Incoming supply dropping to 66 volts
No matter where you are you have a problem that needs to be addressed by your power company. In both US split-phase or EU 240v at the incoming terminals, reading more than 10% below is an indication of a problem.

Since you can not turn the power off before the main breaker, the utility needs to find the problem. It is probably a loose or burned connection, I have seen fuses on the service do this when a carbon path provides some voltage unloaded,  then a load coming on causes the voltage to dive. Many times it is a loose or burned off wire connection, so contact your utility and tell them you have an outage.