What Will The Action of MCB Aginst Voltage Variation.

You are right that the long-time tripping elements work on current, or actually the heating effects of current flow. The instantaneous trips (if any) act on the magnetic field effects of current too. So if your voltage increases but your current does not, there would be no effect on the tripping functions of the breaker. But at some level, an increase in voltage will affect the interrupting capacity of the device, because that is primarily based on the energy in the circuit at the time it attempts to interrupt a fault. More voltage = more energy at the same current. MCBs are not designed as particularly robust devices, most have very low interrupting capacity, i.e. 5kA at their maximum rated voltage, i.e. 415V. If you increase to 830V, the interrupt capacity of the device may only be 1kA (no way to know, they don't test that above rated voltage). So in a 5kA fault at 2x rated viltage, the MCB may actually explode violently because the physical construction cannot handle the mechanical stresses imposed by the higher energy involved.