TT earth rod need to be visually verified?

[odigi]

Hi all,

just did my first PIR on a TT system and found an earthing conductor heading from the MET, under the floorboards and into the ground away from the ccu towards the house. Ze gave me 3.9 ohms.

Question is: do i need to visually verify connection to a rod (if there is one!)?

thanks

odigi

 

[IQ Electrical]

Is it going to a gas pipe or a central heating pipe under the floor?

You don't know and 3.9 Ohms is an extremely low reading for an earth electrode.

If I can't verify the means of earthing, let alone see if there's a secure connection with a BS 951 clamp, it's a code 2 on my report.

 

[odigi]

Thanks

The wooden floor is raised a foot above the ground level. The earth conductor disappears into the ground level. I put a code 2 also. Also, the upstairs flat is using the same conductor - didn't see a problem with this,but is there?

 

[IQ Electrical]

Thanks

The wooden floor is raised a foot above the ground level. The earth conductor disappears into the ground level. I put a code 2 also. Also, the upstairs flat is using the same conductor - didn't see a problem with this,but is there?

Is it a separate installation?

 

[odigi]

Yes it has its own distribution and cut out fuse

 

[Engineer54]

Yes it has its own distribution and cut out fuse

Then you have your answer odigi!!!

 

[odigi]

I see, should I advise moving the isolator, meter upstairs also? what if the upstairs refuse to get their own earth? should i connect as i found and note on PIR?

 

[Engineer54]

I see, should I advise moving the isolator, meter upstairs also? what if the upstairs refuse to get their own earth? should i connect as i found and note on PIR?

You asked if there was a problem with this arrangement, there clearly IS!! What you do to to correct the situation is totally down to you and your customer. It's no good getting upset, because you don't like the asnswer mate....

 

[odigi]

Wasn't aware that I said I didn't like the answer. I only continued to ask what could be put on PIR if anything at all.

 

[Engineer54]

Sorry, i misread your post then mate...

First things first then, have you checked all possible accessible places where connection is possible to extraneous pipework in this flat?? Is it not possible to undertake installing/driving in your own rods for this installation?? As it stands at the moment, your customer is getting a code 2, and maybe a code 1 for this unverified earthing/bonding source if left in it's present situation.

As for this shared connection with another flat, ...have you tried talking to these neighbours, explaining the situation?? Maybe by quoting a competitive price for rectifying the situation!! Think you need to talk to both parties together, before deciding on a course of action

 

[odigi]

Oh I think I see how you read it now! I wasn't being sarci those where genuine questions :!

thanks for your advice - I was hoping to drive one rod but as you suggest I could quote for two. But the question is still: if they say no, should reconnect them to the downstairs or leave them disconnected? I would see this as leaving an installation worse than i found it, albeit not the installation I started on!

 

[vitoboy]

yes i agree ..advise for a new rod in a good place for your customer....advise the other flat on the situation if you can...quote them...of course..for a new rod also...but other wise its up to the customer what they do...
i find if its an option then the other party will just ignore it..
had one with no earth at all...still didnt want it sorted.that was few years ago and still like it now....we can only try

 

[Engineer54]

Though you would in effect be disconnecting a connection, albeit of unknown origin, you wouldn't i hope disconnect this cable from the neighbours installation without informing them before hand, there-by giving them prior notice, to either make there own arrangements, or to employ yourself to make the necessary rectifications.

This is a PIR job your doing at the moment i understand, so in all honesty you could just record your findings and leave it at that, with recommendations. The problem comes if your customer wants you to rectify their installation. It's then that you would have to disconnect that unknown earthing/bonding source from the upstairs flat installation....

 

[vitoboy]

but 54
why will he have to disconnect the other flat?
just put in new for your customer and leave the other flat as is.

 

[Engineer54]

Because no-one knows if this is a bonding connection or a rod connection, ...at 4ohms it is more likely to be a bonding connection. Now you can't leave a main bonding connection from one properties installation that's also a part of another totally independent installation.

 

[odigi]

never occurred to me to leave the earthing for upstairs and advise a new rod for downstairs - makes sense now! nice cup of tea with both parties if they decide to rectify. cheers for help with this

 

[odigi]

As long as I don't disconnect it when the time comes. And inform them what I 'assume' is there i.e. bonding

 

[Engineer54]

Bit of wishful thinking there odigi, that earthing cable that disappears under the floor is much more likely to be connected to a gas or water pipe in the downstairs flat than any earth rod...

 

[Engineer54]

Can't have a bonding connection for one property, that's located in another property, .....and it's Still inaccessible!!!

 

[odigi]

true, but as said earlier they have the last say

 

[domestic spark]

had a similar problem last week, TT system 6mm green, going off somewhere, looked for earth electrode nothing, front/back, under slabs plant pots, in bushes, couldnt find it anywhere,
ZE of 0.30ohms pefc of 609a

hmmm, taking main earth from pipework. had no way of lifting boards upstairs, but 6mm green went up through the ceiling and along the house. MUST have gone to central heating pipe(s)

 

[domestic spark]

havent really dealt with TT though. always TN-CS OR TN-S

 

[Golden_Boy]

When I bought this house it was 2 flats, each with its own concentric overhead feed ( from diffrent supply poles ), cutout ,meter and fusebox. BUT a shared earth rod. Its all one house now but there are still flats up and down my road with exactly the same setup.

 

[Engineer54]

When I bought this house it was 2 flats, each with its own concentric overhead feed ( from diffrent supply poles ), cutout ,meter and fusebox. BUT a shared earth rod. Its all one house now but there are still flats up and down my road with exactly the same setup.

What are you saying here, ....are you saying that this arrangement is OK then??

 

[Golden_Boy]

What are you saying here, ....are you saying that this arrangement is OK then??

Not at all. Just that it was how things where here when I bought the property, and was like that for ages before I made a big house out of the 2 flats, then of course I only have one of everything. Still the same rod though ( Ze 14.7 Ohms )

 

[Engineer54]

That's a good value Ra you have there for a single rod. What sort of soil type do you have there a clay type soil or a sort of marshy land soil?? I reckon if you supplement your existing rod with another you stand a very good chance of a single figure value...

 

[domestic spark]

Engineer, the good book says earth rod ok no greater ZE of 200ohms. However i would be happy with a rod reading that high, and i want what's safest for customer, what max value would you expect or accept. You seem extremely clued up on TT systems. Thanks I'n advance

 

[IQ Electrical]

Engineer, the good book says earth rod ok no greater ZE of 200ohms. However i would be happy with a rod reading that high, and i want what's safest for customer, what max value would you expect or accept. You seem extremely clued up on TT systems. Thanks I'n advance

200 Ohms is an excellent reading-remember that your actual maximum is 1667 Ohms and the 200 Ohms figure is to allow a massive margin for soil drying and freezing etc.

A deeply driven earth electrode with an Ra of 400 Ohms will be far more stable and effective than a shallow driven electrode with an Ra of 100 Ohms because the freezing/drying effects do not penetrate the ground past the first 300mm or so.

You have to understand the purpose of the electrode when looking at Ra values, there is absolutely no point in trying to achieve TN system impedance/resistance readings on a TT system and even if you did get down that low, can you imagine trying to explain your theory of stability?

 

[vitoboy]

wire it
my nic inspector told me just this week that he would accept a tt system with a 450 ohm plus ze.
that is being in a problem area and having taken steps to try and improve ..ie adding another rod.
i can say i was amazed by this...but happy....loads of my jobs have 300-500 ohm earth stakes

 

[Engineer54]

The reasoning behind my philosophy (which i totally stand behind) on having as low a Ra value as possible, is that the lower you can get that value, the better chance you have of forcing a protective device to trip, even if it is outside the required disconnection times. Your not going to achieve that with a single 1.2 3/8'' rod bunged in the ground i might add!! lol!!

So yes, an Ra of 200 ohm is very good (tongue in cheek ...lol!!) , but ONLY while an RCD device is functional!!! Totally useless without that RCD protection. And we should all know, that RCDs aren't the most reliable bits of kit at the present time, you only have to look at the threads in these forums, ...almost every day there is at least one thread or another complaining about failing RCD devices... I'd hate to totally rely on a RCD for my families protection, well i just wouldn't, full stop!!

Another of my hang-ups with TT systems, is the lack of understanding of what makes a good, or stable TT system. Most TT systems these days rely on a single 1.2m 3/8'' rod, bunged in the ground where ever it's easiest to drive it, and normally right by the side of an outside wall where all the old building rubble is normally found!!!

There are many ways of getting a TT system in a ''stable'' state and keeping it that way, not always practicable in domestic installations i might add!!! Which is why they should be considered as a last choice of earthing an installation, it is always better to have a TT systems supply PME'd if it is available on your DNO's local network....

IQ, ...I personally know of domestic TT systems in parts of Essex that by careful placing, 2 x 3m 3/4'' rods (extended) achieve an Ra equal or bettering a standard TN-S earthing arrangement, and that is stable year round without any soil conditioning, i jest not!! I'm sure that there are other areas in the UK, where similar results can be attained..

 

[Golden_Boy]

That's a good value Ra you have there for a single rod. What sort of soil type do you have there a clay type soil or a sort of marshy land soil?? I reckon if you supplement your existing rod with another you stand a very good chance of a single figure value...

THe rod is well away from the propery and the earth is almost all damp soggy clay.

 

[IQ Electrical]

The reasoning behind my philosophy (which i totally stand behind) on having as low a Ra value as possible, is that the lower you can get that value, the better chance you have of forcing a protective device to trip, even if it is outside the required disconnection times. Your not going to achieve that with a single 1.2 3/8'' rod bunged in the ground i might add!! lol!!

So yes, an Ra of 200 ohm is very good (tongue in cheek ...lol!!) , but ONLY while an RCD device is functional!!! Totally useless without that RCD protection. And we should all know, that RCDs aren't the most reliable bits of kit at the present time, you only have to look at the threads in these forums, ...almost every day there is at least one thread or another complaining about failing RCD devices... I'd hate to totally rely on a RCD for my families protection, well i just wouldn't, full stop!!

Another of my hang-ups with TT systems, is the lack of understanding of what makes a good, or stable TT system. Most TT systems these days rely on a single 1.2m 3/8'' rod, bunged in the ground where ever it's easiest to drive it, and normally right by the side of an outside wall where all the old building rubble is normally found!!!

There are many ways of getting a TT system in a ''stable'' state and keeping it that way, not always practicable in domestic installations i might add!!! Which is why they should be considered as a last choice of earthing an installation, it is always better to have a TT systems supply PME'd if it is available on your DNO's local network....

IQ, ...I personally know of domestic TT systems in parts of Essex that by careful placing, 2 x 3m 3/4'' rods (extended) achieve an Ra equal or bettering a standard TN-S earthing arrangement, and that is stable year round without any soil conditioning, i jest not!! I'm sure that there are other areas in the UK, where similar results can be attained..

Oh I believe it, soil resistivity varies enormously.

I hear what you're saying on the RCD reliability point but you have to remember that on faults of negligible impedance, RCD's have a virtually identical type tested failure rate to BS EN 60898 devices.

Imagine if we could test MCB's in the same way we can test an RCD, just at In rather than the usual fault values of a negligible impedance fault, I think we'd see a rather frightening figure then!

A TT system installed as per BS 7671:2008 is perfectly safe and after 25 years in the electrical business, I have yet to hear of a death or injury as a result of a failed RCD on a TT system.

 

[Engineer54]

IQ,

Not so sure at all, about MCBs having a similar failure rate as RCDs, the technology is much older and most of the earlier problems with MCCBs and MCBs have been well and truly sorted over the years. I'm sure in the course of time, RCDs of all types will also be just as reliable as the modern breakers. From my own experience they most certainly are Not at the present time!!!

Never been one to put much credence to reports and surveys, after reading many of them, i can normally pull them to pieces. You can make these things say, or steer them towards whatever you want them to say, very much in the same way creative accounting can make a poor set of figures say, ....All Is Good!! ...or Vicea Versa lol!!!

As for deaths and injuries from failed RCDs on TT systems, who knows, it's not news worthy news these days, these events (if they happen) just wouldn't attract media attention. It would probably take several days to officially determine cause one way or the other, which would further cloud the waters...lol!! Same as PME really, there are plenty of sparks that'll stand-up and tell you they have known of several cases of neutrals being lost, but have they, and have anyone been injured or killed in these so called known instances??? Again who knows ...lol!!

What i do know, is that i personally will always do my uppermost to get as lower Ra as possible. As far as i'm conserned total reliance on an RCD in a domestic situation is something to be avoided whenever and wherever possible. I know and am aware, that in the present economic climate, it can be hard to justify the time and expense needed for installing a decent TT system, especially on a domestic. But in some circumstances, such as these caravan site installations on recent threads, i would definitly be going that extra mile to get those lower Ra values....

 

[Engineer54]

THe rod is well away from the propery and the earth is almost all damp soggy clay.

Your luckier than most GoldenBoy, the electrician took the time to find a decent area to drive his rod, and well away from any builders rubble!! As i said mate, if you choose to drive another rod outside of the area of influence of the existing rod, you stand a very good chance of bringing your Ra down to a single figure.... well worth the effort in my opinion...

 

[IQ Electrical]

Your luckier than most GoldenBoy, the electrician took the time to find a decent area to drive his rod, and well away from any builders rubble!! As i said mate, if you choose to drive another rod outside of the area of influence of the existing rod, you stand a very good chance of bringing your Ra down to a single figure.... well worth the effort in my opinion...

And a waste of time in mine

We'll agree to differ on this one I feel!

 

[Chr!s]

Oh I believe it, soil resistivity varies enormously.

Imagine if we could test MCB's in the same way we can test an RCD, just at In rather than the usual fault values of a negligible impedance fault, I think we'd see a rather frightening figure then!

A TT system installed as per BS 7671:2008 is perfectly safe and after 25 years in the electrical business, I have yet to hear of a death or injury as a result of a failed RCD on a TT system.

From several reports ive seen, its been a case where the Tester has failed and not the RCD, many people seem to misunderstand how installation faults affect RCDS not only upon testing but also how they may be affected in operation.

If people tested the RCDs in isolation i think far fewer would complain of failures.

I hear what you're saying on the RCD reliability point but you have to remember that on faults of negligible impedance, RCD's have a virtually identical type tested failure rate to BS EN 60898 devices.

Its a very interesting and valid point IQ.

 

[Engineer54]

And a waste of time in mine

We'll agree to differ on this one I feel!

We will indeed!! lol!!

 

[Engineer54]

From several reports ive seen, its been a case where the Tester has failed and not the RCD, many people seem to misunderstand how installation faults affect RCDS not only upon testing but also how they may be affected in operation.

If people tested the RCDs in isolation i think far fewer would complain of failures.

Its a very interesting and valid point IQ.

The last 3 projects or so that i've been on, the contractor was sending boxes of these things back for replacement. Several different types, all top of the range MG stuff too, from Vigi units to RCBOs, RCCDs to S types, all from single pole to four pole. ....All or most of them went through the contractors test benches before being shipped back. MCB and MCCB failures were next to none in comparison... As i say, i prefer to go by personal experience, than anything in a report or survey...