Backfeed on Neutral on Residential Service (Storm Conditions)

[buckyswider]

Hi all, in my avocation as a fire officer we've had occasional situations (yesterday being another) where trees/limbs (the usual culprit) cause a primary and neutral line on the pole secondary to contact one another, resulting in a charged neutral in a residence. By the time we get there the initial hazardous situation has self-mitigated, either by the transformer tripping or the tree continuing to fall and severing a wire. So when we get there we see charring in a panel, possible melted components in a panel, odors of burning from fixtures, etc.

I have a few questions regarding this. First, at times like these the POCO is usually swamped, and they're not on-scene to help/advise. Pretty much all we can do is make sure nothing's burning and check with a hot stick to make sure that the panel or nothing else is charged at that point. And tell the homeowner to get an electrician ASAP. Does it help to inform the POCO of our findings? By the time I cleared yesterday the POCO was out at the initial problem so I made my way over there to tell the lineman of our findings. The information didn't seem to be of much concern to him- I guess before re-energizing the secondary circuit they would have repaired and inspected everything anyway. Could re-energizing a residence after such a failure be hazardous? Not sure what all could be compromised in a panel with that fault. The POCO forbids us to pull meters anymore- but if I had my druthers I would've done that before leaving yesterday (in addition to opening the main breaker).

Secondly- is there not some kind of device that could prevent damage in this situation? Thinking about my own house now. I've seen this maybe a dozen times so far in my 21 years. You would think that it's a common enough occurrence to warrant some sort of prevention device, but I can't find anything at all that addresses this.

Thanks!

 

[kwired]

Hi all, in my avocation as a fire officer we've had occasional situations (yesterday being another) where trees/limbs (the usual culprit) cause a primary and neutral line on the pole secondary to contact one another, resulting in a charged neutral in a residence. By the time we get there the initial hazardous situation has self-mitigated, either by the transformer tripping or the tree continuing to fall and severing a wire. So when we get there we see charring in a panel, possible melted components in a panel, odors of burning from fixtures, etc.

I have a few questions regarding this. First, at times like these the POCO is usually swamped, and they're not on-scene to help/advise. Pretty much all we can do is make sure nothing's burning and check with a hot stick to make sure that the panel or nothing else is charged at that point. And tell the homeowner to get an electrician ASAP. Does it help to inform the POCO of our findings? By the time I cleared yesterday the POCO was out at the initial problem so I made my way over there to tell the lineman of our findings. The information didn't seem to be of much concern to him- I guess before re-energizing the secondary circuit they would have repaired and inspected everything anyway. Could re-energizing a residence after such a failure be hazardous? Not sure what all could be compromised in a panel with that fault. The POCO forbids us to pull meters anymore- but if I had my druthers I would've done that before leaving yesterday (in addition to opening the main breaker).

Secondly- is there not some kind of device that could prevent damage in this situation? Thinking about my own house now. I've seen this maybe a dozen times so far in my 21 years. You would think that it's a common enough occurrence to warrant some sort of prevention device, but I can't find anything at all that addresses this.

Thanks!

Not a lot you can do to prevent this from happening. Surge protection helps, but how much depends on the situation when something does happen. The closer your house is to where the fault occurs the higher the possible fault current that finds a path through your home becomes. Keeping trees near the lines is a start, but you could still have line attachment to an insulator fail and have ungrounded line drop on secondary conductors.

Having underground service may be one of the best methods to help minimize this, but still can be some transient voltages if such a crossover connection occurs close enough to you.

Nearby town once had a 34.5 kV line drop onto 4160/2400 distribution line. There was damage reports all over town from that incident, mostly failed electronic devices. I had service calls that had burned up boards in computers and appliances, and nearly every GFCI in the house was not working either.

 

[electrofelon]

Another option would be to have an insulated neutral on the service drop, but I don't see the utility industry/regs making that change. Interestingly, the NEC acknowledges this is an issue and it is given as one of the reasons for having electrodes at the structure to help clear utility faults.

 

[buckyswider]

Thanks guys. So I guess it is what it is. Any tips for responses to residences where we suspect this to be the case? Or is what we do no pretty much all we can do?


For my personal house...I wonder if the power company would let me and my 5 neighbors who share a transformer fund the installation of an insulated neutral? Nah, I didn't think so!!! :happysad:

 

[kwired]

Insulated neutral helps should a high voltage line fall on your service drop, but if it falls on their bare distribution MGN it could still send some surge down your service conductors, could even enter the secondary system via the neutral, feed through the secondary coils and raise voltage in relation to earth on the ungrounded secondary conductors. Don't think the 120 volt to ground secondary voltage is much of a counter against 7200 volts if it finds a way into the secondary.