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Topic Title: High (er) Zs reading on TT system
Topic Summary: Should I be worried?
Created On: 07 December 2012 05:25 PM
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 07 December 2012 05:25 PM
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BrucieBonus

Posts: 666
Joined: 20 February 2007

Hi everyone

Hope you can help. Went to an old job today to change some lighting transformers. A few years back I did a board upgrade - some one before me had put in an earth rod, but not upgraded the board to RCD protection. The Ze was around 150 Ohms from memory. Not great, but job in London and not much option as to where to put the rod (actually, like NO option...)

Anyway, out of interest today I took loop tests at a couple of sockets on different circuits and got 400 Ohms ish. Now I know that 1666 Ohms is the 'max' but should I be worried about the change???? All circuits RCD protected obv.

thanks

BB
 07 December 2012 05:29 PM
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Parsley

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Brucie

I think you need to investigate further.

regards
 07 December 2012 05:40 PM
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leckie

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GN3 says above 200 ohms is unstable, maybe this is an example of this?
 07 December 2012 06:03 PM
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mikejumper

Posts: 1706
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Originally posted by: BrucieBonus

The Ze was around 150 Ohms from memory. Not great, but job in London and not much option as to where to put the rod (actually, like NO option...)

Anyway, out of interest today I took loop tests at a couple of sockets on different circuits and got 400 Ohms ish. Now I know that 1666 Ohms is the 'max' but should I be worried about the change???? All circuits RCD protected obv.

Did you take a Ze reading as well this time?
If it's that high with all the bonding connected I'm just wondering whether your main earth may have come adrift somewhere and what you're seeing is an earth through your main bonding conductors.
 07 December 2012 06:20 PM
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daveparry1

Posts: 6165
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As it's in London isn't PME available? i'm not anti-TT, I have lots of it around my area but being in London it might be worth enquiring,

Dave.
 07 December 2012 06:24 PM
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slittle

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PME should be available in London as it's a UKPN area and they told me last year that the whole network was PME capable,.

I'd do a Ze or Ra test and check what the rod is giving in case it's not there. I had a case on a farm this week where the cable to the electrode had been broken .

Stu
 07 December 2012 06:26 PM
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OMS

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How long is the electrode - if it's a standard 4' long 3/8" diameter copper clad job in London clays and sands, I'd expect it to vary considerably over the year and from year to year. Particularly if it's in a "poor" location (rain shadow, made ground etc etc)

Effectively it's a shallow electrode and will drift significantly with moisture, temperature and prevailing salt content - less so if it's deep driven (say minimum of 2.4m and ideally more)

If you are passing that way after Xmas, check it again - but generally speaking, it's absolutely fine - I wouldn't be concerned over it (particularly as you're measuring it with a loop tester anyway)

Regards

OMS

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 09 December 2012 09:01 PM
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BrucieBonus

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Hi everyone
Thanks for the replies - even if they do contradict a bit!

I didn't do Ze as I couldn't switch off as client working at home - just did a passing loop test out of interest

I'm sure PME is available but when I did the job EDF were quoting figs of £2-3K.... customer decided a rod was a bit more cost effective

The rod is standard length in made up ground and less than 1 metre from the house (the only position available) - so yes I'd expect the readings to be not great and variable
 09 December 2012 09:51 PM
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slittle

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Didn't know London has Sand OMS, in that case I wouldn't be panic.

I thought most of it was clay the same as much of good old Essex which generally behaves nicely in a TT situation.

I've got a couple of sites which sit on a sand/gravel bar and both are hovering around the 200 ohm mark. On the basis that the installation has a 30ma RCD incomer (don't ask) and I know why the reading is high, it's not something I'm going to loose sleep over (or spend weeks banging lots of rods down to try and cure)


Stu
 10 December 2012 12:23 PM
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OMS

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I guess it depends where you are Stu - plenty of sand and gravel deposits over the clays, you can find "brickearth" in many places - London bricks are made from it.

A few centuries of man made junk and infill is always a possibility as well.

As I said, with such a short electrode I wouldn't worry about a couple of hundred ohms variation - although it might just be worth checking that no one has done any work to the water main in the intervening period - like swapped it for plastic - or "forgotten" to put the 951 clamp back on after a few modifications

Regards

OMS

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Failure is always an option
 10 December 2012 01:23 PM
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Ricicle

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Originally posted by: OMS
Effectively it's a shallow electrode and will drift significantly with moisture, temperature and prevailing salt content - less so if it's deep driven (say minimum of 2.4m and ideally more)



If


Wouldn't TFL be worried about the electrode scraping the paint off the top of their tube trains with deep driven rods ?

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Empty barrels make the most noise.
 10 December 2012 01:59 PM
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OMS

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Nah - it would only happen once !!

Perhaps not that deep would be good enough

Regards

OMS

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Failure is always an option
 10 December 2012 05:38 PM
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BrucieBonus

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thanks again for replies all - I'm likely to be back there spring next year, so will take another reading, double check the bonds and the rod connection
 10 December 2012 08:08 PM
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peteTLM

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Originally posted by: Ricicle

Originally posted by: OMS

Effectively it's a shallow electrode and will drift significantly with moisture, temperature and prevailing salt content - less so if it's deep driven (say minimum of 2.4m and ideally more)


If




Wouldn't TFL be worried about the electrode scraping the paint off the top of their tube trains with deep driven rods ?


I do know a civil engineering job a friend was running where they knocked a bit out of a panel of the piccadilly line 20 years ago!

Land near the thames (or where it used to run in its old course) is mainly ballast with a bit of soil. The rest is london clay.
You have to go some to hit the thanet sands deep under london though, only foundations of the tallest of buildings get anywhere near that stuff!

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 10 December 2012 10:03 PM
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OMS

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LoL - Thanet sands are a different ball game Pete - they'll be deep. Sands and gravels within the top 10m though, are not unusual depending on where you are in the great Metropolis.

Regards

OMS

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Failure is always an option
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