Multimeter - Cable Inspection Tips
Finding Breaks in Cables with a Multimeter
When there is a disconnection fault inside the cable or the cable, the exact location of the disconnection is not easy to determine due to the wrapping of the outer insulation. The traditional method is to use a multimeter to find the breakpoint of the cable section by section, which not only wastes time, but also damages the insulation of the cable to a large extent. This problem can be easily solved with a digital multimeter.
Specific method: Connect one end of the wire (cable) with a breakpoint to the phase line of the 220V mains, and the other end is suspended in the air. Pull the digital multimeter to the AC2V gear, start from the phase wire input end of the wire (cable), hold the nib of the black test lead with one hand, and slowly move the red test lead along the insulation of the wire with the other hand, at this time The voltage value displayed on the display screen is about 0.445V (measured by the DT890D meter). When the red test pen moves to a certain place, the voltage displayed on the display suddenly drops to 0.0 volts (about one tenth of the original voltage), and the wire is about 15cm forward from this position (phase line access end). (cable) where the breakpoint is. When using this method to check the shielded wire, if only the core wire is broken but the shielding layer is not broken, then this method is powerless.
This method can also be used to find the breaking point of resistance wires such as faulty electric blankets.
Use a multimeter to check if the cable is damp
When maintaining the line, if the input level of the measuring amplifier is lower than the level loss value of the specified cable length, you can use the pointer multimeter to measure the resistance of the cable (R×1 or R×100), if the needle is slow like a capacitor charging Slow rise, this phenomenon indicates that the cable is seriously damp or there is a lot of water in the cable.
Generally speaking, the resistance value of an intact cable should be infinite. If the cable is severely damp and has a lot of water, the resistance value of the cable is generally around several hundred ohms; if the cable is full of water, its resistance value is basically equal to zero, which is equivalent to a short circuit. , and the hands of the watch are slowly swinging upwards like charging a capacitor. In this way, it can be completely concluded that the fault is caused by the cable being affected by damp and water; at this time, the TV signal received by the TV receiver is very poor, and even cannot be watched.
Remove the two ends of the faulty cable with open circuit or short circuit, and separate the inner and outer conductors of the cable. If there is a short circuit fault in the cable, the pointer will definitely point to the zero position; Check, if the hands still do not swing, it proves that the cable is disconnected.
Due to construction damage to the plastic sheath of the outer conductor of the coaxial cable or due to poor production quality, the thickness of the plastic outer sheath of the cable is different. net) corrosion. When the fault is checked with the multimeter resistance file, if the measured circuit resistance value is much higher than the original cable circuit value, it proves that the outer conductor is severely corroded. If the measured circuit value is infinite, it means that the outer conductor is disconnected after corrosion. In short, there are many detection methods for coaxial cables. As long as we carefully analyze and judge, and master the accurate detection methods, any fault can be detected.
