Baker Hughes Masoneilan SVIII AP, Installation And Maintenance Manual

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Masoneilan SVI II AP Installation and Maintenance Manual =
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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 107 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 48 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.