WARNING!
These alarm outputs should not be used
for critical safety applications such as a
shut-down system.
Fig 10 Alarm outputs
Programmable functions for each alarm include
adjustable hysteresis, alarm delay and alarm
accept.
11.1.1 Solid state output
Each alarm has a galvanically isolated single pole
solid state switch output as shown in Fig 11. The
output is polarised and current will only flow in one
direction.
Fig 11 Equivalent circuit of each alarm output
11.1.2 Intrinsic safety
Each alarm output is a separate galvanically
isolated solid state switch with certified output
parameters less than those specified for
simple
apparatus
in Clause 5.4 of EN50020. This allows
the alarm terminals 8 & 9 and 10 & 11 to be
connected to any intrinsically safe circuit protected
by a Zener barrier or galvanic isolator providing the
intrinsic safety output parameters of the circuit do
not exceed:
Uo
=
30V dc
Io
=
280mA
Po
=
0.85W
The maximum equivalent capacitance and
inductance between each set of alarm terminals is:
Ceq
= 40nF
Leq
= 20µH
These figures must be subtracted from the
maximum permitted cable capacitance and
inductance specified by the system certificate of
the circuit connected to the alarm.
Fig 12 Typical application of alarms
11.1.3 Programming & adjustment
When an alarm card is added to a BA378C
transmitter the main programme menu is extended
as shown in Fig 13. The additional functions
'ALr1', 'ALr2' and 'AcSP' appear between 'Cond'
and 'COde' in the programme menu. For simplicity
Fig 13 only shows the functions available on alarm
1, but alarm 2 has identical facilities.
The alarms relate to the digital display and the
setpoints are shown in the same units as the
display.
The following table summarises each of the alarm
programme functions and includes a cross
reference to more detailed information. Again only
the functions on alarm 1 are listed, but alarm 2 has
identical facilities.
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