Unwanted ground loops can cause inaccurate sensor readings by negatively affecting instrumentation signals
Having problems with your process controls and electrical instrumentation? The source may be a grounding disturbance known as a ground loop. According to the ANSI/ IEEE Standard Dictionary of Electrical and Electronics Terms, a ground loop is a "potentially detrimental loop formed when two or more points in an electrical system normally at ground potential are connected by a conducting path such that either or both points are not at the same ground potential." In other words, a ground loop develops because each ground is tied to a different earth potential, which allows current to flow between the grounds by way of the process loop, as shown in Fig. 1.
Basically, ground loops cause problems by adding or subtracting current or voltage to or from a process signal. As a result, the receiving device can't tell the difference between the wanted and the unwanted signals. So it can't accurately reflect process conditions.
The probability of establishing multiple grounds and ground loops is especially high when you install new programmable logic controllers (PLCs) or distributed control systems. With so many connections referenced to ground within a facility, the likelihood of establishing more than one point is great.
For instruments like thermocouples and some analyzers, you may not be able to completely eliminate ground loops because these instruments require a ground to make accurate rate measurements. Also, all analog control loops are grounded at one or more points, which can result in a ground loop that can upset the proper functioning of instruments. You also may have to ground these instruments to ensure personnel safety.
How an instrumentation signal loop works. Suppose you have an instrumentation loop, as shown in Fig. 2. As you can see, it's basically a DC system that operates at a specific voltage (24V in our example) to a master ground reference called a signal ground. The instrumentation signals vary within a range of 4mA to 20mA, depending on the value of the variable (temperature, pressure, etc.) seen by the sensor.
Let's say a precisely calibrated circuit 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 the voltage reads directly in °F, °C, psi, etc.
The how and why of signal isolators. So what do you do if you can’t eliminate the conditions for ground loops? You can use a signal isolator, as shown in Fig. 3, to break the galvanic path (DC continuity) between all grounds while still allowing the analog signal to continue throughout the loop. These devices also eliminate the electrical noise of AC continuity (common-mode voltage).
Signal isolators use one of two techniques to do the job:
- Analog signal isolation, which uses an isolation transformer to chop, isolate, and reconstruct the signal.
- Discrete signal generation, which chops, transmits optically, and reconstructs the signal. Signal isolators that use this technique are called "opto-isolators."
The choice between the two depends on your circuitry requirements.
Regardless of the isolation method you choose, make sure your isolator provides input, output, and power isolation. If you don't have this three-way isolation, 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. You can power the 2-wire type from either the input or output loops.
The input loop type makes it possible to isolate a process signal when you don’t have line power or output loop power available. The output loop type solves the problem of interfacing non-isolated field signals with systems like a computer, PLC, or distributed control system. These systems provide loop-power to their output devices.
You can find a signal isolator to suit almost any application, including the following:
- Resistance input isolators for use as RTD, slidewire, strain, and potentiometer transmitters.
- Millivolt isolators for use as thermocouple and millivolt transmitters.
- Current/voltage isolators for use as alarm tripping, deviation alarm notification, and other special application transmitters.
Sidebar: An Instrumentation grounding primer
Almost every piece of equipment used in a control instrumentation strategy uses a common signal ground as a reference for its analog signals. Introducing any additional grounds into a control circuit will almost certainly cause ground loops to occur.
To minimize the danger of introducing these loops into a complicated network, you should use a dedicated instrumentation system ground bus (see Fig. 4) and connect grounds from the signal common, cabinet ground, and instrumentation AC power ground to it.
You should tie the instrumentation system ground bus to earth via the building ground and plant ground grid. But, this can be much more complicated than it appears. For example, rarely will you have just one instrumentation loop. In fact, you could have hundreds or even thousands.
Many vendor-supplied instrumentation system cabinets contain a DC signal common bus and power supply common bus. The manufacturer normally ties these busses 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. It also provides 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.
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 (usually the grounding electrode).