A Huge Puzzle

[bernardgreen]

Ex government building so unlikely and no evidence of rewiring after changing hands that I know of.

Any possibility of an alternative supply feed to the building. Not unknown for some very important buildings to have two supplies each from different sub-stations to improve resilience but that would not explain the voltage being above 250.

 

[ericmark]

My thought at the moment is something surrounding a UPS, but I don't know if they have any installed

I would also be looking at UPS. If they are not synchronised with the mains supply then any voltage produced could add to the mains voltage.

In theroy there should be no back feed but in practice with solid state switching it can happen and it may be less current than the limits set.

I remember years ago getting a TV and with an AVO Mk8 I measured voltage at the aerial socket. The voltage varied according to range selected so clearly very low current but when I phoned manufactures to complain I was told that to measure voltage at the aerial socket was normal but the maximum current was less than 1 mA so was permitted.

Considering that if there are small UPS for many items then with each leaking some voltage back out of phase then it could add up.

 

[JohnW2]

I would also be looking at UPS. If they are not synchronised with the mains supply then any voltage produced could add to the mains voltage. ... In theroy there should be no back feed but in practice with solid state switching it can happen and it may be less current than the limits set.

Indeed. One of my initial suggestions related to some 'extraneous source' that was out of phase with the grid supply - although, as we've both said, of the UPS(s) were correctly installed and functioning, that shouldn't really affect what voltages exist in the normal installation.

My other thought about UP(s) is that, although we don't know the exact time scales, I would have thought it quite likely that the UPS batteries would have been exhausted by the the time the testing was done (all CUs were switched off, and presumably had been for some period of time).

It's an intriguing situation, but I rather fear that an explanation might never be found - which would be intellectually frustrating. Unfortunately, now that the problem has been rectified, there is much less scope for investigating (e.g. if the observed voltages may have been artefactual due to the use of high impedance meters).

Kind Regards, John

 

[AdrianUK]

When faced with such problems, one of my first moves is to find another point of reference. When investigating a situation which might involve an earth fault, one must consider that the 'earth' terminal has been shifted away from true earth by the fault and can't be used as a reliable reference.

My solution is to either bang in a temp earth rod or use my biggest screwdriver pushed into the earth as deep as possible & as far away from the installation as possible. I now have a 'true earth' reference that I can use to take voltage readings against while I try to figure out what has happened.

Westie - do you have any knowledge of protection system theory? In particular, do you understand how Neutral Voltage Displacement detection works on a protection relay? It is possible, under the 'wrong' conditions for the Neutral/earth voltage to rise up to twice the phase/phase voltage. This is a pretty obscure fault but can happen. I wonder if something similar happened here?

Adrian

 

[JohnW2]

... It is possible, under the 'wrong' conditions for the Neutral/earth voltage to rise up to twice the phase/phase voltage.

Is there any fairly simplistic way in which you can explain the mechanism of that? As I said in my initial post, my thinking was that (in the absence of 'external influences') one cannot get a p.d. greater than the (intended) phase-phase voltage on the secondary side of an (intact!) tranny, regardless of what has happened to the wiring - but, from what you say, that is not necessarily the case.

Kind Regards, John

 

[JohnW2]

Any update/diagnosis in relation to this puzzle?

Kind Regards, John

 

[westie101]

It is possible, under the 'wrong' conditions for the Neutral/earth voltage to rise up to twice the phase/phase voltage. This is a pretty obscure fault but can happen. I wonder if something similar happened here?

Possibly, but no other customers on the same circuit were affected.

Any update/diagnosis in relation to this puzzle?

No all has gone quiet and there has not been a repeat.

It will remain one of life's mysteries

 

[JohnW2]

Any update/diagnosis in relation to this puzzle?

No all has gone quiet and there has not been a repeat. ... It will remain one of life's mysteries

Thanks. I suppose a repeat is unlikely since, whatever was the mechanism of the high measured voltages, there is presumably no doubt that the actual incident was precipitated by loss of your neutral, which was rectified.

I still can't help but wonder whether the 'puzzle' may not have been primarily related to artefactual/erroneous voltage measurements - since I'm not convinced by what you've told us that anyone undertook measurements using a low input impedance (or shunted) meter.

Kind Regards, John

 

[Spark123]

The only other theoretical explanation I can think of is series resonance, but things would need to be fairly finely tuned to do this or one of the inductors / capacitors is varying to the point of causing resonance and staying there.
I'll vote for a UPS back feeding!!