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I have a problem in my house. If I test the incoming supply on top of the main circuit breaker I get 220 volts with the breaker set to OFF position, but if I test with the breaker ON, I get 66 volts. What is the problem here?

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    Where are you on this planet?
    – ThreePhaseEel
    Mar 11, 2020 at 11:41

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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.

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