Electronic – Are thermal fuses polarized

The maximum interrupting current and maximum voltage rating are two of the most important characteristics of fuses. (All tables below from Littlefuse, a major supplier of fuses and resettable polyfuses).

A cheap glass fuse might be able to interrupt some tens of amperes at 250V. It's thus not very suitable for a fault on the mains, which is why good (as in safe) multimeters intended for use on mains use massive cartridge fuses with 10,000A interrupting capacity. Even a residential electrical system can supply more than 100A under fault conditions.

Here's a cheap glass fuse:

It can interrupt only 35A at 250VAC, and drops 1.1V at rated current (quite a bit).

A (possibly) better ceramic type (still 5 x 20mm) can interrupt 1500A at 250VAC. Much safer- but look at that voltage drop- 2.8V.

Finally, I didn't see any polyfuses with both 250VAC rating and 250mA hold current, but the below chart shows one with 180mA hold and 250VAC rating- it can only interrupt 10A.

If you exceed the interrupting current or voltage on any of these devices, there is no guarantee the device will actually open- it may arc away causing severe damage or safety issues. I've made glass fuses literally explode sending shards of glass and molten metal from the element everywhere.

Bottom line- unless you really, really, really, know what you are doing and have all the information the original designer had, it's best to not muck with the protection devices.

Also, philosophically, you should not be blowing fuses often enough to care. You should find out why it's happening (if its not obvious) and figure out how not do that anymore. You may be damaging your meter and shifting the calibration, for just one consideration. Some meters have an audible alert that tells you if you've left the probe in a current socket and switched to volts, but really it's best to get into good habits that don't depend on tricks in the meter.

Your calculation gives the power dissipated in the load. A fuse blows by the power dissipated in the fuse - not the load. The fuse resistance will be very low and the power to blow the fuse will be correspondingly low.

• The current rating is that nominal value which will blow the fuse in a certain time.
• The voltage rating is the maximum voltage that can be reliably interrupted without forming and sustaining an arc across the burnt-out fuse wire.

Figure 1. GDC fuse time vs current. Source: Cooper Industries.

Note in Figure 1 how the 1 A fuse can sustain about 1.8 A indefinitely (1). On the other hand for a 1 s trip time (2) the fault current would have to be 3 A (3).

How about the resettable fuses (with button) ? Is this the same ? because they are voltage rated too.

They will have the same limitations. Find a datasheet and study it in the light of my Figure 1. If you take my approach you will learn the new words and practice pronouncing them then start to figure out what they mean from the context in the datasheet and further reading. Read, read, read. It's worked well for me.