Power Failure Relay Switching Problems

You can read about the different types of UPS's here.

Just in case, have you noticed that many UPS devices have several outlets, and that commonly only one to a few of those outlets are actually connected to the UPS? Those that are will commonly have a label that says something like 'Battery Backup', while the rest will say something like 'Surge Protected'. Only the former few have power during an outage.

Secondly, make sure that the total power required by all protected devices is well under the output capabilities of the UPS. Non-critical devices such as printers (especially laser printers) should NOT be plugged in to a UPS, except in those 'surge protected' outlets.

Their are two types of power failure devices.

UPS and Stand-by PS.

The UPS is, as it's name implys, a device that uses the incoming wattage to charge a battery. The Battery wattage is transformed back to 120 Hz, usually a square wave, but, in more expensive models, a "pure" sine wave.

The Stand-By PS has an undervoltage relay, that switches from line to the Stand-By PS, in the event of a power failure. High Speed Solid State relays can reduce the switching time; however, there will still be a dwell, where the voltage will drop to near zero.

The Stand-By PS is chaper than the UPS, for a reason. It will keep an incandescent bulb, or a filiament, lit; however, digital devices don't react well to power interuptions. In fact, when the relay opens, it generates an electrical spike which tends to eat sensitive devices.

Where did you get the 120Hz from?

I think there are some mistakes here.

Did you not mean 120 volts at probably 60Hz?

If you are using an inverter for back up power this may cause a problem with the ups.

Some modified sinewave inverters are famous for this.