What is an example of a grounded AC system that is permitted under the regulations?

A corner-grounded transformer connection is an example of a grounded AC system permitted under the regulations because it meets specific criteria established in the National Electrical Code (NEC). In a corner-grounded system, one phase conductor of a three-phase transformer is intentionally grounded at one corner, which helps to stabilize the system voltage and reduce the likelihood of dangerous overvoltages during ground faults.

This method of grounding reduces the risk of electrical shock and equipment damage by ensuring that fault currents have a clear and effective path to ground. Moreover, it provides a degree of protection for the system by allowing overcurrent protective devices to operate correctly in the event of ground faults.

The other options, while they involve electrical systems, do not fit the specific criteria for grounded AC systems under the regulations in the same way. For example, a residential electrical panel typically uses a different grounding method, while a portable generator may not have a permanent ground but rather relies on a grounding method appropriate to its temporary use. Similarly, a power supply for residential lights usually pertains to a standard installation that does not encompass the corner-grounding concept.