The ABB GSH204 AC 4P is a four-pole, Type AC, Residual Current Circuit Breaker (RCCB) from the ABB System PRO M compact® (GSH200 series). This device is designed to provide life-saving protection against electric shock and fire hazards caused by earth leakage currents in three-phase electrical installations that include a neutral conductor. It monitors the vector sum of the currents in all live conductors (L1, L2, L3, N) and instantly trips to disconnect the entire circuit if an imbalance (residual current) is detected, indicating a leakage to earth. The "AC" type signifies sensitivity to sinusoidal alternating residual currents only.