APC AP7921 ATS Failure or...?

[BillP]

In the past couple of weeks I experienced a mains power failure, which had some unexpected impact at the time which I have started looking further into.

My setup is the following:

1. Mains feed directly running into SourceA on my ATS
2. Mains feed also running into my APC SMT1000I UPS which feeds SourceB on my ATS

The idea is that I can run on mains power OR UPS, and have one or the other supplying power.  This also allows me to take the UPS out of service if required.

The total loss of mains power event two weeks ago caused all devices on the ATS (router, switch, wireless controller and a server) to fully power cycle also.  I suspect this may have been because I didn't have the ATS set to "High" sensitivity.  So I have since rectified this.  The frequency deviation is set to 3Hz now but that made no difference.

However since making this change my ATS is frequently (every 2 hours or so) going into a "spasm" state where for anything between 2 - 10 seconds it fails over between SourceA and SourceB in very rapid succession.  It sometimes switches between A and B 20 or so times during this window and makes noisy clicking sounds and then comes good.  Fortunately during this period the devices connected all stay online.  But it's not a happy type of noise.

Mains power here in NSW Australia is overall very good.  We usually have less than 1 outage each year and the data from the ATS shows a very stable 240 +/- 3 volts with 50Hz +/1 1 for very long periods of time.  Now I could almost accept if the unit occasionally switched away from mains power, but it's switching to and from UPS feed which makes me think it's the unit and not the supply which is the cause.

I have power cycled the ATS but this made no difference.  The ATS is under 2 years old and was bought new, but the serial number lookup tells me that the UPS serial number can't be found (it's an ATS...).

Firmware is the latest - and yes when I upgraded it I followed the instructions about having only feed A actually supplying.

Where do I go next with this?  Has anyone else experienced this before?

Thanks,
Reuben