6
Fig 2b Loop powered by a galvanic isolator
4.2 Remote indication
The indicators may be driven via an intrinsically
safe interface from a 4/20mA safe area signal to
provide a remote display within a hazardous area.
The type of intrinsically safe interface is not critical,
either a Zener barrier or a galvanic isolator may be
used, providing that Ui, Li and Pi of the indicator
are not exceeded and the voltage capability of the
4/20mA signal is sufficient to drive the indicator
plus the interface.
When a high integrity earth connection is already
available, a Zener barrier is usually the least
expensive option. If an earth connection is not
available or isolation is required, a galvanic isolator
is the correct choice.
If one side of the 4/20mA current loop may be
earthed, a single channel Zener barrier provides
the lowest cost protection. If the 4/20mA signal is
not isolated, then two Zener barriers, a two
channel Zener barrier or a galvanic isolator must
be used.
Fig 3 shows the alternative circuits which may be
used.
Fig 3 Alternative circuits for remote indication
in a hazardous area
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