Lost Neutral

Depends if 5he building has a 3 phase supply then its a very real possibility that some loads meant for 240 volts could see anything up to 415 volts which would result in lots of damage possible fires  and other unwanted effects. If its a single phase supply then you could either have total power loss or only partial if there are other return paths via metal pipes etc

Large voltage offset between N and E, and L-N voltage either high (anywhere up to about 400V) or low, and varying with loading.

If you have a 3 phase supply, as one L-N voltage goes down, the others go up.

Even on single phase supply- if the lost neutral is out in the street where it is 3 phase, then you are trying to balance the neighbours loads, and  extremis, exploding light fittings and disappointment all round.  Act quickly to save the installation.

mike

Thanks guys. The system in question is 3- phase. Sub-circuit protection did not operate. Can anyone advise why not?

What protective measures on the affected sub-circuits - MCBs and fuses probably won't operate on a lost neutral, even if the phase to flying neutral  voltage goes up to 300+. You may have more joy with an RCD, but even then if the fault comes on gently, as far as the RCD is concerned , L and N current are still in balance.

The only sure fire safety mechanism would be shunt trip whose actuating coil is operated by an over-voltage detector wired L-N.

Mike.

Sub-circuit protection did not operate. Can anyone advise why not?

Most circuit protection is based on overcurrent rather than overvoltage. While an increased voltage will often result in an increased current current being drawn - the increased current would typically be in proportion to the increase in voltage - i.e. a factor of probably less than 2 - i.e. it would appear to the protective device to be at worst a moderate overload rather than a fault - so even where the protective device's rating is close to the normal rating of the load disconnection time would likely be many minutes or hours rather than seconds. In the more likely case where there's a decent margin between the rating of the circuit and the connected load, chances are the current will never reach a level to trigger overcurrent protection.

   - Andy.

How much damage has it done?