200 Ohms

securespark · 1 Apr 2006

The On Site Guide advises that a Zs value in excess of 200 Ohms merits further investigation - does this apply to all systems?

Adam_151 · 1 Apr 2006

I's most certainly say yes, with TT its because its likely to be unstable (though I've heard that in some places you can't do a damn thing about it because of the nature of the soil around the place), and the same is true of the other systems, with TN-S where sub 1 ohm is the norm and you don't usually have to use an RCD to protect against indirect contact, if you are getting 200 ohms plus it'll almost certainly be due to the lead covering of old cables corroding, and its only going to get worse, with TNC-S the lights are either going to be dimming every time you try to make a brew, or the main earth conductor isn't properly connected

securespark · 1 Apr 2006

I'm a bit puzzled by this one, as, in theory, at least, with a 30mA RCD you could have a Zs of up to 1666 Ohms.

In practice, Adam, what in your experience is the average Ze on a TT system?

Adam_151 · 1 Apr 2006

securespark said:
In practice, Adam, what in your experience is the average Ze on a TT system?

Unfortunatly I'm not in a position to comment there, I don't do this for a living (not yet anyway), I just get involved in understanding the physics and maths behind it, from my armchair atm, in with the plugwash, ban, etc, crowd

comms · 1 Apr 2006

I'm on a TT system Ze was 230 ohms, after replacing the old 2ft steel pipe that had been used as a rod for two brand spanking new rods doubled up to get past the gravel bed thats 4ft down, i've got the Ze down to 41 ohms.

plugwash · 1 Apr 2006

securespark said:
The On Site Guide advises that a Zs value in excess of 200 Ohms merits further investigation - does this apply to all systems?

i presume the comment is aimed at TT systems as on a TN system you really shouldn't be having Zs anywhere near that high (its afaict considered bad practice to rely on rcds for earth fault disconnection on TN systems).

securespark · 2 Apr 2006

Yeah, I thought so too, plug, but look at P 22 of the brown OSG. (Para 3.6.3. In TN systems........)

Jim2287 · 2 Apr 2006

"If" RCD is used to provide protection... then the "Recommended" value shouldnt be above 200ohms

something like that ... dont have OSG to hand

james 29 · 2 Apr 2006

Securespark,

We install a lot of earthing systems/lightning protection and its worth mentioning earthing falls under BS7430 and the ze reading of a tt system should not be more than 100 ohms or it is deemed unstable. The value used to be 200 but an ammendment now requires 100 ohms. The niceic endorses the 100 ohm rule but ithink the IEE are still 200. Another case of the powers that be being crystal clear as usual. When you are entering higher values than 100 ohms amtech usually flags this up aswell.

Regards

James

securespark · 2 Apr 2006

Cheers, chaps.

ban-all-sheds · 2 Apr 2006

Adam_151 said:
I's most certainly say yes, with TT its because its likely to be unstable (though I've heard that in some places you can't do a damn thing about it because of the nature of the soil around the place)

There's always something you can do.

You may barf at the price, but there is always something you can do.

securespark · 3 Apr 2006

james 29 said:
the ze reading of a tt system should not be more than 100 ohms or it is deemed unstable. The value used to be 200 but an ammendment now requires 100 ohms. The niceic endorses the 100 ohm rule but ithink the IEE are still 200.

Have today talked to a geezer at the IEE.

He says the 200 Ohm figure relates to Zs, not Ze. He also told me the 100 Ohm figure you talk about is correct, but that it relates to the impedance of the consumers earth rod only.

The 21 Ohms mentioned in the OSG relates to the impedance of the DNO's earth rod.

james 29 · 4 Apr 2006

Secure,

ZE was what i was referring to but i was also trying to point out with a 100 ohm maximum ze how many circuits are going to have a zs of anywhere near 200 ohms?

You seem to be interested in tt systems based on your previous posts so i thought it useful to point this out!

The 200 ohm figure i quoted was for a prescribed maximum ze previous. Im almost certain this is right.

securespark · 4 Apr 2006

J

Wasn't doubting what you said!....just relaying what I'd been given by the IEE.

james 29 · 5 Apr 2006

Absolutely no offfence taken,

Just thought id explain the post better

james

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200 Ohms

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