Schneider Electric ST-540 Capacitor Trip Unit

The Square D/Schneider Electric ST-540 Sure-Trip Capacitor Trip Unit is a dedicated capacitor‑trip module designed to retrofit compatible Square D circuit breakers, enabling automatic disconnection of capacitor banks to protect the power system. It operates from 120 V AC input and delivers a 45 joule energy event to trip the breaker when a fault is detected in the capacitor circuit. DC shunt‑trip voltages supported by the ST‑540 family include 150/177 VDC, 68–95 VDC, and 82 VDC, with some configurations shipped for 480 VAC service. The ST‑540 is cataloged as part number 9810 ST‑540 and is widely referenced in Square D literature as the Sure‑Trip Capacitor Trip Unit for Square D equipment.