A real moan tonight...

[SUNRAY]

Sunray, does it look like this installation is off a smallish transformer which supplies only a very few customers?

Yes, there is a HV pole route across the fields [3 wires arranged horizontally so I'll guess 11KV] and a small floor standing sub, a lot smaller than the 1MW transformers I've installed so I'll guess 250KW, feeding 4 houses and 5 or 6 farms on what appears to be a single 400V 3ph OH pole route.
Other than those there are no properties as far as I'm aware for a mile or 2 in each direction.

 

[SUNRAY]

Sunray, does it look like this installation is off a smallish transformer which supplies only a very few customers?

Yes, there is a HV pole route across the fields [3 wires arranged horizontally so I'll guess 11KV] and a small floor standing sub, a lot smaller than the 1MW transformers I've installed so I'll guess 250KW, feeding 4 houses and 5 or 6 farms on what appears to be a single 400V 3ph OH pole route.
Other than those there are no properties as far as I'm aware for a mile or 2 in each direction.

 

[SUNRAY]

I've been back this morning , so far no tripping since yesterday pm when I did some basic tests and reinstated power. I've now had every termination exposed, done silly amounts of tests and logged all the readings in their log book, all of my installation and PAT results are very close to the previous several sets of tests so I now assume it's a moisture issue [the weather was horrible on Thursday].

I've Jerry rigged a Wylex fuse board [possibly what was in use before the Merlin CU] powered by the shower MCB to run the shower, pool lights and PIR flood lights so they can isolate all of the 'wet areas' next time there's a problem. However what is running through my head at the moment is for the few minutes between calling DNO/doing the light bulb test and the power going off I had everything except the shower running - The swimming pool pumps, the koi pumps & filter, towel rails, radio and all lights including the building, pool & floods. All without tripping and my big regret is I should have tried reconnecting the 2 earths as that was the only thing that had changed.

 

[JohnW2]

That fault on a ring of sockets does not leave parts of the house in darkness until the fault is found.

True - although a fault on a sockets ring might leave, say, a freezer not functioning without anyone realising for a good while.

However, in terms of a 'lighting ring', the risk-averse amongst us might be concerned about the (extremely small) possibility that a 'potentially dangerous fault' (e.g. a theoretical 'fire risk') might persist 'unknown' for a long time, if the fault in the ring results in no lights failing to work, or just one (perhaps rarely used) one failing to work. If a substantial section of lighting stops working, it's far more likely that the fault would be investigated and rectified fairly quickly.

Kind Regards, John

 

[Adam_151]

Is the voltage between the TT earth and the suppliers earth/neutral (that lit the lamp) still there?

 

[JohnW2]

THE L - N voltage was constant, the CPC [DNO] to TT was all over the place, varied between 50-200V roughly. ... The lamp bulb would not have been out of place at a mobile disco.

I'm getting a bit confused by some of this story. Forgive me if any of these questions are ones you've already answered but ...

1... Are we sure that the DNO supply is TN-C-S?
2... Do I take it that the L-N voltage is not only constant, but is roughly 230-240V?
3... Have you measured L and N voltages relative to the DNO 'earth' (CPCs) - if so, what are the figures?
4... Similarly, have you measured L and N voltages relative to the TT earth - if so, what are they?

Kind Regards, John

 

[SUNRAY]

Is the voltage between the TT earth and the suppliers earth/neutral (that lit the lamp) still there?

I can't actually check that now as the end of the SWA is well insulated and inside the 32mm galve coupling.

However when I swapped the RCD and isolator back [RCD in house , isolator in outbuilding] I did do additional final tests including checking the voltage between earth bar in the house and ground that was there on Thursday had gone. AND on top of that I used a LED screwdriver to double check metal parts - oil pipe, radiators, taps etc which all lit up on Thursday.

 

[SUNRAY]

I'm getting a bit confused by some of this story. Forgive me if any of these questions are ones you've already answered but ...

1... Are we sure that the DNO supply is TN-C-S?
2... Do I take it that the L-N voltage is not only constant, but is roughly 230-240V?
3... Have you measured L and N voltages relative to the DNO 'earth' (CPCs) - if so, what are the figures?
4... Similarly, have you measured L and N voltages relative to the TT earth - if so, what are they?

Kind Regards, John

No need for forgiveness John, It took a while for me to get my head around it and used 6 pages of their log book.
1. No. It is an assumption, there are 2 wires coming from the pole, down the wall into a meter box. I have not opened this as it is difficult to get to and muddy. No external signs of an eart cable, there are 3 wires entering the house, probably directly from the back of the meter box to an isolator and earth bar.
2. Yes L-N totally acceptable mid 230's but moves a bit more than I'd like. Not untypical on a very small network
3. Yes voltages are consistant with TN-C-S, ie 230V L-E & 0V N-E with E - ground being 50-200V. Ground being a small spike in the ground.
4. Yes took me a while to understand that one and the main reason I hadn't discussed L or N voltages in detail to avoid too much incorrect supposition. N-E was consistant with DNO E - TT E ie 50-200V but L- TT E seemed to vary very differently, roughly 0-300V, which I have put down to the true earth being dragged around the polar diagram. I monitored with 2 different meters with different update speeds and sometimes higher sometimes lower than N.

This really was one of those ocassions where I wished I had a couple of similar analogue meters with me, I think I'd have got my head round it quicker. Or even better an oscilloscope.

Don't get me wrong with the speed changes, I'd see readings around the same voltage for a good number of seconds then a switch change where I assumed an appliance was switched on or off but the L voltage could change from higher than, to a lower than the N voltage etc

 

[Risteard]

In many countries it is compulsory and, as I said, it was only at a very late pre-publication stage that the requirement of a TT earth for all electrical installations in the UK was removed from the final draft of the current version of BS7671.

One of the practical problems that was extensively discussed when we thought it was going to become a requirement was the potentially dangerous situation that can arise, temporarily, when some, but not all, properties using the same part of a distribution network have their TN earths connected to local TT ones.
You mean the DNOs (local) distribution network? If so, they surely can't allow that situation to persist (if it's true), can they?

Kind Regards, John

An electrode in a TN installation is not a “TT Earth” - it is still entirely TN.

 

[JohnW2]

An electrode in a TN installation is not a “TT Earth” - it is still entirely TN.

How pedantic. I know I often get criticised for saying it about what others have written but "you know exactly what I mean".

 

[JohnW2]

2. Yes L-N totally acceptable mid 230's but moves a bit more than I'd like. Not untypical on a very small network
3. Yes voltages are consistant with TN-C-S, ie 230V L-E & 0V N-E with E - ground being 50-200V. Ground being a small spike in the ground.
4. Yes took me a while to understand that one and the main reason I hadn't discussed L or N voltages in detail to avoid too much incorrect supposition. N-E was consistant with DNO E - TT E ie 50-200V but L- TT E seemed to vary very differently, roughly 0-300V, which I have put down to the true earth being dragged around the polar diagram.

That's largely what I expected you to say, so it still leaves me rather confused!

If L-N is about 235V, N-CPC is 0V (assumed due to it being TN-C-S) and CPC-earth_rod (winston has forced me to type a few extra characters ) is 50-200V, then one would expect (assuming everything is in phase, which surely it must be?!) that L-earth_rod would be 285V - 435V, wouldn't one?

If that were the case, your 0-300V measurement of L-earth_rod would seem very low; I can't see any reason to suspect that the earth rod would ever be at a higher potential than the DNO-derived CPCs, so the very lowest L-earth_rod voltrage one would expect to see would be the L-N voltage (235V or whatever), wouldn't it? ... and, even at the very bottom of your range of observed CPC-earth_electrode measurements (i.e. 50V), the expected L-earth_rod voltage wouldn't (at 285V) be much under 300V (which you say is roughly the highest L-earth_rod voltage you've observed.

Kind Regards, John

 

[Risteard]

How pedantic. I know I often get criticised for saying it about what others have written but "you know exactly what I mean".

It's not pedantic - it's actually critical to understanding it. A TN-C-S installation with an Earth electrode (as in most of the world) does not have any part of it as a TT installation. It's simply another Earth reference for the PEN conductor. Just because your comment was completely wrong does not render my response "pedantic".

 

[JohnW2]

It's simply another Earth reference for the PEN conductor. Just because your comment was completely wrong does not render my response "pedantic".

I'm not sure why you picked on me. If you look back at the four pages of this thread, you will see that all I was doing (at least partially to avoid confusion) was continuing to use the same language as had everyone (including the OP) - namely to talk of the 'earth' derived from a local earth electrode as a 'TT earth'. Everyone, including you, knew exactly what everyone involved in the discussion meant, and I'm sure that none of them thought, or think, that adding what are effectively extraneous-c-ps to a TN installation turns it into a 'TT' one.

 

[EFLImpudence]

I haven't read all the thread, but isn't it the point that if the DNO neutral is lost it will then be a TT installation?

 

[JohnW2]

I haven't read all the thread, but isn't it the point that if the DNO neutral is lost it will then be a TT installation?

I hadn't really thought of that but, yes, I suppose you're right - it certainly can't be TN-anything when there is no 'N'

Edit: on reflection, had it not been for the (presumably unintended) connection between the two 'earths', once the DNO neutral had been lost the main installation would presumably not even have been TT - it would simply have been 'unearthed', wouldn't it?

Kind Regards, John