Control System Grounding - Part 1: Single-Point Ground Techniques

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SYSTEM GROUNDING
To ensure best performance and reliability of your control system, an adequate electrical ground network must be installed. Poor or faulty grounds are among the most common causes of control system faults. The extra time and effort spent in installing a good ground network is rewarded by easier system startup and more reliable system operation.

Single-Point Ground Techniques
The ground network is an organized system of ground wiring that terminates in a single, dedicated point on the plant ground grid. With single-point ground, you can achieve a clean reference for the control signals in your control system, thereby ensuring increased system reliability over systems with poorly designed ground networks. Figure 4-1 illustrates a typical, single-point ground network for a small control system.

Figure 4-1 Typical, Single-Point Ground Network for a Control System Housed in a Single-Enclosure

The system illustrated in Figure 4-1 usually consists of a single enclosure with the isolation transformer included on the enclosure. For heat control purposes, the transformer is usually mounted outside of the enclosure. The transformer only provides power for the equipment in the enclosure. Its AC ground is connected to the control system ground (CSG).

Figure 4-2 illustrates a typical, single-point ground network for either a large control system housed in several adjacent enclosures or for a system spread over a local area. It is important that the term "local area" is well understood in relationship to power and grounding.

Figure 4-2 Typical, Single-Point Ground Network for Control System Housed in Multiple Enclosures

Each local area is defined as having one isolation transformer, a ground reference for the area, and cable-feet limitations. By this definition, all instruments powered by the transformer are located in the same local area, whether the instruments are located in one enclosure, in multiple adjacent enclosures, or spread in remote enclosures. All instrumentation and enclosure grounds, and the transformer ground, terminate at one ground reference for the area. Additionally, each power and ground path is typically limited to 200 cable-feet.

By using a separate isolation transformer, an isolated common ground reference for each area, and cable length limitations, each local area is electrically isolated from other areas. Therefore, ground loops between areas are eliminated, electrical noise is minimized, and common reference voltage is maintained.