It’s possible bigger fuses get a head start in clearing a fault.Ĥ. Consider what happens when a fault occurs to a heavily loaded, heated 100A fuse and to an unloaded, cold 15A fuse. The heat injected into the fusible element causes it to blow.ģ. A high-resistance connection at or near the main fuse heats its fusible element by thermal conduction to its melting point. This could be the case with dual-element, time delay, 15A fuse and renewable fusible element-type 100A fuse.Ģ. The two fuses’ time current characteristics curves aren’t coordinated. Here are some reasons a main fuse (say 100A) blows instead of a branch fuse (rated at 15A, for instance):ġ. should connect a recording ammeter to each line-side phase conductor feeding the panel, for at least one week, to see this effect. The loading on each branch circuit always remains below its respective fuse “minimum melting time.” You can’t detect it using a multimeter, so R.A.M. This is particularly true if the branch-circuit fuses are of the time-delay type, and the main fuse(s) are not of this type. Under this condition, a “small” short-time simultaneous increase on more than one of the branch circuits will push the total panel loading past the “minimum melting time” limit of the main fuse, causing it to blow. Can anyone tell me why? -R.A.M.Ī.Each panel is probably loaded to within 80% of its main fuse size.

I check every branch circuit in the panel with a multimeter and find no problem. I’ve often been perplexed on trouble calls when I find a main fuse blown instead of a 15A or 20A fuse, and I find nothing wrong.

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