
2408
i Indicator Engineering Manual
2.2
Alarms
Alarms
are used to alert an operator when a pre-set level has been exceeded. They are normally used to switch
an output (see section 2.2.2.) – usually a relay – to provide external actions to the process.
Soft Alarms
are indication only and do not operate an output.
Events
are generally defined as conditions, which occur as part of the operation of the plant. They do not require
operator intervention and, therefore, do not cause an alarm message to be displayed. They can be attached to
operate an output (relay) in the same way as an alarm.
2.2.1
Types of Alarm Used In The 2408i
This section shows graphically the operation of different types of alarm used in the indicator. The graphs show
changes in PV plotted against time. The PV may be derived from input 1, input 2 or the main PV which is derived
from input 1 & 2.
Full Scale Low
On
On
Deviation Low
On
On
Deviation High
On
Deviation Band
On
On
On
Full Scale High
On
Rate of Change
On
Rate of change alarms
detect if the rate of change in PV,
set as units per minute or per second, exceeds the
setpoint value. An alarm setpoint set + will detect
positive rates of change. An alarm setpoint set - will
detect negative rates of change. Therefore, if it is
required to measure the rate of change in both
directions then two alarms must be configured. Since
rate of change alarms are calculated over a period of
time a small delay may be apparent before the alarm is
indicated. This is generally only noticeable if the PV
changes very quickly.
Deviation Alarms. The setpoint used for deviation
alarms is normally derived as a remote input from
another device - for example, a temperature
controller. The setpoint can also be internally set
within the controller - in this case called the local
setpoint value.
Delay a settable time between an alarm occurring and
it being displayed on the indicator
Hysteresis
is the difference between the point at which
the alarm switches ON and the point at which it switches
OFF.
It is used to prevent relay chatter.
Latching Alarms
see 2.2.6
Blocking Alarms
only occur after the start up phase
when the alarm has first entered a safe state. The
alarm is only indicated the next time it is active. It is
used, for example, to ignore start up conditions which
are not representative of running conditions.
2.2.2
Alarm Relay Output
Alarms can operate a specific relay or logic
output. Any individual alarm can operate an
individual output or any combination of alarms
can operate an individual output. They are
either supplied pre-configured in accordance
with the ordering code or set up in configuration
level.
Time
Setpoint (SP)
Full Scale High
Full Scale Low
Deviation High
Deviation Low
Deviation
Band
PV
Rate of Change
Process Variable (PV)
Alarm Type
Output State
dIG
Logic or
Relay output
Any combination of alarms can operate the relay or logic output.
The diagram shows typical alarms
Alarm 1
Alarm 2
Sensor break
Input fail
Alarm 3
OR
SEnS
nor
inv
Part Number HA027240 Issue 5.0 Feb 2015
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