Best to select the correct fuse for the test tool

2022-08-01
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Select the correct fuse for the 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 cause an open circuit. In this way, we must enhance our confidence in the road, theory and system, so as to prevent us from electric shock and fire caused by overheated wires. However, some fuses can protect us from more serious accidents

why do testers need fuses

there are various testers in the market, ranging from simple voltage probes to very precise and complex digital multimeter (DMM). The tester for measuring voltage has high input impedance, which makes it unlikely that overcurrent conditions will occur. Therefore, the input of voltage measurement is generally not designed with fuse protection function, but with overvoltage protection function. However, if the same tester is designed to test current, a fuse is required

current measurement input usually adopts a simple shunt circuit, and the measured current flows through the circuit. The impedance of the shunt circuit is approximately 0.01 ohms. Add the impedance of the test line (approximately 0.04 ohms), and the total impedance is less than 0.1 ohms. When the user connects the circuit in series with other loads to measure the current of the circuit, the impedance is sufficient. However, when you jumper the circuit to a voltage source, such as the 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 using the test line connected to the current input jack to measure the overcurrent, the user will try to measure the voltage and forget that the test line is connected to the ampere jack, which actually causes a short circuit at both ends of the voltage source

many years ago, when the analog instrument was the only optional instrument for these measurements, this error could almost destroy the mechanical parts of the instrument (the pin on the top plug), not to mention the internal circuit

according to the requirements of the Convention

in order to prevent this kind of common occurrence, the instrument manufacturer first connects a fuse in series with the test line socket of the instrument as a cheap and effective measure to prevent this low-level error

nowadays, most manufacturers still use fuse protection function in current measuring circuit when designing their instruments. With the development of science, fuse design technology has also made great progress. Although the instrument designers are well aware of the overall impact of fusing, most instrument users know little about it

when such a low-level mistake is made and the voltage is connected across the current jack, people may be glad that the instrument is not damaged. However, they will feel depressed because they have to replace the fuse before measuring the current again. What is more troublesome is that when many people share the instrument, someone burns out the fuse and puts the instrument aside, causing unsuspecting users to encounter faults

select the severity of the wrong fuse

adjust the level to within 0.5/1000 with a level gauge at the horizontal bearing surface of the jaw support. The ampere, interrupting current and voltage ratings required for the replacement fuse will be listed in the manual (usually on the instrument). If you choose a fuse that does not meet these indicators, or worse, connect the fuse directly with wires, whether you believe it or not, you are making a heat grenade. All you need is the conditions to detonate the grenade

when you use a printer, computer, copier or a device with its own power supply (cat I), it may not explode. Even when working in the Branch Road (cat II), it may not detonate the grenade and get away with it. In these two environments, it is a very low-energy environment, and often has built-in fuse protection function, circuit breaker and overcurrent protection circuit. However, this is not a good way to work safely

when you switch to the distribution cabinet (cat III) or the primary feeder (cat IV), the protection circuit changes obviously. On the switchboard, there is a circuit breaker between you and the power company, and the power company has a circuit breaker rated for hundreds of amperes on the branch instead of 15, 20 or 30 amperes. When the voltage is measured at the input side of the circuit breaker panel of the residential building, the protection device is located behind the utility pole or substation, and these circuit breakers can carry thousands of amperes of current before the open circuit, and the time required to create the open circuit is much longer than that of the branch circuit breaker. Therefore, when you inadvertently connect the test line to the ampere jack, and connect the instrument test line to one of these voltage sources, and use an instrument without appropriate fuse protection function, you are putting your life at the gate of death

plasma fireball

in this case, the short circuit circuit formed by the wrong fuse (or the wire connecting the fuse to the circuit) and the test line will generate 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 suddenly opened due to the impact of explosive force. When encountering infinite oxygen, plasma fireballs will be formed. The test line may also start to melt, and very soon, fire and hot metal will touch your hands, arms, face and clothes. The time when the instrument receives energy, the amount of oxygen available, and protective equipment (such as masks and protective gloves) will determine the extent of your injury

all this happens in a few milliseconds, and the time to respond to errors is very limited. If you are lucky, you will get rid of the test line or instrument and disconnect the circuit. However, such accidents can not be completely avoided by just relying on luck and using appropriate fuses

use appropriate fuse

the specially designed "high energy" fuse is used to avoid the energy generated by the electrical short circuit in the fuse cover, so as to protect users from electric shock and burns. This high-energy fuse is designed to limit the time it takes to supply energy and the amount of oxygen available for combustion. The fuse can not only open circuit under the specified constant current, but also form an open circuit in case of instantaneous high current. This high current is referred to as the "minimum interrupting current". Fluke uses fuses with a nominal minimum interrupting current of 10000 amps and 17000 amps in its test tools

if a cat III 1000 V instrument is used, and the test line is connected to the amp jack, there will be about 0.1 ohm of series resistance between the test lines (0.01 ohm for shunt circuit, 0.04 ohm for test line, and 0.05 ohm for fuse and circuit board lead). Now, if you inadvertently connect the test line to a 1000 volt voltage source, according to Ohm's law, 10000 amperes of current will be generated (e/r=i, 1000/0.1 = 10000). At this time, a fuse that can be disconnected under this current is required, and the action should be fast

except for specially designed fuse devices, high-energy fuses are filled with sand. Sand can not only absorb the impact energy generated by explosive components, but also the high temperature (up to 10000 ° f) generated by this energy will melt the sand and make it form glass. The glass can cover the device and inhibit the fireball by cutting off the available oxygen, so that the user and the measuring instrument are protected from damage

as you can see, not all fuses with the same ampere and voltage rating ecological functional polyurethane products will have a large increase in demand. For your own safety, you need to ensure that the fuse you use is the fuse designed by the Engineer for the test instrument. Refer to the user manual of the test instrument or confirm with the manufacturer of the test instrument to ensure that the correct fuse is used. The value of life is much higher than the cost of buying the right fuse for the test instrument. (end)

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