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I was thinking; If a relay is used to power a circuit AND itself, if the circuit is shorted - the relay should lose its own power and turn off - almost like a resettable fuse.

To power it up, I would have to bypass the relay temporarily.

Is this a valid use case? I was actually planning to use it as a "suicide switch" for my robot, but it seems this is an additional benefit?

PS: I'll keep my main fuse as well.

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    \$\begingroup\$ Certainly it's valid if you can guarantee the fault current during a short-circuit will not exceed the rating of the contacts (especially as they open). \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jun 9, 2014 at 19:54
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    \$\begingroup\$ You also seem to be assuming that the impedance of the power source is sufficient so that the fault current will be enough to drag the voltage down to something less than the drop-out voltage of the relay. It probably wouldn't operate very reliably, and the "main fuse" will probably blow first anyway. \$\endgroup\$
    – Dave Tweed
    Commented Jun 9, 2014 at 19:58
  • \$\begingroup\$ @SpehroPefhany My main fuse (15 A) will blow before the relay is destroyed (rated at 40 A), but I was thinking that by adding a resistor I could "tune" the relay to shut down long before the main fuse blows. \$\endgroup\$
    – frodeborli
    Commented Jun 10, 2014 at 9:06
  • \$\begingroup\$ @frodeborli This is actually quite ingenious. Though you have to make sure you don't go over the ratings of the relay coil or you will blow that as well. And I'm also skeptical of how fast a relay will switch, and if the gap distance between common and NO is when common is at NC. \$\endgroup\$
    – Bradman175
    Commented Aug 15, 2016 at 0:16

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But the circuit controlled by the relay is isolated from the controlling circuit. A short in the controlled circuit does not necessarily cut power in the circuit which powers the coil which closes the relay contacts.

Even if the relay and the shorted load circuit share the same power supply, there is no guarantee that a short in the load leaves insufficient current for the relay to stay closed.

Of course, circuitry could be added for this. In fact relays are used for protection; for instance in some audio amplifiers, to cut speakers off from the amplifier in case of a fault that places a large DC voltage on the output. Why a relay might be used for this purpose is that it is transparent (does not add distortion to the signal).

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