Fuses – the mystery.
“I am very impressed – someone is actually trying to educate the huddled masses about things technical! I am very curious – how did this get started?”
It’s a long story. It was just one of those nutty ideas at the right time and place. Now we have almost 30,000 subscribers!
“O.K. – now on to my REAL question: The continuous load on a fuse in an enclosure should not exceed what percent of it’s rating? The question does not indicate enclosure size, which in my experience has a profound effect, nor does it indicate whether or not the enclosure is vented.”
Nor does your question really specify what you mean by enclosure. My house is an enclosure. I presume you mean speaker enclosure, but even that is rather vague, as you suggest. What size? What types of drivers? Is there a passive crossover in play? You also don’t specify the properties of the “continuous load.” Is it DC, sine wave, uncompressed/compressed music? What exact malady are you trying to prevent? One would assume you are trying to prevent too much current/power from blowing drivers, but you could also be trying to prevent a shorted driver from harming a power amp (old amps didn’t have the protection circuitry of today’s modern designs). A fuse wouldn’t be much good for anything else, but that doesn’t mean we can assume this. So lets just explain what a fuse does and maybe we’ll stumble upon the answer you need while helping everyone.
All fuses respond to a characteristic Time versus Current Curve. Cross the curve (meaning too much current flow for too long a time) and the fuse opens, which breaks the circuit and hopefully protects some device downstream. Generally, in a properly configured system, the fuse is there to prevent some failure from harming other equipment. The engineering question is, how fast do I need the fuse to respond to a fault condition? This is why you see slow blow and fast blow fuses. The question of how fast they blow is just as important as how much over current makes them blow.
If your concern is speaker protection then you want to have fuses that rate a safe amount under the point at which the speaker will fail. But do you protect against brief spikes (like a dropped microphone), or do you protect against relatively long term over power events (vocalist screaming, feedback, etc.)? The type and value of fuse used varies dramatically depending upon what exactly you are trying to accomplish. The bigger question is, why use a fuse in the first place? There are other ways of protecting against this type of thing without inserting a device that fails and needs to be replaced. Circuit breakers and, believe it or not, light bulbs work very well to protect speakers. A limiter on the front end of the power amp is a good solution too, depending upon the circumstances. This is one of those questions with many possible answers that depend upon the exact circumstances and concerns at hand. For those wishing to include protection in homemade speaker cabinets we recommend you consult with a qualified audio or electrical engineer and explain exactly what you are trying to accomplish.