The Voltage Sensing Switch (VSS) is designed to prevent a short circuit, below the VSS tool, caused by power going to ground and preventing operation of a Owen Instrumentation Ballistics Delivery System. This would generally happen after a gun has been fired.
The VSS is generally set up to allow any positive voltage (Owen Instrumentation Ballistics Delivery System tools operate on -82vdc) applied to its upper head to pass through while any negative voltage is blocked until it reaches -130vdc. If more than -130vdc is applied it will pass through the tool. If required it can be set up to block positive voltage and by pass negative.