Very Hi ZS Reading on TT system? any advice

Post new thread

Discuss Very Hi ZS Reading on TT system? any advice in the Australia area at ElectriciansForums.net

mechelec said:
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
Click to expand...
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?...
 
Last edited:
srl-8 said:
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.....?
Click to expand...

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!
 
IQ Electrical said:
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!
Click to expand...

In mine too nowadays which I'm blaming on Alzheimer's. :(
 
Marvo said:
You're scaring me Eng54. :aureola: This isn't some kinda new years resolution is it?
Click to expand...


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!!!
 
Engineer54 said:
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!!!
Click to expand...
how about a lovely gas pipe..lol....
 
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..


Engineer54 said:
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!!!
Click to expand...
 
srl-8 said:
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..
Click to expand...

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!!!
 
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 !!!
 
GLENNSPARK said:
how about a lovely gas pipe..lol....
Click to expand...

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.
 
Engineer54 said:
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!!!
Click to expand...
spend effort getting it as low as you can.....much lower than this magic 200 ohms....can be done...deep rod/s.....
 
DNS1 said:
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.
Click to expand...
542.2.4.....
 
DNS1 said:
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.
Click to expand...

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
 
Last edited by a moderator:
Engineer54 said:
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!!!
Click to expand...

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.
 
wirepuller said:
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.
Click to expand...

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.
 
MarkieSparkie said:
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
Click to expand...
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.....
 
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.....
 
wirepuller said:
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.
Click to expand...

IQ Electrical said:
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.
Click to expand...


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!!
 
Engineer54 said:
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!!
Click to expand...
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.....
 
Specialist said:
Geezus, is this still going. It's getting a bit like War & Peace:tounge_smile:
Click to expand...

There's far more fiction here than there ever was in War & Peace, I back up my arguments with all available statistics, British Standards and years of experience installing perfectly safe TT installations that meet the requirements of those standards.

Of course, the requirements are guided minima and if anyone wants to fill up a garden with copper to slavishly attempt to attain Ra values akin to a TN system then go for it but at least ponder beforehand the futility and costs of the task!
 
IQ Electrical said:
There's far more fiction here than there ever was in War & Peace, I back up my arguments with all available statistics, British Standards and years of experience installing perfectly safe TT installations that meet the requirements of those standards.

Of course, the requirements are guided minima and if anyone wants to fill up a garden with copper to slavishly attempt to attain Ra values akin to a TN system then go for it but at least ponder beforehand the futility and costs of the task!
Click to expand...
anybody would think you were going to "fill a garden up" with gnomes ...then link em all up in series............whatever happened to depth???......
 
GLENNSPARK said:
anybody would think you were going to "fill a garden up" with gnomes ...then link em all up in series............whatever happened to depth???......
Click to expand...

You'd need some depth to get to TN values in most UK soils!

I'd take a 400 Ohm Ra reading on a 2.4M rod anytime rather than 50 Ohms on a 1M rod though, depth is indeed king!

E54 keeps mentioning this 1M rod as if that's all anyone here uses!
 
IQ Electrical said:
You'd need some depth to get to TN values in most UK soils!

I'd take a 400 Ohm Ra reading on a 2.4M rod anytime rather than 50 Ohms on a 1M rod though, depth is indeed king!

E54 keeps mentioning this 1M rod as if that's all anyone here uses!
Click to expand...
agreed on the depth thing IQ......better to have a higher Ra ...with stability...than a lower one...but only a shallow rod......until the sun pelts down of course..lol...watch it rocket..lol...
 
GLENNSPARK said:
agreed on the depth thing IQ......better to have a higher Ra ...with stability...than a lower one...but only a shallow rod......until the sun pelts down of course..lol...watch it rocket..lol...
Click to expand...

And that's where depth comes into it, you are seeking depth to get past the level of drying and freezing, both of which will compromise your Ra values.
 
That's what all the girls say, can't beat a bit of length.
 
Last edited:
GLENNSPARK said:
so why do we keep seeing these shallow rods ....all connected in series...as if thats gonna do owt..lol.....
Click to expand...

Rods are available in a few lengths to suit a variety of applications, there could be cases where a rock shelf prevents good depth so multiple rods have to be used, not ideal but it does happen, you are also not just restricted to rods as an electrode...
 
Last edited:
IQ Electrical said:
There's far more fiction here than there ever was in War & Peace, I back up my arguments with all available statistics, British Standards and years of experience installing perfectly safe TT installations that meet the requirements of those standards.

Of course, the requirements are guided minima and if anyone wants to fill up a garden with copper to slavishly attempt to attain Ra values akin to a TN system then go for it but at least ponder beforehand the futility and costs of the task!
Click to expand...

So where's the fiction?? You back up your arguments with ''Statistics'' haha!!! Who's statistics?? have you never heard that old saying about Statistic's, it basically tell's you Never to trust statistic's and for very good reason. The very same figures, can be made to say anything the authour wants them to say, ....An that's a well proven ''Fact!!''

Are you really trying to convince me that a 1m rod with a Ra of 200ohms is and will remain safe for the duration of it's lifespan, even when the paralleled bonding pipes are replaced with plastic services?? Come on IQ pull the other leg, it's got bigger bells on!!

Now, no-one is saying you should slavishly attempt to obtain TN values from a TT system, or filling a garden with copper, that's twisting what i have said here, they are Your words trying to be clever, NOT mine!! What i will say is, in generally found garden British soils, i could in most circumstances achieve a below 10ohm Ra with a maximum of 2 twin rod positions. If that is filling a garden with copper, so be it ...lol!!
What i'm also saying is, Not even trying to improve, a crap Ra value with a deeper rod, or at a second location, to me doesn't say much for an electrician that's installing/creating a TT system. Also a bad Standard, is a bad Standard, whatever way you like to dress it!!

If you have, with your years of experience, installed TT systems that are anything like those that seem to be the norm these days in the UK, ie, a single 1m rod with an Ra of around the 200 ohm mark, i'm sorry, your fooling yourself big time, thinking that's perfectly safe!! That would only be safe as such, while a the prime protection RCD was functional... It sure ain't gonna be safe when it isn't functional!!!

The only way i would feel even half happy leaving such a TT system in place, is if the CU in question, had an up-front 100mA S type RCD and all the circuit protective devices were RCBO's. At least then you have 2 RCD devices covering your bum, unlikely that both will fail, but then again, it's not unheard of either...
 
IQ Electrical said:
You'd need some depth to get to TN values in most UK soils!

I'd take a 400 Ohm Ra reading on a 2.4M rod anytime rather than 50 Ohms on a 1M rod though, depth is indeed king!

E54 keeps mentioning this 1M rod as if that's all anyone here uses!
Click to expand...


That's right, would you like to put any money on this one IQ, i've been looking at several well known wholesalers in the UK, and that is the ONLY length they stock, be they 3/8'' or 5/8'' !! I haven't seen one, that stocks 3/4'' rods of any length!!

So that's exactly what everyone is using, unless they are purchasing direct from people like Furse!!!
 
I have installed and inspected many small-medium TT systems in the south east UK over the 30+ years I have been in the trade.Most of those have utilised 1m rods.Off the top of my head I cant recall an Ra greater than about 250 ohms on an existing at any time of the year. Last year I did a temporary site hut install on the north downs...(chalk)...in the height of summer on very dry ground and got an Ra of around 350 ohms with a single rod. Linking two 1m rods got it down to around 120 ohms...more than acceptable.
Given that 30ma RCD protection was provided, in practice the 350 ohms Ra would have provided satisfactory earth fault protection even though it exceeded the UK recommended maximum.
No doubt E54 will be appalled and call me everything from incompetant to downright dangerous. But like IQ,I have never heard of an incident on a UK TT which could be attributed to a UK TT Ra which would have been prevented by an Ra of the level E54 advocates.
For that reason I am content to adhere to Bs 7671 requirements for TT systems,and have no doubts over the safety of such systems.
It is my belief that E54's posts on Ra values are misleading to those who seek advise on TT requirements...which is why I continue to open this particular can of worms.
 
IQ Electrical said:
Rods are available in a few lengths to suit a variety of applications, there could be cases where a rock shelf prevents good depth so multiple rods have to be used, not ideal but it does happen, you are also not just restricted to rods as an electrode...
Click to expand...
i agree that there are other means of earthing (tt) than rods ...such as tape/s and/or plate/s....
 
Engineer54 said:
That's right, would you like to put any money on this one IQ, i've been looking at several well known wholesalers in the UK, and that is the ONLY length they stock, be they 3/8'' or 5/8'' !! I haven't seen one, that stocks 3/4'' rods of any length!!

So that's exactly what everyone is using, unless they are purchasing direct from people like Furse!!!
Click to expand...


Do they not also sell a coupler that fits the 1.2M rod giving at X2 the 2.4M that I mentioned a few posts back?
 
Haha, the last person i need to teach me how to get a TT system to protect an installation, or how to create a TT system is you!! If anyone is misleading anyone here on Ra values and stability it's those that advocate BS7671!! You open these can of worms, because you are satisfied with an inadequate system based on an inadequate section in the BS7671 that has been in need of revision for too long..

Frankly i don't give a dam what you are prepared to walk away from on a TT system, but instructing those that are inexperienced, to ignore sound advice to improve a crap Ra value based on what is a nonsense Ra value because you happen to be happy with it, beggars belief!! As i thought you use 1m rods like everyone else and even after all these years of experience ,still actually believe what you have installed is a good, stable and reliable TT system,. ....NO Comment!!!
 
IQ Electrical said:
Do they not also sell a coupler that fits the 1.2M rod giving at X2 the 2.4M that I mentioned a few posts back?
Click to expand...

Come on now, and just how many actually buy those couplers or come to that, the second rod... NO need IQ, ...they only need 200 ohms ...and what's stability all about, when it's at home, do we really need it .... It rarely happens, and You know it doesn't!!!
 
New posts

Reply to Very Hi ZS Reading on TT system? any advice in the Australia area at ElectriciansForums.net

Similar Threads

N
TT System High Ze leading to possible Zs fails
Doing a lot of EICRs at the moment and have came across what I'm sure is a common enough problem. In an off grid rural cottage I have a TT system...
Replies
20
Views
3K
davesparks
davesparks
Simthespark
3 lead low Err5 code Fluke 1664FC
Hi All New to this forum, have read the posts on here from google but only recently signed up. I'm having some issues and some input would be...
Replies
13
Views
2K
timhoward
timhoward
S
Curious fault after recent move
Hi All Let me start by saying I have my 2365 level 3 but never worked in the industry so my knowledge is limited. My partner and I recently...
Replies
22
Views
2K
UNG
U
M
Advice on safety of new Consumer Unit Install
Hi all, new to posting but been reading for years. I inherited a house from my grandmother who passed in 2021, I intend to renovate and rent out...
2
Replies
97
Views
10K
nicebutdim
nicebutdim
W
Connecting together 6mm cables
Hi.... First time I've posted here. I own a property which is rented out. Recently the Gas Oven failed a gas safety test and the Gas Engineers...
Replies
4
Views
2K
Wilf999
W

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

YOUR Unread Posts

Tags

advice aus cable circuit course earth electric electrical electrician hello led light lighting lights looking new please question rcd socket supply switch testing training wiring
This website was designed, optimised and is hosted by untold.media Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock