The wiring in each mobile home must undergo a dielectric strength test at __________ volts, with all switches closed.

The correct answer is based on industry standards for testing the dielectric strength of wiring in mobile homes. A dielectric strength test at 900 volts ensures that the insulation and overall integrity of the wiring systems are within safe operational limits. This test is crucial for verifying that the insulation can withstand the voltage it will encounter during normal operation, effectively preventing electrical failures, short circuits, and potential hazards.

Typically, a dielectric strength test is conducted with all switches closed to ensure that the entire circuit and its components are tested as a unified system. This simulates the actual operational conditions of the mobile home and provides a realistic evaluation of the wiring's ability to resist breakdown under expected voltage levels.

The other voltage options, while they may be relevant in different contexts, do not align with the accepted practice specifically required for mobile homes. The consensus in the industry is that a 900-volt test is adequately stringent to ensure safety and compliance with electrical codes.