According to safety guidelines, when can fire personnel enter a structure?

Fire personnel can enter a structure when they are trained, equipped, and authorized because these criteria ensure that they are prepared to handle the risks associated with entering a potentially hazardous environment. Proper training ensures that personnel understand the dangers they may face inside a structure, such as structural instability or toxic smoke, while being equipped refers to having the right protective gear and tools necessary to perform their duties safely. Authorization typically comes from a command structure, indicating that entry is part of a coordinated effort in managing the incident effectively and safely.

The other options do not encompass the comprehensive safety protocols necessary for fire personnel. Simply being directed by a supervisor does not account for whether the individual is equipped or trained, which are critical for safety. Entering a structure based on the presence of smoke alone ignores the strategy and preparation needed to mitigate risks. Lastly, only entering when the fire is not visible could lead to dangerous situations, as hidden fires or hazards may still exist. Overall, the emphasis is on a holistic assessment of readiness and safety compliance.