Home generator safety

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  #1  
Old 09-04-11, 08:48 AM
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Home generator safety

We just went through an extended period without electricity. Home generators flew off the shelves at the local home centers. It's too bad that common sense wasn't issued with the generators.

I was having coffee today with some of the town PW guys that had been clearing downed trees for the last 5-6 days. He told me of two instances where downed wires that had been pronounced safe by the POCO were in fact hot. One was from a local greasy spoon that had hooked a 13KW generator to their service entrance and was backfeeding 220 volts into the power line. The other was a small home generator doing the same thing. The enterprising homeowner built a suicide cord and plugged the generator directly into an outlet in his kitchen.

Fortunately, no one was injured although I'm surprised the restaurant guy isn't visiting the dentist. Apparently he refused to turn off the generator when asked. It got turned off for him.
 
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  #2  
Old 09-04-11, 10:03 AM
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I read about such occurrences all the time but I have never read of one from an AUTHORITATIVE source. I asked the question once on the Electrical forum and one of the moderators stated he would find me such an article but he never did.

While I have a proper transfer switch on my home installation and I always point out the necessity of a properly wired transfer switch I also wonder how often it really works out that some line worker gets fried due to some idiot using a suicide cord or improper switch. The reason why I have my doubts is that any little (and by little I mean any generator that a homeowner might install) generator that is connected to a local distribution system would be so heavily overloaded it would either instantly trip the attached circuit breaker or it would kill the engine.

Now the fact that I have doubts does NOT mean that I in any way advocate any kind of connection of a personal generator to any utility distribution system. All generators used need to be properly connected via an approved transfer switch or transfer panel in a manner that absolutely precludes the possibility of energizing any utility wiring from the generator. I don't even like the kits that allow for interlocking circuit breakers.
 
  #3  
Old 09-04-11, 12:26 PM
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I don't know, Furd, it happened here during Hurricane Ivan. One of our POCO guys was working on a "known" dead line. Maybe not enough precautions were taken, I'm not sure. But 120 volts turned into 15k really quick and killed him. I don't doubt it happened, because he was a local guy. I don't doubt the transformer was doing all the work, not the generator.
Don't let me near an idiot who knowingly wires up a generator to where it will endanger someone's life. As Wayne said, basically, you can't legislate stupid. The restaurant guy should have been prosecuted. He knew better. He HAD to know better.
 
  #4  
Old 09-04-11, 04:42 PM
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Furd - I don't have any first hand knowledge 'cause I wasn't there, but I know these PW guys, it's a small town and I've lived here for 25+ years. One of them was also the town fire marshall, a guy I served with in the Navy. If they say a line was backfed by a generator I believe them 100%.
 
  #5  
Old 09-04-11, 05:39 PM
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Here's my thinking. The power companies always tell us to turn off all "heavy users" (or everything) when a power failure occurs but how many people actually do? So let's say that the "average" home will be using around 1,500 watts when the power goes out. This would be a few lights, a television, DVD player and computer. Maybe a few kitchen appliances or the blower on the central furnace or any of a hundred different appliances.

Let's go on to say that a typical secondary distribution transformer serves four homes, I think that is what applies where I live. Now if the average load among these four homes is 1,500 watts the total load is going to be around 6,000 watts. Understand that depending on circumstances the total load could easily be double or more. All of this is on the 240/120 volt side of the transformer.

Now the village idiot gets out his handy-dandy suicide cord and plugs it into a receptacle on a 20 ampere circuit. He fires up his 3,000 watt gennie and plugs the suicide cord into the receptacle on the generator. The maximum sustained current flow cannot be any more than 20 amperes or else it will trip the circuit breaker on the branch circuit of the house. More likely as not it will also trip the circuit breaker on the generator since generator-mounted receptacles are commonly protected to 20 amperes. Certainly the 20 ampere circuit breakers CAN sustain a momentary current flow perhaps twice their rating but the higher the current flow the quicker the breaker will trip.

Now because Mr. Idiot forgot to open the main breaker in his service panel he is feeding not only the 1,500 watt load of his own home but also the combined 4,500 watt load of three neighbors for a total of 6,000 watts (50 amperes at 120 volts) on his puny little 3,000 watt generator. The result is going to be an almost instant tripping of at least one of the circuit breakers, either the generator or the branch circuit. And we haven't even gotten to the other side of the distribution transformer yet.

But let's up the stakes a bit. This time the idiot has a bigger generator, one rated at 7,500 watts continuous with a 10,000 watt surge. To be certain of being able to utilize all the power of his new gennie Mr. Idiot makes up a suicide cord to go from the generator to the kitchen range (electric model) receptacle. He makes sure that the range branch circuit has a 50 ampere circuit breaker so that the entire output of the gennie is available at his service panel. For an instant (a second or two) his new generator is capable of outputting maybe 15,000 or 20,000 watts. Certainly he has enough power to maintain his own 1,500 watt load along with his three neighbors. He also has plenty of reserve to go through the utility transformer where the voltage is boosted to anywhere from 7,500 volts to as high as 26,000 volts or maybe even higher. This high voltage will then energize the next secondary distribution transformer serving another four homes with their average load of 6,000 watts and you can easily see that even this larger generator is very quickly overloaded. All of this is virtually instantaneous and will either trip the generator circuit breaker or the branch circuit circuit breaker or just plain stall the engine.

Don't believe me? Try it yourself if you have one of those "Interlockkit" circuit breaker blocks on your service panel. Turn on EVERYTHING in your house including all the burners of the electric range and draw down enough hot water to cause the element in an electric water heater to become energized. Now start the generator, open the main circuit breaker, throw the interlock and close the generator circuit breaker. How long does the power from the gennie stay on?

Remember that I said it was virtually instantaneous but there IS a momentary surge of power that goes through the transformer, is raised to the higher voltage and does travel down the utilities distribution wiring. That surge IS quite capable of killing anyone working on the wires. And for THAT reason the utilities have procedures in place to minimize the hazards. I don't know about the eastern (mostly investor-owned) utilities but the publicly-owned utility I worked for some 38 years ago REQUIRED all lines to be considered live until a solid grounding strap was attached. Once the grounding strap is attached (across all three phases and to neutral/earth ground) any voltage impressed upon the wires (like from Mr. Idiot's generator) is a direct short circuit and would draw so much amperage that it would immediately trip the circuit breakers all down the line from the source to the grounding strap.


None of this should be read that I in any way, shape or form endorse connecting a generator to a utility-supplied system without a properly wired transfer switch.
 
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Old 09-04-11, 06:11 PM
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Furd, I agree with your theory that the generator would have to satisfy the low side demand, and could possibly trip out before that demand was met. IMO, if ANY of the feedback reached the transformer, then the transformer would begin doing the work, taking the load off the primary generator and creating the deadly backfeed that kills. It wouldn't take much. I know it happened, I know why it happened, but theories do abound, I guess.
 
  #7  
Old 09-04-11, 06:38 PM
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No, the generator does not "meet the low side demand" first, it supplies the ENTIRE demand instantly. My point is that the entire load the generator "sees" is sufficient to trip the circuit breaker almost instantaneously. It isn't a matter of the generator straining to meet the load for a few minutes, or even a few seconds but almost instantly tripping the circuit breaker or stopping the engine.

I'm more inclined to believe that the line crew didn't follow proper safety procedures and then tried to cover up their mistakes by blaming a homeowner's generator. Certainly if there had been no generator the hazard would not have existed but if proper grounding of the "dead" wires took place BEFORE any other work the hazard would have been nil.
 
  #8  
Old 09-05-11, 06:45 AM
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People also need to learn not to plug everything into a 16AWG extension cord and 2 or 3 daisychained plug strips.
 
  #9  
Old 09-05-11, 12:47 PM
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I'll try to get the specifics on what happened but I know this - the guys working on clearing the trees were not POCO guys. They had been told by the POCO that the lines were dead. Hell the entire state was dead. My guess - and that's all it is - is that the line may have been tested dead when the generator was off.

I do know that the town public works guys would not clear trees and limbs that were tangled with downed wires until they were verified dead by the POCO. We had a whole buttload of closed roads because of downed wires and nobody could find anyone from the power company to clear them for days.

The greasy spoon is in a small strip center with a closed real estate office, a defunct liquor store and a beauty salon - also closed. A tree took down the pole holding the transformer. I have a photo of the mess that I'll try to post. My guess is that there was little load outside the restaurant.

I agree with the public/private comment. There are several small municipal POCOs in the state. Two days in, they had restored power to 90% of their customers. Our bottom line driven mega POCO
had no clue.
 
  #10  
Old 09-05-11, 01:28 PM
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Ahh, the plot sickens. When I worked for the utility it didn't matter what the power dispatcher said about lines being dead, the line crews still treated them as live until they had all the ground straps attached.

As for the restaurant...had I been the line crew chief I would have asked once in a nice manner. If the restaurant owner refused I would have simply cut the connection between the restaurant and the feeding transformer, either overhead drop or underground lateral. I would have cut a section out of the wires so that a new drop or lateral would have been required and made a note that this business was at the end of the list to have power restored. No sense in getting into a fight.
 
  #11  
Old 09-05-11, 06:01 PM
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The sad thing is the resturaunt owner could've probably afforded a transfer switch.
 
  #12  
Old 09-19-11, 09:10 PM
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All of this is very sad. When I first got my gen, I bought like 200 ft of extension cords, and ran them through the window to different rooms that needed power. After like the third power outage, which lasted maybe a half hour, and I was still running around with extension cords, I look into getting the gen wired to the house. At first I was just going to trip the main breaker when I needed the gen. But I was told even thou the main breaker was trip, there was still one line out of the three wires still hooked up, and could pass power, and kill some one. Also if the linemen happened to make a mistake, and hooked the wrong wire, I could burn up my gen. A transfer switch, installed, is like a 1/3 of what the gen cost. And I don't have to worry about hurt or killing some one or burning up the gen, and maybe loosen my garage when it goes. The right way is the safest and cheaper in the long run.
 
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