Do I misunderstand fuses?

Question

A statement that I often read here goes: "Fuses protect the wire, not the load."

However, in basically all cases I use fuses, I think they are there to protect the load - I.e. the device on that side of the fuse that is opposite to the energy source.

• In line with power input for power converters, usually recommended by surge immunity. Protects the converter.

• In line with signal input ports. Either as actual fuses (e.g. in multimeters) or as polyfuses. Protects the decice from excessive overcurrent.

Why would I care about the wires leading to my devices? I often don't even have control over them.

Answer 1

It's important to stress that a fuse is meant to protect from catastrophic outcomes, such as fires, not to protect devices from breaking. In fact, in most cases, when a fuse blows something has already failed, causing a overload such as a short circuit and the fuse prevents this from turning into a blaze.

You are right, you have no control over the wiring leading upto your device. That's why fuses should be rated to protect whatever comes after the fuse. The electrician who installed the wiring in your house knew how much current those wires can safely carry and he installed appropriate fuses in your breaker box to protect them from overheating when something tries to draw more current then that.

As the designer of your device, you (should) know how much current the components of your device, the wires, the connectors, the PCB traces, etc... can carry. So you can work out what fuse to use to protect against your device from causing a fire should anything break.

Answer 2

I know where you get "fuses protect the wires".

That applies to the AC mains utility wiring inside your house. The main threat is either a malfunctioning appliance or receptacles where someone has plugged in more loads than the circuit can bear (real easy to do in 120V-land, where plug-in heat appliances are limited to 1500W=12.5A per UL... yet circuits are 15 and 20 amps per NEC.

If your appliance doesn't have consumer-swappable modules, then you don't have the variable-load problem. So your fuse is only there for conditions which should not ever happen per your design, meaning a hardware failure has occurred, the appliance is possibly a write-off, and now your goal is to keep the appliance from starting a fire.

The UK has a nice answer for that, with fuses in the appliance plug, in which case the fuse protects both the appliance and its cord. (Good thing too: UK circuits are 32A!) North America also does this with a few items, such as Christmas light strings.

Some systems of appliance certification allow you to rely on the branch circuit breaker for your internal fusing, if you can prove that will trip before the appliance does anything destructive to other than itself. That is why North America/NEC requires a match between large appliance rating and branch circuit breaker, (30A appliances on 30A branch circuits only)... yet Australia allows any plug to fit any equal or larger socket.

Answer 3

In a sense, you are right. Fuses are sometimes used for specific reasons by device designers inside the device. But they seldom act by themselves fast enough to protect silicon. You need something else to help buy time. For example a fuse or a polyfuse followed by a shunt Zener can provide pretty good protection against input over-voltage if everything is sized correctly.

Also sometimes fuses are put in place in such a way that they blow after some piece of silicon fails first, with the intention that they make the failure mode relatively safe. Maybe without the fuse, after the silicon failure, the whole device would catch on fire. But with the fuse, the device fails in a safe fashion.

But you are right that the fuses in a good multimeter are intended to protect the meter (and they do). Those are uncommon and very special fuses, though.

Nonetheless, it is an important concept of safe design that every wire has a fuse or breaker in series sized so that the circuit will open before the wire reaches an excessive temperature. This is basic electrical and fire safety and applies to boats, cars, etc.

You may not care about the wire, but you probably don't want a faulty appliance to burn your house down. And the people who write electrical code (and building inspectors) don't normally care if your device stops working.

Answer 4

I think you've found the exceptions to the rule.

If you think about the larger fuses in homes, offices or factories, then the fuses are primarily to prevent fires in the wiring. If an appliance has shorted out, then you're protecting the wires from the appliance. If the wiring has shorted, then that wouldn't damage the appliance, but could cause a fire.

Answer 5

Just to add that fuses also, to some extent, protect the supply before the fuse. Without fuses an overload could damage the supply transformer / battery / generator / supply wiring. If the fuse blows on a faulty device then overloading the supply is prevented.

Answer 6

There are subtle differences between different countries on how domestic fuses are handled and installed. In Sweden the situation is that the "house" fuses are there to protect against fires from the wires. In contrast to eg UK in Sweden the appliance plugs never has fuses.

On the intake to a an apartment or a house you will find fuses controlled by the utility company setting how many amperes you are allowed to draw from the incoming electricity source. The values of the fuses, say 16A, may be connected to the cost of your contract.

Inside the house or apartment there is a distribution central with fuses and from the central position wires goes out in a star to the different connections. (This is different from what you find in eg UK, where the distribution tends to be a ring distribution). Each wire from the central is protected by a fuse, say 10A. The maximum fuse value allowed is regulated depending on the size of the wires going from the central to the outlet. A 1.5 mm2 wire can be protected by a maximum fuse of 10A, a 2.5 mm2 wire with a maximum fuse o 16A and so on. Here, clearly the fuse protects the wire.

Additionally a lot of installations contains ground fault circuit interrupters. These are there to protect humans against electrical shocks when faults when a fault in an appliance may induce dangerous current. Typically they trip on around 10mA ground fault.

Some appliances may contains one or more fuses, some not. The fuses can be of different types but generally are to protect either parts of the appliance from cascading expensive faults or protecting for the appliance creating dangers such as fires. I have seen three different kinds of fuses used in appliances: tripping when the appliances draws too much amperes (protects from cascading errors as well as some fire hazards), tripping when the appliance gets too hot (protects from fire hazards), tripping when input voltage is too high (say from a lightning hitting a power line, protecs the appliance from over voltage). In each case the fuse may one-time or resettable, sometimes resetting itself after a time or when the external conditions are back to normal.

Answer 7

I’d quoted the phrase a few times recently, so it might be me who precipitated this question!

With the responses we’ve had here the phrase is clearly not a universal truth - it forms the basis of electrical power systems. Change of wire size must have the appropriate protection for the wire.

Probably a more accurate phrase would be that the ‘ fuse protects the insulation’. Without functional insulation, it is a fair chance bad things are going to happen - witness kapton failure in airplane wiring.

In some instances the fuse does actually protect the load or the source by limiting the maximum current that can flow.

Nevertheless it is a useful and easy to remember ‘rule’ that applies to many questions we see here. My recent quoting of it related to things like 60A power supplies with multiple strings of leds wired with wire suitable for 5A max. 60A into 5A wire is a gross overload that probably wouldn’t end well. Hopefully this ‘rule’ will serve another generation of people who play with electricity.

Answer 8

The idea of the classic fuse is simple - it is a deliberately weakened section of the electrical circuit that "sacrifices" itself in the name of the other elements of the circuit by interrupting the current when it becomes unacceptably high. This makes the place of interruption predictable. Because the current in a circuit of series-connected elements is the same, they are all protected. The fuse is irreparably damaged and must be replaced... but it is a cheap element.

So the "motto" of a fuse is, "If something will break, let it be me!"

However, there are still people who solve the problem of short circuit by strengthening the fuse...

Answer 9

Sometimes the fuse is meant to protect the wire after the fuse. Sometimes the fuse is there to protect something else from becoming a fuse. I had a friend who used to wrap the fuse for his guitar amp with aluminum foil because it kept blowing. It kept blowing because he tried to get more volume from the amp than the amp was designed for. His technique was successful. The fuse no longer blew. But the power transformer (the next most vulnerable wire) did. In effect, the transformer became a fuse. Now as others have said, there could be a design where no wire opens. That's a situation where the magic smoke escapes.