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Re: [Phys-L] ceramic fuses

Those ceramic fuses are typically high-rupture-capacity (HRC) fuses, and they're in there for a verrrrry good reason. Transient currents in various situations can be hundreds, thousands or even tens of thousands of amps (several hundreds of thousands in industrial settings). Fuses have a hard time doing their job---stopping that current---at especially high currents, and may arc-flash. The glass fuses very likely have a "breaking capacity" of 10^4 amperes (this is a requirement in North America); anything above that, and they can arc-flash and cause serious burns, or explode the meter and kill people. 

   Ceramic fuses are designed to NOT do that, at least up to currents that students are likely to encounter.

   If your institution has an electrical safety officer, or equivalent, please check with him or her. If students have ANY chance of dragging those multimeter leads past a large cap (say, near a power supply), or anywhere near a power amplifier, I would stick with the ceramic fuses. You're protecting yourself, as well as them. Time constants of a few nanoseconds or less aren't unheard of in these things, which gives /massive/ discharge currents (much higher than that 10^4 A requirement) through the meter leads, through a shunt resistor, and through those fuses.

   I've worked with el-cheapo multimeters before, even on some high-power circuits. After going through one of the National Labs' safety programs, and seeing what the different ratings mean, I'm not doing that again. I'll still use cheap meters on low-power circuits, but I'm not going to let students use them unattended on anything.

From: "" <>
Sent: Monday, August 20, 2012 6:38 AM
Subject: [Phys-L] ceramic fuses

I have new multi-meters with, of course, different fuses than I have in
stock. The students have already blown the fuses provided as replacement.
The meter literature specifies ceramic 1/4 amp fuses. They run $3 each
Glass fuses are cheaper. So the question - Could I get away with using
glass fuses instead of ceramic? To complicate things I am "encouraged" to
order from specific venders so even if I find a vendor with real cheap
ceramics I could not order (ebay is NOT allowed)

Don Mathieson
Tulsa Community College
Forum for Physics Educators