What safety feature is required on portable fire extinguishers?

The requirement for a safety pin on portable fire extinguishers is a critical safety feature designed to prevent accidental discharge. The safety pin acts as a locking mechanism that keeps the extinguisher inactive until the operator is ready to use it. When the user pulls out the safety pin, it allows the handle to be pressed down, releasing the extinguishing agent. This feature is particularly important in preventing unintended use, which could lead to ineffective firefighting efforts or create hazardous situations.

Other options, while they may pertain to equipment or operational considerations, do not fulfill the same essential safety function as the safety pin. Features like a metal case or rechargeable batteries may offer durability or maintenance aspects but do not directly contribute to preventing accidental discharge. An automatic alarm is not a standard requirement for portable extinguishers and is more associated with fire detection systems. Thus, the safety pin remains the key safety feature mandated for portable fire extinguishers.