Earth Fault Loop Impedance Test - 3 Lead Low or 2 Lead High - Test Explained In this video Marcus explains what happens when we would set our MFT tester to 3 lead low, 2 lead high and even 2 lead low in order to carryout an earth fault loop impedance on a circuit that has RCD protection. So we can carryout a earth fault loop impedance test it is important to select the correct setting on an MFT tester but do you understand why we often select 3 lead low setting when testing for earth fault loop