What happens when the pencil breaks the fuse?
When a digital multimeter is used instead of a measuring pen to judge the zero line and the live line, the range switch can be adjusted to the AC voltage of 200V, and the black stylus touches the ground or the wall, and the red stylus touches two wires respectively. If the red stylus touches the zero line, the voltage displayed by the multimeter is very small, close to 0 V. If the red stylus touches the live line, the voltage displayed by the multimeter is generally above tens of volts. Using this method to measure AC high voltage, unlike the pencil, there will be current flowing through the human body, which is safer.
(1) very dangerous!
However, the danger does not occur when the fire line is measured with a test pencil. The high resistance resistors used in measuring pens are solid-core resistors, also called volume resistors, which are divided into two categories: organic synthetic solid-core resistors and inorganic synthetic solid-core resistors. This kind of resistor is made of granular conductive material, non-conductive powder material used as filler and adhesive by mixing and pressing. The granular conductive materials are carbon black and graphite; Non-conductive powder materials are mica powder, quartz powder, glass powder, titanium dioxide powder, etc. Some adhesives use organic adhesives, and some use inorganic adhesives. The two categories of organic solid-core resistors and inorganic solid-core resistors mentioned above are distinguished according to the types of adhesives. This kind of resistor has strong overload resistance, high reliability and is not easy to be damaged. In case of damage, it is also in an open circuit state, that is, although the neon tube is not damaged, it does not light up, but the person holding the test pencil to measure the fire line will not get an electric shock.
(3) the danger of electric shock occurs in the case that there is electricity in the live wire, and the resistance of the test pen is damaged, so it is easy for people to accidentally touch the live wire and get an electric shock!
If the resistance of the stylus (mostly solid pressed resistance, the resistance value is several m) is broken, it can be divided into two situations:
First, it is disconnected (there are many such cases). After the resistance is disconnected, the measuring circuit is disconnected, and the neon bulb of the measuring pen has no current, and the neon bulb does not shine, which is easy to misjudge the charged situation of the tested circuit;
The second is short circuit (which is rare), and the neon tube of the test pen is broken down due to instantaneous high voltage (the normal working voltage of neon bulb is 70V). At this time, the neon tube plays an insurance role and will not cause harm to people.
