Choosing the Right Fuse for Your Test Tool

As we all know, a fuse is just a fuse. We know that when the current exceeds a certain value, the fuse will open the circuit. This can protect us from electric shock and fire caused by overheating of wires. However, some fuses can protect us from more serious accidents.

Why do testers need fuses?

There are a variety of testers on the market, ranging from simple voltage probes to very sophisticated digital multimeters (DMM). Testers that measure voltage have high input impedance, making it unlikely that overcurrent conditions will occur. Therefore, the inputs for voltage measurements are generally not designed with fuse protection, but with overvoltage protection. However, if the same tester is designed to also measure current, a fuse is required.

Current measurement inputs usually use a simple shunt circuit through which the current being measured flows. The impedance of this shunt circuit is about 0.01 ohms. Add to that the impedance of the test leads (about 0.04 ohms) and the total impedance is less than 0.1 ohms. This impedance is adequate when the user connects this circuit in series with other loads to measure the current in the circuit. However, it is a completely different story when you connect this circuit across a voltage source, such as a socket in your living room.

This is the most common mistake made by people who measure voltage and current at the same time. After measuring current with the test leads connected to the current input jack, the user will try to make a voltage measurement, forgetting that the test leads are connected to the ampere jack, effectively shorting the voltage source.

Years ago, when analog meters were the only option for making these measurements, this mistake could almost destroy the meter's mechanics (the pins on the top plug), not to mention the internal circuitry.


To prevent this frequent occurrence, meter manufacturers first connected a fuse in series with the meter's test lead jacks as a cheap and effective way to prevent this simple mistake.

Today, the vast majority of manufacturers still design their meters with fuse protection in the current measurement circuit. As science has advanced, so has the art of fuse design. While meter designers are well aware of the full effects of a blown fuse, most meter users are less aware.

When this simple mistake is made and a voltage is placed across the current jack, one may be relieved that the meter has not been destroyed. But then comes the frustration of having to replace the fuse before measuring current again. Even more troublesome is when a meter is shared by multiple people and someone blows a fuse and leaves the meter alone, leaving unsuspecting users exposed to the problem. [page]

The Seriousness of Choosing the Wrong Fuse

The manufacturer lists the ampere, interrupting current, and voltage ratings required for replacement fuses in the manual (and often on the meter itself). If you choose a fuse that does not meet these specifications, or worse, run a wire directly to the fuse's connections, believe it or not, you are creating a thermal grenade. All you need is the conditions to set it off.

When you work on a printer, computer, copier, or device with its own power source (CAT I), you probably won’t get away with an explosion. You might even get away with not setting off a grenade when working on a branch circuit (CAT II). In both of these environments, they are fairly low energy environments and tend to have built-in protection with fuses, circuit breakers, and overcurrent protection circuits. However, this is not a good way to work safely.

When you move to a distribution cabinet (CAT III) or primary feeder (CAT IV), the protection circuits change significantly. At the distribution panel, there are circuit breakers between you and the power company, and the power company has circuit breakers rated for hundreds of amps instead of 15, 20, or 30 amps on the branch circuits. When the voltage is measured on the incoming side of the breaker panel at a home, the protection is behind the utility pole or substation, and these breakers can carry thousands of amps before opening, and it takes much longer to open the circuit than a branch circuit breaker. So when you accidentally connect the test leads to the amp jack and the meter test leads to one of these voltage sources, and you use a meter without the proper fuse protection, you are putting your life in danger.

Plasma Fireball

In this case, the short circuit formed by the wrong fuse (or the wires connecting the fuse to the circuit) and the test leads generates almost unlimited energy. The metal components in the fuse (or wires) are rapidly heated and begin to vaporize, forming a small explosion. When the wrong fuse is used, the fuse cover will be shocked open by the explosive force, and when it encounters unlimited oxygen, a plasma fireball will be formed. The test leads may also begin to melt, and very quickly, flames and hot metal will contact your hands, arms, face, and clothing. The time the meter is energized, the amount of oxygen available, and the protective equipment (such as a mask and protective gloves) will determine the extent of your injury.

All of this happens in a matter of milliseconds, leaving very little time to react to an error. If you're lucky, you'll break the test leads or meter, breaking the circuit. But luck alone won't do, and using the right fuse will completely prevent this.

Using the Right Fuses

Specially designed "high energy" fuses are designed to protect users from electric shock and burns by preventing the energy from an electrical short inside the fuse housing. These high energy fuses are designed to limit the amount of time the energy is supplied and the amount of oxygen available to burn. Fuses are designed to open not only at a specified constant current, but also at a momentary high current. This high current is called the "minimum interrupting current." Fluke uses fuses with minimum interrupting current ratings of 10,000 and 17,000 amps in its test tools.

If a CAT III 1000 V meter is used, and the test leads are connected to the amp jack, there will be about 0.1 ohm of series resistance between the test leads (0.01 ohm for the shunt, 0.04 ohm for the test leads, and 0.05 ohm for the fuse and circuit board leads). Now, if you accidentally connect the test leads to a 1,000 volt source, Ohm's law will produce 10,000 amps (E/R=I, 1,000/0.1=10,000). You need a fuse that can open at this current, and do it quickly.

In addition to specially designed fuse devices, high-energy fuses are filled with sand. Not only does the sand absorb the impact energy of the exploding components, but the high temperatures (up to 10,000°F) generated by this energy melt the sand, causing it to form glass. The glass covers the device and suppresses the fireball by cutting off the available oxygen, protecting the user and the measuring instrument from harm.

As you can see, not all fuses with the same ampere and voltage ratings are the same. For your own safety, you need to make sure that the fuse you are using is the one that the engineer designed for use with the test instrument. Consult the test instrument's user manual or check with the test instrument's manufacturer to make sure you are using the correct fuse. The value of a life is far greater than the cost of purchasing the right fuse for your test instrument.