Calculate: An electric iron rated 1100 W, 220 V is operated for 5 hours.
the minimum rating of the fuse required.
Topic: Fuse rating
Answer
Given: electric iron rated 1 100 W, 220 V
Working current: I = P ÷ V = 1 100 ÷ 220 = 5 A
A fuse must carry the normal working current without melting, so its rating has to be just ABOVE 5 A. The next standard fuse rating above 5 A is 6 A.
Minimum rating of the fuse required = 6 A
Two steps, and the second is the one that gets missed. Work out the current the appliance actually draws, then choose a fuse rated a little higher.
A 5 A fuse would sit exactly at the working current and would blow during normal use, since the surge when the iron is switched on briefly exceeds the steady value. Answering "5 A" is the common mistake.
Nor should the fuse be much too large — a 15 A fuse would let a dangerous fault current flow before melting. The fuse should be the first thing to fail, which means just above the working current: 6 A here.