More on Why Main Fuses Blow Before Branch Fuses

Jan 1, 1999 12:00 PM, Edited by Robert J. Lawrie

Q. Why does a main fuse blow instead of the 15A or 20A branch fuse? After checking every branch circuit in the panel with a multimeter, I find no problem. —R.A.M.

A. Fuses melt in response to heat. The total heat in a fuse element is the sum of the heat generated by the current passing through the fusible link. Heat generated elsewhere also migrates to the link. If generated near the main fuse, the heat will migrate to the main fuse—quicker than to the more remote fuse. In other words, if the fuse clip on the main fuse is loose or corroded, or if a lug is loose on the incoming circuit, a resistance develops that creates heat on one end of the fuse. This causes it to become hotter than the lower-rated branch-circuit fuse. To avoid this problem, you should remove fuses, cleaning and reinstalling periodically. Infrared scanning can identify impending problems. —W.M.L.

A. There may be several reasons why the main fuse blows instead of the branch-circuit fuse. Such sources seem to occur outside the panel. This indicates there is an external surge or overload causing this problem—not a fault on the board or any board circuits. R.A.M. proved this with the multimeter. Thus, the main fuse took the overload from an incoming problem, not from anything on the board. The main fuse did exactly what it was designed to do, if such were the case. This occurrence may fall into the category of “it will remain a mystery, because nobody wants to spend the money to hang a recording instrument on the panel to see what happens.” —B.B.B.

A. To answer this question, we need more information. (i.e. What is the size of the load, size of the main fuse, load on each branch, and which class of fuses are used?) However, I will present three possibilities based on the information provided.

First, perhaps the circuit had a fault upstream from the branch fuses and downstream from the main fuse. In this case, the 15A or 20A branch fuses wouldn’t see the fault—and not react.

Second, fuses shouldn’t operate at more than 80% of current rating (with the exception of Class L, medium-voltage, E-rated, and Class T fuses over 600A). R.A.M. may be running the main higher than 80%, possibly causing it to open before the branch fuses.

Third, to ensure the branch fuse opens first, you want the branch fuses clearing I2T (thermal energy released by the fuse as it clears) to be less than the main fuses melting I2T (thermal energy required to melt the element of the fuse). —P.W.B.

A.My suggestion to R.A.M. is to get copies of the fuse curves for the specific fuses on the installation and overlay the two. Perhaps the main fuses are seeing the fault first. Get a set of branch fuses that clear the fault faster, or a set of main fuses that clear the fault in longer time. If you choose the latter, make sure not to compromise any other protection scheme.

I remember a waste-to-energy facility had a rooftop fan unit tripping out a motor control center (MCC) and bringing down the boiler. We learned that during high humidity, a ground fault was occurring in the fan unit. This, in turn, was tripping out the main feeder breaker for the MCC. Possibly a problem between the main and branch-circuit protective devices, I plotted the curves for the MCC’s main breaker with the ground-fault settings along with the characteristics of the MCC’s feeder breaker, branch-circuit breaker, and fuses in the local disconnect. The ground fault sensors were set to trip the MCC feeder protection before the branch circuits could “see” the fault and before the fuses cleared the fault. After a few time-dial setting adjustments and a new set of fuses, the nuisance tripping problems were resolved.—J.J.P.

A. When a short circuit occurs, it’s not uncommon that 20~ times the expected current will flow in the circuit. The maximum current-handling ability of standard nonrenewable cartridge fuses is some 10,000A—before it’s destroyed. If two fuses are in series, both rated at the same short-circuit ampacity, then the connected load becomes a factor. A branch circuit’s load is small compared to the main’s, causing the main to blow first. If the main were increased to one with a higher short-circuit ampacity, then the branch circuit fuse would blow, while the short circuit flows through the main without interruption. —E.D.