In the example above, the power line voltage, “Vs”, causes current, “Iz”, to flow through the load, “Z”. At any non-zero instance, Current “Iz” flows into “Z” through one power line wire and returns through the other power line wire. E.M.I. voltage, “Vnc1”, causes current “Inc1”, to flow through the load “Z”. Similarly, E.M.I. voltage, Vnc2 causes current “Inc2” to flow through the load “Z”. Because the E.M.I is generating both “Vnc1” and “Vnc2” – the two voltages tend to be in phase. There is very little current flow between them. Current “Inc1” does not flow through both power line wires. It flows through one power line wire and through the ground path. Similarly, current “Inc2” does not flow through both power line wires. It flows through one power line wire and through the ground path. In this example only “Vnc1” produces electrical noise across load “Z” because the “Vnc2” end of “Z” is grounded. In practice, the effective ground point could occur somewhere between the two ends of load “Z”.
Figure 3 below illustrates the same application with a Common Mode Choke.
The common mode choke has two windings. Each winding of the common mode choke is inserted between the end of a power line wire and the load.