With no functioning RCD, does/did a TT electrode achieve anything?

[aptsys]

Exactly - and even with your "30A", no ordinary domestic installation would have had anything approaching "between 3 and 6 x 1.2 meter rods".

In reality, it would probably take at least 45A or so to blow a 30A fuse ever (don't forget that even an 30A MCB would need around 34A for it ever to trip, and around 43.5A to trip in 1 hour) - so the earth rod would have to be around 5Ω or less - I would think essentially unknown in an ordinary domestic installation.

I think the majority of domestic TT electrodes are, like mine, around 50Ω under wet conditions, higher during 'droughts'.

Kind Regards, John

5 ohms is impossible. The TT electrode at the transformer end is part of the path and never that low.

 

[JohnW2]

5 ohms is impossible. The TT electrode at the transformer end is part of the path and never that low.

Good point. I was thinking/talking about the local TT electrode itself but, as you say, the electrode at the other end is also part of the path.

That really all goes to reinforce what I've been saying/thinking - and makes me wonder about eric's "8Ω" one, as well!

Kind Regards, John

 

[ericmark]

Remember I was measuring the rod resistance not the loop resistance, one would need to do both to know the DNO rods resistance.

 

[JohnW2]

Remember I was measuring the rod resistance not the loop resistance, one would need to do both to know the DNO rods resistance.

That's exactly the point ...

... even if 30A would blow a 30A fuse (which it obviously wouldn't), your "240/30=8Ω" calculation would have determined the required loop impedance, so if you wanted any hope of the fuse blowing in a response to an L-CPC fault, the impedance of your rod would have to be near zero (or maybe even, hypothetically, 'negative' ).

Kind Regards, John

 

[RF Lighting]

5 ohms is impossible. The TT electrode at the transformer end is part of the path and never that low.

I’ve had 1 ohm EFLI on a TT supply. Granted it wasn’t a standard setup - way in excess of any domestic realms, but with enough metalwork in the ground it’s perfectly achievable.

Also have achieved around 5 ohms many times using four or five rods attached together.

 

[Adam_151]

5 ohms is impossible. The TT electrode at the transformer end is part of the path and never that low.

I wouldn't say impossible (I know of several sites that are certainly sub 2 ohm) , rather the ability to get a reading that low is dependant on the impedance of the DNOs earthing setup, while a single rod at the transformer end might well 'almost' never be low enough, but if you have a distribution network that's set up for PME, there can be a very low impedance between the TX earth and the ground on which we stand.

I'm reminded me of an interesting fault I looked at a while ago. Shocks reported from radiators in a farming cottage, supply TNC-S. Measured Ze at approx 0.20ohm (give or take). over 190v measured between MET and known good earth with main switch off. Taking out the suppliers earth from the MET made this disappear (with the suppliers earth remaining at the elevated voltage). If you want to have a stab at it (or ask clarification), PM me

 

[JohnW2]

I’ve had 1 ohm EFLI on a TT supply. Granted it wasn’t a standard setup - way in excess of any domestic realms, but with enough metalwork in the ground it’s perfectly achievable.

Fair enough. However, as aptsys said, you are at the mercy of the earthing at the transformer. You could have a million local rods and the EFLI still couldn't be 1Ω if the electrode(s) at the transformer had a greater impedance than that.

Back to context of this thread, I keep stressing that I am talking about 'standard domestic TT electrodes', and I suspect that I'm probably not far off by saying that they are uncommonly a lot below about 50Ω, and almost never below 10Ω-15Ω. Am I wrong?

Kind Regards, John

 

[RF Lighting]

No they seem like pretty expected figures.

As others have said, I don’t think I’ve come across a TT supply designed to operate without some form of earth fault disconnector.

I’ve found plenty which have been incorrectly modernised, but that’s a different topic.

 

[JohnW2]

As others have said, I don’t think I’ve come across a TT supply designed to operate without some form of earth fault disconnector.

You never saw (in the pre-RCD days) domestic installations with a TT rod but no VOELCM or RCD? As I've said, I lived in two of them in my early days. Furthermore, as has been said, I believe that Electricity Boards once checked for the presence of an earth electrode before they would connect a supply, even in the absence of any relevant protective device..

Kind Regards, John

 

[Adam_151]

As far as I understand it, at the time you are talking about the idea of Zs wasn't very well known, and the testers were very primative and expensive, everyone was happy as long as metal parts had continuity to earth, and the fuse fitted was sized to protect the circuit from overload. That's why we still find boards full of type 4 breakers installed upto about the early 80s with zs out of spec on a lot of circuits...the US still does this (and introduced AFDDs as a sticking plaster when breakers didn't trip)

 

[flameport]

Earth electrodes have been required since at least 1939 (11th edition), with the intention that a fault creates enough current to operate fuses or circuit breakers.



In the same edition, earth leakage protection is required unless certain conditions are met, one of which being earthing via 'an urban system of underground metallic water mains having metal to metal joints'.

 

[RF Lighting]

You never saw (in the pre-RCD days) domestic installations with a TT rod but no VOELCM or RCD? As I've said, I lived in two of them in my early days. Furthermore, as has been said, I believe that Electricity Boards once checked for the presence of an earth electrode before they would connect a supply, even in the absence of any relevant protective device..

Kind Regards, John

No, but maybe the different regional electricity boards might also play a part in this. Up until maybe the 60s or even later possibly, virtually all supplies in my area were PILC fed TN-S, or PILC fed with water main earth. It was very rare to see an earth rod anywhere with the exception of rural sites which were fed via pole mount TXs and overhead singles, but I’ve never found one without some sort of VOELCB or COELCB / RCD.

 

[JohnW2]

As far as I understand it, at the time you are talking about the idea of Zs wasn't very well known, and the testers were very primative and expensive, everyone was happy as long as metal parts had continuity to earth, and the fuse fitted was sized to protect the circuit from overload.

Fair enough but, per my OP, in the absence of a VOELCB, what was the perceived point of the "continuity to earth"?

Kind Regards, John

 

[JohnW2]

Earth electrodes have been required since at least 1939 (11th edition), with the intention that a fault creates enough current to operate fuses or circuit breakers.

. Interesting - but I can't help thinking that the "every endeavour" was rather by way of 'wishful thinking'

In the same edition, earth leakage protection is required unless certain conditions are met, one of which being earthing via 'an urban system of underground metallic water mains having metal to metal joints'.

Fascinating As I've said, even decades later I virtually never saw VOELCBs. The first, and only, one I ever 'owned' was the one I inherited here in 1987.

Kind Regards, John

 

[Risteard]

5 ohms is impossible. The TT electrode at the transformer end is part of the path and never that low.

It's not impossible. Even sub-1 Ohm isn't impossible.

The last TT installation I did the impedance was 8.77 Ohms. The one before that was around 40 Ohms.