The purpose of an alarm system (annunciator) is to bring attention to an abnormal or unsafe operating condition in the plant. Traditional annunciators used discrete alarm modules for this purpose. These dedicated hardware units are diminished in numbers but they are used in installations where simplicity is desired or where separation from the basic process control system is required for safety reasons.
The annunciators were compact, reliable, and because of the hermetically sealed relay logic modules, they could also
It is mounted in certain hazardous areas in addition to the general-purpose control rooms. In order to be mounted in
Class 1 explosion-proof areas, they required purging. Miniaturization of instruments and the use of graphic control panels initiated the development of remote annunciator systems, consisting of remotely mounted relay cabinet connected to alarm lights installed at appropriate points in the graphic or semigraphic diagram.
PRINCIPLES OF OPERATION
The annunciator system consists of multiple alarm points. Each alarm circuit includes a problematic contact (alarm switch), a logic module and a visual indicator. The individual alarm points are operated from a common power source and share a number of annunciator system components, including an audible signal generator (horn), a flashing light and confirmation and test buttons. In normal operation, the annunciator system and the individual alarm points are inactive. The problem contact is an alarm switch that monitors a particular process variable and is activated when the variable exceeds the preset limits. In electrical annunciator systems, it is usually a switch contact that closes (creates) or opens (breaks) the electrical circuit to the logic module and thereby initiates the alarm condition. In the alert state, the annunciator turns on the visual indicator of the particular alarm point, the audible signal and the flashing light for the system. The visual indicator is usually a plate with the backlight mark engraved with an inscription to identify the variable and the abnormal condition, but it can also be a bull’s-eye with an identification plate. The audible signal can be a horn, a bell or a bell. The flashing light is common to all individual alarm points and interrupts the circuit to the visual indicator when that point enters the alert condition. This causes the light to continue flashing intermittently until the abnormal condition returns to normal or the operator confirms it.
The horn confirmation button is provided with a momentary contact: when operated, the logic module circuit is changed to silence the audible signal, stop the flashing light and turn on the “stable” visual indicator. When the abnormal condition is corrected, the problematic contact returns to normal, and the visual indicator automatically turns off. The test button of the lamp with its momentary contact tests the burnt lamps in the visual indicators. When activated, the button closes a common circuit (bus) to each visual indicator in the annunciator system, lighting those lamps that are not lit as a result of an abnormal operating condition.
A wide variety of sequences are available to define the operation of an individual alarm point in the normal, alert,
acknowledged, and return-to-normal stages in the annunciator sequence. The five most commonly used annunciator
sequences are shown below.These sequences were specified by the ISA-recommended practice RP-18.1, which has since been revised and updated into standard ISA 18.1.
Because the old sequence designations are still used in some plants, some of their more common versions are listed
in figure and also described below. The sequence designations of the present standard ISA 18.1 will also be discussed below.
ISA Sequence 1B, also referred to as flashing sequence A, is the one most frequently used. The alert condition of an alarm point results in a flashing visual indication and an audible signal. The visual indication turns off automatically when the monitored process variable returns to normal.