When you encounter malfunctions and gross measurement errors, particularly on newly installed or modified systems, ground loops and misconnections usually are implicated. As you study the wiring drawings, keep in mind the specifications of your sensors, control components and final control devices. It is always hard to relate the drawings to the jungle of sensors, wires and components on the plant and inside the control enclosure. You may have to re-identify zones, terminations and components, then mark up the drawings.
Consider the nature of ground loops, their causes and remedies. Figure 1 shows part of a typical control system with the controller handling various incoming and outgoing signals. Each of the four control components will contain its own DC power supply for the internal electronics. For now, assume that no part of the circuit is grounded.
If any one point is grounded -- the tip of the thermocouple, for example -- it will not disturb the controller's indication or other functions. But, if a second point of the circuit is grounded -- any one of the terminals on any of the four control components -- and if there is an internal conductive path from this point to the controller input, you have a ground loop, which will cause a wrong indication on the controller and upset all control functions. I use the term ground, but the guilty connections are just as likely to be to any common reference line such as the negative line of an internal DC supply or the common logic output line of the PLC.
You will not see this problem if all components have inputs and outputs that are electrically isolated from each other, from ground and from common lines. These are sometimes called floating inputs or outputs. Control equipment manufacturers usually specify which terminals are isolated and the maximum voltage they will tolerate relative to ground and other terminals. These details are necessary.
There are controllers in which the thermocouple input is not isolated from the outgoing control and logic signals. Avoid such controllers. However, if you have to use one, make sure that the outgoing signals only go to isolated devices.
Another ExampleIf the PLC in figure 1 has logic outputs that have one side grounded with other outputs or inputs, make sure that the controller's logic inputs are isolated from the thermocouple input and from everything else.
You can measure and control temperature difference using back-to-back connected thermocouples fed into a single controller. You can even ground one hot junction, but if you ground both hot junctions, a major error appears. An isolated instrument input will not solve this problem. It is good practice to isolate both thermocouples from ground and from common metal parts. While this is a cheap and easy method, I'm not fond of this arrangement because thermocouple wire-to-sheath clearance and thermal expansion make it vulnerable to leakage and short circuits. This arrangement was mentioned in a previous series of columns on master/slave control (link at bottom of page).
If you are stuck with components that have nonisolated inputs or outputs that would cause conflict, you can use one of the many varieties of stand-alone isolators or converters. The isolation technique in these components is to pass a representation of the signal through a transformer or optical isolator.
Another advantage of converters is that they can be used to amplify low millivolt signals up to a level where they can be transmitted in copper over long distances with minimal interference. In the case of thermocouples, converters save the higher cost of thermocouple extension cable.
It rarely is necessary to ground working signals, but it is advisable to ground signal cable shielding to prevent capacitive pickup of AC interference.