What is defined as a ground loop?

A ground loop occurs when there are multiple grounding points in an electrical system, creating a situation where different parts of the system can pick up varying voltage levels. This can lead to unwanted noise or interference, particularly in sensitive applications like neurophysiological monitoring.

In the context of this question, the option that describes two separate devices attached to the same patient with separate grounding points accurately captures the concept of a ground loop. When multiple devices are grounded at different points, they can develop potential differences due to ground impedance. This can introduce noise into the readings, compromising the accuracy and reliability of the monitoring.

The other options do not describe a ground loop effectively. For instance, having two devices share one ground point (the first option) would likely help mitigate the issue rather than create a loop. A single device with multiple grounding points (the third option) can also lead to issues but is not the same as two separate devices with independent grounds. Lastly, connections involving different patients (the fourth option) do not constitute a ground loop, as the definition relies on multiple grounding points associated with the same circuit or reference point.