15
A2.2 Loop powered backlight
The backlight is segregated from the measuring
circuit and has been approved as a separate
intrinsically safe circuit under the entity concept. As
shown in Fig 9 the backlight may be connected in
series with the BA304C indicator and powered
from any certified associated apparatus such as a
Zener barrier or galvanic isolator having output
parameters equal to or less than:
Voc, Vt
=
32V
Isc, It
=
200mA
The backlight & indicator 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 backlight.
A2.3 Nonincendive applications
For nonincendive applications the separately
powered backlight may be connected to a dc
supply between 18 and 30V without the need for a
Zener barrier or a galvanic isolator. The wiring
must be nonincendive or 'Division 2 suitable'' as
defined in the National Electrical Code
ANSI/NFPA70.
Similarly the loop powered backlight may be
connected in series with the 4/20mA measurement
loop without the need for a Zener barrier or
galvanic isolator - see A1.2
