Understanding Circuit Protection (Fuses & CBs)

[AKar]

While we are, as of now, lacking confirmation whether and to what extend the circuit protection devices, the fuses and CBs, are modeled in A2A planes, it might be a good time to write a little introduction on what those things actually are and what they do and don't. While many may have extensive knowledge about electrical installations, many others do find electrical systems somewhat confusing and hard to gasp - this short introduction is targeted mainly to that audience. This is to avoid misunderstandings about fuse/CB function and purpose, as false knowledge applied to real life purposes can be dangerously misleading.

So, to summarize this entire post to a single sentence:

Circuit protection devices protect the electrical circuit from dangerous effects of overload or short circuit.

The very important thing to understand right away is that the fuse or CB is not there to protect electrical equipment directly. They won't trip or blow should, for instance, spikes of over-voltage get induced into the system - those will blow your semiconductors regardless of CB being present!

What the device does instead, is that it protects the electrical wiring and other components should some fault cause excessive current to flow through the system, for example, due to faulty equipment (like shorted capacitor or diode) or shorted wiring. Those currents, when much over the design load of the wiring can melt the insulation off the wires and actually create a fire hazard.

What is that excessive current? It depends on the duration of that current! Another common misunderstanding is that a 10-amp breaker trips if you pass an 11-amp current through it for a few seconds or minutes. It might do, but likely it won't. A typical breaker (or fuse) allows a momentary overload to flow through it. The tripping characteristics of a CB for instance is described by a trip curve, shown in time to trip vs. current. For a typical breaker, there are two musts: the breaker must continuously pass its nominal, rated current, and the breaker must trip within one hour at 145 % loading. Fuses are similar, though they have some important differences - important mainly to the system designers.

In general, the wiring must be able to withstand a current 145 % of the rated one (generally more than that with fuses) for this very reason: the protection devices are not necessarily protecting it from the lesser currents.

Why do breakers trip (or fuses blow)? The first reason is obvious: a fault condition exists in the system! Typically, pilots should not just go and reset the breakers unless they know what they're doing. Often that translates into "they are instructed to do so". There are several reasons for that, one being that the original fault and overload condition could have caused some damage and melted some short into the system that again passes some current through it, but the short circuit current after the reset is not enough to trip the breaker again immediately. (The electrical circuits are actually designed to provide large enough short circuit overload current to reliably trip the protective device - but not too high to exceed the device's interrupt capacity. The damaged circuit might not provide enough current after a few resets however, leading into localized overheating in the short.)

Also, both CBs and fuses are temperature sensitive devices. Many nuisance trips are caused by overheating combined to high (but within limits) current. This happens in airplanes mainly due to cramped conditions in the CB/fuse panels and bad thermal design: often the panels are subject to heating from other sources and when not taken into account it can cause something called resettable faults. A successful resetting may require a cool-down period, sometimes instructed in the manuals. Some have mentioned that these are more common in airplanes with avionics updates and like: the modifier may not have taken into account the environmental conditions at the location of their protective devices. Even original manufacturers do relatively bad job on this, but arguably they err to the safe side because the protective devices in airplanes must be able to operate reliably even in very cold conditions. This is why the devices generally must be under-designed some.

Moreover, one can find tripped CBs on the ground. A common reason for this is that often CBs that can be tripped manually are used to isolate systems during maintenance. Similarly, fuses may be removed for the same reason. Such breakers should be identified by a flag, but in any case, one should confirm that no unfinished maintenance is being done to the system in question before resetting the breaker. The breakers won't trip, nor do the fuses blow, on their own in unpowered airplanes, at least without a serious fault condition.

To see the properties of a CB that is used commonly in 182 for example, see the data sheet here [pdf].

-Esa

[Levkovvvv]

As for the difference between fuses and circuit breakers, as you probably know, but just to clarify, fuses use a metal filament that is enclosed in glass or ceramics casing that melts when the current passing through it exceeds its design specification, and circuit breakers are mechanical resetable devices that interrupt flow of current when their trip conditions are met. Fuses react faster than circuit breakers, so they are sometimes used for protecting more sensitive equipment (and not just the electrical installation), but they are not suitable for devices that have short high current surges during normal operation. Esa explained operation of circuit breakers in detail, I will just add that sometimes, a circuit breaker that has exceeded its maximum interrupt current will trip, and you will be able to reset it, but it will be damaged and unable to trip again under any circumstances. That is one more reason why you shouldn't reset tripped breakers on your own accord.

And I will mention a rather dangerous practice that may or may not be common in the US, but here in Serbia, it is so common we even have a verb for it. Fuse bridging or "licnovanje" is replacing the filament of a blown fuse with another piece of conductive material. It can be accomplished by disassembling the fuse casing and putting a piece of metal (usually a thin wire) where the fuse filament once was, or, an even more dangerous version, putting a piece of wire on the outside of the fuse casing thereby bridging the contacts in the fuse slot. You should never attempt such repairs because filaments are designed to trip at specific currents that you can never match with diy materials, and if you put a wire on the outside of the fuse casing, when it trips, pieces of melted wire can short circuit that fuse.

[DC3]

... and circuit breakers wear over numerous trips to the point they no long trip at the rated current. The circuit breaker will continue to trip at lower currents until it can not be reset. A circuit breaker that has been reset many times should be replaced. Also, I can confirm the U.S. is not immune to, or unfamiliar with, the aforementioned fixes for "faulty" fuses...

[AKar]

The CBs have a specific resettable interrupt current (see the datasheet) for which they are guaranteed to be resettable. When quoted, it is lower than the absolute interrupt current. Short circuit currents may or may not exceed the resettable interrupt current in some conditions, but must never exceed the absolute one. Also, they may have a mechanical endurance quoted in cycles for which they are supposed to operate reliably. In practice it depends on the amount of current that is interrupted due to inevitable arcing damage that accumulates internally.

Comparison of fuses and CBs by their speeds is a bit difficult, because different types of both exist. Some CBs have different thermal and magnetic interrupt characteristics, allowing them to cut gross overload almost instantly while some fuses are actually filled with sand to cool the filament and to slow down the interruption. However, as mentioned, typical thermal CBs indeed are bit delayed when compared to a standard fuse of similar rating. Fuses are generally less sensitive to ambient conditions, which may or may not be preferable, it depends. Generally, the difference that is important to given application is that the CBs trip reliably ("certainly") at lower current (145 % for guaranteed 1-hour trip time is a typical value), while fuses may require something like 160 % or more for guaranteed interruption - that means the system wiring must be designed to carry similarly higher currents when using fuses, making CBs typically preferable for overload protection, especially when the wiring is necessarily minimized. However, maximum interrupt capability of some fuse types are better than those of typical CBs, making them good for short circuit protection in high current applications, such as in main fuses. For airplane 'end users' the differences mean a little, it may be good to know however that overload and short circuit protection are in principle different functions though often done with the same device. Sometimes fuses and CBs are used in series to achieve both functions reliably.

Both fuses and CBs should be considered unrepairable, any opened device is not to be reinstalled. In my opinion any use of such should warrant the installer some re-education by the authority.

-Esa

[Oracle427]

I had a scary reminder about how fuses protect from a current overload but not a voltage overload.

I was waxing my pinball machine playfield and noticed a part that I forgot to remove. I grabbed a screwdriver and started unscrewing it from under the playfield. It was a bit dark under there and being lazy I flipped on the power of the machine. Everything was fine until I touched the screwdriver against a solenoid line and sent 80 volts down a path where normally 12 volts would go.

I instantly heard a little pop as the screwdriver momentarily touched the solenoid and shut down the machine immediately.

I checked all the fuses and checked all for continuity when I couldn't find any burned ones.

I powered the machine back up and ran a test of every circuit discovering that am optical switch circuit stopped functioning. After testing all the switches I traced the problem to the LM339 logic chip. I couldn't find any visible sounds of damage but the chip was definitely toast. I was thankful when it turned out to be only a $5 repair.

[Levkovvvv]

Another reason why a typical cb is slower than a fuse is that once it reaches its trip condition, for it to actually interrupt current, mechanical parts need to move, and they have inertia, so that delays the actual circuit interruption. And we are talking miliseconds here people, just to be clear, that difference in speed is something the end user never actually notices
And Oracle, that should teach you to never work on powered devices, unless you are actually testing some portion of them