21
9.3.1
Solid state output
Each alarm has a galvanically isolated single pole
solid state switch output which as shown in Fig 14.
The output is polarised and current will only flow in
one direction.
Ron
=
less than 5
Ω
+ 0.7V
Roff
=
greater than 1M
Ω
Fig 14 Equivalent circuit of each alarm output
9.3.2
Ex nA certification
Each alarm output is a separate galvanically
isolated Ex nA circuit with the following input
parameters:
Ui
=
30V dc
Ii
=
200mA
This allows each alarm output to switch any dc
circuit providing that in normal operation the
maximum supply voltage is not greater than 30V
and the switched current is not greater than
200mA.
Fig 15 shows a typical application in which a
BA304NG or BA324NG indicator is displaying the
output from a 2-wire transmitter in Zone 2.
Alarm 1 is switching a solenoid valve in Zone 2 and
alarm 2 is switching a sounder located in the safe
area.
The BA304NG and BA324NG indicators have
Ex nA protection but when mounted in Zone 2 the
alarm outputs may be used to switch Ex nC
Protected Sparking and Ex nR Restricted
Breathing apparatus in addition to Ex nA
apparatus.
Fig 15 Typical alarm application
(Shown without recommended screened cables)
To strictly comply with the requirements of
EN 60079:14
Electrical Installation in Hazardous
Areas
, each of the wires entering the hazardous
area should be individually fused and contain a
means of isolation. However, in practice
instrumentation energised by a current limited
power supply that can be switched off is often
considered adequate.
