Masoneilan SVI II AP Installation and Maintenance ... - GE Energy

Output Channel Isolation
HART Communications with
Output Channel Isolation
The designer of the signaling circuit where the SVI II AP is to be installed must consider
the 8 design rules in Wiring Guidelines (see “Wiring Guidelines” on page 106 of this
manual). In particular, the control system output interface has analog output channels
that are galvanically isolated and share a common ground or are separated from
ground by the current control transistor or sense resistor.
If the outputs are isolated a single channel zener diode barrier can be used.
If the outputs share a common ground a single channel zener diode barrier can
be used.
If the outputs are separated from ground a dual channel zener barrier is required.
Controller outputs are separated internally from ground by a current sense resistor or a
control transistor. Dual channel barriers apply excessive loop resistance and cause
compliance voltage problems. An Intrinsically Safe galvanic isolator operates with all
three types of output channels, isolated, grounded or separated from ground, and
provides sufficient compliance voltage. The galvanic isolator must be certified by the
manufacturer to be HART compliant if the HART connections are supported on the safe
area side of the isolator. See Figure 69 on page 124. Consult barrier and isolator
manufacturer for devices rated for use with the SVI II AP I.S. entity parameters in
Hazardous Area Approvals.
HART Filter Requirements
The control system output interface must allow the HART frequencies to coexist with
the precision 4 - 20 mA DC signal. Circuits that are not designed for HART may need a
HART filter. Consult the controller or DCS manufacturer for interfacing to a particular
system. The HART communications can cause a non-HART compliant output circuit to
malfunction, in some cases. In other cases the HART communications tones are
disabled by the control circuit.
The SVI II AP can be used with non-HART compliant output circuits but remote
communications functionality are not enabled.
Use pushbuttons for all operation and maintenance. If remote maintenance is desired
always isolate the control valve from the process and disconnect the non-compliant
controller before connecting a current source for power and a HART master device.
If a HART filter is required, then its voltage drop must be considered in calculating the
compliance voltage.
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