A method of determining an effective resistance between a power sourcing equipment and a powered device, the powered device exhibiting an interface and an operational circuitry, the method comprising: prior to connecting power to the operational circuitry of the powered device, impressing two disparate current flow levels (I.sub.1, I.sub.2) between the power sourcing equipment and the powered device; measuring the voltage at the powered device interface (V.sub.PD1, V.sub.PD2) responsive to each of the impressed disparate current levels; measuring the voltage at the power sourcing equipment (V.sub.PSE1, V.sub.PSE2) responsive to each of the impressed disparate current levels; and determining the effective resistance between the power sourcing equipment and the powered device responsive to the V.sub.PD1, V.sub.PD2, V.sub.PSE1, V.sub.PSE2, I.sub.1 and 1.sub.2.