7
5.1.2 With Galvanic Isolators
A galvanic isolator can supply a higher voltage than a
Zener barrier to a loop powered transmitter and is
able to power a BA474D transmitter fitted with an
optional display backlight which has a minimum
operating voltage of 15.5V. Although an isolator is
more expensive than a Zener barrier it does not
require a high integrity earth. For small systems
where a high integrity earth is not already available a
galvanic isolator may reduce the overall installation
cost. Fig 4 shows a typical temperature transmitter
loop.
Fig 4 BA474D powered via galvanic isolator
Any ATEX certified 4/20mA repeater power supply or
transmitter power supply galvanic isolator may be
used. There are three requirements when designing
a BA474D loop incorporating a galvanic isolator:
1.
The intrinsic safety output parameters of the
galvanic isolator must be equal to or less
than:
Uo
=
28V
Io
=
200mA
Po =
0.85W
2. The isolator must supply a minimum output
voltage at 20mA of:
BA474D without backlight
9.0V
BA474D with backlight
15.5V
3. If HART
communication is to be used the
galvanic isolator must be suitable for powering
‘smart’ transmitters, i.e. it must be transparent
to HART
signals.
5.2 Use as associated apparatus
WARNING
When used as associated apparatus
without Zener barriers or galvanic
isolators the BA474D transmitter must
be installed in a safe, non-hazardous
area.
When installed in a safe area, the associated
apparatus certification allows the BA474D Indicating
Temperature input terminals 1, 2, 3 & 4, to be directly
connected to an earthed or floating sensor in a
hazardous area without the need for Zener barriers or
a galvanic isolator. Fig 5 shows a typical loop. The
sensor may be a passive device such as a
thermocouple or resistance thermometer complying
with the requirements for simple apparatus, or a
voltage source. See section 4.5 of this instruction
manual for full details of what may be connected.
The 4/20mA loop must be designed such that the
voltage between terminals 5 & 6 remains between
the following limits at all currents.
without backlight
9.0 to 28V
with backlight
15.5 to 28V
To comply with the associated apparatus certification,
terminals 5 & 6 should not be connected to any
apparatus containing or generating a voltage greater
than 250V rms or dc.
The intrinsically safe input wiring connected to
terminals 1, 2, 3 & 4 must be segregated from the
non intrinsically safe 4/20mA output wiring connected
to terminals 5 & 6. See section 6.2
Fig 5 Use as Associated Apparatus
