Q&A on UPS

Check the codes for power supply back up requirements

Therefore, if your Internet gateway (router-switch) is “listed” (merely for safety, not necessarily for fire alarm use), then a standard plug-and-go UPS system may be used. If your customer already has one of these in use, check it for capacity. If you are asked to provide this UPS, you should know that there are two basic types of UPS units in general use. “Illustration A” shows a standard, off-line UPS permitted by NFPA 72, for the customer’s on-site communication equipment. The AC sensing function causes the trickle-charged batteries to be switched into use should the primary power path be interrupted. A more typical application is to use the UPS to power the customer’s router/switch employed to access the Internet.

The true, online UPS unit in “Illustration B” is connected to the float-charged batteries and charger during normal operation and continues working during local power outages. The only switching to the backup path occurs when the charger, batteries, or DC to AC inverter fails. In those instances, the UPS sensor will switch in an attempt to use the commercial light and power source. However, NFPA 72 stipulates that the UPS be a “Type 0” meaning zero seconds transfer time during a failure of the commercial light and power. An online type (or “true”) UPS is therefore required for compliance with “10.5.3 Power Supply Sources.” It must also be noted that by stipulating a NFPA 111 compliant “Class 24” UPS (“Class 24” means 24 hours of standby time), this new section doesn’t seem to require five or 15 minutes of alarm time after the initial 24 hours of emergency operation, nor does it provide for an additional 20 percent battery capacity. I suspect this new primary power option will need to be cleared up in future editions of 72.



Greg Kessinger is SD&I’s longtime resident fire alarm and codes expert. His column is the longest, continually running contribution in the 34-year history of the magazine. He can be reached at greg@firealarm.org.