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I bought a nice big assorted set of fuses ranging .1A to 20A. All of them have 250V on them.

The question is can I use them in circuits with 3, 6 9 or 12 V?

If yes, does lower voltage changes the rating amp of the fuse?

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    \$\begingroup\$ yes, you can use them, the amp rating does not change. the 250V rating means that they are safe to use in 250V circuit, but may arc at higher voltages. \$\endgroup\$ – jsotola Jan 6 '18 at 17:03
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    \$\begingroup\$ @jsotola, good work, please convert comment to answer. \$\endgroup\$ – TonyM Jan 6 '18 at 17:21
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The design of a fuse conductor material gives rise to the PTC characteristic which causes a thermal runaway effect above rated current so that resistance rises rapidly with temp until it melts the conductor. The thin shape ensures that when this occurs, it explodes the material with sufficient gap that at least 1 mm air gap exists so that ~1kV spikes can be blocked on a 250Vac line.

It has no effect on low voltage circuits other than being relatively high resistance when conducting.

You also might want to consider getting an assortment of polyfuses.
- Why?
- Fuses do not provide great protection for low voltage circuits, because of the thermal mass of the transistor junction is much smaller than the fuse link so the fuse is too slow to protect semiconductors. It is effective for some low voltage applications such LiPo battery protection.

Thus 100mA fuses of this type rated for 250Vac are relatively useless for low voltage due to the maximum voltage drop possible when operating below the specified fusing limit.

enter image description here

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  • \$\begingroup\$ When I'd read the "a 250Vac line" part, I was hoping for a mention about the use of fuses for DC and how the fuse has to blow well enough prevent an arc forming... maybe with an amusing, if scary, anecdote. \$\endgroup\$ – Andrew Morton Jan 6 '18 at 19:16
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    \$\begingroup\$ I should have at least pointed out that there are no DC specifications offered by Eaton, because of the issue @AndrewMorton mentions with inductive loads. At least with AC the zero crossing current has a chance to extinguish the arc. I do have an amusing story of a gas tube surge suppressor that a Sr design Eng once used without a fuse. The field failure came back after no direct hit but the follow-on current vaporized a huge crater in the PCB . \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 6 '18 at 20:14
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You can use them at lower voltages (they will be safe) but you may see excessive voltage drop for your application.

You did not specify whether your proposed application is AC or DC. In general you can use AC-rated fuses for DC voltages beyond 12V (maybe up to 32V), but read the datasheets. High voltage DC (such as 125VDC) is a horse of a different color- generally not safe at all.

This also assumes that your fault current cannot exceed the interrupting capacity of the fuse. For the small glass fuse it's 35A.

For example, a bog-standard 100mA 5x20mm glass fuse from Bel drops about 1.63V so you'll lose more than half your voltage at full current. At 1.6V across them they may never open at all.

This is not a consequence of the fuses being rated for high voltage- it's because they actually require enough power (mW) to heat the fuse element to melting. There are one-time (even SMT one-time) and self-resetting 'fuses' that may be more appropriate for low voltage but there will always be some voltage drop.

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I have used 250V fuses on 12V circuits without relevant voltage drop (less than 0.5V). But the quality of the fuses may differ. I can confirm that they were working as some of these fuses blew instantly when short circuited. The best way is to measure the voltage before and after the fuse and see if it's acceptable. However, for less than 12V, or if you need a very precise voltage level use fuses specificaly adapted.

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Yes, you can use them,

The amp rating does not change.

A 1V power supply that can deliver 25A will blow a 20A fuse if shorted (a 25A fuse would probably glow (light up)).

The 250V rating means that they are safe to use in 250V circuit, but may arc at higher voltages.

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