utility power supply interrupted

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kevlar54

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San Pablo, CA
Two days ago at about 7pm my customer had a tree fall on a residential power line near his house. Several neighbors were also without power. The utility came out to repair and power was restored in a few hours. My customer now has failed circuit boards in the furnace and tankless water heater. Both were in use at the time of power failure. He also lost a microwave oven that was plugged in but not in use, the newer GFI was tripped but could be reset. He lost an older GFI receptacle that had the dishwasher plugged into it, the dishwasher was not in use and is OK. Every light bulb that was on in the house burned out, including several LED recessed. Light bulbs that were not on did not burn out. There is more but I think that is enough for this question, what happened? So far from asking neighbors, not one has lost a single item. Seems to me that the voltage went high perhaps by losing the neutral but since his service is bonded to earth that would allow for a secondary route so maybe something else happened. Why him and not anyone else? The tree fell close to the local transformer, I did not see the event or the damage. My customer is the farthest house from the transformer, is that relevant?
 
two most likely reasons first has already been mentioned - the high voltage line on primary side of transformer contacting a low voltage secondary line, but since none of the neighbors had any damage this is likely not what happened. Second is loss of neutral conductor. Just because there is a grounding electrode at this house doesn't mean the resistance between that electrode and earth is low enough to allow "normal operation" of the neutral through earth. That is not why we install electrodes, we primarily do so for help in dissipating high voltage transients from lightning or other transients.

POCO generally is not responsible for damage in this kind of case as it is an "Act of God or nature" because it was a falling tree branch that caused the problem. Their homeowner insurance hopefully covers the damages.

Edit to add: I don't know if I needed to explain what happens when a neutral is lost or not I can go into further detail if you want.
 
kwired said:
two most likely reasons first has already been mentioned - the high voltage line on primary side of transformer contacting a low voltage secondary line, but since none of the neighbors had any damage this is likely not what happened. Second is loss of neutral conductor. Just because there is a grounding electrode at this house doesn't mean the resistance between that electrode and earth is low enough to allow "normal operation" of the neutral through earth. That is not why we install electrodes, we primarily do so for help in dissipating high voltage transients from lightning or other transients.

POCO generally is not responsible for damage in this kind of case as it is an "Act of God or nature" because it was a falling tree branch that caused the problem. Their homeowner insurance hopefully covers the damages.

Edit to add: I don't know if I needed to explain what happens when a neutral is lost or not I can go into further detail if you want.
Click to expand...

kwired -
Thanks for your input and I do understand all too well what happens when a shared neutral is lost though more info is always welcome. Still, my event scenarios do not explain the clean neighbors. I just found out my customer also lost an outdoor latest generation GFCI receptacle at the meter-main that I installed 2 weeks ago. He also lost 2 GFCI recepts in a bathroom, a third was tripped and was reset, a fourth was still viable, all the same age (3years?) and type. I need enlightenment.
 
kevlar54 said:
kwired -
Thanks for your input and I do understand all too well what happens when a shared neutral is lost though more info is always welcome. Still, my event scenarios do not explain the clean neighbors. I just found out my customer also lost an outdoor latest generation GFCI receptacle at the meter-main that I installed 2 weeks ago. He also lost 2 GFCI recepts in a bathroom, a third was tripped and was reset, a fourth was still viable, all the same age (3years?) and type. I need enlightenment.
Click to expand...

A lost (or even high resistance) neutral could affect just the one house if the fault is located between the shared transformer and his house only. This is particularly consistent with him being at the end of the line for that transformer.
A primary side fault which imposed a high voltage on the ground system could have elevated his neutral relative to everybody else's based on the high impedance neutral and a solid ground/neutral bond at his house.

Any suspicion of a neutral problem can be at least partially checked by looking at whether the failed devices were all 120V loads connected to the same phase line.
 
GoldDigger said:
A lost (or even high resistance) neutral could affect just the one house if the fault is located between the shared transformer and his house only. This is particularly consistent with him being at the end of the line for that transformer.
A primary side fault which imposed a high voltage on the ground system could have elevated his neutral relative to everybody else's based on the high impedance neutral and a solid ground/neutral bond at his house.

Any suspicion of a neutral problem can be at least partially checked by looking at whether the failed devices were all 120V loads connected to the same phase line.
Click to expand...
correct, plus the messenger wire of overhead cable is the neutral and will be the first conductor to fail when something takes it down.
 
GoldDigger said:
A lost (or even high resistance) neutral could affect just the one house if the fault is located between the shared transformer and his house only. This is particularly consistent with him being at the end of the line for that transformer.
A primary side fault which imposed a high voltage on the ground system could have elevated his neutral relative to everybody else's based on the high impedance neutral and a solid ground/neutral bond at his house.

Any suspicion of a neutral problem can be at least partially checked by looking at whether the failed devices were all 120V loads connected to the same phase line.
Click to expand...

GD -
Thanks much for the feedback and food for thought. I am told that the POCO fault, fallen tree, was very close to the transformer location. That would put 6 houses closer to the event. My customer's 200 amp service is only a few years old and the bonding is clean. The neighborhood is 1920's so the other houses might have higher resistance but all are definitely a shorter distance.

All of the damage at his house was done through 120volt circuits. In his house only the oven draws 240volt and it still works. I had checked to see if all of the damaged devices were on the same phase but, no, both phases of 120volt circuits were affected. He lost every light bulb that had been in use but only some of the dimmers! One thing that rattles me is that the microwave and the dishwasher were plugged into separate GFCI receptacles on separate circuits, not sharing neutrals after the sub panel. Neither appliance was in use. The DW receptacle, older, failed and the DW survived. The MW receptacle, newer, was tripped, could be reset, yet the MW is lost. Both circuits come from the same sub panel but I do not know if they are on different phases. I am still in the dark.
 
131125-0751 EST

kevlar54:

From your description the tree limb broke the circuit completely near the transformer such that all neighbors on the transformer lost power. No neighbor of your customer had any equipment or light failures after restoration of power. Your customer is at the end of the line.

Unlikely that your customer's problems occurred at the time of interruption. More likely the problems occurred after restoration of power. Based on no neighbor problems this would imply that hot to hot voltage did not deviate grossly from 240 V except during power loss when it was 0, and the neutral to all neighbors was and is in good condition.

The described problems imply excessive voltage to the items damaged.

What are the conditions at the home presently?

What are the present voltages line to line, each line to neutral, and each line to a screwdriver in the earth some distance from the home, 20 feet or so away?

Where could the neutral from the closest neighbor to your customer have been damaged? And was this repaired at some point in time after the original line break?

How many days since the tree branch incident to now? Is the customer's service OK at the present time?

.
 
Another possibility (not knowing how things lay out) is that the neutral first became disconnected somewhere at the far customer's end leaving that one customer with an open neutral and the remaining customers still had a neutral connection until something happened to remove power completely. If the neutral were to have broke at the source end then they all would have been subject to the open neutral. Simply put the open neutral only effected the end customer somehow. Do you know specifically what was broken or just that the line came down and was then repaired?
 
kevlar54 said:
...One thing that rattles me is that the microwave and the dishwasher were plugged into separate GFCI receptacles on separate circuits, not sharing neutrals after the sub panel. Neither appliance was in use.
Click to expand...
Many appliances even when not in use have active circuitry connected to the grid and running, e.g. system clocks.
 
ggunn said:
Many appliances even when not in use have active circuitry connected to the grid and running, e.g. system clocks.
Click to expand...

ggun -
Yes and that is what puzzles me since both the DW and GD are newer models that have a constant minor draw. The only difference is separate 120 volt circuits, not sharing a neutral, not sure if they are on the same phase. Also they are each on a GFI receptacle but of two different generations. I will go back tomorrow.
 
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kevlar54 said:
ggun -
Yes and that is what puzzles me since both the DW and GD are newer models that have a constant minor draw. The only difference is separate 120 volt circuits, not sharing a neutral, not sure if they are on the same phase. I will go back tomorrow.
Click to expand...

They are sharing the same neutral out to the transformer, as well as everything else in the house that operates at 120 volts.
 
kevlar54 said:
kwired -
Yes sharing a neutral as you described. I meant not sharing a neutral downstream of their individual circuit breakers.
Click to expand...

I don't see that as being all that relevant as that was likely undisturbed, the neutral conductor somewhere on the utility side of the service was what was subjected to open circuit conditions, and if that happens everything in the house that operates at 120 volts is subject to possible over/under voltage, load conditions at the time is what will determine just what those voltages will be. Unless there is equal load impedance on each ungrounded conductor there will be an unbalance in voltage, a low resistance path like a metal water pipe to another building is all that would remain to stabilize the voltage, ground rods usually have a high enough resistance they will do little to help with this.
 
kevlar54 said:
kwired -
Yes sharing a neutral as you described. I meant not sharing a neutral downstream of their individual circuit breakers.
Click to expand...
What does that mean? Neutrals do not pass through OCPDs. Everything in the house that is a 120V load is connected to the same neutral bus back at the service. Whether or not the same wire services both the outlets the devices are plugged into doesn't mean much.
 
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gar said:
131125-0751 EST

kevlar54:

From your description the tree limb broke the circuit completely near the transformer such that all neighbors on the transformer lost power. No neighbor of your customer had any equipment or light failures after restoration of power. Your customer is at the end of the line.

Unlikely that your customer's problems occurred at the time of interruption. More likely the problems occurred after restoration of power. Based on no neighbor problems this would imply that hot to hot voltage did not deviate grossly from 240 V except during power loss when it was 0, and the neutral to all neighbors was and is in good condition.

The described problems imply excessive voltage to the items damaged.

What are the conditions at the home presently?

What are the present voltages line to line, each line to neutral, and each line to a screwdriver in the earth some distance from the home, 20 feet or so away?

Where could the neutral from the closest neighbor to your customer have been damaged? And was this repaired at some point in time after the original line break?

How many days since the tree branch incident to now? Is the customer's service OK at the present time?

.
Click to expand...


gar -
All good questions, thanks. I will investigate the possibility of multiple repairs by POCO.
It is now 3+1/2 days since the event. I did a cursory inspection of the electrical system at the home 36 hours after the event and found all to be in good condition, only the devices and appliances had been compromised. Everything at the house is OK and the lost appliances are slowly getting replaced and returned to service. The 3wire residential service has 240volt from hot to hot and 120volt from each hot to the bonded neutral. I have not checked hot to earth more than 20 feet away, I will do that tomorrow when I return. What info will that possibly gain?

I agree that a high voltage condition caused the damage. There was a small standard incandescent bulb in a table lamp that had a hole blown through the glass. I do not know if that happened at disconnect or reconnection of power. I also do not know if one hot leg went high or if the neutral got energized. It seems more likely that the neutral got energized because the damage was to devices on both hot phases. And to me that pushes the question of why no neighbors seem to have been affected yet my customer had extensive injury. I will pursue that more, maybe there was some peripheral damage and we have not discovered it yet.
 
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kevlar54 said:
gar -

I agree that a high voltage condition caused the damage. There was a small standard incandescent bulb in a table lamp that had a hole blown through the glass.
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That is not very likely at all to happen from just open neutral conditions. Max voltage that lamp will see in such an event is 240 volts, which should make it burn really bright for a few seconds at the most until the filament fails.
 
131125-1408 EST

kwired has a good point. It may look like more than 240 V was the source.

Some wild thoughts.

How could this happen? If the neutral failed between the home and the neighbors, and some large voltage source occurred across the neutral break, then high voltage could occur on all circuits in the home. But how would this correlate with a tree branch falling on the circuit near the transformer? Not likely.

kevlar54:

Have you opened any of the failed devices to see how extensive the damage is and what may have been damaged. GFCIs have an input line to neutral MOV. Is this damaged? The light bulb with a blown hole implies a lot of energy.

Any load with circuitry across the line and not switched by a mechanical switch is likely to have failed. Loads that were off by virtue of a mechanical switch were apparently not damaged. This probably means a voltage less than 10,000 to 20,000 V. I can generate several thousand volt line transients by switching a magnetic ballast 8' Slimline fixture on and off. Never caused any damage to ordinary devices. I assume the energy that caused the damage was much greater than a 100 W magnetic ballast.

You do have very much of a mystery.

.
 
gar said:
131125-1408 EST

kwired has a good point. It may look like more than 240 V was the source.

Some wild thoughts.

How could this happen? If the neutral failed between the home and the neighbors, and some large voltage source occurred across the neutral break, then high voltage could occur on all circuits in the home. But how would this correlate with a tree branch falling on the circuit near the transformer? Not likely.

kevlar54:

Have you opened any of the failed devices to see how extensive the damage is and what may have been damaged. GFCIs have an input line to neutral MOV. Is this damaged? The light bulb with a blown hole implies a lot of energy.

Any load with circuitry across the line and not switched by a mechanical switch is likely to have failed. Loads that were off by virtue of a mechanical switch were apparently not damaged. This probably means a voltage less than 10,000 to 20,000 V. I can generate several thousand volt line transients by switching a magnetic ballast 8' Slimline fixture on and off. Never caused any damage to ordinary devices. I assume the energy that caused the damage was much greater than a 100 W magnetic ballast.

You do have very much of a mystery.

.
Click to expand...

Thanks to everyone for your interest, it is helping me a lot. I just found out the the original event created what he called "boom time". My customer was in the kitchen with the hot water running and likely the furnace as well. All power went out, after dark, so no lights until the power returned a few hours later, a smooth event, lights turning on evenly and only once. He is certain that all of the lights were lost at the beginning, not on power restart.

I will replace a few GFCI receptacles, dimmers, and lights tomorrow. The supply wiring for the 4 outlets already replaced seemed to be in perfect condition, no burn or evidence of overheating, insulation still soft and pliable. There was one receptacle that had a small transformer plugged in for a computer speaker system. The cover plate had a black charred mark but no damage to the house wiring. The plug-in transformer showed no signs of overheating but failed to perform, likely not so good on the inside.

However, he did find a power strip in the storage/garage that had scorched the carpet. I will investigate further tomorrow.
 
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