4. Maintenance-related failures
While the periodic maintenance of UPS systems may seem limited, it is absolutely essential and requires a rigorous set of procedures. Failure to maintain equipment on schedule as required is a major cause of malfunctions.
For example, some UPS systems are neglected for extended periods, leading to instability and requiring repair. Upon opening the casing, a thick layer of dust is often found on the circuit boards and components. Simply cleaning this dust with a hairdryer and vacuum cleaner restores normal operation. Another example is a UPS repair where the technician connected mains power to the input but forgot to start the inverter or close the battery switch. When the mains power failed again, the UPS shut down because the inverter failed to start.
5. Experience-based faults
Even experience gained from handling several malfunctions cannot be applied rigidly. For example, a user experienced with a Type A UPS tried to operate a newly installed Type B UPS without consulting the manual, attempting a DC start based solely on experience. He was familiar with UPSs that could DC start, but this particular machine lacked this function and therefore wouldn’t start. He then opened the chassis and started poking at every relay with a screwdriver. The inverter started, but immediately began smoking, and the power transistors were damaged. He didn’t realize that a UPS with DC start function follows a specific startup procedure: after the DC start switch is turned on, the control circuit operates first, then drives the inverter—a normal startup. However, this UPS lacked DC start capability; the control circuit was working while the inverter was starting, and the unstable transition caused both power transistors in the inverter to conduct simultaneously, burning out.
Some maintenance personnel, believing they have mastered the principles of UPS systems during training, only require learning troubleshooting methods. However, short-term training can only teach simple fault diagnosis methods; specific faults require specific analysis, and experience can only be used as a reference. For example, there are many reasons why a UPS AC input fuse might blow, such as rectifier breakdown, filter capacitor breakdown, inverter breakdown, short circuit in the RC network across the input rectifier diodes, and damage to the control circuitry caused by these issues. All of these can lead to fuse burnout. Of course, experience is useful for diagnosing and troubleshooting some simple faults, but it must be applied flexibly and tailored to each specific situation.