Circuit breakers are used for overcurrent / short-circuit protection. At overvoltage conditions, what will happen to the circuit breaker if the line current stays within the rated current? For example, consider the circuit breaker of this link. According to the datasheet, it will trip above 1A (as the curve is type Z). According to the datasheet, its rated voltage is 240 Volts AC. What if I apply 250 volts 0.5 Amp? Will it blow up or something else?
What happens to a circuit breaker at overvoltage condition?
From the data sheet for your device the...
...rated impulse withstand voltage is 6 kV conforming to EN/IEC 60947-2
So, I doubt it'll have any problems with 250 volts.
But, as with any mains AC supply line, it can typically rise and fall by 10% quite often and, 10% of 240 volts is 24 volts hence, you can expect to see periods of time when the supply is as high as 264 volts AC AND, any breaker worth its salt will handle this slight over-voltage without batting an eyelid (if it had eyes).
You might have heard of 264 volts before; it's quite common for SMPs units to work across a so-called universal range of 85 volts to 264 volts. This is how the 264 volts is founded i.e. 10% higher that 240 volts.
Additionally, one answer suggests that a circuit breaker has no knowledge of the circuit voltage and that is true until the breaker has reason to to trip; then the opening contact will be subject to the full AC or DC supply and, if the applied voltage is too high, this will certainly cause problems.
Circuit breakers and fuses are normally inserted in the Hot/Live wire, with no connection to the Neutral/Zero volt wire, so they have no knowledge of the circuit voltage - they only trip on over-current.
After the breaker trips it will have the full circuit voltage across it. The voltage rating of a breaker or fuse is the maximum circuit voltage that it can reliably break.
Most circuit breakers are designed to tolerate higher voltages within certain limits.
The supply tolerance is, or was, +10% -6% for 230V. So a high of 253V is considered within the normal range. That means the circuit breaker should not open at the 250V you stated.
Circuit breakers for households are designed to detect overcurrent and/or current leakage, not overvoltage. UK house installations don’t have a device to check for overvoltage.