I can't believe this!

[Walter R.]

Two weeks ago, my new a/c condenser stopped running. When the technician checked the condenser, he found that one of the fuses in the disconnect box had blown. The circuit breakers did not trip.

He said the clips that hold the fuse did not make good contact with the fuse. Consequently, the fuse contacts got hot and the fuse blew. He replaced the fuse and it has been working fine ever since.

How can mere heat in the fuse clips blow a 30 amp fuse? This seems to refute everything I know about electricity (albeit, not much).

[EXT]

If they got hot enough they may have melted the fuselink inside the fuse.

[Ralph Mowery]

A fuse works by melting the element inside it. The element is designed with a certain resistance and when the current through it gets the element hot enough the element melts. If the fuse is not making good contact with the holder (high resistance contact point), you get heat that is transferred to the fuse and the element melts. You can "blow" a fuse in the frying pan and never have electricity touch the fuse.

[Thomas G. Marshall]

Ralph Mowery said something like:

Similar issue I discovered: My circuit breaker was throwing every time I started the A/C. After several trips the repair man concluded that I needed a new motherboard in the system. Expensive.

I asked him to take a look at the circuit breaker itself: the screw connecting the load wire to it was very very loose. Beats me how that happens. But in any case, it had discolored the brass screw and when I tightened it, the breaker no longer threw.

From what you suggest, that is a heat only issue and not a change in the load, like he speculated.

[Walter R.]

Thanks for the clarification. Never too late to learn something new!

[Al Bundy]

"Thomas G. Marshall" wrote in news:fryyg.20$AX.5@trndny07:

Similar thing. In a brand new house, after some months all the lights would dim slightly whenever anything that drew a decent amount of current went on.

Asked electrician who did the work about it. Said to tighten the screws on the bus. Problem solved.

[Tom The Great]

When enough heat is generated fuses melt, breaking the circuit. typically the heat is generated by the amps passing through the filaments, but case, the filiament was heated by the amps through it, and through the clips. So they seem to have blown prematurely.

later,

tom @

[Bill]

A good connection becomes very important when there is a lot of amperage. Such an electrical connection can be "wrench tight" and not be good enough!

Large machinery, main electrical connections, etc. frequently require that the main connections be torqued with a torque wrench. Not tightening the connection enough can result in a warm or hot (to the touch) connection (Don't touch these connections BTW!).

A common connection found in a home which requires using a torque wrench is a main breaker panel (main connections). The label on the panel door will sometimes specify ft. lbs. for tightening these connections.

[HeatMan]

What seems like several decades ago.......

I was 'Refrigeration Manager' for a country club. One re-occurring problem was the heat lamps on the kitchen hot line would pop the circuit breaker (CB) 2-3 times a shift.

I was called to look at the problem as I had never really looked at it. The panel was a Square D 60 amp panel that served the heat lamps, a refrigerator and maybe something else. The 4 heat lamps were fed by 15 amp, 1/2" breakers. Just as a thought, I suggested moving the 15 amp heat lamp breakers to the full size 1 inch breakers just to see what they would do.

Never another trip.

I figure that the half inch breakers were getting weak from the constant resetting but also couldn't dissipate the heat generated by the heat lamps. Also, by moving the wires, the screws got tightened down securely.