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12 / Supplement S1400
1. Multi-Cabinet Instrument Grounding using Multiple Supplies.
The instrument ground
(24VRET terminal) of each DPC in a cabinet must connect to a terminal block within that
cabinet that is electrically isolated from the cabinet frame. This terminal block must
provide termination for all DPC instrument grounds within that cabinet and include
termination for a #4 gauge (or greater), multistranded, insulated wire that will connect
to the zero reference point of the facility. This #4 wire will be run through metal conduit
(pipe) to the same termination point in the other cabinets. Only the #4 wire will be
containe in this conduit. This conduit must also be connected by bonding strap to the
cabinet and facility frame as described in the NEC.
2. Setting DPC Power Jumpers.
If the DPC is a Model 3335 or 3310, jumpers W1A and W1B on
the System Interconnect Board must be removed to isolate the chassis connection from the
24V RET connection (see Figure 8). If it is a Model 3330, jumpers W1A, W1B and W1C on the
System Interconnect Board must be removed. Series 3308 Gas Flow Computers, if used with
these systems, provide an isolated instrument ground without setting jumpers.
3. AC Power Source.
The 24 Vdc power supply requires a 120 Vac power source. The ac power
terminals of this supply are identified in Figure 8. The 120 Vac wiring for this supply must
be contained in cable trays along with the power grid grounding wire. Figure 9 illustrates
the cable tray layout and grounding points of a typical facility. The cabinet frame and the
DPC chassis must be connected with bonding strap at points specified in the NEC.
4. Grounding of Peripheral Equipment.
Some peripheral devices such as printers, CRTs,
personal computers, etc., have their internal logic ground connected to the chassis. This
configuration will cause loss of isolation between the DPC's instrument ground and chassis.
These devices may still be used providing that opto-isolated circuits, galvanic isolation, or
other types of circuitry between the device and the DPC are added to maintain the integrity
of the DPC's "instrument ground to chassis" isolation.
Multiple DPC Cabinets Powered by Single Power Supply
All of the DPC cabinets in this installation operate from a single power supply which may be
installed in one of the cabinets or in a separate cabinet. The following procedures apply:
1. Multi-Cabinet Instrument Grounding using Single Supply.
The instrument ground (24VRET
terminal) of each DPC in a cabinet must connect to a terminal block within that cabinet that
is electrically isolated from the cabinet frame. This terminal block must provide termination
for all instrument grounds within that cabinet and include termination for a multistranded,
insulated, #4 gauge wire (or greater).
2. Routing of Instrument Ground and 24 V Power Wires.
The #4 wire, along with +24V and
24VRET wires, are run through conduit trays to the power supply. The #4 wire connects to
the 24VRET terminal of the supply and to the zero reference point of the facility. The +24V
and 24RET wires connect to corresponding terminals on the power supply.
3. Cable Trays.
Figure 9 illustrates the cable tray layout and grounding points of a typical
facility. The cabinet frame and the DPC chassis must be connected with bonding strap at
points specified in the NEC. The NEC also applies to the connection of the 120 Vac power
supply terminals to the line.