Trouble Shooting Ground Loops
Ground Loops can be a Mystery, from a “buzz” in your home stereo to malfunctioning electronic equipment. A definite answer cannot always be given to why they occur, but several suggestions can be made to locate and minimize ground loop effects.
A ground loop very basically, is two or more electrically grounded points at different potentials. Because you have different potentials, current will flow causing electrical noise known as ground loops. Specifically in process equipment it can cause readings to offset from the actual value, it could saturate the input causing the readings to peg up scale or down scale, or it may lock up at one value even though the process has changed.
There is a simple but not fool-proof test for ground loops in process equipment.
- First with the system in normal operating position, place the sensor in a plastic or glass beaker filled with a known solution, and note the reading (this isolates the sensor from the process).
- Next place a wire into the beaker with the sensor and take the other end of the wire and lead it into the line or tank where it can contact the process water. This will bring the interference into the beaker if one exists, and the reading on the analyser will change. This test will determine if there is an electrical interference in the process water that is causing the shift in the reading.
- If the tank or the line is metal, ground the earth ground of the analyser to the tank or the line the sensor is mounted in. Sometimes this can eliminate ground loops.
A definite answer on what caused the ground loop cannot always be given but here are a few suggestions. Other units connected to the analysers analog outputs such as recorders or computers may have their own grounding scheme. These may be different from electrical or earth ground. Other problems may be improper wiring such as shields terminated at both ends or shields connected to other circuit-lows (not ground). It may also be as simple as moisture or corrosion in a junction box.
Some of the ground loop solutions are obvious but most require a little more work. The best solution for a ground loop is to ground all shields and electrical grounds at one stable ground point, but this is not always practical. For ground loops caused by analog outputs, an instrument with isolated outputs or an isolation device may be necessary. The bottom line is finding causes and solutions for ground loops is largely trial and error, but this covers the majority of ground loop problems.