Precision Digital: Ground Loops & Non-Isolated Commons

31 August 2017

Any installer of industrial process control equipment will tell you that ground loops are one of the most frustrating signal connection errors to diagnose and repair. The steps required in order to troubleshoot them are often equated to something as mysterious as magical incantations. Issues caused by shared, non-isolated commons are viewed with a similar outlook. Difficulties with shared signal returns are often even confused with ground loops. Ground loops and shared commons can both cause unpredictable signals and render your current loop unusable.

The best and most practical way to repair these signal issues is to prevent them from occurring in the first place by planning proper device wiring and following specific best practices. However, if you suspect that you are having signal issues related to ground loops or shared commons in an existing network, no need to pull out the spell Ground Loops & Non-Isolated Commons book and magic wand, there are some predictable symptoms which you can look for in order to diagnose the problem.

First of all, you need to know the definition of ground loops and shared commons. A ground loop is the flow of current from one signal ground to another because of a voltage differential between the two grounds. This can happen if two devices in the network are grounded at separate locations and one of the locations causes the signal ground there to experience a higher voltage potential. As any electrical engineer will tell you, any voltage differential will result in a current flow. It is this current flow that causes the symptoms of a ground loop.

A shared, non-isolated common can become problematic when it is improperly wired. Multi-input and multioutput devices, especially those with more than one loop going through them, are notorious for difficulties related to shared commons. These are commonly referred to as “ground loops” because of the similarity of their symptoms, but are not true ground loops because they are not caused by grounding issues. These kinds of problems arise when nodes are created, intentionally or not, prior to reaching all of the applicable devices on the circuit requiring a clean, predictable signal. This will result in mixed current flow and signal averaging, which will produce an unusable process signal.

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