Unwanted ground loops can cause inaccurate sensor readings by negatively affecting instrumentation signals.
Today's modern process plants are highly dependent upon their electrical instrumentation for the quality of the end product. The information supplied by thermocouples, RTDs, and other temperature sensors is used by manufacturing instrumentation not only for control purposes but also to prevent runaway reactions.
The same applies to other manufacturing parameters, such as pressure and flow. As such, maintaining accurate instrumentation output is critical to safety as well.
Instrumentation grounding
Any equipment used to implement a control instrumentation strategy (see sidebar, on page 96) makes use of a common signal ground as a reference for analog signals. Any additional grounds introduced into the control circuit will almost certainly cause ground loops to occur.
To minimize the danger of introducing these loops into a complicated network, you should employ a dedicated instrumentation system ground bus. This bus ultimately receives grounds from the signal common, the cabinet ground, and the instrumentation AC power ground. The bus is tied to earth via the building ground and the plant ground grid. Fig. 1 shows a typical configuration of interconnecting these various grounds.
Further complicating the picture is the fact that a facility rarely has just one instrumentation loop; it may have hundreds or even thousands. Many are packaged together in vendor-supplied instrumentation system cabinets. Generally, these cabinets contain a DC signal common bus and a power supply common bus; these busses normally are tied together within the cabinets at a master ground bus.
The cabinet ground is a safety ground that protects equipment and personnel from accidental shock hazards while providing a direct drain line for any static charges or electromagnetic interference (EMI) that may affect the cabinets. This cabinet ground remains separate from the DC signal ground until it terminates at the master ground bus. Also, the cabinet ground must meet all applicable NEC requirements.
The AC service ground is a single-point ground termination of the system AC power. This ground connects to the neutral-to-ground bond at the main AC power isolation transformer. It also terminates at a single point on the plant ground grid (the grounding electrode).
Ground loops
In a ground loop, a circuit as shown in Fig. 2 develops because each ground is usually tied to a different earth potential. This condition allows current to flow between the grounds by way of the process loop.
Ground loops cause problems by adding or subtracting current or voltage from the process signal. The receiving device is unable to differentiate between the wanted and unwanted signals and, thus, can't accurately reflect actual process conditions.
The probability of multiple grounds and ground loops being established is especially high when new programmable logic controllers (PLCs) or distributed control systems are installed.
With so many connections within a facility referenced to ground, the likelihood of establishing more than one point is great. Thus, if an instrumentation system seems to be acting strangely or erratically, and the problem seems to point toward ground loops, the chore of eliminating all unintended ground connections becomes overwhelming.
Ending ground loops
All analog control loops are grounded at one or more points. While a single ground poses no problems, multiple grounds can result in a ground loop. This phenomenon can upset the proper functioning of instruments.
Keep in mind that eliminating ground loops just isn't feasible for some instruments, such as thermocouples and some analyzers, because they require a ground to obtain accurate rate measurements. Also, some instruments must be grounded to ensure personnel safety.
When ground loops can't be eliminated, the solution to instrumentation ground loops lies in the use of signal isolators, as shown in Fig. 3. These devices break the galvanic path (DC continuity) between all grounds while allowing the analog signal to continue throughout the loop. An isolator also can eliminate the electrical noise of AC continuity (common-mode voltage).
Signal isolators use either of two techniques to achieve their function. Analog signal isolation usually is achieved with isolation transformers. Discrete signals usually employ opto-isolators. Both have their own advantages and disadvantages. The choice between the two depends upon the circuitry requirements.
Regardless of the isolation method used, an isolator must provide input, output, and power isolation. If this three-way isolation is not provided, then an additional ground loop can develop between the isolator's power supply and the process input and/or output signal.
Isolators, like most other transmitters, come in 2- and 4-wire versions. The 4-wire type requires a separate power source and is partially suited for back-of-panel mounting. The 2-wire type can be powered from either the input or output loops. The input loop type makes it possible to isolate a process signal when line power or output loop power isn't available. The output loop type solves the problem of interfacing nonisolated field signals with systems, such as a computer, PLC, or distributed control system, that provide loop-power to their output devices.
RELATED ARTICLE: HOW AN INSTRUMENTATION LOOP WORKS
A typical instrumentation loop is shown below. Basically, it's a DC system that operates at a specific voltage (24V in this example) to a master ground reference called a signal ground. The instrumentation signals vary within a range of 4mA to 20mA, depending upon the value of the variable (temperature, pressure, etc.) seen by the sensor. A precisely calibrated circuits takes this mA signal and converts it into a 1V-to-5V signal for a chart recorder. At 4mA, the voltage measured by the recorder is 1V (250 ohms x .004A). At 20mA, the measured voltage is 5V. Normally, the recorder scale is calibrated so that the voltage reads directly in [degrees] F, psi, etc.