I have a group of slave clocks which receive a brief pulse once a minute from a Raspberry Pi.

In case of the unlikely event of the pulse getting 'stuck' on and damaging the clocks, I mounted a Schurter 50mA slow-acting trage fuse in series with the clocks.

111mA of current flows through the clocks when switched on, and the fuse should last for about 2min. at this current according to the datasheet. However the fuse randomly blows after a few days of the clocks operating and needs replacing.

Does anyone have any ideas why? The pulses the clocks receive are about 1 second, much less than 2 min., and the Pi's logs show the pulses haven't been getting stuck on.


EDIT: Here's a link to the datasheet: http://docs-europe.electrocomponents.com/webdocs/141c/0900766b8141c57c.pdf

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    \$\begingroup\$ Where's the full part number and link to the fuse datasheet, man? Add it into your question. Maybe it's a tragic fuse. \$\endgroup\$ – Transistor Oct 7 '17 at 11:00
  • \$\begingroup\$ Are you sure it's not doing it's job? Is the output perhaps getting stuck on in a way that wouldn't be detected by the log? \$\endgroup\$ – pericynthion Oct 7 '17 at 13:38
  • \$\begingroup\$ @Transistor I've added a link to the datasheet! :) \$\endgroup\$ – user2878409 Oct 7 '17 at 14:49
  • \$\begingroup\$ @pericynthion I'm fairly certain it would get picked up by the log. In any case, I have a separate script which ensures the outputs are switched 'off' at points when the clocks aren't running, just to ensure they definitely remain off when they should be off. \$\endgroup\$ – user2878409 Oct 7 '17 at 14:50

I strongly doubt that the datasheet is guaranteeing that the fuse won't blow for at least two minutes! Usually such curves are showing the worst-case blow times -- in other words, the fuse will blow within two minutes at most.

If your typical current is 111 mA, then your fuse should be rated for at least 200 mA. Or find a different way to limit the current.

Is this the fuse in question? Note that the datasheet only guarantees 200 ms minimum holding time between 1.5× and 2.75× overload.

  • \$\begingroup\$ The reason I chose this fuse is because I want the clocks to run very briefly, but I want the current to be cut altogether if the clocks stay on. I understood that this sort of fuse would be suitable for that? \$\endgroup\$ – user2878409 Oct 7 '17 at 12:09
  • \$\begingroup\$ Just seen your edit with that link. My fuse is this one: uk.rs-online.com/web/p/non-resettable-wire-ended-fuses/2845192 \$\endgroup\$ – user2878409 Oct 7 '17 at 14:51
  • \$\begingroup\$ Same idea. That one only guarantees 400 ms holding time between 1.5× and 2.75× overload. \$\endgroup\$ – Dave Tweed Oct 7 '17 at 15:18
  • \$\begingroup\$ But seeing as my load is 111mA, which is 2.22x overload, wouldn't the holding time be about 1,000ms (according to the graph)? The pulse for the clocks isn't longer than 300ms. \$\endgroup\$ – user2878409 Oct 7 '17 at 15:30
  • \$\begingroup\$ You said 1 second previously -- you need to get your story straight. And no, you can't interpolate the graph and consider that a guarantee -- it only represents "typical" behavior. Whenever you operate a fuse above its rating (plus a design margin chosen by the manufacturer), it will eventually fail, as you are discovering. If you want to impose a time limit, you need to find a different way to do that. \$\endgroup\$ – Dave Tweed Oct 7 '17 at 15:34

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