Work stations in your plant's administrative area have point-of-use UPS boxes, most of which are rated 500VA. These units provide ride-through during brownouts. They also allow people to save their work and conduct an orderly shutdown in case of a blackout.

During recent sag and blackout events, these boxes didn't stop workstations from shutting down. Everyone agreed that the obvious problem was that the batteries had outlived their usefulness and needed to be replaced. But even with new batteries, the problem persists. What's going on?

While it may seem the UPS boxes aren't doing their job, probably the opposite is true. Upon power loss, large loads can kick spikes into the premises distribution system. During sags, large loads can draw enough current to saturate transformers.

The UPS boxes see transients and open the circuit to protect the loads. So, they've saved you a few workstations, monitors, and other office equipment. You don't have a UPS problem; you have a power quality problem.

To solve it, start with these steps:

  • Test your power correction capacitors.
  • Put a power analyzer on the panel that supplies an area that's been reporting trouble. Put power analyzers on all large loads on site. If analysis shows transients, design a multi-stage transient reduction system.
  • Conduct a thorough, methodical inspection of the transformers, panels, and large inductive loads for conformance to Art. 250, Part V.