UPS for PLC 1

This is a pretty small UPS... there a number of vendors who provide these at this size, APC is one that we use... check with your local computer store, they might even have one on the shelf that will fit you needs...

Depending upon the operating environment (model required), the cost should be fairly reasonable...

Thanks for the reply, I know that there are many ups available at the store, the problem is finding one that can be switched on/off externally. I do not want a computer to control the ups with a usb or serial port.

Look at APC web site they have a lot different types with different features, rack mountables etc. I used APC with plc it is very good. Another is Tripp lite "smart pro rack mounted" but I have not used this one.

Why not just put a relay on the output of the UPS and use that to switch your load?

APC makes a Remote Power Off (RPO) device, maybe that's what you need: .

Since PLCs work on 24 V DC its pretty stupid to provide a UPS on the mains. You can buy power supplies for PLC-application which provide power directly from a battery to their output in case of an outage.

rtads,
Sola/Hevi-Duty has a full range of UPS. Their 2000 series models range from 350 VA to 700 VA and are line interactive. We have been using these to back up our industrial PC's and PLC racks for about 2 years and have proven to be very reliable in a harsh industrial environment. Their 3000 models are more expensive but have more features. Check out their website.
We have had some bad luck with the APC units in the past. (Two of them have failed)

APC has alittle-known industrial product called the SP500DR, which mounts on a DIN rail and has screw terminals for input and output power.

Unfortunately, it has an RS-232 port that works with their PC software instead of a status contact. We have connected a 120V relay coil in parallel with the input and connected its contact to a PLC input to signal the PLC that normal power is gone and it's running on backup.