Very Hi ZS Reading on TT system? any advice

[srl-8]

Hi guys,

i have changed a fuse board today in a outbuilding/large shed (it has a kitchen and shower and lounge etc.. a )

It is a TT system with earth rod.

My ZE reading was 187 ohms - now the regs state I am ok upto 200, so I assumed this was ok.

All the R1 + R2 and insulation resistance came back normal.

When I carried out the ZS tests, the reading were very high, for example:

Shower circuit on a 40a MCB - 2.6 omhs !
Kitchen Ring on a 32a mcb - 2.7 omhs !

same for lounge and bedroom rings.

I have checked to regs and these figures are over the maximum allowed.


Any advice ???

 

[Specialist]

sorry if i read that wrong mechelec but why would 2 rcd`s in series slow the disconnection time

I don't think Mechelec was talking about it slowing Disconnection times actually mate, he was saying he was too slow to beat me posting the question of Discrimination.

 

[biggles]

doh, brain not in gear.

 

[mechelec]

sorry if i read that wrong mechelec but why would 2 rcd`s in series slow the disconnection time

It wouldn't "slow the discrimination time? " but it would mean there was no discrimination between the RCD's and it would be pot luck which would trip first. inconvenient at best, dangerous at its worst. First one should be time delayed or S type

 

[srl-8]

So to confirm,

ZS readings of 2.6 ohms or there abouts is acceptable, providing the system and all circuits are protected by a 30ma RCD. (which they are)

??

Ok then your maximum allowed Zs reading for each 30mA protected circuit is 1667 Ohms, problem solved in 31 posts.

Your RCD at the supply end should ideally be an 'S' type time delayed to achieve discrimination with the RCD at the remote end but I wouldn't loose sleep over that one.

 

[srl-8]

i got confused earlier, because a member said the my ZS reading could never be lower than my ZE...

My ZS of 2.6 etc is lower than 187 omhs.....?

So to confirm,

ZS readings of 2.6 ohms or there abouts is acceptable, providing the system and all circuits are protected by a 30ma RCD. (which they are)

??

 

[Octopus]

To save me wading through all the posts, for clarity when you say 30ma RCD do you mean 1 RCD or multiple RCD's?

 

[srl-8]

1x 30ma rcd which is part of the new fuse board.

The main supply also has a 30 ma RCD at the house end.

To save me wading through all the posts, for clarity when you say 30ma RCD do you mean 1 RCD or multiple RCD's?

 

[Marvo]

I refuse to get involved!! lol!!!

You're scaring me Eng54. :aureola: This isn't some kinda new years resolution is it?

 

[GLENNSPARK]

It wouldn't "slow the discrimination time? " but it would mean there was no discrimination between the RCD's and it would be pot luck which would trip first. inconvenient at best, dangerous at its worst. First one should be time delayed or S type

yeah well this one cropped up here a while back......2 RCDs..both 30mA...both in series...one feeds the house..the other the garage/outbuilding...theres a fault in the garage/outbuilding...which lets go first?...

 

[IQ Electrical]

i got confused earlier, because a member said the my ZS reading could never be lower than my ZE...

My ZS of 2.6 etc is lower than 187 omhs.....?

It's often lower because of parallel paths from bonding conductors etc. I explained in post 4.

Your 187 Ohms is Ra NOT Ze on a TT system, this is all in the On-Site Guide that you already mentioned, PLEASE read it thoroughly, there are large gaps in your knowledge and someone could be hurt because of this!

 

[Geordie Spark]

It's often lower because of parallel paths from bonding conductors etc. I explained in post 4.

Your 187 Ohms is Ra NOT Ze on a TT system, this is all in the On-Site Guide that you already mentioned, PLEASE read it thoroughly, there are large gaps in your knowledge and someone could be hurt because of this!

In mine too nowadays which I'm blaming on Alzheimer's.

 

[Engineer54]

You're scaring me Eng54. :aureola: This isn't some kinda new years resolution is it?

Not really, just that there are more than enough posters on this thread already!! lol!!

Does seem strange though, that the OP is more than satisfied with his Ra of 185 ohms or whatever, ...and for his own house!! Seems he didn't even give a thought to adding a couple of rods at the workshop end, or adding on a rod, at the existing rod for the house.... Oh well, his house and his family!!! lol!!!

 

[GLENNSPARK]

Not really, just that there are more than enough posters on this thread already!! lol!!

Does seem strange though, that the OP is more than satisfied with his Ra of 185 ohms or whatever, ...and for his own house!! Seems he didn't even give a thought to adding a couple of rods at the workshop end, or adding on a rod, at the existing rod for the house.... Oh well, his house and his family!!! lol!!!

how about a lovely gas pipe..lol....

 

[srl-8]

just to confirm, I never said I was satified with my high RA, just giving the facts and hope to get good information from the forum.

I will be looking to lower the RA, however it is deemed as ok according to the Regs..

Not really, just that there are more than enough posters on this thread already!! lol!!

Does seem strange though, that the OP is more than satisfied with his Ra of 185 ohms or whatever, ...and for his own house!! Seems he didn't even give a thought to adding a couple of rods at the workshop end, or adding on a rod, at the existing rod for the house.... Oh well, his house and his family!!! lol!!!

 

[Geordie Spark]

how about a lovely gas pipe..lol....

Aye, I was reading a post by Tony a while ago in which he said that iron / steel pipe was better than lead as a conductor - which is what the old gas pipes were - but it goes all to hell with the plastic stuff we use nowadays.