Square D ST-540 Sure-Trip Capacitor Trip Unit

The Square D ST-540 Sure-Trip Capacitor Trip Unit is a capacitor-based trip device designed to work with Square D Class 9810 protection assemblies, delivering a fast, energy-storage trip when fault conditions are detected. Operating from a 120 V AC input, it stores energy in a 45 joule capacitor to enable a rapid, reliable trip, supporting the protective function of the associated circuit breaker. Electrical data specify a DC trip threshold of 82 Vdc, with a load output of 150/177 Vdc and a no-load output range of 68–95 Vdc, reflecting the unit's behavior during trip and normal conditions. The ST540 is cataloged as ST540 and is part of Square D's Sure-Trip capacitor trip units lineup, offering a dedicated solution for precise, capacitor-assisted tripping in 120 VAC circuits.