The M710 has three operating modes. The first two modes harness the electromagnetic field associated with an earth fault passing through the overhead line on its way to the fault site.  The first mode measures the absolute level of earth fault current while the second mode looks for changes to that current   as a result of changes to the system conditions (change in capacitance, inductance or faulted phase earthing). Both these modes require the earth fault to be (re)energised. The third mode works on tripped lines by injecting a pulsed signal which the M710 then readily detects. In this mode, the first measurement is typically taken at the head of the faulted feeder to provide a definitive datum against which all subsequent measurements are compared.