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In addition to the user-selectable alarm relays, all pH 500
and mV 600 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. This is an important feature since with most meters
the alarm terminals close only when an abnormal situation
arises, however, due to line interruption, no alarm is sounded,
causing extensive damage. On the
other hand, software is employed to set
off the alarm in abnormal circum-
stances, for example, if the dosing
terminals are closed for too long a
period. In both cases, the red LED’s
will also provide a visual warning sig-
nal.
The Fail Safe mode is accomplished by connecting the exter-
nal alarm circuit between the FS•C (Normally Open) and
the COM terminals. This way, an alarm will warn the user
when pH goes over the alarm thresholds, the power breaks
down and in case of a broken wire between the process meter
and the external alarm circuit.
Note
In order to have the Fail Safe feature activated, an external
power supply has to be connected to the alarm device.
• ALWAYS OFF if pH > setpoint;
Using a similar format, the second relay may be set up ac-
cording to the diagram below.
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
Note
If the power supply is interrupted, the relay is de-energized as
if in alarm condition to alert the operator.
High setpoint mode
alarm
ON
OFF
0
Se
Deviation
Setpoint
14
ON
OFF
Low setpoint mode
0
alarm
Setpoint
14
Setpoint–
Deviation
FS•O = NC (Normally Closed)
De-energized Relay
COM
FS•C = NO (Normally Open)
Energized Relay