If, for economic reasons, the reactor instrumentation and control system is designed for use on a single d-c source, then the power-source system is simple If a single d-c power source is not satisfactory, the choices of power-source configurations increase greatly. Factors to be considered include alternating source voltage and fre­quency stability, harmonic distortion, plant auxiliary-power outage time and allowable transfer time, maximum allow­able rate of change of voltage and frequency, and load power factor. A specification that is overly restrictive, just to be safe, results in an unnecessarily high cost for the power supply and should be avoided.

Momentary loss and surges and dips of voltage are relatively frequent in plant auxiliary-power systems Voltage dips and surges are caused by switching, failure of equipment remote from the critical bus, and starting large motors on the same power system The duration and intensity of the undesirable transient depend on its proxim­ity and the clearing time of the protective equipment ahead of the faulted section. A single cycle can be severely distorted without a power interruption. (Such distortion might occur when heavy loads are placed or pulsed on a radial feeder remote from the critical bus ) Wave-shape distortion can also result from momentary faults that, m fact, become equivalent phase shifts Large banks of capacitors and intermittent reactive loads can give rise to wave-shape distortion

The most widely accepted method of avoiding or minimizing these effects is to insert a noninterruptible power source to act as a filter between the plant auxiliary — power supply and the critical instrumentation load In addition, the system cable routing and installing must be designed to segregate the redundant systems and methods so that the clean output from the noninterruptible power source will not be contaminated by induced noise from cables in other noncntical systems The imposed static and dynamic seismic loading postulated for the specific area within the nuclear plant in which the noninterruptible power supply is to be located must be taken into account The seismic criteria should be incorporated within the equipment specification. In addition, proper seismic design of equipment foundations and anchors is essential, electric cables and conduit between all critical items of equipment must be flexible.

The reliability of a noninterruptible power system is only as good as the weakest part of the total installed system. This point should be emphasized not only during the design and installation phases but also throughout the life of the system. A continuous and conscientious mainte­nance and testing program is essential