse spur for feeding socket outlet what should the max ze be please

 

[Sintra]

According to the powers that be it is a ridiculous Ra figure of < 100 ohms.

 

[telectrix]

if your circuit is RCD protected at 30mA, the the max. Zs is 1667. bloody joke.

 

[brightspark2010]

really 1667 ohms the some are coming up like 180 odd ohms was worried but our manager said it was fine with the 30ma protection and 1667 is what he said we work to I always thought a ze on tt had to be less than 21 ohm

 

[Sintra]

What is the Ra reading now?

 

[Geordie Spark]

really 1667 ohms the some are coming up like 180 odd ohms was worried but our manager said it was fine with the 30ma protection and 1667 is what he said we work to I always thought a ze on tt had to be less than 21 ohm

Did your manager not tell you that on a TT it isn't ze .... it's Ra ??

And, whatever you do DON'T get Engineer54 started !!!

 

[Sintra]

Lets give him 5 mins from now lol

 

[Geordie Spark]

Lets give him 5 mins from now lol

Ok ............ are we using your stop watch or mine?

 

[itselectric]

The 1667 is calculated through ohms law and a max volts through human of 50v .

50v x30ma = 1667

 

[brightspark2010]

I am still confused my zs can be upto 1667 ohm at the socket but what about my Ra from electrode to incoming supply to spur from rec fuse would tis be 21ohm or 100ohm or 200ohm?

 

[monkeyelectric]

I went to a job the other day to change a broken plug socket and a zs revealed a reading of 480ohms, it was on a tt but they didn't want me to do anymore as they were having an extension done in a couple of months time so it could get sorted then! I didn't feel overly comfortable leaving it but it was all protected by a single 30mA rcd and i advised them of the danger if the rcd were to fail but they didn't seem that concerned, what can you do

 

[ipf]

if your circuit is RCD protected at 30mA, the the max. Zs is 1667. bloody joke.

Aye 1667, one up from 'the great fire of London'

 

[itselectric]

The lower the better but I seem to recall on a regs course that the preferred maximum is no more than 200ohms.
once job is complete do a rcd test with an approved meter if it trips out in the required parameters then ok.

 

[Engineer54]

6 Hours later!! lol!!

The lower the better but I seem to recall on a regs course that the preferred maximum is no more than 200ohms.
once job is complete do a rcd test with an approved meter if it trips out in the required parameters then ok.

Another Reg bible basher!! with no understanding of a TT system!!

I am still confused my zs can be upto 1667 ohm at the socket but what about my Ra from electrode to incoming supply to spur from rec fuse would tis be 21ohm or 100ohm or 200ohm?​

​

21 ohms is the max for sub-station earthing and the like, but you'll never see a value anywhere near that high at a sub-station.

The 100 and 200 ohm values that are thrown about by the NICEIC and BS7671 are one almighty joke, and good for basically nothing!! Always aim for a low double figure. the old accepted standard was 10 ohms, but that was with decent sized and lengths of earth rods. Just as important as low Ra values is the stability of the earth rod position(s) The deeper the better, for both stability and Ra values...

 

[Geordie Spark]

6 Hours later!! lol!!




Another Reg bible basher!! with no understanding of a TT system!!




21 ohms is the max for sub-station earthing and the like, but you'll never see a value anywhere near that high at a sub-station.

The 100 and 200 ohm values that are thrown about by the NICEIC and BS7671 are one almighty joke, and good for basically nothing!! Always aim for a low double figure. the old accepted standard was 10 ohms, but that was with decent sized and lengths of earth rods. Just as important as low Ra values is the stability of the earth rod position(s) The deeper the better, for both stability and Ra values...

Better late than never bonny lad!!