Is it true that both sides of the secondary must be connected to earth ground when using a control transformer?

In the context of using a control transformer, it is not necessary for both sides of the secondary winding to be connected to earth ground. Grounding requirements for control transformers typically focus on safety and system stability. The primary function of grounding in such systems is to provide a reference point for voltage levels, protect against electrical faults, and ensure that the system remains stable under various conditions.

Control transformers are used mainly to step down voltage for control circuits. It's common practice to ground only one side of the secondary winding, usually the neutral side, which helps establish a ground reference for the control circuit. This single grounding point aids in reducing noise and improving the performance of the control devices connected to the transformer.

Other options address scenarios that either suggest unnecessary grounding of both sides or specific situations that do not generally apply to standard practices for control transformers. Hence, the understanding that it is false to assert that both sides must be grounded aligns with the established practices in electrical safety and control systems.