David Carter:

Living in a rural Area I've had both ultra extremes


1 Walked into house late one night and switched on light to be rewarded with a soft dim glow rather than normal illumination levels. Checked voltage with my plug in power/volts meter to find (as I recall) it was around 130Volts.

Sigh, do nowt  and go to bed - someone had fixed it by next morning.


2. Working on computer with GLS desk lamp on.

Suddenly desk lamp goes a LOT brighter and UPS on the computer goes nuts signifying its intervening to prevent high voltage.  Yikes say I....Check voltage and its steady as she goes at 278Volts - and remained at that for at least a couple of hours+ despite me ringing them - before it was switched off totally.  Meanwhile I was running round the house switching off any operating electrical item I could.




Sparkingchip:

I have noted readings of 256 volts at home when checking meters in the past, my old Robin multifunction tester would lock out occasionally and refuse to do a loop test.


Generally it stays lower, obviously it depends who’s doing what and where.


 Andy Betteridge 

 

 

If I remember it correctly the 130 volts is due to an incoming phase being lost at the DNO transformer, usually due to a fuse blowing, due to it being connected delta 

.

Driving down a road at night you will typically see every third street light and house or pair of semis being affected resulting in them being lit dimly.


Andy B.

if one phase goes open circuit at the substation then if all the loads are single phase, every third light or house goes out.

However, more industrial loads with things connected t 3 phases are more fun. Spinning 3 phase motors, if they keep spinning, will try and generate the missing phase. If this does not happen then 400v  loads connected phase to phase now see 230 instead of 400, and with the wrong phase relationship.

In practice, I agree 130V sounds like phase off, but it could have been almost any voltage less than 230.

It is nastier if the neutral falls off, as then the 400V between phases is retained,  delta wired loads do not really notice,  but it is bad for single phase and star wired loads,  which see the neutral move from the centre and being equally 230v from each phase to almost anywhere in  the triangle. In principle if the single phase loads are a very much out of balance then you could have one phase of near zero volts to ground/ local neutral , and the other two at more or less 400V - as the fuses tend to disconnect loads on the over-volted side this is an unstable condition, and can do a lot of damage.


There is also a similar problem with faults from phase to true terra-firma earth with too low an electrode impedance, if the fault earth is less resistive than the  electrode resistances  at the transformer end of things - not impossible if perhaps the fault is to the frame of a steel piled building, and the transformer feeding it is a small pole-pig with 10-20 ohms of rod electrode resistance.

In that case the phase gets grounded, the bulk of the fault voltage is dropped at the transformer electrode, so the transformer star point and everything that uses that as an earth gets pulled off ground. If for example the L to ground fault is 3 ohms, and the transformer earth is 10 ohms  then this neutral voltage  lift is 10/13 of 230v, or about 180V relative to any electrode or other earthed metal outside the near field of the transformer earth electrodes. The faulted phase however ends up about 60V off true ground. Unless there is an RCD,  or the supply is lightly fused, this condition is likely to persist for some time before it is noticed, as nothing will trip - in this example 230V /13ohms , or about 16 amps, will flow from the faulted phase to ground so the electricity bill will be high, but that is about all that will happen. 

There are more fault modes than the obvious shorts to the CPC or occasional  broken phase or PEN to consider.

The low voltage incident I had in mind due to a fault at the transformer was about ten years ago and a couple of streets away from home.


A lady phoned to say her lights were dim, I drove around to her house and every third lamppost in her road was dimly lit, as was roughly every third house.


Obviously if such an event occurs then the EV chargers need to drop out.


Andy Betteridge

I came across a reduced voltage due to one of the HV phases being lost - unfortunately by then CFLs had taken over from GLS lamps so the symptoms were far less obvious - some lights appeared to work normally, others not at all and a few just flickered. If I recall correctly two-thirds of the houses in the street were affected - only one third having normal voltage.

  - Andy.

Is the cable still in use and live?


How old is the cable, is it more than twenty yo thirty years old?


Andy Betteridge

Yes Andy, that is an unfortunate "false positive" for this kind of device.


But options are really limited for many single-phase installations from PME supplies.


Further, providing a separate TT earthing system for the charging point in such installations may achieve nothing, or introduce other hazards.