On which type of system is a surge-protected device not permitted unless specifically listed for them?

A surge-protected device is typically not permitted on impedance-grounded systems unless it is specifically listed for such use due to the unique characteristics of these systems. Impedance-grounded systems have a ground reference through an impedance, which helps limit ground fault currents but can create challenges in terms of surge impedance and ground fault detection. Surge protection devices can introduce additional capacitance or other impedances that may interfere with the system's operation or protective functions, potentially compromising safety and system integrity.

In contrast, solidly grounded and grounded through a transformer systems allow for certain surge protective measures because they maintain a direct ground reference that is conducive to receiving such devices without derailing the intended grounding scheme. Ungrounded systems, while presenting specific risks, have been developed to accommodate certain types of surge protective devices that are designed to work within their unique operational constraints. Thus, the requirement for special listing for surge protection in impedance-grounded systems is rooted in the necessity to ensure compatibility and safety within the specific electrical design of those systems.