A double pole single throw (DPST) switch is electrically similar to a pair of single pole single throw (SPST) switches. The difference is that in a DPST switch, both contacts must be in the same position, either on or off. By contrast, two SPST switches may be operated independently. The most common use for a DPST switch is to control a 240-volt appliance, where both supply lines must be switched, while the neutral wire may be permanently connected.
Disconnect power to the circuit. Identify the four connections on the switch. There should be a diagram embossed on the switch body. Double-check the switch in both on and off positions, using the multimeter.
Loosen all four screws with the screwdriver. Connect supply lines 1 and 2 to the two input connections on the switch. Tighten the screws.
Connect input lines 1 and 2 from the appliance to the two output connections on the switch. Tighten the screws.
Connect the neutral wire from the appliance to the neutral supply wire. Reconnect power to the circuit.