ECN Forum Forums Code Related Discussion NEC & other Code issues Do Transerfer Switches Break the Neutral?

Thanks guys...
So to reiterate... Both methods are acceptable according to the NEC? That is.. Breaking the Neutral or not...?

That's really the back bone of my questions... the other thing was a tangent... but then I digress... again...lol

I'm aware that the BEST method is to use a UL approved transfer switch... But I'm still in question as to who has the better homemade system (remember my neighbor and I are both qualified electricians and we keep an eye on our unorthodox contraptions...) He with the backfed breaker... or myself with the elaborate ( and fail-safe , if I may add.....) cord and plug system...Easy on the flames... neither of us would approve a customer getting such a setup...but I do want to know what you think.

Sparky,
Unless there is some kind of mechanical interconnect between the main breaker and the back fed breaker at your neighbor's house, his installation is in violation of 702-6. If there is an interlock then both systems are OK, except that the grounded conductor to the generator frame must be removed in his and must be inplace in yours. You must also have a grounding electrode system connected to the grounded conductor at your generator.
Don(resqcapt19)

Sparky,

Of the two, I would sleep better with your arrangement. His is a lot simpler, but more prone to accidents. It is potentially a very dangerous situation. I probably don't have to elaborate on ther dangers if the main was left on or there was some leakage through the main back to the pole.

Does anyone know what would happen to the generator if it was hooked up, breaker was turned on and utility power was also present?

This Topic was moved from the General Discussion Area to the NEC Discussion Area.

[This message has been edited by Webmaster (edited 12-03-2000).]

Great question, Bill...
In fact, I was going to ask that myself...
Ya beat me to it.

What could happen if the generator and grid tried to occupy the same system?

Would the timing of the phases be critical in this? I realize that any timing would be short-lived with the flucuations of a portable generator...

And if so, would the potential be anywhere from 0 to 480V (given a 120/240V system) depending upon how the phases lined up?

Thanks again for your input guys...and thanks for taking the time to "spell it out" for me...

Sparky,

We have several people that frequent this board that should know the answer to that, hopefully they'll pop in soon and tell us about it.

If you try to connect the generator output to the utility, there may be serious damage to the generator. When you tie the generator in parallel with the utility the utility will instantly try make the phase angle of the generator match that of the utility. If the phase angle between the 2 sources is not within about 5 degrees, the utility power will try to make the generator mechanically match the utility phase angle instantaneously. This will result in as a minimum of a tripped breaker, but may actually cause the generator shaft to break. When generators are run in parallel with a utility source or other generators, special paralleling equipment is used to match the phase angle before closing the parallel switch. I am not talking about phase rotation here, but rather the actual point on the sine wave where the two sources are when they are tied together. It goes without saying that the phase rotation and voltage must be matched in addition to the phase angle. All most all utilities require reverse power switching in any generator that operates in parallel with them. This is to prevent back feeding their system. Most of the time it you try to back feed a dead utility system you will just stall out the generator due to the high load, but under some conditions you will create a very hazardous situation for any utility personnel working on the lines.
Don(resqcapt19)

Thanks Don,

That was interesting, I had never heard anything on the subject but assumed that it wasn't good. What happens if the Utility power is fed into the Generator? (Generator not running or stalled)

Wow... You're answering my questions now before I ask them!

Seriously...

I'm to understand that a generator connected to the mains would backfeed the transformer sending line voltage out on the line...

I've always felt that the loads on the rest of that line would be too much for a small generator to compensate for...and as you said, except for some circumstances...it will.

Thanks again for such great and thorough answers, Don.

Just this fall I went to a call, a homeowner had a small 100A RONK dpdt t-switch between the meter and his main panel at his home. He had a small 5kw generator made into it. At his insistence, he had me watch as he cranked up his generator, and threw the t-switch. The house panel died, and the generator simply wound down rather quickly and stalled. The owner exasperated, claimed that was all he'd been getting repeatedly.On investigation, you guessed it, the t-switch was miswired, sending the utility right into the generator. So I corrected it, metered out the generator and ran it to his satisfaction. I suppose, reading some of the reply's here, that it was a rather lucky situation, especially in that I was metering said unit while it was croaking....