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I live in the UK. The fuses that I have always seen are the glass fuses which look like this:

enter image description here

When they blow they need to be replaced. They are present in all AC plugs. They could also be present in the wall sockets.

I recently came across "polymeric positive temperature coefficient device (PPTC) resettable fuse". The device is also known as a multifuse or polyfuse or polyswitch.

Why isn't the PPTC based resettable fuse used in all applications since it does not need to be replaced? I have never seen one of these yet so it makes me think that perhaps, these PPTC based fuse devices are actually not often used.

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    \$\begingroup\$ "They could also be present in the wall sockets." They aren't. \$\endgroup\$
    – Transistor
    Nov 20, 2023 at 19:59
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    \$\begingroup\$ Does this answer your question? PTCs on AC/mains line \$\endgroup\$ Nov 20, 2023 at 20:02
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    \$\begingroup\$ "I live in the UK. glass fuses ... are present in all AC plugs." Have another look. You won't find 5x20 mm clear glass fuses in UK BS1363 plugs. UK plugs are fitted with ¼"x1" sand-filled ceramic tube fuses to BS1362, examples here. \$\endgroup\$
    – Graham Nye
    Nov 20, 2023 at 23:59
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    \$\begingroup\$ @Transistor '"They could also be present in the wall sockets." They aren't.' Indeed not. But BS1362 fuses can also be found in fused connection units in UK practice, which can be wall-mounted. (I'm not familiar with practice in Ireland.) \$\endgroup\$
    – Graham Nye
    Nov 21, 2023 at 0:09
  • \$\begingroup\$ @GrahamNye Glad you mentioned the "glass fuses" point. The distinction is that the ceramic ones- from reputable suppliers- don't shatter explosively. \$\endgroup\$ Nov 22, 2023 at 10:41

2 Answers 2

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The PTC devices are fine for low voltage (up to about 60V, with some exceptions), low breaking current (up to some tens of amperes) applications.

They are not good for applications with potentially high fault current, high voltage, or applications where 'leakage' in the off-state could cause problems. They also have higher losses than fuses (roughly double, according to Littelfuse).

The leakage current of a tripped PTC device might be >100mA. The equivalent would be less if a higher voltage device was available, but likely not a safe level for human contact.

Fault currents in mains circuits can be perhaps <100A to 10,000A depending on whether it's an industrial circuit or a home circuit. A typical 5x20mm fuse might be able to break 100A at 250VAC and 10kA at 125VAC. They will literally explode if called upon to break an industrial 240VAC/60Hz direct short.

Generally, the designer of a product does not include the fuse with the expectation that it will require frequent replacement. Often it's a safety device that prevents fire or other damage when something else fails permanently. Similar to the one-time thermal fuse in many appliances that will permanently disable the power in case there is overheating that exceeds whatever 'softer' means exist to limit it (eg. a temperature sensor or microcontroller circuit fails and the applicance begins to heat to the point where the enclosure is softening).

As far as fusing wall sockets, all the characteristics of the PTC resettable fuses are show-stoppers or disadvantages for that application except the resettability. For that, we have per-circuit circuit breakers in the 'fuse panel', at least in North America we do. In really old houses you can still find actual screw-in fuses like this:

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    \$\begingroup\$ UK domestic installations will typically have circuit breakers too (and again, in old houses you'll find fuse boxes with actual wire fuses...). \$\endgroup\$
    – psmears
    Nov 21, 2023 at 13:28
  • \$\begingroup\$ @psmears I've had rewirable fuses up to 45(?)A, and also used a fuse box with cartridge fuses up to 30A in the UK \$\endgroup\$
    – Chris H
    Nov 21, 2023 at 13:32
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Polyfuses usually have a higher on resistance than normal fuses, so there is more voltage drop. They are more expensive and the trip point is more sensitive to changes in ambient temperature.

Polyfuses are used in prototyping boards like Arduinos because you can easily plug anything in and draw more current than the onboard regulator can supply, but for appliances and any "finished" products, it's more likely that an overcurrent condition means the device is truly broken.

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