The Measurement and Analysis of Dynamic Response of Alarm Units

Abstract

Alarm units are used to monitor some measured parameter (e.g., flux, pressure), and actuate an alarm and, if necessary, channel or reactor trips when abnormal values occur. If the measured parameter is above (or below for low alarms) the alarm setpoint, the output relay of the alarm unit opens. Alarm units are not simple comparators, they contain some type of internal amplifier driving a comparator, and the output relay has an inductive coil whose current cannot change instantly. It takes some time, called "response time", after the input signal passes the setpoint before the relay opens. In reactor safety analysis, it is important to allow for the dynamic response time of the instrumentation, including that of the alarm units. The ISA standard S67.06 defines methodologies which may be used to measure response times of the various components in the nuclear safety channel. The most popular tests are ramp tests and steps tests. This report reviews the theory of operation of alarm units and shows how to apply ramp tests and step tests to an alarm unit to obtain ramp response time, time constant and fixed delay. This report also discusses how the time constant and fixed delay derived from step tests can be cross-checked against the ramp response time and how a first-order approximation to a higherorder system should be made. Finally, the ramp and step response time measurement methods are applied to a test circuit representing a stand-alone alarm unit having known time constants and fixed delays. The results of these tests are evaluated as a means of assessing the practicality and accuracy of the test methods.