That is why you have the Power Failure Detection Module. Without it your machine might 'coast through' a very brief blip, but it might shut down and kill you tool or job.
Your local voltage may be running a bit low making it more likely that the detector circuit will trip. When does it happen? Any time no matter whether the machine is running or not, or just when the machine is starting up or running under a heavy load.
Sometimes when the supply voltage is marginal the machine will trip out due to the starting draw pulling the line voltage down a bit. In this case it is possible to change the Parameter that controls the maximum spindle acceleration during start up.
An open mind is a virtue...so long as all the common sense has not leaked out.