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Topic Title: Earth rod resistance
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Created On: 27 January 2013 12:12 PM
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 27 January 2013 12:12 PM
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Avatar for colinhaggett.
colinhaggett

Posts: 358
Joined: 08 July 2004

Hi, last week I installed a single earth rod for a garden light/gym project. The resistance measured 58 ohms, but the ground is very much water logged and covered by snow this time of the year, so I was thinking how much would the resistance increase come summer. Must say I normally install two rods screwed together. Would a deeper earth rod be more stable across the seasons? Has anyone here measured a earth rod resistance though the year?

Thanks Colin

 27 January 2013 12:29 PM
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OMS

Posts: 19531
Joined: 23 March 2004

Deeper is usually more stable as it tends to mitigate gainst significant soil wetting/drying/freezing/thawing.

They will vary depending on what the ground is doing (often quite significantly - I know of several that treble the resistance from summer to winter)) - common sense says that testing them every 13 months (akin to LPS electrodes) gives you a seasonal/annual picture over time.

At 58 Ohms and a (presumed) 30mA RCD I wouldn't be particularly worried though

Regards

OMS

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Failure is always an option
 27 January 2013 12:30 PM
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aargeitakis

Posts: 141
Joined: 14 July 2005

Hi Colin,

its hard to guess what the resistance will be when the weather gets drier(of course it will be higher). It depends from the ground to, how hard or soft is.
You can go and measure it later in the year and if the result is not satisfactory you can add another rod, but at the moment your reading is ok. I believe you have an RCD protection in place already.

Regards

Paul
 27 January 2013 12:46 PM
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colinhaggett

Posts: 358
Joined: 08 July 2004

Yep, Dual 30ma RCD Hager consumer unit fitted. They had these on special offer so thought would try one.
 27 January 2013 01:37 PM
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slittle

Posts: 3482
Joined: 22 November 2007

I'm with OMS on this one Colin,

At 58 ohms in the current weather conditions I can't see it going silly high in the summer. Ok it's going to change but I don't suppose for one minute it's going to become dangerously high.

We know from the maths that the 200ohm "limit" that's frequently talked of is only really a good practice thing and with a 30mA RCD you should in theory be good up to 1500 Ohms. Obviously the lower the better but...

What sort of ground are you on ?, The clays around here don't tend to vary too much, but the sand/gravel belts do tend to dry out more readily

Stu
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