Very Hi ZS Reading on TT system? 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?

 

[GLENNSPARK]

what or who...lol....no seriously he`s a VERY well respected member in here matey.....although when i explained to my cousin (from hull) that the gentleman i was buying the books from`s avatar was "widdler"...well he thought that really funny.......good deal though so i aint complaining.....

 

[GLENNSPARK]

you`v got a PM eng mate....

 

[Engineer54]

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.

And the above is basically why, the UK has crap TT systems and always will do, with the perpetuation of some of the stupid comments such as these two examples...

200 ohms from a 1m rod is about as useful as using a bent screwdriver to tighten a screw, be it for the Ra or it's stability! The UK's obsession with this crazy figure of 200ohms (which is based only on stability) is about as far from logical as you can get. No other country that i know of, has or mentions such a high limit, of anywhere approaching that figure. Yet both of you are trying to defend it!! It is indefensible and that section of the Reg's is in dire need of a revision. ...In fact there was talk of a lowered figure being pushed for inclusion into the last batch of amendments...

No-ones trying to obtain disconnection times equal to a TN system, but that's not by any means an impossibility either. But the lower an Ra is, the better the chances of having a protective device trip in a relatively decent enough time, not to cause fire or maybe possible injury, despite a non-functioning cure-all RCD... Great if you have parallel earths from water/gas bonding to help you along, well until they come along and replace with plastic that is...

And yes your quite right, RCD's DO fail and a lot more often than either of you will, or care to admit too. Just because YOU haven't read anything about an RCD causing fire, injury or even a death, doesn't mean it hasn't happened, can't happen, won't or will happen!! Smugness like this, just doesn't become you!!!

What a sorry state the UK TT systems are these day's, ...in the past it was nothing to find many if not most old TT systems with Ra's 5 ohms or lower, even 10 ohms was considered to be on the high side, (even the Yanks talk about trying to improve a 20 ohm Ra with an additional Rod!!) Yet here we are talking in the realms of 200 ohms, and even experienced electricians trying to defend this as all fine and dandy. Why, because we now have the cure-all RCD devices, Oh, ...and it's written in there all seeing all knowing bible, so it must be good-un!! ...lol!!

 

[Specialist]

Geezus, is this still going. It's getting a bit like War & Peace:tounge_smile:

 

[GLENNSPARK]

And the above is basically why, the UK has crap TT systems and always will do, with the perpetuation of some of the stupid comments such as these two examples...

200 ohms from a 1m rod is about as useful as using a bent screwdriver to tighten a screw, be it for the Ra or it's stability! The UK's obsession with this crazy figure of 200ohms (which is based only on stability) is about as far from logical as you can get. No other country that i know of, has or mentions such a high limit, of anywhere approaching that figure. Yet both of you are trying to defend it!! It is indefensible and that section of the Reg's is in dire need of a revision. ...In fact there was talk of a lowered figure being pushed for inclusion into the last batch of amendments...

No-ones trying to obtain disconnection times equal to a TN system, but that's not by any means an impossibility either. But the lower an Ra is, the better the chances of having a protective device trip in a relatively decent enough time, not to cause fire or maybe possible injury, despite a non-functioning cure-all RCD... Great if you have parallel earths from water/gas bonding to help you along, well until they come along and replace with plastic that is...

And yes your quite right, RCD's DO fail and a lot more often than either of you will, or care to admit too. Just because YOU haven't read anything about an RCD causing fire, injury or even a death, doesn't mean it hasn't happened, can't happen, won't or will happen!! Smugness like this, just doesn't become you!!!

What a sorry state the UK TT systems are these day's, ...in the past it was nothing to find many if not most old TT systems with Ra's 5 ohms or lower, even 10 ohms was considered to be on the high side, (even the Yanks talk about trying to improve a 20 ohm Ra with an additional Rod!!) Yet here we are talking in the realms of 200 ohms, and even experienced electricians trying to defend this as all fine and dandy. Why, because we now have the cure-all RCD devices, Oh, ...and it's written in there all seeing all knowing bible, so it must be good-un!! ...lol!!

I had one the other day on an EICR...RCD let go alright ..on test that is....but came to the manual (push button) test at the end of the 6.....wouldn`t let go......an instant condemnment.....