What should be done to the secondary neutral in a 3 phase 4 wire and a single phase 3 wire system?

In both a three-phase four-wire system and a single-phase three-wire system, grounding the secondary neutral is essential for safety and system stability. Grounding provides a reference point for the electrical system, ensuring that fault currents have a safe path to the ground. This can help to prevent electrical shock hazards and minimize the risk of electrical equipment damage.

When the secondary neutral is grounded, it helps to stabilize the voltage during normal operation and allows for effective operation of protective devices. In the event of a fault, the grounded neutral facilitates the clearing of the fault by ensuring that overcurrent protection devices, such as circuit breakers or fuses, can operate properly.

While other options like disconnecting or insulating the neutral might seem plausible in certain contexts, they fail to provide the necessary safety and operational benefits that come with proper grounding. Grounding is a fundamental practice in electrical systems to ensure that all equipment remains safe and functional.