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How To Choose The Correct Fuse For The Tester
- Jan 16, 2018 -

Why does the tester need a fuse?

There are all kinds of testers on the market, from simple voltage pens to very sophisticated digital multimeter (DMM). The measuring instrument for measuring voltage has a high input impedance, which makes it less likely to overflow. So,

The input of voltage measurement generally does not have the function of fuse protection, but the function of overvoltage protection. However, if the same tester is designed to be able to test the current, the fuse is needed.


The current measurement input usually uses a simple shunt circuit, and the measured current will flow through the circuit. The impedance of the shunt circuit is about 0.01 Omega. In addition to the impedance of the test line (about 0.04 omega), the total impedance is less than 0.1 Omega. When the user connects the circuit with other loads to measure the current of the circuit, the impedance is sufficient. However, when the circuit is connected to a voltage source, such as a socket inserted into the living room, the situation is completely different. This is one of the most common mistakes made by people who measure both voltage and current at the same time. After measuring the overcurrent through the test line connected to the current input socket, the user will try to measure the voltage, and forget that the test line is connected to the ampere socket, causing the short circuit of both ends of the voltage source actually. Many years ago, when the analog instrument was the only optional instrument to carry out these measurements, the error could almost destroy the parts of the instrument, such as pins on the top plug, not to mention the internal circuit.


In order to prevent such things from happening frequently, the instrument manufacturer first connects the instrument test line socket with a fuse, which is a cheap and effective way to prevent such a low grade error.


Today, most manufacturers are still using fuse protection in current measurement circuits when designing their instruments. With the development of technology, the technology of fuse design has also made great progress.  Most instrument users know little about the overall impact of fusing, although the designers are very aware of the overall effects of fusing.


When this kind of low error is made, people may be relieved that there is no damage to the instrument when the voltage is crossed over a current socket. However, it would be depressing to be forced to replace the new fuse before measuring the current again. And more troublesome is that when a number of people share the instrument, someone burns the fuse but puts the instrument aside to make the unsuspected user malfunction.

Use the appropriate fuse

The specially designed "high energy" fuse is used to avoid the energy produced by an electrical short circuit in the fuse hood, thus protecting the user from electric shock and burn.  These high-energy fuse designs are designed to limit the time of supply of energy and the total amount of oxygen available.  The fuse can not only break under the prescribed constant current, but also form a circuit breaker at a moment of high current. The high current is called the "minimum interruption current". For example, the Fluke Corporation uses a fuse with a minimum interrupt current nominal value of 10000A and 17000A in its tester.


If you use the CAT III 1000 V instrument and the test line is connected to the ampere socket, there will be about 0.1 ohm series resistance between the test lines (the shunt circuit is 0.01 ohms, the test line is 0.04 ohms, and the fuse and circuit board leads to 0.05 ohms). Now, if you accidentally connect the test line to a voltage source of 1000V, according to Ohm's law, it will generate 10000A current (E/R=I, 1000/0.1 = 10000), which requires a fuse that can be disconnected under this current and act fast.