What is the most common cause of an emergency generator not shutting off after normal power is restored? Could it be something hanging up in the transfer switch?

...Is there a latching relay??

I'm not certain, please explain.

Some have a time delay so you have to have power restored for a pre-determined time. It could also not be recognizing the return of normal power. Who knows? It might not be an automatic.

I'm going back to it early tomorrow morning. I'm also going to check for stable utility power. I've just read if utility power is not stable it will not switch back.

You don't say if this is a service call or a new install. I would get the manual out and be sure the clock is programmed correctly and the dip switches are set right. Check the wiring diagrams too.

You may also want to check your return to normal delays (could be extended, I prefer 5 minutes minimum) and your cool down cycle, I prefer 15 minutes on larger units of over 100 kw. I have seen factory defaults of 1/2 hour each meaning an hour for retransfer and cool down, IOW it may be shutting down, just taking one heck of a long time to do it, some customers don't understand these things and panic.

We have had two very new (less than a year) Onan transfer switches cause generators to run when normal power was fin.

Both times a electronic board in the ATS went bad.

One Generator ran for 126 hours the other for TWO WEEKS! they where natural gas units so fuel was not an issue.

I also agree that it is probably a relay or control board.

Bob,
Were these nat. gas units life safety? Do they allow nat. gas units for life safety in your area?

...Well, we had a customer who put his in "test", and left it on all weekend, then couldn't figure out why it didn't turn off

Boy have I had, and had, and had, the argument about Natural Gas. When I got my present job I noticed we had about 80% (of 45 gensets) that were natural gas.

I then checked and we were not installing battery back up lights, so the gens were the back up.

I asked who had approved the use of natural gas since it was a violation. 700-12 states you will have a 90 minute supply on site, and natural gas cannot meet that requirement. I got no reply, but did notice they quit fighting me about diesel generators.

We just went over part of this in this thread: https://www.electrical-contractor.net/ubb/Forum18/HTML/000062.html

In case you can't tell, I don't believe Nat. Gas should be used for emergency backup gens, but limited to NON (Optional) emergency standby use. Should some disaster occur such as earthquake, tornado, or act of terrorism, you will lose the gas pretty quick.

One of my men found a defective time delay relay this morning. It's an ONAN brand emergency generator and ATS approx. 25 years old.

Natural Gas is the most common fuel for small to mid sized life safety generators in this area.

A few times we have had to remove diesel sets and replace them with natural gas sets.

This was necessitated because the storage of diesel fuel near wet lands around here is a major problem.

The NEC provides an exception to the on site fuel requirement.


I have never heard of the gas being shut down and I guess the AHJ has not either.

iwire,
Glad to hear about your good luck.

When I was at the Prison, an alarm would sound, and we had 20 minutes to switch our boilers to #2 fuel before the gas was shut down. Happened about 4 times a year.

We live in seismic zone 1, the weakest, but we do have 'em occasionally, and the seismic monitors shut it down.

Part of it was shut down during Isabel.

A line rupture some years ago shut it down for days to a significant portion of the region.

I quoted the exception in the post linked, but when I was the AHJ, it was not going to be allowed, and with design review and acceptance, it still won't be allowed.

George If it can be demonstrated that the gas service in a particular area is that unreliable it makes more than good sense not to count on it.

That is not the case in the area I work, gas service is very reliable.

I did not follow your statement about starting times, the natural gas sets we install fire in less than 3 seconds and if they have not started in 10 they are not going to.

Of course these units are generally inside a building and have block heaters that keep the water temp above 120 F all the time.

If the water temp falls we get a low temp alarm.

We also set them up to run once a week for 30 to 60 minutes.

One of the company's customers has over 340 of these natural gas units ranging in size from 30 to 100 KW. Right now we have been contracted to go to about 100 of them and connect them to a monitoring station for Gen. Run, Gen. Fail and Power fail.

This will, when done, verify the generator runs when the exercise cycle comes, if not a generator technician will be dispatched.

These are article 700 generators for a large retail chain in the North East.

Bob

If it is a Guardian/Generac generator, that is is in auto and you know utility is back on, check F1 and F2 fuses to see if thefe it 240vac going out to the generator transformer. If the PCB doesn't "see" the utility voltage return, the genset will keep running.

Just spotting this topic, and reading the 90 minute fuel aspect, and thinking... Well that's 90 minutes of fuel, minimum! Then thinking Natural Gas, it won't go out. Now, I'm remembering a job we did last year, where the gas did go out, and all the time! It was on a busy road, and had a new gas siezemic shut off valve.

Every time a large truck would go by, the gas would shut off! It was required by and provided by the utility. Meant to operate in an earthquake. It had a little ball that would fall off a holder, and into a valve that would shut off the gas. So, I guess natural gas might not be such a wise idea after all.

Most likely it's the PC board.