SSL

Start-Stop Logic (SSL) is a control scheme that automatically disconnects the main power supply to a machine or subsystem when it detects inactivity, then restores power when demand returns. Unlike a simple on-off switch, SSL uses sensors and timers to make this decision without operator intervention. The logic typically monitors parameters like production rate, cycle completion, or command signals; when none arrive within a preset window, a contactor or relay trips the main circuit breaker or disconnects the supply to non-critical loads.

SSL differs from soft-start or reduced-voltage schemes, which ramp power up gradually to avoid inrush current. SSL is a hard cutoff, meaning full supply voltage drops to zero. This is valuable in energy-intensive industries: a hydraulic press, injection molding machine, or large motor-driven pump can draw substantial standby current even when idle. By severing supply entirely for minutes or hours of inactivity, SSL can recover 10 to 30 percent of operating energy in facilities with cyclic or intermittent demand.

Implementation and Variants

A basic SSL circuit uses a timer module (solid-state or electromechanical) wired to sense machine status. Common sensors include cycle-complete switches, current transformers on the main feed, or logic inputs from a programmable controller. When the timer counts down from its setpoint (often adjustable from 5 to 60 minutes) without a reset signal, it de-energizes the contactor coil, dropping the load. A manual override or "keep-alive" button lets operators hold power live during setup or maintenance. More sophisticated versions integrate with energy management systems and log idle periods for auditing.

The main limitation is restart delay: motors and pumps need 2 to 10 seconds to accelerate back to operating speed after SSL cuts power, so SSL suits processes with natural gaps between production runs rather than high-frequency cycling. Thermal shock is a secondary concern; sudden loss of coolant circulation in a spindle or heavy induction motor can cause expansion stresses if the system lacks cooling-down time. Some designs include a brief coast-down period, energizing a blower or maintaining low-speed circulation for 30 seconds before full cutoff.

SSL is most common in metal fabrication shops, plastics processing, and sawmills, where machines run in batches and idle between jobs. It is less useful in continuous-process industries like petrochemicals, where equipment rarely stops. The term originated in industrial electrical control and appears most often in retrofit energy-efficiency projects and in the specification of programmable logic controllers and variable frequency drives that offer integrated idle-shutdown modes.