Connection between switch threshold and reset threshold
on
TRD
(trend monitor):
The trend feature has the task of monitoring the change of the
input signal over a specific time span. The time base over
which the signal is to be monitored can be set up in menu
group
PARAM
in address
TRDT
. The calculation is the difference
between the start value MW
A
and the end value MW
E
of the
interval. If the calculated value is larger than the value set up in
SETP
then the relay is deactivated. The relay is powered up
again once the value has dropped below that set up in
RESP
.
The direction of the difference (rising or falling) is set by the
prefix. A new value is calculated every second (moving interval).
Example:
The filling height is monitored for its rate of change.
In menu group
LIM
address
MODE
is set to parameter
TRD
. The
switch on threshold value is set in
SETP
and has the value
3
,
the reset value in
RESP
is set to
-2
. The time cycle tm is set up
in menu group
PARAM
in address
TRDT
.
In this example the relay is deactivated when the increase in
filling (MW
E
-MW
A
) has exceeded a value of 3/time unit. The
relay is only powered up again when the lowering of the level
has reached a value of 2/time unit.
Operation of the alarm feature
ALARM
:
If the parameter in
MODE
is set to
ALARM
, then the relay operates
as a system alarm relay. The relay is only deactivated in an unit
fault condition, this can be:
- Cable open circuit as well as short circuit on 2-wire
transmitters
- Sensor fault on a 2-wire transmitter
(<3.6mA or >21mA)
- Cable open circuit on RTD or thermocouples
- Certain hardware and software faults
(see fault finding)
Measurement value
t
À
Relay deactivated (no power), yellow LED off
Process transmitter
62
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