In current daily applications, clamp meters have been widely used, so here I will introduce the working principle classification of clamp meters and simple use precautions. The working principle of clamp meter The clamp meter is a combination of a current transformer and an ammeter. The iron core of the current transformer can be opened when the wrench is tightened; the measured current can pass through the opening of the iron core without cutting off, and the iron core is closed when the wrench is released. The measured wire passing through the core becomes the primary coil of the current transformer, and the passing current induces current in the secondary coil. So that the ammeter connected to the secondary coil has an indication-----measure the current of the circuit under test. The clamp meter can be changed to different ranges by shifting the shift switch. However, it is not allowed to operate with power when shifting gears. The accuracy of clamp meters is generally not high, usually 2.5-5. For the convenience of use, there are switches with different ranges in the table for measuring different levels of current and measuring voltage. The classification of clamp meters is divided into two types: transformer type and electromagnetic type according to its structure and purpose. Commonly used is a transformer type clamp ammeter, which is composed of a current transformer and a rectifier system instrument. It can only measure AC current. The deflection of the movable part of the electromagnetic system instrument has nothing to do with the polarity of the current, so it can be used for both AC and DC. How to use the clamp meter and matters needing attention 1. Read the manual carefully before use to find out whether it is AC or AC/DC dual purpose. 2. The voltage of the circuit under test cannot exceed the value indicated above, otherwise it is easy to cause a grounding accident or cause an electric shock hazard. 3. Only the current of one phase wire can be measured at a time. The wire to be tested should be placed in the center of the clamp window. Multi-phase wires cannot be clamped into the window for measurement. 4. Before measuring, the clamp meter should first estimate the size of the measured current, and then decide which range to use. If it is impossible to estimate, you can use the maximum range first and then change it to a smaller one to get an accurate reading. It is not possible to use the low current file to measure large currents. To prevent damage to the meter. 5. The jaws should be closed tightly during measurement. If there is a noise after closing, open the jaws and close it again. If the noise cannot be eliminated, check whether the joint surfaces of the magnetic circuit are smooth and clean. Wipe clean if there is dust. 6. Do not operate with power during maintenance to prevent electric shock. 7. Due to the low accuracy of the clamp meter itself, when measuring small currents, the following method can be used: first wind the wire of the circuit under test for a few turns, and then put it into the jaw of the clamp meter for measurement. At this time, the current value indicated by the clamp meter is not the actual value being measured. The actual current should be the reading of the clamp meter divided by the number of turns of the wire.