When the sample temperature is warm enough so that the Joule heating takes over the environment's radiation heating as the dominant heating means, thermal runaway follows and flash sintering is triggered. This condition accurately predicts the reported onset temperature T-on of all the constant-field (E) experiments on flash sintering, performed under a constant heating rate. The predicted linear ln(E-2/T-on(4)) versus T-on(-1) relation determines the activation energy and the relative ranking of the electrical conductivity of the tested materials.