Generators work

[4x4freak]

I got to test the backup generators at the local hospital this evening.

Had a call this morning from the maint. man. He didnt think the kitchen AC was blowing like it should and it kept throwing the belt and wasnt cooling good. I met up with him to check it out; first thing I saw was that the pullies werent quite lined up and the belt wasnt tight enough. I adjusted the mount a little to get it all lined up and tight, flip the disconnect back on and the 3 hp ($$$) motor sizzled and burst into flames. Ok, I guess the windings were bad and that's why the blower wasnt running fast enough. I had to go out of town to do another call, and could swing by the parts house and get a new motor. He gives me the go ahead and asks me to check the rest of the unit when I come back. I tell him to reset the breaker and make sure the thermostat is set to run.

Fast forward about 4 hours. I get the new motor put on, everything is working fine except the 2nd stage comp isnt running. Turns out it hasnt been hooked up since the unit has been on the building (new building opened this spring, unit wasnt that old so it was reused from the old building). It had a little stub of wire on the terminal that had been cut off, didnt match up with any of the spare wires; I just assumed the morons didnt hook it up after it was moved. It doesnt even have a two stage thermostat on it. I put a jumper from Y1 to Y2 and soon found out why it wasnt hooked up. Nothing like dead silence then the roar of the generators. Compressor was shorted and tripped the main breaker to the building.

I'm beginning to think this thing doesnt like me.

This unit is only 4 years old and looks like it was drug out of the scrap pile after they got done with it. It was previously setup for a side draft application and is now on a curb. This was their solution for a replacement panel.

[yorktek37]

I had one building that when a grounded compressor occures it trips the main(gotta love gfci mains) and the generator comes online but there is a problem. The mail is still tripped and the generator runs with no load. They were sensing the power from a location on down the line. Guess the electricians decided to re-engineer the transfer switch. Bottom line,,, Gotta love gfci mains. Loose a fan motor or anything 2.5 amps to ground and loose the building.

[Tiger93rsl]

I was on a call yesterday at a pump station that housed 6" booster pumps for the water main. They had a 6 ton trane split system with shorted windings. The maintainace guy told me the day before the compressor took out the entire pump station. I ohmed out the windings and took his word for it but that must have been one hell of a short concidering that the compressor was on it's own 40 amp breaker that was also tripped, after it tripped the 600 amp main.

[Some Dude]

Gonna take you awhile to clean that one up.

[reef10]

I don't have a lot of experience with auto backup generators but it seems dangerous. So you have a short... you blow the breaker..... genset starts.....fire/electrocution continues. Did the branch breaker blow as well thus killing the power to the unit? Or was there a problem with the breaker not tripping. (over amped?)

[jdblack]

This is a big problem with GFCI breakers on switchgear. I've had several fan motors go out to ground and I lose 1200amps of buss duct.

[4x4freak]

I don't have a lot of experience with auto backup generators but it seems dangerous. So you have a short... you blow the breaker..... genset starts.....fire/electrocution continues. Did the branch breaker blow as well thus killing the power to the unit? Or was there a problem with the breaker not tripping. (over amped?)

I dont think it tripped the breaker to the unit. I know everything on the roof shut down when the main tripped and didnt come back on until the main was reset. AFAIK, the generators on this building only serve critical equipment and a few lights.

[tbirdtbird]

A sparky I know, actually a very good one, was in an electrical room of a major hotel when somehow a 1φ branch circuit shorted. Did the branch breaker trip? Did the 3φ main trip? Naw why would that happen. The utility co. overload on the pole opened. He must have seen this before since he immediately threw the main himself so the whole place wouldn't single-phase. Breakers do *not* always do what they are supposed to.

[stebs]

I don't have a lot of experience with auto backup generators but it seems dangerous. So you have a short... you blow the breaker..... genset starts.....fire/electrocution continues. Did the branch breaker blow as well thus killing the power to the unit? Or was there a problem with the breaker not tripping. (over amped?)

Most Generators have their own overload protection circuits. In the event the main service entrance breaker trips, there will be a brief delay before the generator gets going and is providing juice again. So in theory, while i haven't seen this personally, if the main trips and the generators fire up, as soon as it transfers the load to gen power, it would probably trip another breaker, either the branch circuit or the gens overload breaker. As for the breaker not tripping, I believe a GFCI breaker will trip way before a regular breaker has a chance to trip. After all, they're designed to help prevent people from getting zapped. That is also why they recommend you not to put refrigerators on GFCI outlets or circuits in kitchens.

I have a 17kw residential one by generac, while its nowhere near as big as the big ones you guys see, i'd think this sequence of events would still apply. When our generator senses loss of power, there is approximately a 7 second delay until the generator begins cranking. Once its started and running, there is a 10 second delay to allow the engine to warm up before the transfer switch transfers the load to the generator. As for the larger generators, those delays may be shorter, especially on critical installations.

[drsmith012]

A hospital I use to work at had their generators go from start to load in 7 seconds or less. Anything over 7 they got serviced. They were kept at operating temp all the time.

[stebs]

A hospital I use to work at had their generators go from start to load in 7 seconds or less. Anything over 7 they got serviced. They were kept at operating temp all the time.

Ah. Good to know