28
If an incorrect security code is entered, or a
button is not pressed within ten seconds, the
instrument will automatically return to the
display mode.
Fig 13 Setpoint adjustment from the display
mode
To adjust an alarm setpoint select the
required alarm prompt and press P which
will reveal the current setting. Each digit of
the setpoint may be adjusted using the Up
and Down push-buttons, and the P button to
move to the next digit. When the required
setpoint has been entered, pressing E will
return the display to the alarm prompt from
which the other setpoint may be selected, or
the BA358C may be returned to the display
mode by pressing E again.
Direct access to the alarm setpoints is only
available when the menu is enabled - see
section 9.3.12
9.4 Display backlight
The BA358C can be supplied with LED
backlighting to improve display contrast
when the instrument is installed in a poorly
illuminated area. The backlight is
segregated from the measuring circuit and
has been certified as a separate intrinsically
safe circuit, but it does not comply with the
requirements for simple apparatus. Systems
certificates for use with Zener barriers and
galvanic isolators have therefore been
issued.
The backlight must be powered from the
safe area via a Zener barrier or a galvanic
isolator as shown in Fig 14. Any certified
Zener barrier may be used, providing the
output parameters do not exceed:
Uo
28V dc
Io
159mA
Po
0.8W
Ci
40nF
Li
20
µ
H
Alternatively a galvanic isolator may be used
In place of the barrier. For UK installations
only the galvanic isolators specified on the
system certificate should be used.
Fig 14 Backlight powered by Zener barrier
The display brilliance depends upon the
current flowing through the backlight. This is
determined by the supply voltage and the
end-to-end resistance of the Zener barrier or
output resistance of the galvanic isolator.
Brilliance will not be significantly reduced
until the current falls below 20mA.
Backlight current = Vsupply -18
End-to-end resistance of barrier#
# or output resistance of galvanic isolator
