Zs testing on TT

[Ben Butcher]

Hello all,

Many Thanks to everyone on previous questions, I am new to forums generally but it sometimes feels like there's a big bus full of keen knowledgeable people outside that can answer any question!

So todays question; Whilst doing some work at a house yesterday I found myself wondering if some results I got could be correct. A TT installation, and firstly I measured Ze as you would elsewhere, I imagine the result I got of 205 ohms would be expected? and is this recorded as Ze or Ra? Secondly whilst doing Zs at the far point I got 83 ohms, this obviously would not comply with the tabulated maximums or are they for TN systems only? Also how can the Zs be lower than the Ze, is it just because of paralell paths and water main etc?

Any guidance would be gratefully received, Ben.

 

[ackbarthestar]

I'm afraid the powers that be, don't agree with you. As far as they are concerned any test reading that includes any type of TT system electrode will be valued as Ra... Storm in a tea cup really, i think everyone knows what OP's mean, when they state testing Ze on a TT system. ...lol!!!

I'd probably agree with that sentiment.
A bit odd though that we all sniff around looking at the minutia of BS7671 statements and then find this one open to any interpretation

 

[Ben Butcher]

Few! nice people with reasonable advice, I thank you all. We are all learning forever (me perhaps more than some at the mo...) but I will continue to ask if I feel it might make my work better, and I don't shy away from criticism, if it's going to help overall bring it on!

 

[Engineer54]

I'd probably agree with that sentiment.
A bit odd though that we all sniff around looking at the minutia of BS7671 statements and then find this one open to any interpretation

BS 7671 is full of holes mate, along with the OSG's. It's been a good many years since i have totally relied on the gospels of BS7671 or the Reg's as they were previously known...lol!!! Not to say that the new guy's and learners shouldn't take a lot more notice of them than say myself!!

 

[Engineer54]

Few! nice people with reasonable advice, I thank you all. We are all learning forever (me perhaps more than some at the mo...) but I will continue to ask if I feel it might make my work better, and I don't shy away from criticism, if it's going to help overall bring it on!

Your more than welcome Ben....

 

[IzzyS]

Ben,

I seem to come across more than my fair share of TT systems where the "rod" is actually the incoming water pipe. Just make sure you can actually inspect the connection - if you can't find it then it's not really BS7671 compliant, regardless of its value. Unless of course it is documented as a maintenance free connection to rod / strip / plate etc..

If it's a new install you've done, then I'd suggest trying to get the reading down a bit. As already stated the NICEIC suggests values >100 Ohms may be unstable. Amazingly this actually agrees with BS7430 (code of practice for earthing) as below:

With I Delta n equal to 30 mA the resistance of the earth connection should not be greater than 100 ohms since a resistance in excess of this value may be unstable and therefore unsuitable.

And don't forget that not all RCDs in a TT install are necessarily rated at 30 mA, so you may need an even lower Ra (or Ze or whatever)

 

[Gavin A]

I'm going to post this as a question rather than a statement as I've not got the regs with me, but aren't we now supposed to be ensuring either a proper TT earthing arrangement, or that the DNO fits a PME in situations where the original earthing arrangement was via the water main?

I believe it used to be classed as ok if the incoming water main was a private supply for x meters outside the property, but I'm pretty sure that this got removed at the last regs updating. I'm partly aware of this because our house is a TT system using the incoming water main as the earth, which was allowed in the regs when I checked a few years back as it was a private supply, but wasn't when I checked again a few months back, so we're now awaiting the DNO to fit a PME.

The reason for the change being the water companies are rapidly going round whole areas replacing the metal pipe with plastic, and the same with the gas, so as soon as they do that the house loses its earth connection. We've come across 2 houses in the last year where this had actually happened in the previous few months, and the Ze readings were well over the limit when we took them even with the bonding connected to water and gas. I'd expect that 90% of that whole estate had actually lost it's earth protection, which we did mention to the DNO, but apparently it's only their problem if they're supplying the earthing arrangements, and it's nobodies problem other than the householders if the original builders didn't pay for a PME connection.

I also thought the bonding to the water and gas pipes was supposed to be disconnected now when taking the Ze readings, presumably to pick up on situations where the water and gas pipes are supplying the earthing.

Happy to be corrected, but I'm pretty sure we double checked the regs on this quite thoroughly.


(fwiw, we're happy enough to leave it as is if the Ze is ok with the water connected, as long as the customer rings the DNO in our presence to arrange for a PME to be installed, but we've had to bang earth spikes in temporarily while waiting for the DNO to come out to fit a PME on the couple of occasions where the pipes had already been done).

 

[Engineer54]

Ben,

I seem to come across more than my fair share of TT systems where the "rod" is actually the incoming water pipe. Just make sure you can actually inspect the connection - if you can't find it then it's not really BS7671 compliant, regardless of its value. Unless of course it is documented as a maintenance free connection to rod / strip / plate etc..

If it's a new install you've done, then I'd suggest trying to get the reading down a bit. As already stated the NICEIC suggests values >100 Ohms may be unstable. Amazingly this actually agrees with BS7430 (code of practice for earthing) as below:

With I Delta n equal to 30 mA the resistance of the earth connection should not be greater than 100 ohms since a resistance in excess of this value may be unstable and therefore unsuitable.

And don't forget that not all RCDs in a TT install are necessarily rated at 30 mA, so you may need an even lower Ra (or Ze or whatever)

Sorry to correct you, but if an installation has no made earth electrode then you can't class it as a TT system. What your relying on is a bonding connection to an incoming service. If there is no chance of a PME service available from the DNO, then you need to make a made earth electrode, normally by means of a driven rod(s). Stability can usually be achieved by driving at least 2 X coupled 5/8'' rods to give you the depth. This will also in most cases give you a far superior Ra value than those given in BS7671 and BS7430...

Both 200 ohm and 100 ohm Ra values are pretty useless as far as providing any form of protection to a TT system, which is the main reason for the requirement of RCD devices on TT systems. Unfortunately RCD's are not the most reliable of protection devices. So i would always recommend the addition of a 100mA S type up-front unit (maybe as your main switch in a CU) as a back-up for a failed or faulty RCD/RCBO....

 

[spark 68]

As Far as I am aware you can still use a private water supply pipe as an earth with the permission of the supply owner, the operative word here is PRIVATE, ie. as in not public , for the reasons you correctly state, gas has never, at least as far back as the 16thed. (may be earlier) been allowed for this purpose.

 

[Amp David]

Blimey, it reminds me of my first week on my 5 week course this,

 

[GLENNSPARK]

you could define it as you like, ze zs depends on where in the system you are, at origin, one sub main is a zs of another, which becomes a ze of another, on most paper work ra is the earth rod with the main earthing conductor, on its own. legal limit 200ohms. But i know in many places this can not be achived, most rcd's guarantee, they will operate at 1000 ohms

what happened to Zdb then?

 

[Engineer54]

you could define it as you like, ze zs depends on where in the system you are, at origin, one sub main is a zs of another, which becomes a ze of another, on most paper work ra is the earth rod with the main earthing conductor, on its own. legal limit 200ohms. But i know in many places this can not be achived, most rcd's guarantee, they will operate at 1000 ohms

If you can't achieve an Ra of 200 ohms, ...then something has to be very wrong!! lol!!!

 

[GLENNSPARK]

If you can't achieve an Ra of 200 ohms, ...then something has to be very wrong!! lol!!!

shallow rod

 

[Gavin A]

As Far as I am aware you can still use a private water supply pipe as an earth with the permission of the supply owner, the operative word here is PRIVATE, ie. as in not public , for the reasons you correctly state, gas has never, at least as far back as the 16thed. (may be earlier) been allowed for this purpose.

I believe it changed in the last year or so, probably when the 17th edition update happened.

I had to check the 2 different versions of 17th to know that it had changed though IIRC, which I only did because it impacted on my house, and I knew for a fact that it used to be allowed.

I'll see if I get time to check though, as this is going to bug me now if I don't.

 

[Engineer54]

Doesn't really matter if it is still allowed or not, you can't in all honesty say that an installation relying on what is a bonding connection, is a TT system. A TT system requires a made electrode, be it by means of a rod, plate, Ufer, or other made electrode type...

 

[Gavin A]

I've just checked, and it's not changed at all - thinking about it, I think I have one of my electricians 17th edition update course to thank for thinking it had changed.

It does have a bit about precautions needing to be taken against it's removal, but I'd think this would be covered by the usual warning tag, so as long as that's in place I'd think it would be fine still.

sorry for questioning people on this.