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

 

[R G]

Fully agree with High Tower on all aspects

 

[Deleted member 9648]

post deleted....didnt load and then did!

 

[Deleted member 9648]

Of course not, you've freely admitted that you've never even tried to achieve better, and that was the basis of this thread!! All too easy to fall in to compromise mode, and rely on a single RCD ....Well while it's working correctly that is!! ..lol!!

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...

Just to make it clear....I have always advocated the use of a back-up 100 ma S type on all the debates I have been invoved in on this subject.
And also lets remember E54's first post on this thread quoted above....a low double figure Ra is relying on an RCD to meet disconnection times whether he likes it or not....which is somewhat at odds with his comments on the other quoted post.

 

[KAS1]

Just a thought does anyone know what ohmic value the suppliers aim for when they install their earthing rod at the supply transformer?

 

[ipf]

How many times do we read on here of failed RCD’s? I wouldn’t like to think I was relying on something that is known to be fallible.

Just found one, on a 'builders supply' unit.

 

[ipf]

Just a thought does anyone know what ohmic value the suppliers aim for when they install their earthing rod at the supply transformer?

<200....lol.

 

[des1957]

Hi it is to the best of my knowledge 21-200 ohms

 

[ipf]

Hi it is to the best of my knowledge 21-200 ohms

Eh ??????? at a sub station? phew! A bit more than rodding involved.

 

[Deleted member 9648]

How many times do we read on here of failed RCD’s? I wouldn’t like to think I was relying on something that is known to be fallible.

Fair enough....lets rely on a 'low double figure' Ra to operate an OCPD then shall we?

 

[itselectric]

Does anyone think these faulty RCDs are because of lack of testing with the test button to the manufacturers recommendation. After all they are mechanical latching. In my years of installing RCDs I don't know of any body who does test them regularly. Just a thought.

 

[des1957]

Eh ??????? at a sub station? phew! A bit more than rodding involved.

didn't read the questionproperly

 

[SPARTYKUS]

Does anyone think these faulty RCDs are because of lack of testing with the test button to the manufacturers recommendation. After all they are mechanical latching. In my years of installing RCDs I don't know of any body who does test them regularly. Just a thought.

no, I think its just rubbish being churned out, most I have found are straight off the shelf.

 

[kevelec]

Max 1667 ohms is so no more than 50 volts can appear on earthed metalwork.
Anything above 200 ohms is classed as unstable and likely to increase in frozen or dry soil conditions.

 

[Bobster]

Just a thought does anyone know what ohmic value the suppliers aim for when they install their earthing rod at the supply transformer?

<1ohm.

 

[Engineer54]

<1ohm.

Actually the max figure is 21 ohms, but you'll never see one anywhere near as high as that. Most will be under an ohm as you have suggested...