Under which circumstances should master stream devices be directed into buildings where firefighters are operating inside?

Master stream devices are powerful tools designed for suppressing large fires, allowing fire departments to apply significant amounts of water from a distance. However, directing master streams into buildings where firefighters are inside poses a serious risk. The correct answer emphasizes that master streams should not be used under any circumstances in such situations.

Using a master stream when firefighters are operating inside can lead to unintended injuries or fatalities. The sheer volume and force of water can cause structural instability, endangering the lives of those inside. Additionally, the sudden influx of water can create dangerous steam and rapid temperature shifts, potentially trapping or displacing firefighters.

In contrast, the other options suggest situations in which it might be acceptable to deploy a master stream while firefighters are inside. However, those scenarios fail to adequately address the inherent dangers involved with this action. Ultimately, firefighter safety must always come first, and this principle underscores the decision to refrain from using master streams when personnel are inside a burning structure.