Emerson Process Management 9110-00A Several DPCs Mounted in Metal Cabinet with Power Supply

increased noise pickup and signal offset errors. If more information is required on this subject, the reader should refer to the publications cited at the end of this document.

The examples that follow describe the grounding techniques for several types of Bristol Babcock systems employing DPCs. Refer to the system description that is closest to your application.

Several DPCs Mounted in Metal Cabinet with Power Supply

A small system can consist of one or more DPCs mounted in a single metal cabinet or rack with a power supply. A power wiring diagram for this arrangement is shown in the example of Figure

8.The following installation procedures apply:

1.Instrument Ground. The instrument ground of the DPCs (24VRET terminal of each DPC) must connect to a terminal block within the cabinet that is electrically isolated from the cabinet frame. This terminal block must provide termination for all in-strument grounds and include termination for a multistranded, insulated, #4 gauge wire (or greater). This wire, which will connect to the "zero reference point" of the facility, must be run through metal conduit (pipe). Only the #4 wire will be contained in this conduit. The 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 24VRET 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 or Correctors, 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 with Local Power Supply in Each Cabinet

This application consists of two or more DPC cabinets either separated or clustered together in a room. Each cabinet has its own 24 Vdc power supply. The following procedures apply:

10 / Supplement S1400