Tech Tip Of The Day: The Mystery Of Fuses
What is meant by "the continuous load on a fuse in an enclosure should not exceed what percent of it's rating"?

December 04, 2013, by PSW Staff

tech tip
Provided by Sweetwater.

Q: I have what may seem like a silly question, but I’ve been racking my brain and I can’t seem to come up with the answer. Well, not really a question, really, but I keep getting confused by this:

“The continuous load on a fuse in an enclosure should not exceed what percent of its rating?”

You see, I’m doing some work, and I noticed that the above 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.Any thoughts you have would be greatly appreciated.

A: Well, using what you’ve given me I’ll do my best. However, your question doesn’t really specify what you mean by “enclosure.” My house is an enclosure. I presume you mean loudspeaker 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 we still can’t really assume this.

So, since it’s unlikely I can directly answer your question, I’ll just explain what a fuse does and hopefully we’ll stumble upon the answer you need while helping everyone else a bit in the process.

All fuses respond to a characteristic known as 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.
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Tech Tip Of The Day: The Mystery Of Fuses