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Power quality and mains disturbance

Electronic equipment functions properly as long as the utility is available and with high quality. Unfortunately not always this is possible because most of the power quality events are caused by factors beyond the control of the Distribution Company. These factors may occur either externally (in the network distribution system) or internally (in the facility when the system is located). External causes may be for example storm, lightning, high wind, tree limb contact, construction activities etc, while the internal causes may be switching of the machine, capacitor banks, fault or maintenance activities. Depending on the events the power quality disturbances may vary in type, duration and intensity. Typical power supply disturbances that cause major bad effects on your load are listen below.

Power outage

Power interruption for a time of more than 3s is called an outage or commonly “blackout”. This is normally caused by atmospheric accidents to the high voltage distribution network or a failure in the power supply equipment.



  • Computer applications. Complete system shutdown with loss of data, possible hardware damage and long periods of employee inactivity.
  • Industrial applications. Huge impact in the production activity with consequent production loss and/or unexpected safety consequence.



It is a voltage reduction in amplitude for a time between 10 ms to 1 s. expressed as a percentage from 10 and 100% of the rated voltage. This is caused for example by short circuits in the resident distribution systems or the large inrush current of the transformers.



  • Computer applications. Overheating of the electronic components with consequent operational breakdown.
  • Industrial applications. Instability of asynchronous motors and loss of synchronization of synchronous motors, opening of contactors (voltage drop > 30%), power-off discharge lamps (voltage drop > 50% for 20-40 ms) with subsequent return of power after several minutes and consequent operational breakdown.



Overvoltage is an increase in voltage for a time of more than 10 ms. Overvoltages may be caused by the disconnection of major loads (interruption in manufacturing processes of industrial companies, reduction in the speed of electric motors, arc furnaces, rolling mills, etc.) or natural events such as lightning.


Faults in all electric/electronic equipment (100%<overvoltage<150%); e.g. damage to boards, power supplies, computers/servers, faults in lighting systems, etc.

Sag and Swell

Any short term voltage decrease (sags) or increase (swell) for a time from half cycle to 3 seconds. It is normally caused by the equipment shutdown, circuit breaker operations, high peak current absorbed by the equipments, or short-circuit.


The major effects may be the loss of data or the lights flicker, very unpleasant for the people exposed to these effects.


Transients are rapid, very high overvoltages of up to 20 kV. They are caused mainly by lightning (which is random in its location, duration and amplitude) but also by faults on the high-voltage network, the switching of inductive loads or the powering of highly capacitive loads.


Transients destroy inadequately protected equipment (melting of wires, perforation of isolation in motors, badly-timed release of protection devices, etc.).


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