Analysis of the tester fuse selection, although small but not careless-EEWORLD

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Although a fuse is just a fusible wire, its effectiveness cannot be underestimated. Especially when protecting circuits or using a tester to test voltage and current, if the tester's fuse is not selected properly, it is easy to cause safety hazards and even serious casualties. So, how to select a fuse for a tester?
A fuse is just a fusible wire, right? We know that when the current exceeds a certain value, the fuse will cause a circuit break, which can prevent electric shock and fire caused by overheating of the wire. Some fuses can protect us from more serious accidents. This article describes the safety hazards of using a tester to perform voltage and current tests without the protection of a fuse designed for the tester. These safety hazards can cause serious burns or even death.

Why does a tester need a fuse?

There are a variety of testers on the market, ranging from simple voltage probes to very sophisticated digital multimeters (DMMs). Testers that measure voltage have high input impedance, making it unlikely that overcurrent will occur. Therefore,

the input of the voltage measurement is 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 0.01Ω or so. Add to that the impedance of the test leads (about 0.04Ω), and the total impedance is less than 0.1Ω. This impedance is adequate when the user connects this circuit in series with other loads to measure the current in the circuit. However, when the circuit is connected across a voltage source, such as a socket in the living room, the situation is completely different. 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 across the terminals. Years ago, when analog meters were the only option for making these measurements, this mistake could almost destroy the instrument's mechanics (the pins on the top plug), not to mention the internal circuitry.

To prevent this from happening all too often, meter manufacturers first placed a fuse in series with the meter's test lead jacks as a cheap and effective measure to prevent this simple mistake.

Today, the vast majority of manufacturers still design their meters with fuse protection in the current measurement circuit. As technology has evolved, so has the art of fuse design. While meter designers are well aware of the full impact 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 to experience the problem. The

Seriousness of Choosing the Wrong Fuse

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

may not get an explosion if you are working with a printer, computer, copier, or equipment that has its own power source (CAT I). You may even get away with not setting off a grenade if you are working on a branch circuit (CAT II). In both cases, these are fairly low energy environments and often have built-in protection from fuses, circuit breakers, and overcurrent protection circuits. However, this is not a good way to work safely.

Plasma Fireball

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

This all happens in milliseconds, which is a very limited time to react to an error. If you are lucky, you will break the test leads or meter, thus breaking the circuit. But don't rely on luck alone, using the right fuse can completely avoid this accident.

Use the Right Fuses

Specially designed "high energy" fuses are designed to avoid the energy generated by an electrical short circuit inside the fuse housing, thus protecting the user from electric shock and burns. These high energy fuses are designed to limit the time that energy is supplied and the total amount of oxygen available to burn. Fuses can not only open at a specified constant current, but also at a momentary high current. This high current is called the "minimum interrupting current". For example, Fluke uses fuses with minimum interrupting current ratings of 10,000A and 17,000A in its testers.

If you use a CAT III 1000 V meter and the test leads are connected to the amp jack, there will be about 0.1 Ω series resistance between the test leads (0.01 Ω for the shunt, 0.04 Ω for the test leads, and 0.05 Ω for the fuse and circuit board leads). Now, if you accidentally connect the test leads to a 1,000 V source, Ohm's law will produce 10,000 A (E/R=I, 1,000/0.1 = 10,000), and you will 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 explosive 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 created equal. For your own safety, you need to make sure that the fuse you are using is the one that the engineer designed for your tester. Consult the user manual for your tester, or check with the manufacturer of your tester to make sure you are using the correct fuse. Your safety is worth far more than the expense of purchasing the right fuse for your tester.

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