Fusing and a Thing Called Hysteresis
It seems, no matter how many times a myth is dispelled, they keep raising their ugly head. One common myth is that power cable fuses protect the radio from damage. They don't. Myth number two is, that a fuse will open instantly as soon as the current exceeds the fuse's rating. It won't. Myth number three is, it's always okay to use fuses designed for 120 volts AC, in a nominal 12 volt DC system. It isn't.
Fuses are there to protect the cabling. For example, the Icom IC-7000 has a 5 amp (system) fuse mounted inside the radio, and 30 amp fuses in the cabling (plus and minus). If you short out a supply connection (pin 3 of the tuner port for example), a circuit board trace and/or switching transistor will fail long before the 5 amp fuse opens. The 30 amp fuses will never open in this particular case. It can be argued that the power cable fuses do protect the radio if something fails catastrophically, a final perhaps, but chances are some other component in the circuitry will be damaged beyond repair before the power cable fuse(s) opens.
The definition of a fuse is relatively simple. It is a wire, that when subjected to too high of a current, melts. When it does, the circuit opens... hopefully. I say hopefully, because if you've chosen the incorrect size for your application, it may not open. Or, it may open after a long delay. In any case, you want the fuse to do its job, well before your wiring becomes its own fuse.
All fuses exhibit hysteresis. This is the time lag between any given ampere overload and when the fuse opens. For example, a nominal 20 amp fuse will handle a 30 amp load for about 90 seconds. It will hold a 100 amp load for about 1 second. This is one reason slow blow fuses are not recommended for amateur devices.
Unless you're an engineer, you don't hear the word hysteresis very often. Hysteresis is used to describe a phenomenon, in which the value of a physical attribute lags behind changes in the effect causing it. I use it here because fuses have a hysteresis effect. This fact figures heavily on our choice of fuse amperage ratings, the wire size we choose as a result, and ultimately how safe our installation is should a dead short occur.
The chart at left is courtesy of Littelfuse, Inc. and I thank them for allowing me to reproduce it here. The chart covers their 299 series fuses (shown below in colors which match their ratings), more commonly called Maxi fuses. They're popular in amateur mobile installations as they are a modern replacement for the older cylindrical style 5ag fuses. They're also available with LED failure indicators.
Note that a 30 amp Maxi fuse will take about 3 seconds to open when subjected to a 100 amp load! The same fuse will carry 40 amps for about 2 minutes! As the static temperature goes up, the vertical scale compresses slightly, and in very cold temperatures settings it elongates slightly. So, hysteresis is the time lag between applying an overload, and the fuse opening to protect the wire. Here is a pdf with more data on the Maxi fuse.
In the mean time, the wire being protected is getting rather warm. If it gets too warm, hot really, it could cause a fire. For a better understanding, let's look at some specific cases.
Most amateur transceivers' DC power cords are built using 10 AWG, or an equivalent (e.g.: Metric 6). Further, most are about 9 feet long, and most are fused with 30 amp fuses. If you subject them to 22 amps of current (nominal input for key down full power), they'll exhibit about a half volt drop (including the drop caused by the internal resistance of the fuse, and that of it's holder). This means the power cable will be dissipating about 11 watts.
If we subject the cable to a load of 100 amps (not a dead short) where the fuse would nominally require 3 seconds to open, our voltage drop is 2 volts, and our wire has to sustain 200 watts of dissipation for 3 full seconds! Now you know one of the reasons why it is so important to choose the correct wire size. To reiterate, the wire must be capable of handling the nominal ICAS load with a minimal amount of voltage drop, and it must also be capable of handling an impressive overload, albeit briefly.
So, if the radio draws just 20 amps peak, why not use a 20 amp fuse instead? Here's why. Subjecting any given fuse to instantaneous loads close to their current rating will eventually cause them to fail due to element fatigue (sometimes referred to as erosion). Depending on the load characteristics (steady or varying), fuses are sized from 25% to 50% larger than their impressed loads.
In some cases, peak loads will exceed the rating of the fuse, like those encountered when starting an electric motor. Depending on the application, the designer may use a slow-blow fuse with an appropriately longer hysteresis rating. However, in an amateur application, it is only necessary to keep the average current draw below about 60% of the rating for any given fuse to avoid element fatigue.
The correct wire size should be based on the peak current, not the average, if you want to keep I2R losses low. In any case, should a dead short occur, the wire size needs to be large enough to carry the current imposed by the fuse's hysteresis time delay without exceeding the wire's temperature rating. After all, you don't want to turn the wire itself into a fuse!
As stated, part of the sizing calculation is the temperature rating the protected wire is designed for, as well as its ambient operating conditions. In other words, the fuse must open before the wire reaches its maximum rating for any given overload. Remember, underhood wiring should have a temperature rating of at least 90C, and preferably 105C.
For any given ampere rating, fuses designed for high voltage (nominal 250 volts maximum) service typically have lower resistance than those designed for low voltage (nominally 32 volts maximum). Thus, their low voltage hysteresis time is elongated, which means they take longer to open under a given overload. While these facts alone don't preclude their use in low voltage applications, the bottom line is, you should select fuses specifically designed for the voltage range in use.
Lastly, fuses protecting your wiring should be as close to the battery as possible. Remember, that short length of wire from the battery to the fuses is NOT protected. It should be mounted out of harms way (in case of a crash), and protected with an outer sheathe such as plastic split loom.
If you're thinking out loud to yourself about now with the admonition, I use circuit breakers, so I don't have this problem, you're kidding yourself! Fact is, circuit breakers exhibit a much longer hysteresis than any fuse except some specially designed slow-blow fuses. What's more, most circuit breakers will fail closed on dead shorts if the current exceeds 2,000 amps or so. A standard SLI (Starting, Lights, Ignition) car battery in good condition can easily supply this amount of current when subjected to a dead short.
One very popular way of powering multiple ancillary devices is West Mountain Radio's RigRunner series. The unit shown has a main fuse, and five separately fused circuits. This particular one has a maximum rating of 40 amps. They use the very-popular Anderson Power Pole connectors (PP), which makes wiring easy.
They're typically wired directly to the battery, just like a transceiver should be. And like a transceiver, their leads (negative and positive) need to be fused close to the battery connections to protect the wiring should a short occur.
In-line fuse holders should be avoided for two reasons. First, they require butt splices which are very hard to solder. Secondly, the supplied wire size is universally too small. Here's a suggestion. The PP15-45 connectors will hold an ATU fuse for loads up to 40 amps. For larger loads, the PP75 connectors will accommodate a Maxifuse perfectly. What's more, the connectors are a lot cheaper than most decent fuse holders.
Speaking of fuse holders, Digi-Key, Fastenal, and Mouser Electronics carry quite a variety. However, I prefer the Littelfuse MAB1 holders (at left), and the only current supplier Fastenal. They cost about $35 with a clear plastic cover (not shown).
If you're not into PP connectors, you might want to look into the Centech line of products. Under the cover are heavy-duty connectors for the primary wire, and a screw terminal block for the secondary wires. There are more photos on their web site.