8- 3.1 General System Categories

Instrumentation and control power systems may be categorized as either interruptible or nomnterruptible.

(a) Interruptible Systems. Interruptible systems are those m which power is obtained directly from the plant auxiliary-power system through suitable filters and regulat­ing equipment, if they are required to improve the power quality. An interruption of plant auxiliary power results in total loss of instrumentation power This type of system is applicable only where the instrumentation and control loads are noncntical, i. e., absolute continuity of operation is not essential. Usually the total interruption time is a maximum of approximately 15 sec, ie., the time in which the plant standby auxiliary power source, several diesel — engine-dnven generators, reestablishes power to the essential auxiliary buses

(b) Noninterruptible Systems. Nomnterruptible systems are those in which any interruption of power results in loss in continuity of operation or m erroneous operation of critical instrumentation and control loads Since nomnterruptible power systems are the type most commonly required for nuclear-reactor instrumentation and control applications, the remaining sections of this chapter concentrate on power-source systems that are noninter­ruptible for a discrete length of time after a plant auxiliary-power failure.

There are two categories of nomnterruptible power systems, nonisolated and isolated

Nonisolated systems are those in which plant auxiliary power is imposed directly on the essential auxiliary-power buses Normally the nomnterruptible power supply is also fed from these essential power buses and is storing up potential energy. If a plant auxiliary system fails or if there is a severe transient, the nomnterruptible power supply releases the stored energy to the critical bus, thus affording a short-time carry-over of stable power

The more commonly used type of noninterruptible power supply is the isolated system This differs from the nonisolated system in that the instrumentation power buses do not receive power directly from the plant auxiliary power system but rather through the nomnterruptible power supply. The nomnterruptible isolated power supply thus acts as a filter or buffer separating the instrumentation power buses from the plant auxiliary-power system and any transients that appear on that system