How many additional test articles are required to be tested alongside the unit that has completed A-28 qualification testing?

The correct number of additional test articles required to be tested alongside the unit that has completed A-28 qualification testing is four. According to the guidelines set forth in ABYC standards, after a unit has successfully completed its A-28 qualification testing, it is necessary to test additional articles to ensure that the results are representative of typical performance and account for any variability in manufacturing or operational conditions.

Testing four additional articles allows for a more comprehensive assessment of the unit's reliability and performance characteristics. This number is typically determined based on statistical confidence levels, ensuring that the sample size is adequate to draw valid conclusions about the unit's overall compliance with safety and performance standards.