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

[JohnW2]

A recent thread about the 'earthing conductor' in a TT installation has got me thinking ...

... in the absence of a functioning RCD (i.e. if there isn't one, or if it is faulty), does a ('standard domestic') TT earth electrode actually achieve anything? .. and that leads to the question of whether such 'earths' ever achieved anything in the days before RCDs (or VOELCBs).

Very few domestic TT electrodes have a low enough impedance to facilitate operation of even a 5/6A OPD in the event of an L-CPC fault, and certainly not for OPDs with higher ratings than that. Provided only that the building is properly constituted as an equipotential zone (i.e. any/all required main bonding is in place), I can't really see why the premises would be any safer (in the presence of an L-CPC fault) with a TT electrode connected than without it?

Am I missing something? If not, what was the thinking behind TT earthing in the pre-RCD/VOELCB days?

Kind Regards, John

 

[bernardgreen]

TT earthing before VOLCB was via the water supply pipe which was in those days metallic. Often the metallic water supply network was connected to the Ground ( Neutral ) at the electrical sub-station.

The fault current through the very low impedance of the water mains would be enough to blow a wire fuse.

The loss of this third "wire" when the water pipes became plastic meant that Earth leakage devices had to be used in domestic installations.

 

[JohnW2]

TT earthing before VOLCB was via the water supply pipe which was in those days metallic. Often the metallic water supply network was connected to the Ground ( Neutral ) at the electrical sub-station. The fault current through the very low impedance of the water mains would be enough to blow a wire fuse.

Indeed so, but that merely reinforces my question - which was about domestic TT electrodes, not water pipes.

As you say, if the CPCs were connected through a low impedance path, such as a water pipe, to earth (hence neutral at substation), that would facilitate operation of a fuse, in which case adding a local (relatively high impedance) TT electrode would make no difference. On the other hand, if the CPCs were, for whatever reason, not deliberately 'earthed' (or bonded) to metal water supply pipes (hence no low impedance path to earth), then (per what I initially wrote), I don't see that a local TT electrode would achieve anything - so, in either case, I can't see what was the thinking behind TT electrodes.

Despite what you say in the first sentence above, I have seen countless (mainly pre-WWII) properties in which, although the water pipes may well have been being used as an earth, also had a (clearly very long-standing) TT electrode - so, again, I wonder why? There either is, or is not, a low impedance path to earth via a water pipe or whatever and in neither case do I see what a local TT electrode achieves/achieved.

Kind Regards, John

 

[EFLImpudence]

A quick search says sockets only had earth terminals introduced around WW2 time and VOELCBs are pre 1950.

So...

 

[JohnW2]

A quick search says sockets only had earth terminals introduced around WW2 time and VOELCBs are pre 1950. So...

My earliest years were spent in two houses which had definitely pre-WWII electrical installations (which remained essentially unchanged well into the 60s) - both had earth terminals on most sockets and both had earth rods which had clearly been there for a long time, and neither ever had a VOELCB. Indeed, I have to say that it was long after the 50s that I first became aware of, or saw, a VOELCB.

Kind Regards, John

 

[conny]

My first house re-wire in the late 70's was a TT supply. The arrangement was I did the work then the local electricity board would test and connect the meter tails.
This they did without any problems but he did go outside to check where the rod was and that it had a tag attached. There was no RCD or VOELCB fitted for any of the circuits. The 'fusebox' was a standard Wylex 6 way with wireable fuses.
3 x 30A for 2 rings and a cooker, 1 x 15A (spare), and 2 5A for up and down lights. The house was a 2 up 2 down with an outside toilet.

 

[ericmark]

Pre the MCB there was no sharp cut off when a fuse would or would not rupture, so for a 30 amp fuse it would rupture over 30 amp just increased the speed so 240/30=8Ω and to get 8Ω on the point of aye gas terminal I needed between 3 and 6 x 1.2 meter rods. Well the water pipe would have been well over 8 meters long so yes the fuses would have ruptured, but as to the earth rod, I have only fitted a few away from point of aye, and most were around the 50Ω mark, which will a RCD is enough, I have never fitted earth rods pre-RCD maybe they did fit multi-earth rods back then, and once in the ground they get better over time.

 

[JohnW2]

My first house re-wire in the late 70's was a TT supply. The arrangement was I did the work then the local electricity board would test and connect the meter tails. This they did without any problems but he did go outside to check where the rod was and that it had a tag attached. There was no RCD or VOELCB fitted for any of the circuits. The 'fusebox' was a standard Wylex 6 way with wireable fuses.
3 x 30A for 2 rings and a cooker, 1 x 15A (spare), and 2 5A for up and down lights. The house was a 2 up 2 down with an outside toilet.

Indeed. I presume that was quite common - but, as you say, they did then 'require' that a TT rod was there and connected - even though, as I've said, as far as I can see it would have achieved essentially nothing.

Kind Regards, John

 

[Harry Bloomfield]

The arrangement was I did the work then the local electricity board would test and connect the meter tails.

Interesting - when did the local electricity board stop doing the inspections on domestic installations?

 

[JohnW2]

Pre the MCB there was no sharp cut off when a fuse would or would not rupture, so for a 30 amp fuse it would rupture over 30 amp just increased the speed so 240/30=8Ω and to get 8Ω on the point of aye gas terminal I needed between 3 and 6 x 1.2 meter rods.

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

 

[conny]

Interesting - when did the local electricity board stop doing the inspections on domestic installations?

I've no idea Harry. This would have been around 1978 or 79
The only time I ever needed them to do it. The few other rewires I did over the years were using the CU that was already fitted. This was in the days when people used their receipt to say that it had been done. LOL

 

[crystal ball]

Interesting - when did the local electricity board stop doing the inspections on domestic installations?

I worked for NORWEB up till 1990 and remember inspecting new builds, very basic though check bonding/earthing and IR the tails with isolator off, I refused to connect one estate as the bonding was not connected, the engineer sent me back as "he knew" the contractor

 

[AdrianUK]

Back in those days..... was an earth electrode common?

The houses that I can remember, the earth was provided by the lead water pipe only, and linked to the MET by 6mm2 or smaller.

I thought earth electrodes only started to appear when voltage operated earth leakage breakers started to be used, since you couldn't then use the waterpipe as the reference electrode.

 

[JohnD]

Interesting - when did the local electricity board stop doing the inspections on domestic installations?

Electricity Board inspected the first house I rewired in the late 1970's. looked at the tails and main bonds, looked at supplementary bond and the general appearance of accessories and visible wiring, took off the CU front and a couple of faceplates, asked why I'd used an FCU on the immersion heater circuit (I was waiting for a 2-element switch to be delivered). he may have tested polarity and earth loop. I believe they worked on the principle that if you've connected one socket and light switch correctly, you've probably done all the others as well.

I subsequently had a disconnect and reconnect done by a PES on my house in a different region in the 90's when I added an RCD onto the tails, I think the guy had a look at what I'd done, would have seen the main bonds, I don't remember him testing anything. House was PME

At around the same time I did some repairs and upgrades for a local voluntary organisation's premises, I was working to an inspection report following damage arising from a lost neutral on the 3-phase supply, it was TNS, and I removed several iron switchfuses and replaced with BS88 I think, and the company inspector spotted additional points not on the original report at his first visit (bonds and grommets, I think) and came back a week later to approve and reconnect. It was a company I later joined.

neither of these were charged for.

 

[JohnW2]

Back in those days..... was an earth electrode common?

Dunno for sure, but ....

I thought earth electrodes only started to appear when voltage operated earth leakage breakers started to be used, since you couldn't then use the waterpipe as the reference electrode.

That would make sense. However, back in the 60s and 70s I saw many houses (like the two I lived in) that had earth electrodes which had clearly been there for a long time, but virtually never saw a VOELCB. In fact, I've seen very few VOELCBs at all - my experience was essentially a jump from 'nothing' to early (whole installation) RCDs. My present house is one exception - when I moved here in 1987, it did (very briefly!) have a VOELCB.

Kind Regards, John