Seven categories of common power problems and their solutions

  • Impulsive transients are sudden high peak events that raise the voltage and/or current levels in either a positive or a negative direction. Electrostatic discharge (ESD) and lightning strikes are both examples impulsive disturbances. Impulsive transients can be very fast, happening as quickly as 5 nanoseconds and lasting less than 50 nanoseconds. For example, an ESD may have a peak of over 8000 volts, but last less than 4 billionths of a second. The transient, however, may still be strong enough to damage sensitive electronic equipment. An approach to solve the problem of impulsive transients is the utilization of a Transient Voltage Surge Suppressor (TVSS). A TVSS is a device that either absorbs the transient energy, or short circuits the energy to ground, before it can reach sensitive equipment.Motors turning on or off commonly cause oscillatory transients for power systems. The voltage quickly rises above its normal level, and then gradually fades back to its normal level over several wave cycles.

  • Interruptions occur when there is a temporary break in the power supplied. There are four types of interruptions: Instantaneous (0.5 cycles to 30 cycles), Momentary (30 cycles to 2 seconds), Temporary (2 seconds to 2 minutes), and Sustained (longer than 2 minutes). An uninterruptible power supply (UPS) can provide shortterm backup power during an interruption.

  • A sag or dip is a reduction of AC voltage at a given frequency for a duration of 0.5 cycles to 1 minute’s time. Sags are usually caused by system faults, and are also often the result of switching on loads with heavy startup currents. Common causes of sags include starting large loads, such as one might see when they first start up a large air conditioning unit, and remote fault clearing performed by utility equipment. Power line conditioners and UPSs can compensate for sags or dips. According to the IEEE, Undervoltage is “… a Root Mean Square (RMS) decrease in the AC voltage, at the power frequency, for a period of time greater than one minute”. An undervoltage is the result of long-term problems that create sags. The term “brownout” has been in common usage in describing this problem, but has been superseded because the term is ambiguous in that it also refers to commercial power delivery strategy during periods of extended high demand. Undervoltages can create overheating in motors, and can lead to the failure of non-linear loads such as computer power supply failures. Undervoltages can overheat a motor or make a power supply fail. Power line conditioners and UPSs can compensate for undervoltages. 

  • A swell, or surge, is the reverse form of a sag, having an increase in AC voltage for a duration of 0.5 cycles to 1 minute’s time. For swells, high-impedance neutral connections, sudden load reductions, and a single-phase fault on a 3-phase system are common sources. A swell is also prevalent when large loads are switched out of a system. Power line conditioners and UPSs can compensate for swells.   According to the IEEE, over voltage is “an RMS increase in the AC voltage, at the power frequency, for durations greater than a few seconds. An Over voltage is common in areas where supply transformer tap settings are incorrectly set, and where loads have been reduced and commercial power systems continue to compensate for load changes that are no longer necessary. This is common in seasonal regions where communities diminish during off-season. Over voltage conditions can create high current draw and unnecessary tripping of downstream circuit breakers, as well as overheating and stress on equipment. Power line conditioners and UPSs can compensate for over voltage.

  • Many different causes of waveform distortion exist. DC Offset happens when direct current is added to an AC power source. DC Offset can damage electrical equipment, such as motors and transformers, by overheating them. Harmonic waveforms are another form of waveform distortion. Harmonics appear on the power distribution system as distorted current. Keep in mind that all equipment that does not have the advantage of modern harmonic-correction features should be isolated on separate circuits.

  • Voltage fluctuation is a systematic variation of the voltage waveform or a series of random voltage changes of small dimensions, namely 95 to 105% of nominal at a low frequency, and generally below 25 Hz. Power line conditioners and UPSs can compensate for voltage fluctuations.


  • Frequency variation is extremely rare in stable, utility power systems, especially systems interconnected through a power grid. Where sites have dedicated standby generators or poor power infrastructure, frequency variation is more common especially if the generator is heavily loaded. IT equipment is frequency tolerant, and generally not affected by minor shifts in local generator frequency.

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