Switches vs Alarms


Thread Starter


What is the difference between a switch (like a TS,PS or LS) and an alarm (TA, PA, LA)? I know that a switch reads the signal from a measuring instrument and reacts when the conditions you set are true. How does an alarm differ?
Under the ISA standar for Instrument Identificaton,

Alarm Is a Passive Function (May not affect the process system)

Switch is an Output Function (Does affect the process system)

Switch is an input function like limit switch. selector switch proximity switch etc.,. But alam is an abnormal condition for example, in a process , the temperature is high, pressure is low. I hope this clarifies your doubt.


Jose Manuel Barros

Swith is an element that generates a digital signal (in our context). An alarm is an action or consecuence derived from the treatment of a signal, digital or analogue and is generally conveyed to the operator in one or other form (light, horn, text on the screen,..). In a P&I, not all swithches generate alarms, but a sensor or transmitter could generate one.