Under what condition must the secondary side of a transformer be grounded?

The correct answer highlights the specific conditions under which grounding of the secondary side of a transformer is necessary for safety reasons. Grounding is essential to ensure that any fault conditions do not result in hazardous voltages that could pose a risk of electric shock or equipment damage.

When the voltage on the secondary side is less than 50 V, it is generally considered to be at a low enough level to not pose a significant risk of electric shock; however, if the primary voltage exceeds 150 V, the induction and potential rise in secondary voltage levels due to various factors (such as transients or faults) could create unsafe conditions. Therefore, grounding the secondary side in this scenario is crucial to mitigate risks and ensure safety. It serves as a means to ensure that the equipment remains safe to operate under fluctuating conditions or possible faults, even when the secondary voltage is low.

This understanding reflects key safety protocols within electrical engineering and transformer operation, emphasizing that grounding strategies are inherently linked to anticipated voltage levels and associated risks.