In electrical testing, what does "in-rush current" refer to?

In-rush current refers specifically to the maximum current that an electrical device draws when it is first powered on, or during start-up. This phenomenon occurs because, at start-up, various electrical components in the device—such as motors, transformers, or other inductive loads—may require a significant amount of current to overcome initial resistance and inertia before reaching their normal operating levels. During this brief moment, the current can exceed the normal running current substantially.

Understanding in-rush current is crucial in electrical testing because it can impact the choice of circuit protection devices, such as circuit breakers and fuses, which need to be rated to accommodate this temporary spike without tripping unnecessarily. Recognizing this concept helps ensure the safe and effective design of electrical systems, minimizing the likelihood of equipment damage and ensuring reliable operation.