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Some very good points and information, Chad.
My personal experience (a number of years flying military aircraft as an electronic warfare officer) has clearly shown you can have circuit breakers trip due to a temporary condition, reset the circuit breaker and have a functioning system - not always, not necessarily the major of times, but it does happen. One thing is absolutely for certain - when a fuse blows there is no resetting.
I also use fuses - so I am not against them by any means. I think there is a place for fuses and a place for circuit breakers. I look at military aircraft, airliners, business jets, etc, and I have yet to see a fuse - only circuit breakers - covering the cockpit walls (I'm certain there are fuses there someplace, but they are certainly not very visible {:>)). I can not help but wonder why these type of high value aircraft use circuit breakers rather than fuses. Circuit breakers are in essence no more than a mechanical switch (we all use those I believe) activated by heat rather than a finger. But, this debate could (has and will) continue into the future, for sure.
I fully agree with you that each approach has pros and cons. I personally will believe in the fuse approach (for crucial system) when -I see all the circuit breakers gone from military aircraft and airliners - and replaced by fuses {:>). Who knows - that may already be happening as I have not been in an airliner since my homebuilt was completed in 1998.
I do use fuses in non-critical areas, but put my trust in good quality circuit breakers in system critical areas. Certainly not trying to convince anybody to follow my lead - we're all capable of making those important decisions in a manner which address our own personal Risk Profile.
.Thanks for your input on the topic, Chad. Different viewpoints are what keeps us all mental alert and reassessing our positions on approaches.
Best Regards
Ed
----- Original Message ----- From: "Chad Robinson" <crj@lucubration.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Friday, October 20, 2006 9:32 PM
Subject: [FlyRotary] Re: Fuse Ratings for Wiring??
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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