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 after device operation, is still connected to the remainder of the power system and remains
energized.
8.2.3.2 10 to 40 Cycles. These disturbances are classed as sags or surges due to the
operation of relatively slow speed switching devices, operation of tap changers on transformers
and voltage regulators, and the starting of motors.
8.2.3.3 0 to 8 Cycles. These disturbances are classed as surges, or more commonly sags,
due to the operation of fuses or high speed switching devices. Inrush currents result from
energizing electrical equipment. A single-phase load on a multi-phase source may cause a surge
on the unloaded phases while causing a sag on the loaded phase.
8.2.3.4 0.001 to 1 Cycles. These disturbances are usually caused by surge arrester
operation, capacitor switching, and short duration faults.
8.2.3.5 Less Than 0.001 Cycle. These disturbances are generally classed as impulses; the
most severe of these are caused by natural lightening, electrostatic discharge, and switching of
nearby loads.
The above paragraphs illustrate that the electrical distribution system may ride through most
disturbances without change. The utilization equipment, however, may be unable to tolerate a
disturbance. It may operate improperly, fail to operate, or operate in a such a way, that if
continued, may cause equipment failure or danger to personnel and operations. These factors
must be taken into consideration at the time of the design and installation of the utilization
equipment. Although this problem is most often encountered with various types of electronic
equipment, it can also effect mundane equipment such as fluorescent and sodium vapor light
fixtures. Since ideal power quality and continuity can seldom be obtained from the supplying
utility, the effects of power supply disturbances can be reduced to acceptable levels using the
following methods:
(a) Modify the design of the utilization equipment to be impervious to power
disturbances and discontinuities.
(b) Modify the power distribution system to be compatible with the utilization
equipment.
(c) Modify both systems and equipment to meet a criterion that is realistic for both.
(d) Interpose a continuous electric supply system between the prime source and the
utilization equipment. This will, however, act as a buffer to external sources of disturbances, but
could increase the magnitude of load induced disturbances.
8-4
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