A real moan tonight...

[SUNRAY]

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

One has to remember this is a 3ph circuit even though there is only access to a 1/3 of it in the form of L&N and for our purposes lets assume we are using the red or brown phase. However if there is no stated reference point we can nominate one. This is usually the star point or N.

As an example:

Let's make some assumptions that all 3 phase voltages are 230V [called R, Y, B] and points 1 & 2 are 100V all referred to N.

Now lets make the reference at point 1 [let's not get into how or why] then N = 100v, red=130V, Yellow= 293V
Likewise
Use 2 as the reference at point [let's not get into how or why] N = 100v, red=293V, Yellow= 130V
Furthermore use Y as the reference point [let's not get into how or why] N = 230v, red=400V, Yellow= 0V

The reference point can be anywhere within the black triangle or even outside it if the loads are very reactive, for your calculations to be correct it would have to be directly antiphase with L, I've shown is as point 3 at 100v from N. Therefore N = 100v, red=330V, and if its of interest Yellow= 200.6V with resistive loads it could never exceed: N=115V, L=345V [& Y would be 200V] unless there is any additional stray voltage, bearing in mind this is derived from HV and could concievably be floating on 11KV

All voltages quoted are rounded to some extent.

 

[SUNRAY]

Double post deleted.

 

[securespark]

Have I misunderstood, or was this purely down to a PD between TT earth and DNO earth?

Which would have been solved by using the DNO earth just to protect the supply cable, then terminating it in an insulated enclosure, using a TT earth for the outhouse?

 

[SUNRAY]

View attachment 203997

Have I misunderstood, or was this purely down to a PD between TT earth and DNO earth?

Which would have been solved by using the DNO earth just to protect the supply cable, then terminating it in an insulated enclosure, using a TT earth for the outhouse?

Yes that's exactly what this thread is about, except the PD between the 2 earths was enough to light a mains lampbulb and, as commented by the DNO guys when they arrived, their earth wasn't earthed.

Yes it did solve it except the neutral for the whole road was floating at some random voltage.
Personally I don't like the brass gland on the end of a SWA when its earth is different to the earth in use at the location, as has been so eloquently demonstrated on this installation it can and does cause problems.
My solution is an insulated gland with the armour being kept well out of harms way.

And yet again I'll ask why even bother with the additional effort and cost of an additional enclosure, why not just gland it directly into its final location?
This would have been so much easier and quicker and lets be honest tidier too.

 

[JohnW2]

One has to remember this is a 3ph circuit even though there is only access to a 1/3 of it in the form of L&N ...

Yes, you're right. I was rather too hasty when I wrote ....

... (assuming everything is in phase, which surely it must be?!)

Kind Regards, John

 

[JohnW2]

Yes that's exactly what this thread is about, except the PD between the 2 earths was enough to light a mains lampbulb and, as commented by the DNO guys when they arrived, their earth wasn't earthed.

I suppose that "their earth wasn't earthed" is one way of putting it, but the actual issue was that the 'neutral' conductor entering the houses was not connected (by conductors) to the neutral of their transformer (although the neutral was earthed at the transformer), leaving the only return path to the transformer as any other 'incidental' connections to true earth that might exist (essentially bonded extraneous-c-ps, including any earth rods like the one at your installation).

Yes it did solve it except the neutral for the whole road was floating at some random voltage.

If the problem (no direct connection to transformer neutral) affected a lot of properties, then IF many/most of those properties had (bonded) relatively 'low impedance' extraneous-c-ps, then all those paths to earth in parallel might keep the 'neutral' potential down to a relatively 'manageable' level, mightn't they?

Kind Regards, John

 

[bernardgreen]

then all those paths to earth in parallel might keep the 'neutral' potential down to a relatively 'manageable' level, mightn't they?

Or, depending on ground conditions, the parallel paths might keep the "ground "potential up to the potential of the broken Neutral (*) with a potential gradient to the point ( at the substation ? ) where the star point of the supply is grounded.

This would not be a significant problem for any one at the affected location as the "Ground" would be at the same potential as the CPCs in the houses.

There would be a problem for any circuits ( such a communication circuit ) from the affected area to an area away from the affected area if both ends were ground referenced. Cables carrying RS232 serial links could be "burnt out" unless one end was opto isolated to remove all ground reference from that end.

(*) The potential on the broken Neutral ( isolated from the substation ) would be the potential of the star point of the loads

 

[SUNRAY]

Yes, you're right. I was rather too hasty when I wrote ....Kind Regards, John

I's very easy to forget the external influences.

I suppose that "their earth wasn't earthed" is one way of putting it, but the actual issue was that the 'neutral' conductor entering the houses was not connected (by conductors) to the neutral of their transformer (although the neutral was earthed at the transformer), leaving the only return path to the transformer as any other 'incidental' connections to true earth that might exist (essentially bonded extraneous-c-ps, including any earth rods like the one at your installation).
If the problem (no direct connection to transformer neutral) affected a lot of properties, then IF many/most of those properties had (bonded) relatively 'low impedance' extraneous-c-ps, then all those paths to earth in parallel might keep the 'neutral' potential down to a relatively 'manageable' level, mightn't they?
Kind Regards, John

i'm not convinced the neutral was compromised as there wasn't [or certainly didn't appear to be] an issue with L-N voltage. This property is about half way along the run and the house lights didn't have any obvious fluctuations but they did have volts on the earth and kitchen taps.

This would not be a significant problem for any one at the affected location as the "Ground" would be at the same potential as the CPCs in the houses.

The kitchen taps had a strong tingle to the touch and I was seeing the 50-200V between earth bar and ground [small spike - AKA screwdriver - in the ground] but stable L-N pd so I'd say it was an earth issue and not neutral.

 

[JohnW2]

Or, depending on ground conditions, the parallel paths might keep the "ground "potential up to the potential of the broken Neutral (*) with a potential gradient to the point ( at the substation ? ) where the star point of the supply is grounded.

Indeed, but you cunningly did not quote my words which preceded the ones which you did quote, namely ...

... then IF many/most of those properties had (bonded) relatively 'low impedance' extraneous-c-ps, ...

... so your "depending on ground conditions" is saying much the same as my (emboldened) "IF" (IF there were relatively low impedance extraneous-c-pcs).

However, on the basis of what SUNRAY has most recently written, maybe we are not talking about a 'broken neutral' but. much more unusually, an intact neutral which has lost most/all of it's usual ('PME') connections to earth, including (maybe only) the one at the transformer.

Kind Regards, John

 

[SUNRAY]

The problem with a fault like this is knowing how much info to put in a post.
If I'd put everything in the OP it would have been stupidly long and most likely very confusing.

However, as is common, this thread has gone a long way away from the salient points I was highlighting, namely:
The silly practices of insisting on using a brass gland when it must be obvious it is not required, and
The even sillier practice of unnecessarily making the installation more difficult by adding a superfluous box.

On this ocassion the inclusion of both could have caused serious injury to the customer, or my bigger concern to me.

 

[JohnW2]

i'm not convinced the neutral was compromised as there wasn't [or certainly didn't appear to be] an issue with L-N voltage. This property is about half way along the run and the house lights didn't have any obvious fluctuations but they did have volts on the earth and kitchen taps. .... The kitchen taps had a strong tingle to the touch and I was seeing the 50-200V between earth bar and ground [small spike - AKA screwdriver - in the ground] but stable L-N pd so I'd say it was an earth issue and not neutral.

I'm still pretty confused ...

If the neutral conductors were intact throughout the local distribution network, but it had lost its connection to earth at the tranny, then the supply would be totally 'floating'. In that situation ...

(a) ... although the pd between true earth and either L or N of the supply would be pretty 'random' (and could be measured as very high), essentially due to capacative and/or inductive coupling, in the absence of a return path to to the tranny via earth you should not have been able to draw significant current between either L or N and true earth - certainly not enough to light a bulb/lamp.

(b) ... if the tranny's secondary was totally floating, then connecting it (anywhere) to true earth (e.g. by PME earth electrodes, or bonded extraneous-c-ps in connected installations would, in fact, simply 'earth reference' the supply (such that neutral potential {hence CPCs if TN-C-S} would everywhere be close to true earth potential, rising slightly above that only due to VD in the neutral conductor) - but, again, without providing any return path to the tranny.

If it was earthed by such a means in one place along the distribution (such that, given that no current would be flowing through the connection, N potential would be exactly equal to earth potential at that place), then, if the neutral conductors were intact throughout, then the only pd one would see between the neutral (hence installation's CPCs, if TN-C-S) and true earth anywhere else in the local network would be that due to the normal VD along the course of the N conductors - which would be pretty small.

Hence, I'm struggling to see how, with an intact neutral and inevitable earth-referencing connections (PME and bonded extraneous-c-ps), loss of its connection to earth at the transformer could result in the neutral conductor anywhere having an appreciable potential above true ground.

What am I missing?

Kind Regards, john

 

[JohnW2]

However, as is common, this thread has gone a long way away from the salient points I was highlighting, namely: ... The silly practices of insisting on using a brass gland when it must be obvious it is not required, and The even sillier practice of unnecessarily making the installation more difficult by adding a superfluous box. ... On this ocassion the inclusion of both could have caused serious injury to the customer, or my bigger concern to me.

As you know, for what it's worth, I agree with you totally about both points, even if some others might disagree (particularly about the 'superfluous box').

Although the discussion has certainly evolved, it hasn't gone off totally at a tangent in the way that so often happens. The issues we are continuing to discuss are very much related to the happening you described at the start of the thread.

One interesting thought is that if you hadn't unscrewed that faceplate, the problem with the supply might have persisted undetected and unaddressed for quite some time - so you might take some comfort in the fact that you weren't "nearly thrown across the room" for nothing!

Kind Regards, John

 

[333rocky333]

Noticed the Black core is Neutral, fairly common in the early days, luckily taped Blue as can easily catch you out if ever reconnecting.

 

[bernardgreen]

However, on the basis of what SUNRAY has most recently written, maybe we are not talking about a 'broken neutral' but. much more unusually, an intact neutral which has lost most/all of it's usual ('PME') connections to earth, including (maybe only) the one at the transformer.

This is what I think is the situation. The Neutral from the substation extends into the area where the ground potential is being pulled by the star point potential to be different from the ground potential that the sub station Neutral is connected to.

The loads at the consumer's location are the totals of several consumers and are not equal to each other.

 

[SUNRAY]

I'm still pretty confused ...

If the neutral conductors were intact throughout the local distribution network, but it had lost its connection to earth at the tranny, then the supply would be totally 'floating'. In that situation ...

(a) ... although the pd between true earth and either L or N of the supply would be pretty 'random' (and could be measured as very high), essentially due to capacative and/or inductive coupling, in the absence of a return path to to the tranny via earth you should not have been able to draw significant current between either L or N and true earth - certainly not enough to light a bulb/lamp.

(b) ... if the tranny's secondary was totally floating, then connecting it (anywhere) to true earth (e.g. by PME earth electrodes, or bonded extraneous-c-ps in connected installations would, in fact, simply 'earth reference' the supply (such that neutral potential {hence CPCs if TN-C-S} would everywhere be close to true earth potential, rising slightly above that only due to VD in the neutral conductor) - but, again, without providing any return path to the tranny.

If it was earthed by such a means in one place along the distribution (such that, given that no current would be flowing through the connection, N potential would be exactly equal to earth potential at that place), then, if the neutral conductors were intact throughout, then the only pd one would see between the neutral (hence installation's CPCs, if TN-C-S) and true earth anywhere else in the local network would be that due to the normal VD along the course of the N conductors - which would be pretty small.

Hence, I'm struggling to see how, with an intact neutral and inevitable earth-referencing connections (PME and bonded extraneous-c-ps), loss of its connection to earth at the transformer could result in the neutral conductor anywhere having an appreciable potential above true ground.

What am I missing?

Kind Regards, john

I'm with you all the way John.
I'm guessing, and this really is guessing without any clues to any facts, I'm guessing it's all down to the accumulation of leakage, capacitance, inductance etc of every part of the network to earth. Not least of which is the number of trees growing through the OH pole route, at one point the main route runs through dense woodland for about 480m and a couple of branches add 350m or so and I estimate another 400 to 500m in patches running along the side of the road. I'll bet it's in almost total contact with trees for over 1Km. Thursday evening was very wet when I took measurements and had been for a number of hours. Various bits of farm equipment of unknown [to me] age and quality. I don't know the whole story, but experience will let me make a few guesses.