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A1.3.1.2 Nonincendive applications
For nonincendive applications a Zener barrier or
galvanic isolator is not required. Each alarm
output may switch any dc circuit providing the
voltage does not exceed 32V and the current is
less than 250mA. The wiring must be
nonincendive or 'Division 2 suitable'.
A1.3.2 Display backlight
Either the separately powered or the loop powered
backlight may be used.
A1.3.2.1 Separately powered backlight
The 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 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
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 15.
A1.3.2.2 Loop powered backlight
This backlight has been approved as a separate
intrinsically safe circuit under the entity concept.
As shown in Fig 16 the backlight may be
connected in series with the indicator and powered
from any certified associate 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 and indicator terminals have a
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.
A1.3.2.3 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 galvanic isolator. The wiring must
be nonincendive or 'Division 2 suitable'.
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 A1.3.2.2
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