| Momentary interruption | Loss of supply lasting from fractions of a second to several seconds. | Server resets, interrupted transactions, corrupted in-process data, and disruption to communications equipment. | Maintains the load during the interruption and provides time for orderly transfer to standby generation when required. | High | IEEE 1159 power-quality event classifications; IEC 62040-3 UPS performance requirements. |
| Voltage sag | Short-duration reduction in RMS voltage, commonly caused by faults, motor starting, or heavy load changes. | Equipment shutdowns, control-system faults, nuisance alarms, and reduced production reliability. | Regulates or reconstructs the output voltage so sensitive equipment receives a stable supply. | High | IEEE 1159 defines voltage sags as reductions in RMS voltage lasting from 0.5 cycle to 1 minute. |
| Voltage swell | Short-duration increase in RMS voltage, often associated with fault clearing or sudden load removal. | Stress or damage to power supplies, reduced component life, and unexpected equipment trips. | Conditions the input and limits the effect of abnormal voltage on connected loads. | Medium to High | IEEE 1159 defines voltage swells as increases in RMS voltage lasting from 0.5 cycle to 1 minute. |
| Complete power outage | Total loss of utility power for seconds, minutes, or longer. | Business downtime, lost productivity, missed service-level targets, interrupted customer access, and potential data loss. | Supplies stored battery energy immediately and bridges the gap until generator power is available or systems are shut down safely. | Very High | UPS autonomy is determined by battery capacity, connected load, operating temperature, and system design. |
| Overvoltage | Sustained voltage above the equipment’s normal operating range. | Excessive heat in power supplies, premature component aging, and equipment failure. | Provides voltage regulation and, in double-conversion designs, electrically separates the load from many input disturbances. | Medium to High | IEC 62040-3 covers UPS output performance and operating characteristics. |
| Undervoltage | Sustained voltage below the equipment’s normal operating range. | Motor overheating, reduced equipment performance, control instability, and unexpected shutdowns. | Boosts or regenerates the output voltage to maintain an acceptable supply for protected equipment. | Medium to High | Power-quality effects are addressed in IEEE 1159 and equipment compatibility practices. |
| Frequency variation | Deviation of the AC supply frequency from its nominal value. | Timing errors, unstable operation of frequency-sensitive equipment, and problems with certain motors or control systems. | Uses an inverter-based output to provide a controlled frequency within the UPS operating specification. | Medium | UPS frequency-performance requirements are specified in IEC 62040-3. |
| Electrical noise and transients | High-frequency interference or short-duration impulses superimposed on the normal AC waveform. | Communication errors, data corruption, false alarms, and accelerated degradation of sensitive electronics. | Filters or reconstructs the power waveform and helps isolate connected loads from disturbances. | Medium | IEEE 1159 identifies high-frequency noise and transients as power-quality disturbances. |
| Harmonic distortion | Waveform distortion caused by nonlinear loads such as switching power supplies, drives, and rectifiers. | Neutral-conductor heating, transformer stress, reduced efficiency, and malfunction of sensitive equipment. | Selected UPS systems reduce the impact of distorted input power and can provide a cleaner output waveform. | Medium | IEEE 519 provides recommended practices and requirements for harmonic control in power systems. |
| Generator or transfer delay | Short interruption while an emergency power source starts and the electrical load is transferred. | Computers, networking equipment, security systems, and industrial controls may reboot before backup generation is available. | Provides no-break or limited-break ride-through, depending on UPS topology and configuration. | High | Emergency and standby power arrangements are addressed by NFPA 110; UPS performance is addressed by IEC 62040-3. |