[megous]

Ok, so you detect voltage drop of say 0.4V@5A on the phone end, so slightly out of spec. How do you decide whether it's a slightly underspecced cable (quite common) and 2W being radiated as heat from a 1m long cable (which is not really an issue), or great cable but high contact resistance (where 2W are concentrated into a tiny space that can't cool it)?

[londons_explore]

You could design a port with multiple contacts onto the same cable pin. One contact does voltage sensing while the other takes the current.

Then any amount of dirt in the connector can cause whatever heating it likes, but the device can always calculate how much heat is being dissipated in the connector.

[megous]

Nice idea. :) Thanks.

[baybal2]

The cheapest option I see is a fusible link from some very low melting point material.