Is this for an electrical job or a homework exercise? Also, I don't quite understand what you are talking about with regard to combined circuit protection elements. If they are in series then you need to ensure electrical discrimination between the circuits, if they are in parallel groups then they have the same maximum operation protection time (load current and load type dependent of course).
You need to consult your local electrical regulations and standards regarding this question as it should be clearly explained in there, including the use of fuses and MCB's as protection elements.
Disconnection Times
Re: Disconnection Times
