Pico fuses

[scottw]

Hey,

I was looking for an alternative to pigtail (wire lead end) fuses and stumbled upon the Pico type fuses. The application is power entry, 120vac/60Hz, to my projects (moderate powered amps/chipamps) and modded CE stuff (cd/dvd players).

The spec I'm not that familiar with is the "interrupting rating" -I know what it means, just don't know what's enough for my application.

Littelfuse 263 series Pico has a rating of 50A at 250vac (and no 125vac rating):

http://www.littelfuse.com/data/en/Data_Sheets/263.pdf

And the common 3AG type (312 series) has a rating of 35A at 250vac and 10,000A at 125vac:

http://www.littelfuse.com/data/en/Da...ts/312_318.pdf

So, does anyone know if the Pico type fuses are appropriate for use on mains power entry into a component?

Thanks,

Scott

[poobah]

Scott,

You have to look at the highest conceivable current that could flow through a short. Ask yourself; are there fundamental impedances in circuit that could limit the current to some value
under the interupting rating of the fuse.

In the case of an input fuse; is it possible for the secondary to short and draw 35 amps? Probably/maybe... you have to look at the saturation behavior of the trans and the resistance in primary and secondary.

Now, is it possible to short the primary, or its wiring, and exceed 35 amps? YES.

The Pico's are very fast-acting and are intended for board mounted applications. These fuses may well blow on power-up do to surges etc... By the time you increase the amperage rating to cure this problem, you have lost much of the protective value of fusing.

The 3AG's, on the other hand, can be purchased in slow-blow varieties. These fuses are much more tolerant of power-up surges and will allow you to size the fuse smaller, closer to the actual current draw of your system.

Your best bet for an input fuse is the larger type.

[AndrewT]

Hi,
the house wiring and the mains incomer both factor into the short circuit current potential when a fuse is close to blowing.
As the voltage goes up the breaking ability of a fuse becomes critical. A mains rated fuse is designed for breaking a mains supply. A board mounting fuse (35A or 50A breaking limit) sounds like it is not designed for mains fusing.

Go look at your distribution board breaking currents. It will probably be specified in tens of kA. Yes 10,000A or 15,000A of breaking limit, because that's what the mains will try to shove into faulty equipment.

[scottw]

Thanks for the replies.

It does seem those Pico's aren't the right choice for power entry. I did email Littelfuse tech support to see if they have the interupting spec at 125vac.

What I liked about the Pico's is they have about half the resistance(cold) as the common glass or ceramic fuses. Also, they solder in directly with fewer joints and connections; even many of the pigtail fuses seem to have a wired cap attached/slapped on over the chrome/nickel endcap.

Another interesting bit I found while pouring over the datasheets is that the "slo-blo" types have higher resistance than fast-acting types, at least at the ratings which I looked (3A).

Scott

[poobah]

Just to clarify,

A fuse with a 250 Volts rating is perfectly acceptable in a 125V apllication... you are not comprimising anything there. All that is required is that the interrupting rating of the fuse is equal to or greater than the circuit voltage.

[RetroAudio]

I have come to realize that power entry fusing is basically worthless. If it worked, then all the power amps with fried output sections with mains fusing intact would not need a trip to repair shops. Maybe this can fuel a debate.

[anatech]

Hi RetroAudio,
The line fuse is there to protect the power cord and socket (if used). I haven't seen a power cord damaged by excessive current yet, so the fuse seems to be doing the job.

The power transformer has a thermal fuse in it to protect the transformer from igniting itself or objects around it. It seems to work well also.

Since all fuses operate thermally, there is a time lag built in. Semiconductors tend to have less of a thermal limitation and ensure current flow by shorting. Heaven forbid the current flow be interrupted before the fuse gets a chance to dutifully blow. I think it's in the union contract.

-Chris

[poobah]

Inlet fuses aren't useless, they don't protect the amp though, they protect the power cord and the primary.

Using a pico to fuse the rails (not the speakers) might not be such a bad idea...

[RetroAudio]

it just so happens i'm using some picos on rails now with some stuff i've been doing and it works pretty well i must say. very, very fast! The narrower range of operation can either be a plus or minus i suppose, so ya need to have your act together a little better I think.

You're right about the mains fusing of course, just is rare and hard to think about saving a cord when so much else is at stake. At any rate, I'd think the house fusing would work in that territory.

[anatech]

Hi RetroAudio,
Actually, fusing is more to avoid legal issues than it is to protect the equipment. Your house fuse / breaker will protect the house wiring up to the outlet by intent. The AC inlet fuse is designed to protect the AC cord and primary wiring from igniting, as well as any other objects that may be in contact with these components. That's all.

Sometimes, and quite by fluke, these fuses may actually protect the components. It is possible that a low impedance load on a large amplifier may burn out the primary fuse, or even the breaker (or fuse) in the panel.

-Chris