why does the supply drop to 50V

[SparkyTris]

we get a lot of powercuts in winter due to poor weather mainly. (not exactly sure what the mechanism there is)
usually the power just goes completely off.
but occasionally, and only this last couple of years, the power dips down to 50VAC. I only noticed this because the remaining filament bulbs were lighting dimly, which caused me to test the line voltage during such a "brownout".

this state has on occasion remained for a few hours. sometimes it will suddenly come back on to normal 240V, sometimes it will go completely off before coming back on.

seems strange. Im just curious, what could be causing this?

 

[DaftPunk]

It's either due to a lost phase on the HV network or a lost neutral on the LV network

 

[SparkyTris]

ok, interesting.thanks.
could you explan a bit further how that actually causes the lower volts appearing on the line conductors in the house?

I thought the networks are now so full of sensors etc that any abnormality causes complete disconnects...

we have 2 overhead wires (11kV single phase?) into the back garden, the trafo is on a pole out there. 3 properties are connected to it.


I think I get how a missing phase on the HV could lead to undervoltage appearing on the LV.
I dont understand how a lost Neutral could cause this though...?

 

[westie101]

It will be a lost phase on the HV, a lot of the switching on those systems is done by un-ganged links or fuses, if one of the "drops out"" because of poor contacts you get the situation described.
It can also occur due to a connection failing

I thought the networks are now so full of sensors etc that any abnormality causes complete disconnects.

Not in these circumstances, they need visited and dealt with on site

 

[plugwash]

I thought the networks are now so full of sensors etc that any abnormality causes complete disconnects...

Afaict while the really big stuff is well instrumented/protected the little 11KV lines that power rural stuff are mostly just protected with old fassioned fuses and instrumented by waiting for customers to complain.

we have 2 overhead wires (11kV single phase?)

Sounds like a single phase feed, but unlike on the LV system when single phase loads are connected PN there is no neutral in the HV system so loads are connected PP.

into the back garden, the trafo is on a pole out there. 3 properties are connected to it.

The output of the transformer may be either traditional single phase or it may be split phase.

You didn't give these details at the start. So i'll cover all the cases.

ok, interesting.thanks.
could you explan a bit further how that actually causes the lower volts appearing on the line conductors in the house?

If A LV neutral is lost in a single phase system you get a complete loss of power.

If a LV neutral is lost in a split phase or three phase system you get undervoltage on some phases and overvoltage on others as the neutral moves arround relative to the lives (which remain fixed relative to each other)

In a HV system if you have three phases A, B and C and you lose phase B then loads connected A-C are unaffected, loads connected A-B after the break end up in series with loads connected B-C after the break resulting in undervoltage (with the exact ratios depending on the ratio of loading after the break).