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 ???

 

[Engineer54]

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..

The Reg's are badly written on this point, it leaves newly trained and inexperienced electricians with the idea that 200 ohms is the figure to comply with, ie the max Ra. It doesn't mean that at all.... They are only referring to stability, which i find is wishful thinking, at anywhere near 200 ohms with a 1.2m rod. You need ''Depth'' to achieve that!!!

 

[Ponty Massive]

IMO. Ra below 200 ohms which you have measured correctly. R1 + R2 sounds about right. If you tested the Zs correctly you would have had the bonding for gas and water in place? Your probably getting such a low reading of Zs because of the bonding.

Test the Ra again but this time with all bonding and earth connected to the MET and you will probably get a reading of around 2 ohms.

As far as your max Zs readings and the BGB, these are the maximum readings you would need to meet your required disconnection times on a TN system. As you are installing circuits on a TT system, then you will need to install a RCD which means your disconnection times will be met providing you don't have a Zs of greater than 1667 ohms.

Hope this makes sense !!!

 

[DNS1]

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

I may be completely wrong (forgive me!) but I thought it was ok to use pipework as a ground rod, as long as the owner was aware that it was being used as such (so knew not to remove/replace it without giving thought to it's electrical function)

Is this true, or complete nonsense that I've picked up somewhere? I guess in most cases the water/gas pipes are owned by the suppliers anyway, not the property owner, so this wouldn't be applicable.

 

[GLENNSPARK]

The Reg's are badly written on this point, it leaves newly trained and inexperienced electricians with the idea that 200 ohms is the figure to comply with, ie the max Ra. It doesn't mean that at all.... They are only referring to stability, which i find is wishful thinking, at anywhere near 200 ohms with a 1.2m rod. You need ''Depth'' to achieve that!!!

spend effort getting it as low as you can.....much lower than this magic 200 ohms....can be done...deep rod/s.....

 

[Geordie Spark]

spend effort getting it as low as you can.....much lower than this magic 200 ohms....can be done...deep rod/s.....

I needed a temporary TT a while ago & used a bit of scrap 15mm copper tube about 5 feet long, bashed into the lawn & got an RA of 80 something Ohms.

 

[GLENNSPARK]

I may be completely wrong (forgive me!) but I thought it was ok to use pipework as a ground rod, as long as the owner was aware that it was being used as such (so knew not to remove/replace it without giving thought to it's electrical function)

Is this true, or complete nonsense that I've picked up somewhere? I guess in most cases the water/gas pipes are owned by the suppliers anyway, not the property owner, so this wouldn't be applicable.

542.2.4.....

 

[DNS1]

542.2.4.....

I'm glad I didn't completely imagine that, even if I got it half wrong!

 

[GLENNSPARK]

I'm glad I didn't completely imagine that, even if I got it half wrong!

no probs matey...the amount of stuff i have got totally wrong in here..lol......

 

[MarkieSparkie]

I may be completely wrong (forgive me!) but I thought it was ok to use pipework as a ground rod, as long as the owner was aware that it was being used as such (so knew not to remove/replace it without giving thought to it's electrical function)

Is this true, or complete nonsense that I've picked up somewhere? I guess in most cases the water/gas pipes are owned by the suppliers anyway, not the property owner, so this wouldn't be applicable.

It used to be acceptable (<=15th edition) to use private water supply pipework as an earth electrode. "Old hands" have probably mentioned it in the past and maybe this is what you have a vague memory of.
Now of course the practice is is prohibited by 542.2.6

 

[Deleted member 9648]

The Reg's are badly written on this point, it leaves newly trained and inexperienced electricians with the idea that 200 ohms is the figure to comply with, ie the max Ra. It doesn't mean that at all.... They are only referring to stability, which i find is wishful thinking, at anywhere near 200 ohms with a 1.2m rod. You need ''Depth'' to achieve that!!!

To the OP.....ignore this.
There is nothing wrong with the UK max Ra of 200 ohms, as long as your system is protected by RCD's,preferably an up front 100ma S type at the origin,and 30ma protection to final circuits...a single rod with an Ra of 187 ohms is perfectly adequate and acceptable.
Unless you achieve a sub 2 ohm Ra you are wasting your time attempting to improve it...ie,if you drive in more rods and achieve 10 ohms,an Ra most would consider very low,your PEFC will be 23a...not a lot of use for most OCPD's,you are still reliant on the RCD for earth fault protection so you may as well have left it at 187 ohms.
As already stated,your Zs readings are low..(not high)...due to parallel paths through bonding etc and are acceptable.

 

[IQ Electrical]

To the OP.....ignore this.
There is nothing wrong with the UK max Ra of 200 ohms, as long as your system is protected by RCD's,preferably an up front 100ma S type at the origin,and 30ma protection to final circuits...a single rod with an Ra of 187 ohms is perfectly adequate and acceptable.
Unless you achieve a sub 2 ohm Ra you are wasting your time attempting to improve it...ie,if you drive in more rods and achieve 10 ohms,an Ra most would consider very low,your PEFC will be 23a...not a lot of use for most OCPD's,you are still reliant on the RCD for earth fault protection so you may as well have left it at 187 ohms.
As already stated,your Zs readings are low..(not high)...due to parallel paths through bonding etc and are acceptable.

Absolutely agreed, what is this weird obsession with trying to obtain TN values on a TT system?

Someone will post a reply regarding unreliable RCDs, I'll ask for just 1 example of a death or injury from this scenario, none can be found!

The reg's, BS 7430 (earthing) and Guidance Note 8 are perfectly clear and logical regarding TT Ra values.

 

[GLENNSPARK]

It used to be acceptable (<=15th edition) to use private water supply pipework as an earth electrode. "Old hands" have probably mentioned it in the past and maybe this is what you have a vague memory of.
Now of course the practice is is prohibited by 542.2.6

unfortunately i`m still on the red book markie which has it as 542.2.4...as i quoted......will be getting the green one shortly.....

 

[JONOXNAM]

unfortunately i`m still on the red book markie which has it as 542.2.4...as i quoted......will be getting the green one shortly.....

Get someone who is a member of the IET to get it for you as its £52 for IET members pls the onsite is £15

 

[GLENNSPARK]

Get someone who is a member of the IET to get it for you as its £52 for IET members pls the onsite is £15

its sorted and will be in the post shortly...courtesy of Widdler...£55 including postage for the BGB and the osg...not bad eh?....

 

[JONOXNAM]

its sorted and will be in the post shortly...courtesy of Widdler...£55 including postage for the BGB and the osg...not bad eh?....

Yep great, whats Widdler?