A ground loop is an undesirable current path in an electrical circuit. Ground loops occur whenever the ground conductor of an electrical system is connected to the ground plane at multiple points. Not only can ground loops induce noise in instrument signal cables, but in severe cases it can even overheat the instrument signal cable and thus present a fire hazard!
The phenomenon of ground loops is illustrated in the schematic diagram below:
There are several causes of ground loops in any instrumentation installation.
- Differences in potential between the points of the ground plane to which the ground terminals have been connected
- Inductive coupling
- Capacitive coupling
- Use of internally grounded instruments inside an already grounded loop
- Cable shields grounded at both ends
- Grounded thermocouples with non-isolated transducers
- Four wire transmitters used as input to a receiver instrument grounded to a different ground connection
There are several methods of curbing ground loops that introduces undesirable noise voltage into instrument signal cables. However, two of the most effective methods of reducing ground loops are:
- Single Point grounding
- Use of Differential Inputs
Single point grounding involves grounding the instrumentation installation at a single point. This approach significantly reduces the noise voltage generated due to ground loops from multiple grounding points. Differential inputs are used to cancel out the noise voltage that may appear in the instrumentation circuit. One very effective way to completely isolate an instrumentation system from ground loops is to use battery powered instruments. However because of the limited life of a battery, they are seldom used.
Impedance Coupling (or Conductance Coupling)
Where two or more electrical circuits share common conductors, there can be some coupling between the different circuits. When the signal current from one circuit proceeds back along the common conductor, it produces an error voltage along the return bus, which affects the other signals. The error voltage is due to the impedance in the return wire. One way to reduce the effects of impedance coupling is to minimize the impedance of the return wire. The second solution is to avoid any contact between the circuits and to use separate returns for each individual circuit.