At what percentage of explosive gases should a transformer be taken out of service according to DGA tests?

The correct answer is based on industry standards and practices for transformer maintenance, specifically focusing on dissolved gas analysis (DGA) tests. When the concentration of explosive gases, such as acetylene, ethylene, and hydrogen, in the transformer oil reaches a threshold, it indicates potential problems such as overheating, arcing, or other forms of insulation failure.

Taking a transformer out of service at 5% and above is essential for safety and operational integrity. Levels at or above this percentage signal a significant risk of explosive conditions, necessitating immediate action to prevent catastrophic failures or safety incidents. Additionally, consistent monitoring of gas levels helps in assessing the health of the transformer, allowing for preventive measures to be employed before reaching such a dangerous threshold.

The other options suggest lower percentages which do not align with the established safety standards that dictate 5% as the critical level for intervention, ensuring that the transformer is handled appropriately before a serious issue arises.