According to the SGM standard, when should sumps not be used as a primary suction point?

The correct choice highlights a critical aspect of the SGM standard regarding the use of sumps as primary suction points. Sumps are often used in mitigation systems primarily for water drainage and can accumulate water, thus creating a potential point of entry for radon gas. The standard indicates that sumps should not be relied upon as the primary suction point when other options are inadequate, as this suggests that there are better alternatives available that could potentially provide more effective radon reduction.

This is particularly important because using a sump as a primary suction point can lead to inefficiencies in radon mitigation if it's not the best choice. It is essential to utilize the most effective methods for reducing radon levels, which could involve evaluating other accessible and more effective suction points. This part of the standard emphasizes making informed decisions based on the unique characteristics of the structure and the nature of the radon problem.

The other options do not align with this reasoning. Accessibility does not inherently make a sump an unsuitable suction point; in fact, it can be a consideration if looking for ease of maintenance. Multiple drainage connections do not automatically disqualify a sump, and locations in dry areas might have different implications depending on local geology but do not directly relate to the efficacy of radon mitigation.