Undervoltage refers to a state in which the voltage level of an electrical circuit or device plunges below the accepted operational level, thereby leading to possible malfunction or breakdown of equipment. A power disruption, anomalies in the electrical wiring, or device failure are among the several factors that may cause this state. Should this be disregarded, the outcome of undervoltage may culminate in weakened performance or even destruction of the equipment.
Overvoltage and undervoltage, usually attributed to severe faults accompanied by 50–100% voltage loss on one or more phases, often result in an outage in some circuit. Faults often involve all three phases and may be the result of a downed pole, a tree, or a crane on the line, a breaker lockout, or an in-line fuse blowing. If the critical load is on the cleared side of the fuse, the disturbance becomes an outage. If it is on the power source side of the fault clearing device, the normal voltage may be restored.
Undervoltage protection is protection intended to operate when the power system voltage is reduced to less than a predetermined value [source: IEC 60050-448-1995].
Where a drop in voltage, or a loss and subsequent restoration of voltage could imply dangerous situations for persons or property, suitable precautions shall be taken. Also, precautions shall be taken where a part of the installation or current-using equipment may be damaged by a drop in voltage.
An undervoltage protective device is not required if damage to the installation or to currentusing equipment is considered to be an acceptable risk, provided that no danger is caused to persons.
The operation of undervoltage protective devices may be delayed if the operation of the appliance protected allows without danger a brief interruption or loss of voltage.
If use is made of contactors, delay in their opening and reclosing shall not impede instantaneous disconnection by control or protective devices.
The characteristics of the undervoltage protective device shall be compatible with the requirements of the IEC standards for starting and use of equipment.
Where the reclosure of a protective device is likely to create a dangerous situation, the reclosure shall not be automatic.
Undervoltage protection is used to protect motors from several damaging conditions: low voltage due to a voltage sag, automatic reclosing or automatic transfer, and power restoration. In a voltage sag, the motor draws more current than normal and has unusually high heating. Excessive heating can be a serious problem in hazardous areas where the motor must stay within its T marking.
When the supply voltage is switched off during automatic reclosing and transfers, the motors initially continue to rotate and retain an internal voltage. This voltage decays with motor speed and internal flux. If the system voltage is restored out of phase with a significant motor internal voltage, high inrush can occur. Such current can damage the motor windings or produce torques damaging to the shaft, foundation, drive coupling, or gears. IEEE Std C37.96-2000 discusses considerations for the probability of damage occurring for various motor and system parameters.
When power is restored after an outage, the starting sequence should be programmed so that all motors on the system are not starting simultaneously. This step is important for the generating equipment, as well as for transformers and conductors. Undervoltage devices are
not normally installed on essential loads such as motors for fire pumps.
