It is an element incorporated between the QST-5 and the measurement cable and is an additional protection for the tester’s inputs, protecting it from incorrect connection. The QST-5 tester can be safely connected to any sensors as long as these sensors are not powered in parallel, for example, from the vehicle’s installation. Connecting the tester’s inputs to a voltage source can damage the tester. Additional protection protects against such a situation by allowing voltage from the vehicle’s installation (up to 24V) to be applied to the input without damaging the device.