When the external injection of power is sufficiently high a tokamak ohmically relaxed state with maximum entropy can no longer be sustained and the plasma becomes resistively unstable. The instability is the result of a peculiar interaction between the edge resistivity, the current and the thermal transport at the edge of the current channel. The thresholds of the instability depend on the existence of a current pedestal, on the detachment of the current channel from the material limiters combined with a sufficiently high ratio between the auxiliary power and the toroidal magnetic field and a sufficiently low toroidal current. In the paper the thresholds are expressed in a form which allows the comparison with the experiments. The theoretical predictions are generally consistent with the observations on the H-transitions in tokamaks, in particular with the scalings observed in DIII-D and JIPP T-IIU.