Some older RCBOs used to be like this, namely the older Square D Qoe range.
However, if it is only an MCB then I'd be inclined to think the MCB is shot or there is a fault on the circuit. If you switch the main switch off, does the MCB hold up? If so, switch it back off again before closing the main switch and seek further assistance. If the MCB will not hold up then the MCB is probably shot at.
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