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Ed Anderson wrote:
One thing I have never really understood regarding fuses (or CB for that matter) is - why you would use a fuse/CB with the rating set to protect the wire!!! If you have a system operating off a power wire, it is highly likely that the wire will flow much more current than the system requires by quite a bit. So if you select a fuse or CB rating that protects the equipment then automatically it will protect the wire.
[Dons asbestos suit...]
EXCLUDING motor loads or items that may have broken limit switches, if something goes wrong IN the device that would cause it to blow a CB or fuse, I don't see many cases where you're actually protecting the device - it's already busted by the time it triggers an external protection mechanism, no? If there's a short in the wire the device will just see 0V, which doesn't hurt it, and a fuse/CB won't protect against over-voltages and spikes. What else would the device see that the fuse/CB would protect it from?
When you need protection, each method has pros and cons.
Fuses are much more reliable than CBs. If you repeatedly blow a CB, or if you blow it with enough current, it can actually weld its contacts. Every time it pops you shorten its lifetime. They're also much lighter, a tiny fraction of a CB's price, and 10-100x (or more) faster.
However, fuses require very careful planning. "Nuisance trips" are common when they're under-sized, and it's not always easy to avoid this - precise data on expected draw for a given device is often sketchy at best, and even if it's available, issues like peak usage or inflow (required to properly size a fuse/wire) are rarely addressed well. A CB is better in an environment where you just really aren't sure it's the right size, because it lets you keep resetting it when you're pushing against the limit.
I try to avoid extremist views. I don't think just one or the other is a really appropriate strategy. A fuse is ideal for non- and semi-critical loads - you can protect more wires without spending a fortune or having a ridiculous panel, so you aren't tempted to cut corners and put all your devices on a single breaker. You can easily fuse every piece of avionics you have with a separate fuse, and it's unlikely any of these will blow unless you have very poor wiring skills. This eliminates single points of failure, where one critical breaker pops and you lose the whole panel. Fuses for these loads also keeps the panel clean, so I can focus on critical items.
On the other hand, motor or other inductive loads that might have a lot of inrush, might have a failed limit switch, might be a pump that could face line blockage, or might be deliberately designed to trip a breaker as the INDICATION of a limit (such as the IVO electric prop) are much better suited for CBs. You also want a CB in any case where you can't identify the precise normal and peak draw of a device.
In defense of Nuckolls, take a look at his actual wiring diagrams, and you'll see that they are NOT clean of CBs. His crowbar over-voltage setup, ground power link, and a few other areas all use CBs, because these are areas where trips are part of the plan. You can read his words whichever way you care to interpret them, but the diagrams speak for themselves.
Regards,
Chad
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