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Why Does the Main Fuse Blow and Not a Lower-Amp Fuse?

Dec 1, 1998 12:00 PM, Edited by Robert J. Lawrie

Q. 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. I check every branch circuit in the panel with a multimeter and find no problem. Can anyone tell me why? —R.A.M.

A.Each panel is probably loaded to within 80% of its main fuse size. 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. 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. 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. should connect a recording ammeter to each line-side phase conductor feeding the panel, for at least one week, to see this effect. —F.M.P.

A. Here are some reasons a main fuse (say 100A) blows instead of a branch fuse (rated at 15A, for instance):

1. The two fuses’ time current characteristics curves aren’t coordinated. This could be the case with dual-element, time delay, 15A fuse and renewable fusible element-type 100A fuse.

2. A high-resistance connection at or near the main fuse heats its fusible element by thermal conduction to its melting point. The heat injected into the fusible element causes it to blow.

3. Consider what happens when a fault occurs to a heavily loaded, heated 100A fuse and to an unloaded, cold 15A fuse. It’s possible bigger fuses get a head start in clearing a fault.

4. A history of current surges damages the fusible element of the main fuse, elevating its trip temperature many times too close to the point of melting, resulting in a “softening” of its element, allowing it to blow.

5. All or some of the above.

Coordination of protective devices (fuses and circuit breakers) in power systems is an important engineering procedure that requires a “fault and coordination” study. Most residential systems, with fuse ratings such as those used in the above example, are inherently coordinated by virtue of the fuse continuous current ratings ratios. These can be as low as 2 to 1 (100A main versus 50A branch fuse = 2 to 1). But watch for old installations where connections have deteriorated and/or fuses have been in service for a long time.

A multimeter isn’t likely to uncover such problems. Troubleshoot connections while under full load with an infrared imaging instrument to find “hot spots.” You can see damaged fuses or loose, heated connections in a thermogram and make repairs. —J.F.F.

A. With more than 40 years of experience in electrical maintenance and construction, I’ve learned loose and overheated main bus connections often cause such a main fuse problem. Frequently, this problem occurs where fuse blocks plug into the bus. Edison and Type S fuse adapters can cause additional problems if the hot bus screw isn’t tight, causing heat to spread or “sink” up the main bus.

I’ve experienced this many times. In some cases, the equipment was badly overheated. Upon tightening, it got better for a short time before overheating and failing again. When this happened, we replaced the service-entrance panel and most of the existing wiring. Also, to find the trouble, we checked and tightened all wiring connections in the facility. —B.B.

A. A main fuse can blow, leaving the branch-circuit fuses intact, with nothing else apparently wrong. This can happen wherever the combined rating of the branch-fuses exceeds that of the main fuse. For example, if the main fuse is rated at 100A, with 10 feeders fused at 20A each, they could all be loaded to 10A to 15A. This is well below the branch circuit rating, however, together they pull enough to cause the main fuse to blow. This commonly occurs when you add a large load to an existing panel not robust enough to handle it.— P.V.

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