Cause of blown fuse?

I have a conventional split system central air conditioner. The condensing unit is protected by a fused disconnect, which contains the properly sized fuses for the unit. The other night I noticed that the unit was not running even though the thermostat was calling for cooling. I pulled the fuse block from the disconnect. One of the two fuses had blown. In 13 years this has never happened. I replaced the fuse and the unit has been working since then. Any ideas on what could have caused this? It was not an instance of the unit being turned on again too soon after being turned off (before pressures have equalized) because a few years ago I installed a delay on break timer that keeps the condensing unit from being energized until 5 minutes after it has been deenergized. I pulled apart the blown fuse and it looked pretty charred inside (as if it had melted from a direct short as opposed to an overload). I looked and didn't see anything in the condensing unit that could cause an intermittent short. Any ideas?

 

Probably age and/or a spike.

Believe it or not fuses and breakers get old and weak just like us. Since they work on the heat caused by the current passing through, stuff wears out.

And since you guys, like us in Central VA had storms roll through this weekend if the power supply fluctuates to much it can cause more current to flow through a fuse or a breaker.

Big storm rolls through here and we'll have a few calls where a fuse is blown but nothing else wrong, that is until the next big storm rolls through.

But, to be on the safe side. Kill power to the unit (and disconnect) and check tightness of all the screw terminals. Connections can get loose over time and actuall increace amp draw. Be careful it's inch-pounds not foot ponds on terminals.

Might get a 1/2 turn or so, tighten everything up.

 

Any thunderstorms in the area shortly prior to the fuse blowing?

 

There had been a thunderstorm earlier in the day, but I could swear that the system had been running after that. I could be wrong, though. On the other point, all the wire connections are tight. Checking those is part of my annual maintenance routine.

 

Ok then. Keep an eye on it, but don't loose any sleep over it.