During a short interruption, the voltage is zero; thus, there is no supply of power at all to the equipment. The temporary consequences are that there is no light, that motors slow down, that screens turn blank, etc. All this only lasts for a few seconds, but the consequences can last much longer: disruption of production processes, loss of contents of computer memory, evacuation of buildings due to fire alarms going off, and sometimes damage when the voltage comes back (uncontrolled starting).

For most sensitive equipment, there is no strict border between a voltage sag and an interruption: an interruption can be seen as a severe sag, i.e. one with zero remaining voltage.


The effect of a zero voltage on an induction motor is simple: the motor slows down. The mechanical time constant of an induction motor plus its load is in the range of 1 to 10 seconds. With dead times of several seconds, the motor has not yet come to a standstill but is likely to have slowed down significantly. This reduction in the speed of the motors might disrupt the industrial process so much that the process control trips it.

The motor can re-accelerate when the voltage comes back if the system is strong enough. For public distribution systems, re-acceleration is seldom a problem.

Also, the setting of the under-voltage protection should be such that it does not trip before the voltage comes back. This calls for coordination between the under-voltage setting of the motor protection and the reclosure interval setting on the utility feeder.

Induction motors fed via contactors are disconnected automatically as the contactor drops out. Without countermeasures, this would always lead to loss of the load.ย In some industrial processes, the induction motors are automatically reconnected when the voltage comes back: either instantaneously or staged (the most important motors first, the rest later).


Synchronous motors can normally not restart on full load. They are therefore equipped with under-voltage protection to prevent stalling when the voltage comes back.ย For synchronous motors, the delay time of the under-voltage protection should be less than the reclosing interval. Especially for very fast reclosure, this can be a problem. We see here a situation where an interruption causes a more serious threat to the synchronous motors the faster the voltage comes back. With most other loads the situation is the other way around: the shorter the interruption, the less severe it is to the load.


Adjustable-speed drives are very sensitive to short interruptions and voltage sags. They normally trip well within I second, sometimes even within one cycle; thus even the shortest interruption will cause a loss of the load.ย Some of the more modern drives are able to automatically reconnect the moment the voltage comes back. But being disconnected from the supply for several seconds will often have disrupted the process behind the drive so much that reconnection does not make much sense anymore.


Without countermeasures, electronic devices will trip well within the reclosing interval. This leads to the infamous “blinking-clock syndrome”: clocks of video recorders, microwave ovens, and electronic alarms start blinking when the supply is interrupted; and they keep on blinking until manually reset. An easy solution is to install a small rechargeable battery inside of the equipment, to power the internal memory during the interruption.

Computers and process control equipment have basically the same problem. But they require more than a simple battery. An uninterruptible power supply (UPS) is a much-used solution.

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