well, it's one way of testing the fuse/MCB.As I said ther is 3 ways that I know how lights are wired you have a loop in loop out so a cable leaves the fuseboard then loops in and out of each light point with the cable from the switch being wired in as well hence why you have 3 cables 3 reds 3 blacks 3 earths so you have the 3 reds one from the fuseboard one from the switch and one going to the next light the 3 blacks is one from the fuseboard one going to the next light and here is the rub one going black to the switch hence why you need to flag or ID this cable because when you take it down you will get mixed up. Also I have lost count on the number of people who switch the power off put all 3 blacks and all 3 reds together and connect the light up if the switch is off and the power is put back on then the light works ok the problem you get is when you flick the switch you get an almighty BANG Here endith the lesson
well, it's one way of testing the fuse/MCB.