Underground fault - lost neutral

[Bob Minchin]

A friend has just contacted me say that he woke to find his domestic circuits were "dead". Once reset, one appliance was showing an overvoltage indication and several white goods and hifi were damaged/non functional. He measured 378 volts live to neutral.
Turned out that that there was a break in the neutral connection underground.
I've heard that some of the latest installations include an SPD but looking at photos these don't seem man enough to rupture a 100amp board fuse so would one of these protected his equipment in any way?
If not what precaution measure can be fitted to a domestic installation to protect against continuous overvoltage casued by such a fault.

Bob

 

[Andy™]

SPD wouldnt work, there not really designed for this. best you'd get would be a contactor feeding everything and use overvoltage relay to drop the contactor coil to isolate the power. it may not be quick enough to avoid damage though, although a slowly increasing voltage may not cause much damage

 

[ProDave]

In a similar situation to this, where it was a DNO fault that resulted in an over voltage due to the failed neutral, the house in question got all the failed items replaced at the DNO's expense, there were a lot of white goods, and other home electronics and an alarm system. I think they also paid for an EICR to confirm none of the house wiring was damaged.

 

[Bob Minchin]

SPD wouldnt work, there not really designed for this. best you'd get would be a contactor feeding everything and use overvoltage relay to drop the contactor coil to isolate the power. it may not be quick enough to avoid damage though, although a slowly increasing voltage may not cause much damage

I wonder if an overvoltage relay could be configured to simulate an earth leakage and trip the rcd to avoid the need for an inline contactor?
Bob

 

[Fleeting]

I suspect an over voltage relay may not react sufficiently to prevent damage.

 

[UNG]

A friend has just contacted me say that he woke to find his domestic circuits were "dead". Once reset, one appliance was showing an overvoltage indication and several white goods and hifi were damaged/non functional. He measured 378 volts live to neutral.
Turned out that that there was a break in the neutral connection underground.
I've heard that some of the latest installations include an SPD but looking at photos these don't seem man enough to rupture a 100amp board fuse so would one of these protected his equipment in any way?
If not what precaution measure can be fitted to a domestic installation to protect against continuous overvoltage casued by such a fault.

Bob

The fault you describe sounds more like a floating neutral where the neutral has lost it's earth reference and the voltage floats relative to the individual phase loads.
I've seen this quite a few times over the years where a phase can be under voltage and another will be over voltage
Depending on the level of over voltage it can create some serious damage to a lot of equipment

 

[ProDave]

The fault you describe sounds more like a floating neutral where the neutral has lost it's earth reference and the voltage floats relative to the individual phase loads.
I've seen this quite a few times over the years where a phase can be under voltage and another will be over voltage
Depending on the level of over voltage it can create some serious damage to a lot of equipment

Just a different way of describing it. It IS a floating neutral, because somewhere the neutral connection to the substation has broken, hence describing it a "broken neutral"

 

[UNG]

Just a different way of describing it. It IS a floating neutral, because somewhere the neutral connection to the substation has broken, hence describing it a "broken neutral"

A neutral that has been broken between the substation and the premises would not cause an overvoltage and would probably and more likely cause a loss of supply on a single phase installation, whereas a loss of the earth reference to the substations transformer secondary windings centre point would cause the windings centre point and therefore the neutral to float at a varying voltage depending on the phase loads and in extreme circumstances the neutral voltage could be as high as the phase voltage giving 400v or more across the live and neutral terminals at the consumers installation. The highest phase to neutral voltage I have seen on an installation with a floating neutral was 385v and it made quite a mess of a lot of equipment

 

[Andy™]

A neutral that has been broken between the substation and the premises would not cause an overvoltage

voltage depending on the phase loads and in extreme circumstances the neutral voltage could be as high as the phase voltage giving 400v or more across the live and neutral terminals at the consumers installation.

first part says it won't happen then second part details exactly what does happen

 

[mort2376]

Could be a loose neutral. Had it on an industrial premises and had 300+ volts on single phase to earth. Cooked a few lights and the fire alarm panel.

 

[UNG]

Could be a loose neutral. Had it on an industrial premises and had 300+ volts on single phase to earth. Cooked a few lights and the fire alarm panel.

I'm curious as to where the loose neutral was that would cause the phase voltage to rise above 300v to earth, what was the phase - neutral voltage at the time. The only time I've seen voltages like that on an industrial site was were the 11Kv transformer auto taps were not set correctly and the voltage pushed up towards 300v when the load dropped because the non 24 hour parts of the site shutdown for the day.

 

[Bob Minchin]

Properties down stream of a broken neutral will experience all manner of voltages with respect to their live phase depending on the loads between the floating neutral and the other phases imposed by subsequent properties down the line up to and including full phase to phase voltage in extreme conditions.

 

[Andy™]

I'm curious as to where the loose neutral was that would cause the phase voltage to rise above 300v to earth,

good old TNCS...