Finding a fuse...

[ximox]

I'm trying to find a fuse for a Triplett 9055, it's a fast blow 660VAC, 500mA, 5x20, part # 3207-136.
Triplett manual lists a part number, so i email them and basically get told to check at Radio Shack & that "they don't stock it"...
I looked around quite a bit but can't seem to find it, looks like it's quite uncommon. I pulled it out from the meter and turns out it's a SIBA 7018040.
Any one here knows how to find this type of stuff? Is it common to have such hard to find fuses in a device?
Thanks,
Antonio

[IanB]

This one might do:

http://m.radioshack.com/radioshack/product/detail.do?itemId=2102756&categoryId=&path=

[ximox]

But it's rated 250V vs 660V isn't that quite a bit of a difference?

[KJ6EAD]

Here's the data sheet. The fuse is a bit unusual for it's size.
http://www.produktinfo.conrad.com/datenblaetter/525000-549999/535530-da-01-de-MULTIMETER_SICHERUNG_F1_5X20MM.pdf

It's an instrument fuse with a high quenching rating.

[IanB]

But it's rated 250V vs 660V isn't that quite a bit of a difference?

It depends what you are going to do with your meter. If you will use it industrially and might connect it to highly energized (i.e. mains power) 400 V circuits, and might accidentally have the meter on the current range when you do this, then the proper fuse might help to prevent the meter exploding in your hands.

On the other hand, if you simply do something dumb like connect the meter across a low voltage supply like a car battery while on the current range, then an ordinary 500 mA fuse will be OK.

The voltage rating on a fuse concerns its ability to interrupt a circuit of a given voltage when the fuse blows. Multimeters have high voltage fuses for safety reasons, but if you don't use your meter in dangerous and life threatening environments then you don't need such safeguards.

[ximox]

Thanks, very clear explanation. I just use it for electronics, always supply power either through a battery or a bench supply and never high voltages.
Thanks again.
Antonio

[Uncle Vernon]

The voltage rating on a fuse concerns its ability to interrupt a circuit of a given voltage when the fuse blows. Multimeters have high voltage fuses for safety reasons, but if you don't use your meter in dangerous and life threatening environments then you don't need such safeguards.

That's not the best bit of advice. An instrument will always be taken at face value if it says CATIII on the outside it's only human nature to assume (yes I know we should never assume) the unit will be fine for that environment. I understand isn't the intended use but the potential in using the instrument could realise at a distant future date.

[ejeffrey]

On the other hand, if you simply do something dumb like connect the meter across a low voltage supply like a car battery while on the current range, then an ordinary 500 mA fuse will be OK.

Actually, car batteries are often harder on fuses than 230 VAC.  Because car batteries are DC and can source hundreds of amps the arc created by a blowing fuse can last quite some time -- as long as it truly is a dead short.  Any series resistance in the circuit will keep the current from rising to such high levels.  Sand filled ceramic fuses help extinguish the arc faster and without shattering the fuse body.  On the other hand, mains voltage has a zero crossing 100 or 120 times per second that helps quench the arc, and has a circuit breaker than will quickly flip if the current draw exceeds 15/16/20 amps.  This obviously only applies to normal outlets.  Measuring inside the breaker box is a whole different ballgame where the current limit may be over a hundred amps in a residential breaker box.

If you looks at fuse ratings, they always have a much lower interrupt voltage for DC than AC.  250 VAC / 24 VDC is common, but you have to keep the peak current less than the interrupt rating as well.

[PetrosA]

On the other hand, mains voltage has a zero crossing 100 or 120 times per second that helps quench the arc, and has a circuit breaker than will quickly flip if the current draw exceeds 15/16/20 amps...

Not true. A breaker has two trip methods - magnetic, which works fast for short circuits and gross overloads and thermal, which takes much longer to trip. Each breaker design will have a trip curve which illustrates how fast it will trip magnetically or thermally. A 100% load on a breaker may never trip it, or take hours to do so. The magnetic trip curve will show what kind of inrush currents a breaker is designed for. A typical breaker in your home may withstand 10 times it's rated trip current for 1/2 a second before it trips. So you can expect a 16A breaker to easily pass 160A  for 0.003-0.75 seconds (class C,D breakers) before the breaker trips.

[Zero999]

I don't thin you should replace it with anything less than original fuse.

With DC the situation is even worse, especially if the circuit is inductive. The battery voltage may only be 12V but if there's some inductance in the circuit, the voltage across the fuse could rise to 100s of volts when it interrupts the current so it's best to use a 660V fuse.