The variable frequency inspection plan shall provide testing of all commercial devices on:

• A. Regular frequency
• B. Variable frequency
• C. Statistical sampling procedure
• D. Any of the above as applied by director

Answer: (B)

The main idea here is that some instruments can behave differently across different frequencies, so a plan that tests each device across a range of frequencies is used to verify performance throughout the operating spectrum. The best choice is that every commercial device is tested at variable frequencies, ensuring coverage over the whole range rather than at a single point. Testing at a fixed frequency would miss frequency-dependent variations, while testing a statistical subset or leaving it to a director’s discretion would not guarantee that every device has been checked across the necessary range.