Earth Rod Resistance (ARW?)

What is considered "good" or "low" for the resistance of an earth rod? Rod to ground as tested using multiple temporary test rods.
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Dave.
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On 15/04/2015 20:49, Dave Liquorice wrote:

It needs to be low enough for a live to earth fault to trip the breaker in the maximum time allowed. If I had the figures to hand it would be easy to work out. I would be aiming for low milliohms rather than ohms.
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On 15/04/15 21:06, Dennis@home wrote:

Sorry to interject, but that is completely incorrect.
a) You will not get an earth rod Ze to low milliohms, even if you bury it in marconite;
b) The aim is to keep the touch voltage down to safe limits up until the RCD (which there must be) trip current is reached.
The exact allowable earth loop impedance is a matter of debate - there is an upper bound which is supply-voltage/RCD-tri-current but a lot of leeway needs to be added as the ground in which the rod is buried is an unstable and slightly unpredictable medium.
Here is a bit of reasoning:
http://www.theiet.org/forums/forum/messageview.cfm?catid 5&threadid"507
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On 15/04/2015 21:06, Dennis@home wrote:

In your dreams...
Sub 10 ohms would be "good" in my book. However even a few hundred will still be adequate to trip a RCD and have some "spare" capacity left over.
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Christsakes Den where is the world you live in eh:?..
Milliohms;?...
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Tony Sayer



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scribeth thus

Just off the A461 in Tipton.
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Adam


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I have no such problem with my fusebox. And my earth is through the neutral to the substation.
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A Smith and Wesson beats four Aces.

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Dave Liquorice wrote:

100 Ohms is just about the limit for a stable resistance, so you really want to get below that. In real life, you can probably get less then 20 ohms with 2 rods either joined together going 2 metres deep, or in separate areas. Of course, it all depends on what your ground is like around the rods.
And if Dennis can get his down to milliohms, then he really should be lecturing on how to do it.
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Alan
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On 15/04/2015 21:21, A.Lee wrote:

!00ohm will give you a fault current of about 2.5A!

20 ohms is going to give a fault current of about 12.5A!
Is it worth having an earth that poor? The breaker will never blow and the "earthed" cases will be live at near mains voltage.

You can always change it.
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On 15/04/15 21:48, Dennis@home wrote:

Dennis - RCD. Or ELCB before that.
No TT installation in the history of man has ever been able to achieve disconnect times with fuses/MCBs along.
Not unless you maybe live in a salt water marsh...
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On 15/04/2015 21:57, Tim Watts wrote:

Yet they existed before elcb or rcds.

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On 15/04/15 22:22, Dennis@home wrote:

Oh do stop talking bollocks.
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On 16/04/2015 11:23, Tim Watts wrote:

Actually he's right, they (TT systems) did /exist/. The trouble was that they didn't actually /work/ until ELCBs came along.
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Andy

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On 16/04/15 21:47, Andy Wade wrote:

Well, I don't dispute that back in 1880, they did dodgy stuff. I was answering more in recent contexts :)
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On 16/04/2015 21:50, Tim Watts wrote:

Ineffective earthing was very common, right into the 1950s and 1960s.
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Andy

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On 16/04/15 22:56, Andy Wade wrote:

I though ELCBs were much older than that - or were they not mandated?
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On 17/04/2015 01:15, Tim Watts wrote:

Our last house was built in the 1980s and just had fuses. Cartridge ones, but still fuses.
I'd actually have preferred wire!
Andy
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On 17/04/15 21:28, Vir Campestris wrote:

The ELCB would have predated the RCD - and only relevant (to this discussion) if you had a TT earth system...
You could use either fuses or MCBs with an RCD (or ELCB) on a TT system
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Why?
I would have said that you had a better install.
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Adam


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Who bought you a Ladybird book of maths?
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