Cross-posted from a Bio-Med list. I took the time to write this so why not share with my fans and clients?
We’ve had a couple instances of losing E-power (switching to generator) and many of our UPS units failing to provide any backup. These power losses are for just seconds. Today we had a stack of new APC 750’s that not only failed to back up but when power was restored it still would not power the computer plugged into it! Has anyone else had this type of experience or know what might be causing it? Does anyone have a recommendation for a rock solid UPS or a way to keep them from getting knocked out?
I’m not a huge fan of stand-by (or line interactive, which are mostly the same thing) UPS’s for emergency power system use. I suspect that’s what you have here.
Most of the time, such UPS’s feed regular power to the load. When an outage is sensed, the inverter switches in and feeds the load. There is some dead time (usually in the 8 – 16 msec range, 1/2 to 1 cycle). This works pretty well for normal power – the utility rarely fails abruptly (typically some drop in voltage over several cycles, as motor loads backfeed the line) and the UPS transfers over cleanly with a minimal impact on the load.
Emergency power systems provide a lot of complications. Most of the time, facilities persons fire up the generator, bring it up to speed / sync, then exercise the transfers switches to “test” emergency power (which is not the most realistic test of the emergency power system, but has minimal impact on the facility). As a result, there is often a short transfer time (from 10 – 50 msec) and based on my power monitoring over the years, that transfer time is abrupt and often has high voltage transients that are going to get past the UPS (not yet switched to inverter) and the TVSS (designed for high voltage, high frequency transients, not relatively low voltage sub cycle transients)
In addition to the unusual outage profile, emergency power can bring to the dance frequency fluctuations, voltage distortion, and out of phase transfers that some UPS have problems with. Again, not what the utility normally does, so perhaps the highly engineered to take cost out UPS’s are not designed to deal with.
If you have access to a power monitor, I might suggest monitoring (a) a normal emergency power test (no UPS) and (b) the output of your UPS. I suspect you will walk away kind of amazed that you are not having more issues.
My advice: for critical system on emergency power, use an online UPS (not stand-by or line interactive) – these will provide no-break power. Yeah, they will cost more. There are a lot of vendors out there who do that sort of thing, and I don’t have a particular favorite – the key is to look for online, double-conversion technology. APC has some of these although maybe not in a small (750 VA) size that we are discussing.
You also might consider that the UPS you are using in undersized. Your load might power up fine while on utility, but the inverter might be collapsing due to load inrush. So the load will not power up when the UPS is on battery. If you mean “generator power” being restored, its possible that the UPS does not think the generator power is good enough (frequency, voltage level, voltage stability, waveform) and so is still running from battery / inverter. I would take a known good UPS and try to power up the load with the line cord unplugged – I suspect the issue is that the UPS is undersized to supply the inrush current while on inverter.
I’m not APC or standby / line-interactive bashing here – that’s what I use in my office for my server / desktop. The price is right, they make a ton of them so reliability is good, they work well for normal computer loads and normal power, and the software interface to my network is good. But in my experience, emergency power systems cause problems for them often enough that I would not consider myself “protected” unless I had been through a few months of emergency power testing without an issue, and had a good idea what the emergency power system testing looked like (power monitor) on the line side as well as the load side of the UPS.