While electrical circuits are diverse in their design and application, they all share a similar need for an electrical supply system. One of the most commonly implemented electrical supply devices is the distribution board or fuse box. Providing power to homes, commercial locations, and even some aircraft, the fuse box has become ubiquitous in modern circuitry. In this blog, we will discuss the design and function of fuse boxes and how they are implemented in aviation.
Fuse boxes are metal receptacles containing up to a dozen fuses. Fuses are safety devices that protect connected circuits from transient spikes in voltage. They are considered sacrificial in that the entire fuse must be replaced if an overcurrent event were to occur. Each fuse is designed with an element that can melt during voltage spikes without setting on fire. Once this piece melts, the circuit is broken to prevent any negative downstream effects. Fuse boxes contain either screw-in or cylindrical fuses depending on the load of the associated devices. Larger loads make use of the cylindrical design, whereas smaller elements can safely be paired with a screw-in fuse.
Since fuses are designed with the capability to fail safely, it is important to understand what circumstances cause failure. The most common event causing a fuse to "blow" is a circuit overload, which is typically caused by too many connected devices being turned on at the same time. While different fuses can handle varying voltage loads, generally anything over six times the normal current will cause the fuse to blow. The other notable cause of fuse failure is a short circuit event, which occurs when a separate electrical pathway forms. Examples of such pathways include water entering the fuse box, damage to the wire insulation, and rogue nails or screws entering the circuit.
Since aircraft have complex electrical systems with devices demanding different operating voltages, fuse boxes are an attractive modality for providing appropriate and safe electricity levels. Aircraft implement fuses as an intermediate between a power source and the target devices. These fuses are typically rated for higher voltage than residential or commercial alternatives. During the late 1900s, fuse boxes fell out of style before being widely used again. Before the smaller, ruggedly-designed fuse boxes we see on aircraft today came out, fuses short-circuited very frequently due to moisture and corrosive damage.
Airline manufacturers choosing to implement fuse boxes must follow particular standards regarding their design and installation. First, the fuse box must be readily accessible so that it may be accessed during the flight if needed. There also must be enough spare fuses on board to cover half the number of fuses in the box. It is crucial to ensure that any spare fuses are compatible with the installed system. Compatibility information is easily found in the manufacturer's instructions.
While circuit breakers have taken over as the preferred current protection strategy in modern aircraft, fuses can still be found in the APU and light transformers. The hydraulic system also implements mechanical fuses, which work similarly to their electrical counterparts, ceasing excess hydraulic flow during a leak. Although fuse boxes are cheaper and easier to install than circuit breakers, they are harder to change during flight, possibly risking long downtime without cabin power.
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