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Are Fuses allowed in the control electronics of an airplane?

Are Fuses allowed in the electronic power paths which supply actuators?

Thank you in advance.

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  • $\begingroup$ Are you asking specifically about fusible links or over-current circuit protection including circuit breakers too? $\endgroup$
    – Jim
    Sep 19, 2022 at 13:44
  • $\begingroup$ No, Jim. Until now I didn't use fuses in the projects I followed. I did so only because i was told it wasn't a good practise and it wasn't allowed. Unfortunately I didn't find this rule written anywhere and here it is my doubt. $\endgroup$
    – UMBRO93
    Sep 19, 2022 at 14:49

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Yes you will find fuses sometimes used where you need instantaneous protection of a circuit from overcurrent, where a circuit breaker, which must heat up to work like the fuse does but needs more time to do so, takes too long to function.

Normally you will see them protecting sensitive electronics where even a short overcurrent state can blow up a $50,000 circuit card within a black box, and the fuse may be built into the box itself.

You can use a fuse in place of a circuit breaker anywhere you want, since they are considered more reliable and faster acting than circuit breakers, but you have practical considerations with fuses, like the need for spares and the difficulty in resetting the circuit, and the fact that in some (or most) circuits, the transient overcurrent tolerance of a circuit breaker is desirable.

So you'll use a fuse where a fuse's technical characteristics are really necessary and aren't available by other means, but otherwise a circuit breaker is a better choice.

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  • $\begingroup$ Thank you John K for your answer. I agree on what you said. Do you know if DO-254 talks about that? $\endgroup$
    – UMBRO93
    Sep 19, 2022 at 14:54
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    $\begingroup$ There’s another use case for fuses, that are quite common in GA aircraft. For instance, in many Cessnas, the Hobbs meter and clock are protected by a fuse, not a breaker, because you don’t want people to be able to turn them off. The battery contactor (what most people would just call a relay) for ground power is also fused. I would add that circuits that are prone to arcing, like a ground power connector, may be better suited for fuses. Circuit breakers will be more resistant to transient voltages, but may not handle an arcing situation correctly. $\endgroup$
    – Max R
    Sep 19, 2022 at 20:14

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