An electrically operated pipe organ shall have both the generator and motor frames connected to the equipment grounding conductor unless ___.

• A. effectively insulated from ground
• B. effectively insulated from ground and from each other
• C. effectively insulated from ground and the motor driving it
• D. both the generator and motor have double insulation

Answer: (C)

The safety concept here is about providing a fault-current path to ground to trip protective devices. You generally bond electrical equipment frames to the equipment grounding conductor so a fault on a frame quickly becomes a ground-fault current.

This exception applies when the generator frame and the motor frame are effectively insulated from earth and there is no conductive connection between them through the drive—that is, the motor that drives the generator does not create a grounding path between the two frames. If both frames are truly isolated from ground and from each other via the drive arrangement, there’s no fault current path to earth, so bonding to the equipment grounding conductor isn’t required.

The other options don’t describe this exact isolation scenario. Insulating merely from ground isn’t enough on its own to guarantee no fault path if the drive or other connections could provide a path. Double insulation is a separate design concept and isn’t the specific condition described here.