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[Part 1 of this two-part article focused on increasing control system reliability with single-point ground techniques.]
Ground Wiring
Proper connections, wire sizing, and overall ground impedance are important to effective grounding.
Local and Isolated Common Reference Points
An organized ground system includes reference points that are local to an enclosure (or groups of enclosures) and also an isolated common reference point for the plant area. A local reference point provides a central termination point for all power supply common connections within an enclosure. The isolated common reference point ties together the local reference points.
Note ... Some figures in this manual show DC Return Reference Grounds connected directly to an isolated common ground reference. This scheme is applicable when there are only a few reference ground returns. Where many reference grounds exist, as in the case of grouped enclosures, use an isolated local ground reference in each group and connect these to the isolated common ground reference for the area.
Good reference points should be highly conductive, copper, copper-clad, or brass bars. Such bars are available from local industrial electronics suppliers, or you can fabricate your own ground bars. The bars mount on isolated brackets inside an enclosure. Bars normally allow screw connections for wiring lugs. Use lugs that accept wire sizes of AWG 1/0 to 4/0 (53.5 to 107.2 mm2).
Figure 4-7 shows the details of a typical ground bar. Table C-2 lists recommended wire sizes based on cable length. If you fabricate your own ground bars, ensure that the following conditions are met:
Copper/copper clad steel or hard brass (B16)
Minimum of 3/8 inch (9.5 mm) thick and 1-3/4 (44.5 mm) inch wide
Holes for lugs
Double-bolted lugs
Bus isolated from mounting bracket with standoffs
Figure 4-7 Typical Reference Point Ground Bars
DC Return Ground Wiring
Ensure that DC return grounds are on separate wires from each power supply to the enclosure local ground reference point. The DC return ground connection at the power supply is the negative (") DC terminal. Where bulk power supplies feed a DC-to-DC controller and I/O power supply, the DC return is placed between the carrier ground terminal and ground reference.
The recommended wiring for these ground points is AWG 8 (8.4 mm2) copper, stranded, insulated with the lengths being as short as possible. The return ground wiring should be insulated to avoid unintentional ground loops that can occur if bare wires touch the metal enclosure frame, or each other. Proper isolation from metal should be verified by electrical measurement after installation.
Marking Grounds
To aid in ground identification, identifiable insulation colors (green or green with a yellow stripe) or some labeling method should be used that meets U.L.-listed connections. All system ground points should be labeled as follows:
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FOR CONTROL SYSTEM GROUND ONLY. DO NOT USE FOR ELECTRIC ARC WELDER CONNECTION OR OTHER ELECTRICAL OR ELECTRONIC CIRCUITS. |
Ground Impedance
A high quality control system ground should provide a ground point that measures one ohm or less to true earth. In some cases, three ohms may be acceptable. In an area where soil does not provide a good ground, it may be necessary to select the best ground impedance available.
There are several methods that can be used to obtain a high quality earth ground system; these methods vary, depending on the soil type and moisture content at the individual location. Refer to Chapter 7 for information about soils and earth grounds. Testing an earth ground per the procedure described in Chapter 7 is highly recommended.
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