Earth rod query

Deleted member 9648 · Jul 14, 2012

Engineer54 said:
The flack came and rightly so, from you advising junior members to bung a twig in the ground and if you saw 200 ohms or less, that's all you needed for a TT system, and not bothering any further...

I think that i have stated many times now that as far as domestic installations go, a couple of coupled 5/8'' rods (coupled) should be the standard installation, coupled with an up-front S type RCD device. Do you really think that's over the top, considering that the TT installation you have installed should be robust enough to last a min of 30 to 40+ years??

Now let's say that driving 2 rods in the ground on a domestic installation gave you an Ra value of say 10 ohms. You know that in all probability, the stability is going to be present. Would you approach the client and give them a choice of adding another rod, thus getting down, pretty much close to the 1 ohm mark (after settlement) or not?? Which is the other point i've been trying to make over the many threads on this subject... Wherever or whenever possible, go that extra distance to get the desired results, and not just rely on what it says in the Reg's...

As for commercial and industrial installations that have a number of employee's etc why wouldn't you be trying to aim for TN values?? A 200 ohm's max in such circumstances, is complete tosh!! Oh, ...and what you call ''Super Rods'' were once the standard sized rods used in the UK and everywhere else and STILL ARE, in most of the countries i've ever been.... I actually saw for the first time, a 1m 5/8'' rod while in an electrical wholesalers in Cyprus while i was home a short time ago. I asked if that was the standard size they were using now in Cyprus. ...What i got was a blank look, he then disappeared out back and came back with a what looked like a 2.4m rod, ''first one of these, ...then one of these'', pointing to the 1m rods!!! So even in Cyprus the one metre rod is classed as no more than an extension piece!! lol!!!

The thing is E54 I have seen many TT installs in my area which are 20+years old with a single 'twig' which are perfectly satisfactory today,and will be for another 20 I've no doubt...I base my opinions on that.
My only argument is with those who turn up their noses at 200 ohms...and yet seem to think they are the dogs danglies with 40 ohms.....I just cant see the logic.....If the installer insists the Bs value is not acceptable then he should get a stable TN value....I have no dispute with that at all.

spark 68 · Jul 14, 2012

I was just looking through the 1995 16th ed.amd.1 OSG, and this recommends the earth electrode should not exceed 220 ohms, and does not appear to be mentioned at all in the same edition of the regs.
There is also no mention of it in the 2004 16th ed.amd.2 regs either

So this looks to have been lowered slightly to 200 ohms in the 17th.

Engineer54 · Jul 14, 2012

wirepuller said:
The thing is E54 I have seen many TT installs in my area which are 20+years old with a single 'twig' which are perfectly satisfactory today,and will be for another 20 I've no doubt...I base my opinions on that.
My only argument is with those who turn up their noses at 200 ohms...and yet seem to think they are the dogs danglies with 40 ohms.....I just cant see the logic.....If the installer insists the Bs value is not acceptable then he should get a stable TN value....I have no dispute with that at all.

Have these twigs been the norm there for 20 years now then?? Wow!! If so, are you pulling these rods up to look then, or just relying on the high Ra value, that you blindly accept as being fine cause it says so in 7671??

If nothing else, those that put a little extra effort in and gets anything from 50 ohm or lower (which would be about the norm for 2.4m's of 5/8'' rod) stable installation IS the dog's dangles compared with those that just bang a twig willy nilly in the ground and think they have actually achieved a TT system. That logic, i will never understand. I'm not however implying that a domestic TT installation should cost mega bucks, it shouldn't. But using the right materials and quantity with a little care, is how i would want any electrician to perform on a standard TT job that i (and i suspect customers) employed him for....

Sorry, but apart from anything else, a stable 1m twig ain't gonna happen...

Finally, why is it that a Standard rod size that had in the past almost always delivered the goods in both stability and Ra values, be superseded by a twig that ''almost always'' (i'm being kind here), will never be able to deliver either??

Deleted member 9648 · Jul 14, 2012

I think I'll change my user name to twiggy.....or twiglet perhaps?

Top Cat · Jul 14, 2012

wirepuller said:
I think I'll change my user name to twiggy.....or twiglet perhaps?

Maybe smear your rod in E45 then everyone is happy!

Silly Sausage · Jul 14, 2012

or E54 changes his name to 'twigpuller'

Top Cat · Jul 14, 2012

Archy Styrigg said:
or E54 changes his name to 'twigpuller'

Wish there was a "like a lot button"!

IQ Electrical · Jul 15, 2012

Top Cat said:
Maybe smear your rod in E45 then everyone is happy!

E54 surely?

Richard Burns · Jul 15, 2012

Top Cat said:
Maybe smear your rod in E45 then everyone is happy!

E45 sends a very different message compared to E54 and unfortunately gives an insght into TC's mind!!:cheesy:

La Poste · Jul 15, 2012

All this talk about earth rods and I have one query.

When measuring an earth rod resistance and trying to get it down to 10 ohms are we talking about the rod resistance itself or are we talking about the EFLI?

If we are talking about the rod resistance being ten ohms then what about the impedance of the transformer rod which could be twenty ohms itself so no matter how low we get our earth rod resistance we are never going to get lower than the impedance of the transformer rod.

Admittedly there will be parallel paths involved through other peoples bonding etc but when talking about a low Ra are we talking just about the earth rod or are we talking about the total earth fault loop?

Thanks.

IQ Electrical · Jul 15, 2012

La Poste said:
All this talk about earth rods and I have one query.

When measuring an earth rod resistance and trying to get it down to 10 ohms are we talking about the rod resistance itself or are we talking about the EFLI?

If we are talking about the rod resistance being ten ohms then what about the impedance of the transformer rod which could be twenty ohms itself so no matter how low we get our earth rod resistance we are never going to get lower than the impedance of the transformer rod.

Admittedly there will be parallel paths involved through other peoples bonding etc but when talking about a low Ra are we talking just about the earth rod or are we talking about the total earth fault loop?

Thanks.

In theory, when we use an earth fault loop impedance tester, the instrument measures Ze because it gives us Ra for the earth electrode and earthing conductor, the resistance of the supply transformer electrode, the transformer winding impedance and the resistance of the supply line conductor from the transformer.

drew35 · Jul 15, 2012

If you take this argument to the extreme, once you start ignoring one regulation why not just ignore them all? Then you don't need to bother with an earth at all!

IQ Electrical · Jul 15, 2012

drew35 said:
If you take this argument to the extreme, once you start ignoring one regulation why not just ignore them all? Then you don't need to bother with an earth at all!

Which regulation was ignored?

Engineer54 · Jul 15, 2012

what about the impedance of the transformer rod which could be twenty ohms itself

You would have to search long and hard to find a sub-station earthing system anywhere near that 20 ohm max. Here they do use multiple rod positions and the 2.4/3.2m rods, for both the MV and the LV side of things!!

The best way to test an earth electrode of any type, is with a dedicated 3/4 pole earth test kit. unfortunately most of the even half decent ones are too expensive for the self employed electrician that only comes across TT system from time to time.

Some MFT's have a facility for earth testing, but then your only getting what you pay for, so the facilities are very basic...