[erics37]

Water treatment plant in the same town as the police station with the UPS and buck boost set up from my other thread is having an issue with their transfer switch.

It happens very seldom, but they called me out to see if I could determine anything.

It's an ASCO switch, 230 amp 480 volt 3 phase, pretty straightforward simple unit with a minimal interface. They do a weekly exercise test on it, where they will run the generator and actually switch the plant over to it for 30 minutes or so. It is supposed to time out on its own and switch back to normal utility power after the exercise period is over. Most of the time it works great, but 2 or 3 times in the past it has stayed on and never switched back to utility power.

According to the plant operator, once he realized, "Hey the generator sure has been running for a while," he went down and looked at the switch and sure enough, the indicator light for "Utility Power Available" was lit up as expected, but for some reason the transfer switch did not transfer back.

The other issue happened just a week or two ago. They ran the generator's exercise like normal but when the exercise period was up, the transfer switch switched back to utility power but all the lights and equipment shut off. Turned out the breaker feeding the utility supply to the transfer switch had tripped right at switchover.

It's not a GFP breaker or anything; just a standard molded case breaker. I did a FOP test on it and measured around 20-35 mV dropped across each breaker pole at approximately 75 - 100 amps per leg. Pretty darn good, I'd say. I don't dare meg the feeder and branch circuits though, it's supplying toooooooooooons of instruments, drives, etc. I can't imagine why that breaker would trip right at switch over. Haven't been able to duplicate it for a recording meter either.

Any thoughts?

 

[Wirenuting]

Does it have an in phase monitor? If so you have to wait till they sync. Older Asco's can take awhile. If the monitor isn't working correctly, older ones were relays, newer are board mount.
Didn't take much to screw them up.
A bad switch over will pop the breaker.

 

[Bbsound]

I agree the inphase monitoring can take forever if the genset is running at 60hz.
We always set them at about 60.5 or so, and try to not use the inphase monitoring.

About the breaker tripping, it is not VERY unusual (but it should not happen) for breakers to trip on transfers. Think about all the motor loads, etc that could be not inphase with the utility.
If the utility breaker tripped why didn't the transfer switch transfer to emergency?

 

[micromind]

If the transfer switch does not have an in phase monitor, it will transfer whenever the time has elapsed.

If this occurs when the utility is far enough out of synch with the gen, inductive loads will be seen as a fault by the utility main. If they don't slam back in synch quick enough, the instantaneous trip unit may very well cause the breaker to trip

If the breaker has an adjustable trip, a slightly longer time delay will likely solve the problem.

If this doesn't solve the problem, you might look into installing a #25 (synch-check) relay in the switch. Depending on the switches control scheme, this can be pretty easy, or a bit more involved.

 

[brian john]

It's an ASCO switch, 230 amp 480 volt 3 phase, pretty straightforward simple unit with a minimal interface. They do a weekly exercise test on it, where they will run the generator and actually switch the plant over to it for 30 minutes or so. It is supposed to time out on its own and switch back to normal utility power after the exercise period is over. Most of the time it works great, but 2 or 3 times in the past it has stayed on and never switched back to utility power.

260 Amp is a more common size.

What is the age of the ATS?
Not sure it has an In-Phase-Monitor- Place a meter across utility and Generator
A to A, or B to B, or C to C and watch the meter as it goes to "0" volts after satisfying all Time Delays it SHOULD transfer.

AS for the CB read what others posted and check inrush current as it transfers. Typically a In-Phase-Monitor should minimize inrush current.

 

[erics37]

260 Amp is a more common size.

What is the age of the ATS?
Not sure it has an In-Phase-Monitor- Place a meter across utility and Generator
A to A, or B to B, or C to C and watch the meter as it goes to "0" volts after satisfying all Time Delays it SHOULD transfer.

AS for the CB read what others posted and check inrush current as it transfers. Typically a In-Phase-Monitor should minimize inrush current.

It's 230 amp, I'm looking at a picture of the nameplate :laughing:

I think the transfer switch is only 1-2 years old.

Thanks for the pointers, I'll try that

 

[Meadow]

Open, closed transition?

It possible the ATS contacts did not make good contactact causing intermittent voltage and motors to drop off line all at once?

 

[Semi-Ret Electrician]

How good is the frequency regulation on the generator.

If it wanders all over the place it may never transfer back automatically as others have stated.

Also, if the transfer switch is getting dirty it may be getting too slow for an "in-sync" transfer.

Or, have any heavy loads been added or high inertia loads removed since it was put in service?

 

[64pvolvo1800]

It's 230 amp, I'm looking at a picture of the nameplate :laughing:

I think the transfer switch is only 1-2 years old.

Thanks for the pointers, I'll try that

So probably a D3003230n1xc catalog?
The 300 series should have a group 1 panel which has an in phase monitor activated by a dip switch. It is not enabled on a stock switch but in our application should be. As someone pointed out, an out of phase transfer could periodically pop the normal side breaker on rexfer. However, if you do it have an electronic governor on the generator set and it is running too fast or slow to sync long enough for a smooth switch, it will do two things: take forever to switch and possibly pop the breaker too.
Check the manual for the dip switch and turn it on. Check the generator freq and look for 60.5 with load. Check n the governor though. If the generator isn't electronic and is too small for the load, you can have some wide swings in freq and see what your seeing.