15
A2.1 Seperately powered backlight
The separately powered backlight is segregated
from the measuring circuit and has been approved
as a separate intrinsically safe circuit under the
entity concept. The backlight may be powered
from any FM certified associated apparatus such
as a Zener barrier or galvanic isolator having
output parameters equal to or less than:
Voc, Vt
=
32V
Isc, It
=
159mA
Po
=
1.2W
The backlight terminals have a maximum
equivalent capacitance and inductance of:
Ci
=
0.03µF
Li
=
0.01mH
These figures must be subtracted from the
maximum permissible cable capacitance and
inductance allowed for the Zener barrier or
galvanic isolator powering the backlight.
Segregation between the backlight and the
measuring circuits should be maintained by
following the wiring methods recommended in
ANSI/NFPA70 article 504.
See Fig 8.
A2.2 Nonincendive
applications
For nonincendive applications a Zener barrier or
galvanic isolator is not required. The backlight
may be powered directly from a dc supply
providing the voltage does not exceed 32V. The
wiring must be nonincendive or 'Division 2
suitable'.
A2.3 Loop powered backlight
This backlight is connected in series with the
4/20mA measuring circuit so that like the BA324D
indicator it is powered by the 4/20mA signal. This
eliminates the need for a separate Zener barrier or
galvanic isolator and associated wiring for the
backlight, thus significantly reducing the installation
costs, but it increases the total voltage drop in the
4/20mA loop from 1.1V to 5.0V
See Fig 9.
Any FM certified Zener barrier or galvanic isolator
may be used, providing the output parameters do
not exceed:
Voc, Vt
=
32V dc
Isc, It
=
200mA
Po
=
1.2W
The indicator and backlight terminals have a
combined maximum equivalent capacitance and
inductance of:
Ci
=
0.05µF
Li
=
0.02mH
These figures must be subtracted from the
maximum permissible cable capacitance and
inductance allowed for the Zener barrier or
galvanic isolator powering the indicator and
Backlight.
A3.4
Nonincendive
applications
For nonincendive installations no barrier or isolator
is required, but the maximum supply voltage to the
combined indicator and backlight must be less
than 32V. Wiring must be nonincendive or
'Division 2 suitable'.