Solving Instrumentation Ground Loop Problems

Nov 1, 1999 12:00 PM, Edited by John A. DeDad, Editorial Director

Unwanted ground loops can cause inaccurate sensor readings by negatively affecting instrumentation signals.

Have you ever had problems with process controls and electrical instrumentation? The source may be ground loops. What's this? According to the ANSI/ IEEE Standard Dictionary of Electrical and Electronics Terms, it's 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 layman's terms, a ground loop develops because each ground is tied to a different earth potential. This condition allows current to flow between the grounds by way of the process loop.

Basically, ground loops cause problems by adding or subtracting current or voltage from the process signal. As a result, the receiving device can't differentiate between the wanted and the unwanted signals. Therefore, 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.

You may have to ground at more than one point. For some instruments (such as thermocouples and some analyzers), it may not be possible to eliminate ground loops. That's because these instruments require a ground to make accurate rate measurements. Also, all analog control loops are grounded at one or more points. And as we've seen, multiple grounds can result in a ground loop that can upset the proper functioning of instruments. You also may have to ground instruments to ensure personnel safety.

So if you can't eliminate the conditions for ground loops, what's your next step? You can use signal isolators. These devices break the galvanic path (DC continuity) between all grounds while allowing the analog signal to continue throughout the loop. An isolator can also eliminate the electrical noise of AC continuity (common-mode voltage).

Signal isolators use one of two techniques to do the job. One is analog signal isolation, which uses an isolation transformer to chop, isolate, and reconstruct the signal. The other is discrete signal generation, which chops, transmits optically, and reconstructs the signal. Signal isolators using the latter technique are referred to as "opto-isolators." The choice between the two depends on your circuitry requirements.

Regardless of the isolation method you choose, an isolator must provide 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 line power or output loop power isn't available. The output loop type solves the problem of interfacing non-isolated field signals with systems such as a computer, PLC, or distributed control system, which provide loop-power to their output devices.

You can find a signal isolator to suit almost any application. Here are just a few.

• 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.

So if an instrumentation system starts acting strangely or erratically, make sure you eliminate all unintended ground connections.

Sidebar: Instrumentation grounding

Almost all equipment used in a control instrumentation strategy makes use of 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 and connect grounds from the signal common, cabinet ground, and instrumentation AC power ground to it. The bus is tied to earth via the building ground and plant ground grid.

But, this can be much more complicated than it appears. For example, you will rarely have just one instrumentation loop. In fact, you could have hundreds or even thousands.

Many are packaged together in vendor-supplied instrumentation system cabinets. Generally, these 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).

Sidebar: How an instrumentation signal loop works

Suppose you have an instrumentation loop, as shown in the figure (in the original article). 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 ohms2.004A). At 20mA, the measured voltage is 5V. Normally, the recorder scale is calibrated so the voltage reads directly in DegrF, psi, etc.