The 209-3-1-66-4-9-50 is a three-pole hydraulic magnetic circuit protector from the AIRPAX 209 series, designed for branch circuit protection in applications such as EDP, air conditioners, panel boards, and lighting controls. This unit features a 60Hz medium delay (65) characteristic with common-trip construction that opens all poles in the event of an overload in any pole, and incorporates the magnetic/ampere turn principle to minimize nuisance tripping due to temperature variations. The breaker is equipped with 1/4-20 stud terminals for back-connected panel mounting and is rated for up to 100 amperes at 240Vac or 125Vdc. It withstands high inrush currents from ferroresonant transformers, lamps, and capacitive filters without tripping or affecting normal delay curves, and includes standard inertial delay for 50/60Hz operation. The unit's compact design and reliable protection make it ideal for industrial and commercial electrical systems requiring accurate overload and short-circuit protection.