TT High Zs

[paulk11]

Evening. Did a Zs test today on a TT lighting circuit and got a reading of 335ohms. I haven't checked Ze yet but I'm presuming that's also in the same ball park. I'm aware of the upper limit Zs of 1667 and the advice contained within the good book that states Zs should be below 200, so I'm in the middle. I've advised homeowner that it should be reduced if possible and I see a couple of options; Add an earth rod or increase the main earth with a 16mm CSA cable (it's currently only 6mm). It's been pouring with rain all week so no point adding water. Any other suggestions? Thanks

 

[Octopus]

Changing the CSA of the cable to the stake isn't going to help that much with a RA that high... best add a 2nd and possibly a 3rd rod, and make sure they go nice and deep!

 

[paulk11]

Changing the CSA of the cable to the stake isn't going to help that much with a RA that high... best add a 2nd and possibly a 3rd rod, and make sure they go nice and deep!

Where do we stand regarding the 200ohm limit though? If max is 1667 then why worry at all?

 

[Rocboni]

It is considered unstable above 200ohms

 

[Dave OCD]

Changing the cable from 6 to 16mm will only make a tiny difference, less than 1 ohm for sure.
Although the high Ze is strictly speaking acceptable combined with 30ma RCD protection I'd try and get it lower.
A decent 5/8 rod even 2 coupled together should make a difference although I know how hard it can be to get a rod in the ground in many places 'down west' in Cornwall.

 

[paulk11]

Yes

It is considered unstable above 200ohms

Yes, I got that bit as its stated in 7671 but can someone elaborate on the 'unstable' bit. What does that actually mean? Why should we worry as the max permitted is much higher? Trying to comprehend it (still). Cheers

 

[paulk11]

Changing the cable from 6 to 16mm will only make a tiny difference, less than 1 ohm for sure.
Although the high Ze is strictly speaking acceptable combined with 30ma RCD protection I'd try and get it lower.
A decent 5/8 rod even 2 coupled together should make a difference although I know how hard it can be to get a rod in the ground in many places 'down west' in Cornwall.

It's on an estate so may be doable

 

[bigspark17]

The max 1667 ohms applies if the breaker controlling the circuit is 30mA. I would suggest a ze test carried out and a reading of sub <200 ohms should be obtained for a stable earth reading.

 

[Andy78]

My first thought is always to enquire if a DNO supplied earth is available.

 

[Ib≤In≤Iz]

Contact the DNO to see if PME is available

 

[westward10]

If it is on an estate probably isn't using the rod as a return anyway. Be interesting to know the Ze with the bonds isolated, could be a case of the earthing conductor doing no more than earthing the electrode.

 

[paulk11]

If it is on an estate probably isn't using the rod as a return anyway. Be interesting to know the Ze with the bonds isolated, could be a case of the earthing conductor doing no more than earthing the electrode.

When I say estate, I mean that in the loosest possible terms!! There's a few houses, big for these parts!!. Ze (or Ra on a TT) is the next move and then perhaps an extra rod. May call dno too to enquire. Thanks very much.

 

[paulk11]

The max 1667 ohms applies if the breaker controlling the circuit is 30mA. I would suggest a ze test carried out and a reading of sub <200 ohms should be obtained for a stable earth reading.

All circuits are protected by a front end 30mA RCD so happy with that.

 

[Deleted member 9648]

Yes

Yes, I got that bit as its stated in 7671 but can someone elaborate on the 'unstable' bit. What does that actually mean? Why should we worry as the max permitted is much higher? Trying to comprehend it (still). Cheers

The fact is that Bs7671 does not give a maximum Ra value and does not state that the value must be stable....only that a value exceeding 200 ohms may be unstable. Nobody has ever given a valid reason as to why it needs to be stable as far as I can recall.
People pluck values out of the air for their preferred maximum..The NIC seem to favour 100ohms,and I recall a poster on here saying he gets twitchy if it's over 80 ohms. Nobody has ever given a valid reason for their preferred value.
If your Ra is around 335ohms then it is in full compliance with Bs7671 and you need do no more, although you may choose to .
In that case pluck a figure out of the air like everyone else seems to do.

 

[Dave OCD]

Mind you 335 at the moment when it's rained heavily for ages and the ground is pretty saturated could go sky high when the ground dries out.

 

[Deleted member 9648]

Mind you 335 at the moment when it's rained heavily for ages and the ground is pretty saturated could go sky high when the ground dries out.

Probably not by around 1300 ohms though!
This is where it gets silly because of 7671's vague requirements, it's left to the tester/installer to decide what is acceptable...10...20...80...100...200 ohms? Whatever you fancy.

 

[Wilko]

Mind you 335 at the moment when it's rained heavily for ages and the ground is pretty saturated could go sky high when the ground dries out.

Yes - my thinking too. I haven't checked the 'above 200 unstable' statement in Table 41.5 but I'm prepared to accept it.

For a really good debate - what code would you give this, if any?

 

[Midwest]

I've read here, that depending on how you carry out your Zs test, you might not be getting an accurate figure. Try bypassing RCD and two wire high current test. See what result you get then? Do Ze test, R1+R2 test?

 

[paulk11]

The fact is that Bs7671 does not give a maximum Ra value and does not state that the value must be stable....only that a value exceeding 200 ohms may be unstable. Nobody has ever given a valid reason as to why it needs to be stable as far as I can recall.
People pluck values out of the air for their preferred maximum..The NIC seem to favour 100ohms,and I recall a poster on here saying he gets twitchy if it's over 80 ohms. Nobody has ever given a valid reason for their preferred value.
If your Ra is around 335ohms then it is in full compliance with Bs7671 and you need do no more, although you may choose to .
In that case pluck a figure out of the air like everyone else seems to do.

Ha ha! That seems to sum it up nicely Wirepuller! I'll have to discuss with the customer and see what she wants to do but an extra earth rod is looking likely. Cheers

 

[davesparks]

Yes

Yes, I got that bit as its stated in 7671 but can someone elaborate on the 'unstable' bit. What does that actually mean? Why should we worry as the max permitted is much higher? Trying to comprehend it (still). Cheers

BS7671 only states that values above 200 ohms may be unstable, it is not a definitive figure or a target value.
You should aim to achieve a value of Ra as low as possible, ideally low enough for ADS.
Instability is often a result of seasonal changes in ground conditions, especially cold weather causing freezing of the ground. A number of deep driven earth electrodes in parallel will provide a far more stable value than a skinny 4' rod.
I usually start with 2x 8' long 5/8" dia rods spaced as far apart as possible for a small installation and take it from there.

If soil conditions prevent driving the rods to a sensible depth then other options need to be explored such as horizontal electrodes.