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9.5 Display backlight
The BA307NE and BA327NE loop powered
indicators can be supplied with a factory fitted
backlight that may be loop or separately powered.
When loop powered the backlight produces green
background illumination enabling the display to be
read at night or in poor lighting conditions. No
additional power supply or field wiring are required,
but the indicator voltage drop is increased. When
separately powered the backlight is brighter, but
additional field wiring is required.
Fig 17 Terminals for optional backlight
9.5.1 Loop powering the backlight
The backlight is loop powered by connecting it in
series with the indicator’s 4/20mA input as shown
in Fig 18, which increases the maximum indicator
voltage drop from 1.2 to 5V.
Fig 18 Loop powered backlight
9.5.2 Separately powering the backlight
The optional backlight may also be powered from a
separate power supply as shown in Fig 19.
Fig 19 Separately powered backlight
The separately powered backlight is a current sink
which will draw a constant 22.5mA when the
supply voltage is equal to or greater than 9V. At
lower supply voltages the backlight will continue to
function but with reduced brilliance and will draw
less current.
The ATEX certificate therefore only specifies the
maximum safety input voltage Ui for terminals 12
and 14.
Ui
=
30V
To comply with the requirements of
BS EN 60079-14
Electrical installations design,
selection and erection
, each of the wires entering
the hazardous area should be individually fused
and contain a means of isolation. These two
requirements are frequently satisfied by using fuse
holders with easily removable fuses and removing
the fuses to achieve isolation. This is a
satisfactory method at the low voltages and
currents common in instrumentation systems.
Clear identification of, and easy access to the
means of isolation is essential for their effective
use. It is also necessary to ensure that the
maintenance procedure makes sure that
unauthorised re-closure of the switches does not
occur. It is not considered necessary to have a
means of isolation or electrical protection for the
screen. Figure 20 illustrates an example of this
type of switch fuse terminal block.
For some application Ex nA instrumentation
energised by a current limited power supply that
can be switched off, is often considered adequate.
