You've hit on the key issue and the nuance of the slash-rated breaker application. Here's a breakdown of why slash-rated breakers are prohibited on non-grounded or impedance-grounded systems: Listing and Testing

The primary reason is, as you mentioned, listing and testing. Slash-rated breakers are specifically designed, tested, and listed according to UL standards for use in grounded neutral electrical systems. This listing includes rigorous testing that confirms their ability to reliably interrupt faults to neutral, which are a common type of fault in grounded neutral systems. Breakers get a UL listing for the specific job it is intended. UL listing cost money to do.

Voltage Stress

In a solidly grounded neutral system (e.g., 120/240V single-phase), the voltage from each phase to neutral is lower (120V). When a fault occurs from one phase to neutral, the breaker is designed to interrupt this lower voltage fault current effectively. In a non-grounded or impedance-grounded system, the voltage from phase to ground can be much higher, approaching the phase-to-phase voltage. This higher voltage stress on the breaker during a ground fault can exceed its designed and tested capacity.

Asymmetrical Fault Current

In grounded systems, the fault current typically has a significant DC offset (asymmetrical current). Breakers are designed to handle this asymmetry and interrupt the current safely. The behavior of fault currents in non-grounded or impedance-grounded systems can be different, potentially exceeding the breaker's ability to handle the asymmetrical components effectively.

Coordination and Selective Tripping

Slash-rated breakers are often used in coordination schemes with other breakers in the panelboard. These coordination schemes are designed based on the expected fault current levels and characteristics in a grounded neutral system. Using them in a non-grounded system can disrupt this coordination, leading to unwanted tripping or failure to trip when needed.

Fault Current Magnitude

You're correct that a breaker capable of handling a phase-to-phase fault can handle a similar fault current magnitude between a phase and a ground point (assuming the impedance is the same). However, the characteristics of the fault and the voltage stress during the fault are different. The breaker's ability to interrupt the fault reliably under those different conditions is what hasn't been verified for slash-rated breakers in non-grounded or impedance-grounded systems.

Insulation Issues

Slash-rated breakers might have internal insulation clearances designed for lower voltage-to-ground conditions. A phase-to-ground fault on a higher-voltage ungrounded system could overstress the insulation within the breaker, leading to insulation breakdown and a potential breaker failure.

Why This Matters (Beyond the Code)

Using equipment outside of its tested and listed parameters is a recipe for potential safety hazards. In the case of slash-rated breakers on ungrounded or impedance-grounded systems, it could lead to:

Breaker Failure

The breaker might fail to interrupt the fault current, leading to equipment damage, fire, or electrocution.

Arc Flash

A sustained arc fault could occur due to the breaker's inability to clear the fault quickly and effectively.

Coordination Issues

The breaker might not trip selectively, causing unnecessary power outages in other parts of the system.

In Summary:

While a slash-rated breaker might theoretically be able to handle the fault current magnitude in a phase-to-ground fault on an ungrounded system, the voltage stress, fault current characteristics, and internal insulation are different. Because slash-rated breakers are not* designed, tested, or listed for those conditions, their use on ungrounded systems is prohibited. The Code's prohibition is in place to ensure safety and the reliable operation of the electrical system. Always use equipment within its listed and labeled parameters. Flag for review