WiremanWireman · UPS alarming or stuck on battery: troubleshooting for electricians · 2023 NEC ·
UPS alarming or stuck on battery
Input power, overload, then the batteries.
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How it works
A UPS (uninterruptible power supply) runs the load from its batteries when its input power is missing or outside the range it accepts. It alarms when it’s on battery, overloaded, or its batteries need attention. A UPS that stays on battery with the power on is usually seeing input it doesn’t like: low or high voltage, or an unstable frequency like some generators produce.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What is the UPS indicating?
- On battery with power onTest: Measure the receptacle the UPS plugs intoExpect: About 120 V (or the UPS’s rated input), steady.
- No voltageNo power at the receptacle
- Low or highInput voltage outside the UPS’s range
- NormalBrief dips or a UPS input fault
- OverloadIs anything plugged into the battery outlets that doesn’t need backup?
- Yes, a printer, heater or similarHigh-draw load on the battery outlets
- No, just computer and network gearUPS too small for the load
- Replace battery or battery faultBatteries worn out
- On battery only when on generatorGenerator power the UPS won’t accept
- On battery with power onTest: Measure the receptacle the UPS plugs intoExpect: About 120 V (or the UPS’s rated input), steady.