TT earth rod need to be visually verified?

[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..