Re: Question on Generator Backfeeding...

Posted by K Effective on January 01, 2017 at 17:46:21 from (162.235.236.141):

In Reply to: Question on Generator Backfeeding... posted by John B. on January 01, 2017 at 15:49:19:

Okay, let's discuss this a little bit, since you have much more experience with generators than I do. Obviously, this situation happens, so I must be missing something. I am using the male term lineman universally here, yes I know there are many female utility workers, it's just less typing.

For argument sake, say my house averages 10 amps load at 240 volts. My 8000 watt generator would supply about 30 amps of 240 volt current via it's built-in 30 amp receptacle. IF I were to accidentally back feed the line with my generator, about 0.4 amps of current is available to loads back on the main line, at 12K volts backwards through the normal step-down transformer on the pole leading to my house. (20A@240V)

ASSUMING there are more than three houses on the line, all averaging the same 10 amps load as my house, I SHOULD very quickly trip the 30 amp breaker on my generator. IF that generator breaker welds itself shut, the 30 amp dryer breaker used to back feed the main box should also trip open (bi-metallic heats in both directions). Past that, I have a 200 amp main breaker, but I ASSUME the generator would fail internally long before that level would be reached (48000W)

The only way I can see a lineman being injured from a back feeding generator is IF I were to bring the generator online and back feed while he is in contact with the previously dead line, AND he has failed to ground the line or take protection from it.

IF I am the only house or there are fewer amps of draw on the main line,and my little generator is actually carrying the whole line by itself, both sides could be energized, but I would assume any lineman is taught to verify that first, or at the very least assume both are HOT. IF he is working the repair HOT, as often is the case, the live line he is attempting to reconnect the dead line to would pose a much greater threat, and he would be protected by his equipment from the high side, and thus from the dead side as well. Simply grounding the "dead" line would trip open any generators on it before he reconnected to the high side, right?

So, technically, any accident nearly has to be caused by the worker failing to treat the fallen line as hot. I hate to say it that way, as it lays the blame on those people who work so hard at being safe and keeping us powered for all our silly wants.

Now, as far as damage to the generators back-feeding the line when the main power is restored, that I can clearly see. Every generator we have at work uses sophisticated synchronizing systems before connecting to the grid, any little generator would be no match for the power of the grid once reconnected, and could not survive the slip of synch.

I have taken measures to make my generator a physical break-then-make system, to protect the generator AND any repair persons.

Thanks for listening, and let's discuss this.

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