23
ALARM RELAY
The alarm relay functions in the following manner:
During alarm condition, the relay is de-energized. When
not in alarm condition, the relay is energized.
Example:
High alarm set at 10 pH
Low alarm set at 4 pH
An hysteresis will eliminate the possibility of continuous
sequences ‘energizing/de-energizing’ of the alarm relay
when the measured value is close to the alarm setpoint. The
hysteresis amplitude is 0.2 pH in HI21 and 30 mV in HI22.
Moreover the alarm signal is generated only after a user
selectable time period (alarm mask) has elapsed since
the controlled value has overtaken one alarm threshold.
This additional feature will avoid fake or temporary alarm
conditions.
Note
If the power supply is interrupted, the relay is de-energized
as if in alarm condition to alert the operator.
In addition to the user-selectable alarm relays, all
HI21
and
HI22
models are equipped with the
Fail Safe
alarm feature.
The
Fail Safe
feature protects the process against critical
errors arising from power interruptions, surges and human
errors. This sophisticated yet easy-to-use system resolves
these predicaments on two fronts: hardware and software.
To eliminate problems of blackout and line failure, the
alarm function operates in a “Normally Closed” state and
hence alarm is triggered if the wires are tripped, or when
the power is down.
