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Under what conditions would a faster, higher rupture capacity, higher voltage-rated fuse be less safe than the equivalently current-rated fuse which was slower, lower rupture capacity or lower voltage-rated? My intuition is that a faster fuse is not always good because it may trigger too easily but that higher rupture capacity and voltage rating is always safer.

As an example, I have a multimeter which has "fast" 250V fuses (one glass 200mA, one ceramic 20A). I presume at least the glass one is not HRC. It is rated for measuring up to 15A/600V DC or AC (two separate probe sockets for low/high current, corresponding to the two fuses). One of the fuses has blown and I am considering replacing both with >600V HRC fuses of the same speed and current rating. It strikes me that in the case of user-error on a 600V circuit, you would want the fuses to be able to cope safely.

It also seems that fuses tend to be rated for AC volts (LBC fuses state "breaking capacity at maximum voltage rating and then state maximum voltage in VAC, HBC fuses often have gaps in the charts for DC like this). Littlefuse implies that you should halve the AC rating, but also suggests contacting them to be sure.

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  • \$\begingroup\$ What does the user manual recommend? \$\endgroup\$ – Andy aka May 10 '15 at 9:11
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    \$\begingroup\$ I want to maximise my understanding and safety rather than just follow instructions - my question is not specific to my current use-case. The manual recommends "of the same type" but the fuse is not fully-specified. \$\endgroup\$ – Sam Brightman May 10 '15 at 9:41
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The "speed", rupture capability, and voltage spec of a fuse are each separate traits that aren't necessarily related. They will be specified according to the nature of the application.

Many cheap multimeters (i.e. <50$us) will only have glass fuses, because they're cheaper. But 'high rupture' capability (the ability to break a circuit up to a rated voltage, when a high fault current is flowing) is important in a multimeter (and in a circuit breaker), especially in a DC scenario, because DC >40V can easily form an arc and continue the current flow, even though the fuse has blown.

For the "20A" fuse in your meter, a HRC type would be wise, and rated for the max V rating of your multimeter, and same speed as the original.

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  • \$\begingroup\$ And the 'speed' of a fuse is about how quickly it responds to, for example, high inrush currents of some circuits - power supplies, motors, etc. \$\endgroup\$ – Techydude May 10 '15 at 9:44
  • \$\begingroup\$ And also about how long it takes to break on an overcurrent of course - it tends to be a curve. In fact, most fuses will take a good chunk of current (50-100%) over their rated one for minutes before breaking according to data sheets I'm seeing. Only at 2-400% over current does the open time drop into seconds-milliseconds. \$\endgroup\$ – Sam Brightman May 10 '15 at 10:16
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    \$\begingroup\$ I corrected the question - the 20A fuse is ceramic and standardised as BS1362 so should already be HRC (although BS1362 supposedly stops at 13A...). Not 600V though. \$\endgroup\$ – Sam Brightman May 10 '15 at 10:36

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