40 Double-pole, single-throw (DPST)

This switch looks like a standard SPST switch from the outside but inside, instead of interrupting only one current-carrying conductor, it interrupts two. This makes it suitable for controlling and isolating loads that utilize two ungrounded conductors and operate at 240V single-phase.

The schematic symbol for a DPST switch has a dotted line connecting the two movable switch components signifying a “mechanical interlock”, meaning that when one switch moves, the other moves with it. This ensures that operating the external handles will either connect or disconnect both line conductors.

These types of switches are often used to control electric heating loads and have a temperature sensing component to allow them to automatically open or close the contacts with a change in external temperature. When used for this purpose we refer to these switches as thermostats.