Limit Alarm Trips and Switches0 pages

June 2006 Something happensa signal peaks or fallsחandyou need to know. A limit alarm trip can trigger the
response needed to maintain normal, and safe,
operations. A limit alarm trip monitors a process signal (such as
one representing temperature, pressure, level or flow)
and compares it against a preset limit. If the process
signal moves to an undesirable high or low condition,
the alarm activates a relay output to warn of trouble,
provide on/off control or institute an emergency
shutdown.While limit alarm trips are best known as a sure way toactivate a warning light, siren or bell when a process
problem occurs, they are also called upon to do much
more. In fact, todays highly flexible and versatile alarm
trips can be found working in a wide range of
applications, under an impressive list of pseudonyms.
Here are just some: >
Independent ғhard alarm trips can be usedto warn of trouble, provide on/off control ortrigger an emergency shutdown.

ԓHard vs. ԓSoft Alarms

Because they are hard-wired into the
process and provide relay outputs,
independent limit alarm trips are often
referred to as ԓhard alarms. This term
differentiates a ԓhard alarm trip from the software-
implemented alarm (a ԓsoft alarm) which is found within a
Distributed Control System (DCS) or a programmable
logic controller (PLC). Hard-Wired AlarmTrip AlarmOn/Off Controller
Limit SwitchTrip Amplifier or Trip AmpRange Alarm
Safety ShutdownLevel ControllerSafety Interlock
Redundant ShutdownTemperature, Voltage or Current AlarmHigh/Low Controller
High Integrity SwitchEmergency ShutdownCurrent or Temperature Switch
Sensor Failure MonitorComparatorTemperature Averaging Alarm
Supervisory AlarmDifferential AlarmRate-of-Change Alarm
Shutdown AlarmFailsafe AlarmFault Monitor
Failsafe ShutdownWindow Alarm >

Why Use ԓHard Alarms?

Most every plant performs alarm functions using ԓsoft
alarms within their DCS or PLC. As such, some might
argue that ԓhard alarms are not necessary. However,
ԓhard alarm trips complement DCS and PLC systems >
Figure 1. Limit alarm trips monitor a process signal and send oneor more relay outputs when a monitored signal exceeds preset highand/or low limits (dual high/low alarm configuration shown). DUAL HIGH/LOW ALARM TRIP Change in Input Signal Change over Time The Interface Solution Experts ԕ www.miinet.com >
2006 Moore Industries-International, Inc. Page 1 size="-1">