Introduction to the use of clamp ammeter
Before measurement
The first step is to correctly select the clamp ammeter according to the type and voltage level of the current being measured. The voltage of the line being measured should be lower than the rated voltage of the clamp meter. When measuring the current of a high-voltage line, a high-voltage clamp ammeter that matches its voltage level should be used. Low-voltage clamp ammeters can only measure the current in low-voltage systems, but cannot measure the current in high-voltage systems.
Secondly, the appearance of the clamp ammeter must be correctly checked before use. Be sure to check whether the insulation performance of the meter is good, the shell should be undamaged, and the handle should be clean and dry. If the pointer is not at zero, mechanical zero adjustment should be performed. The jaws of the clamp-type ammeter should be tightly connected. If the pointer shakes, the jaws can be opened and closed again. If the shaking still exists, check carefully and pay attention to remove debris and dirt from the jaws, and then measure.
Because the clamp ammeter contacts the circuit being measured, the clamp ammeter cannot measure the current of bare conductors. When measuring with a high-voltage clamp meter, it should be operated by two people. During the measurement, insulating gloves should be worn, standing on the insulating pad, and other equipment should not be touched to prevent short circuit or grounding.
When measuring
First, when using it, press the wrench tightly to open the jaws, put the wire under test into the center of the jaws, then loosen the wrench and close the jaws tightly. If there is noise on the joint surface of the jaws, it should be opened and closed again. If there is still noise, the joint surface should be processed to make the reading accurate. In addition, do not clamp two wires at the same time. After reading, open the jaws, withdraw the wire under test, and set the gear to the highest current gear or OFF gear.
Secondly, the appropriate range of the clamp-type ammeter must be selected according to the size of the measured current. The selected range should be slightly larger than the measured current value. If it cannot be estimated, in order to prevent damage to the clamp ammeter, measurement should be started from the maximum range and gradually change gears until the range is appropriate. It is strictly prohibited to switch the gear of the clamp ammeter during the measurement process. When changing gears, the conductor under test should be withdrawn from the clamp before changing gears.
When measuring a current less than 5 amps, in order to make the reading more accurate, if conditions permit, the current-carrying wire to be measured can be wound several times and then put into the clamp for measurement. At this time, the actual current value of the measured wire should be equal to the meter reading divided by the number of wire coils placed in the jaws.
When measuring, attention should be paid to keeping a safe distance between all parts of the body and charged objects. The safe distance for low-voltage systems is 0.1 to 0.3 meters. When measuring the current of each phase of a high-voltage cable, the distance between the cable heads should be more than 300 mm and the insulation should be good. The measurement can only be carried out when it is deemed convenient. When observing the timing of the meter, special attention should be paid to keeping a safe distance between the head and the live parts. The distance between any part of the human body and the live body shall not be less than the entire length of the clamp meter.
When measuring the current of low-voltage fusible fuses or horizontally arranged low-voltage busbars, each phase fusible fuse or busbar should be protected and isolated with insulating materials before measurement to avoid short circuit between phases. When one phase of the cable is grounded, measurement is strictly prohibited to prevent ground breakdown and explosion due to low insulation level of the cable head, which may endanger personal safety.
After measurement
After the measurement, if the clamp ammeter is measured with a common magnetoelectric clamp meter, the indicated value will be greatly different from the actual value being measured, or there will even be no indication. The reason is that the magnetoelectric clamp meter The meter head of the clamp meter is connected to the secondary coil of the transformer, and the voltage at the meter head is obtained from the secondary coil. According to the principle of electromagnetic induction, it can be seen that the mutual inductance electromotive force is E2 = 4.44fWФm. It is not difficult to see from the publicity that the size of the mutual inductance electromotive force is proportional to the frequency. When this kind of clamp meter is used to measure the rotor current, due to the low frequency on the rotor, the voltage obtained on the meter will be much smaller than the voltage when measuring the same power frequency current (because this meter is based on AC 50Hz Power frequency design). Sometimes the current is so small that it cannot even turn on the rectifier element in the meter, so the clamp meter has no indication, or the indicated value is very different from the actual value.
If you choose an electromagnetic system clamp meter, since the measuring mechanism does not have a secondary coil and a rectifier element, the magnetic flux generated by the current being measured passes through the meter head and magnetizes the static and moving iron pieces of the meter head, causing the meter head pointer to deflect and interact with the measured The frequency of the current does not matter, so the value of the rotor current can be accurately indicated.
(2) When measuring a three-phase balanced load with a clamp ammeter, the indicated value of the current when two-phase conductors are placed in the clamp is the same as the indicated value of the current when one phase is placed in the clamp. When measuring a three-phase balanced load with a clamp ammeter, a strange phenomenon will occur. That is, the indicated value when two-phase conductors are placed in the clamp is the same as the indicated value when one-phase conductor is placed. This is because the three-phase balanced load is In the circuit, the current value of each phase is equal, and the following public expression is used to express Iu=Iv=Iw. If one phase conductor is placed in the jaws, the clamp meter indicates the current value of that phase. When two phase conductors are placed in the jaws, the value indicated by the meter is actually the sum of the phasors of the two phase currents. According to the principle of phase addition, I1+I3=-I2, so the indicated value is the same as when one phase is put in.
