Have you ever had problems with process controls and electrical instrumentation?
The source may be ground loops. What is this?
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.” Unwanted ground loops can cause inaccurate sensor readings by negatively affecting instrumentation signals.
A ground loop exists when a circuit is connected to earth ground at two or more points. Because the potential of the earth varies from point to point, two or more connections to the ground cause currents to flow. If the current flows through a signal carrying wire, the result is a noisy, offset signal.
The classic symptom of a ground loop is a sensor that reads correctly in buffers but gives a reading grossly in error when placed in the process liquid. In a typical process measurement, the pH sensor is connected through the process liquid and piping to the earth ground. If the circuitry in the pH analyzer becomes connected to a second earth ground, the current will flow through the reference electrode. A voltage proportional to the current and the electrode resistance develops across the reference electrode. Because the voltage is in series with the other cell voltages, the ground loop current causes the pH reading to be substantially different from the expected value. The currents created by ground loops are often unstable, so pH readings affected by ground loops are often noisy.
Checking for a Ground Loop
If an instrumentation system starts acting strangely or erratically, make sure you eliminate all unintended ground connections. Or if your readings fluctuate when you touch a cable or move the sensor. These can occur when you – add or change a motor or agitator. Any electrical item that is worked on – may upset the balance and needs to be checked once more.
Use the following procedure to check for ground loops:
- Remove the pH sensor from the process liquid.
- Calibrate the sensor in buffers. Be sure there is no direct electrical connection between the container holding the buffer and the process liquid or piping.
- Strip back the ends of a heavy gauge wire.
- Connect one end of the wire to the process piping or, better, place it in the process liquid. Place the other end of the wire in the container with the buffer and sensor. The wire makes an electrical connection between the process and the sensor
If the pH reading changes or becomes noisy after making the connection, a ground loop exists. If no symptoms develop a ground loop probably does not exist.
The presence of ground loops is not merely something that distorts the readings but rather something that also polarizes and damages the sensor. The polarization of the sensor can lead to erroneous readings even once removed from the ground loop condition. The polarization may dissipate in time to return to a more normal response although recalibration may be required because on account of it. In time the presence of a ground loop where the sensor is installed will completely destroy the responsiveness of the sensor and lead to premature failure. This situation requires immediate corrective action.
What is next?
It is much easier to avoid ground loops during installation and project planning than to diagnose and resolve them in the field after installation.
Often not the same ground and often separated by distance
Not always just in the 4-20 mA loop
Consider non-isolated RS-485 of signal wires
Consider non-isolated power/output input power grounds
The ground potentials are NOT equal
RGND caused by multiple factors such as:
Resistance of ground path
Poor initial power rail installations
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). There are a couple of ways to do this – but 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.
Stopping Ground Loops in the future
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 the 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 a 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).