I manage a business network closet and want to improve its uptime during power outages. Originally, two UPS units provided about seven minutes of runtime. After reusing two older UPS units and optimizing the equipment load, the closet now runs for roughly an hour, and I have a fifth UPS available.
Because UPS units should not be daisy-chained, I considered using an automatic transfer switch or ATS PDU to switch between them. I purchased an inexpensive AC120V, 15A automatic transfer switch with NEMA 5-15 connections and have done some basic testing. It appears to work, and none of the connected equipment exceeds its current rating, but I later noticed that the device is not UL or ETL listed. My main concern is whether using it could create a fire or electrical-safety hazard.
Would an ATS be an appropriate way to use multiple UPS units, or would an expandable-battery UPS, a properly sized enterprise UPS, or a generator be a better solution?
4 Answers
I would avoid the inexpensive unlisted ATS. The current rating alone does not establish that the device is safe for continuous critical power use. Look for a properly certified solution, but in most cases the better answer is to replace the collection of UPS units with one appropriately sized UPS that supports external battery packs. That also makes monitoring, battery replacement, and testing much simpler.
A more conventional network closet design uses two properly installed power circuits and two PDUs, with dual-corded equipment connected across both sides. One or both circuits can be backed by a monitored, expandable UPS. Single-cord devices can use a certified ATS where appropriate, but that should be a deliberate part of the electrical design—not a cheap switch used to combine spare UPS units. Any work on the circuits should also follow proper isolation and lockout procedures.
Multiple UPS units create several problems beyond the transfer switch. Older batteries may fail without warning, and a standby UPS may not be carrying a realistic load or may not reveal a fault until it is needed. Many UPS batteries need replacement every three to five years. Measure the actual load with a power meter, then select a single enterprise-grade UPS with expandable battery capacity. A generator is worth considering if you truly need several hours of runtime.
UPS systems are generally intended to bridge short outages or keep equipment running until a generator starts. If outages commonly last for hours, adding battery capacity or installing a generator is more dependable than stacking five UPS units. Also consider an automatic, monitored shutdown plan for equipment that does not need to remain online for the entire outage.
A longer runtime is usually achieved by adding approved battery modules to the UPS, not by putting several unrelated UPS units behind an ATS. The UPS should also be able to report its condition and runtime through a network management interface.