Precautions before and after use of the clamp ammeter
before measurement
The first is to correctly select the clamp ammeter according to the type and voltage level of the measured current, and the voltage of the measured line 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 selected. The clamp ammeter of low voltage level can only measure the current in the low voltage system, and cannot measure the current in the high voltage system.
The second is to correctly check the appearance of the clamp ammeter before use. Be sure to check whether the insulation performance of the meter is good, the shell should not be damaged, and the handle should be clean and dry. If the pointer is not at zero, perform mechanical zero adjustment. The jaws of the clamp ammeter should be tightly connected. If the pointer shakes, you can open and close the jaws again. If the shaking still exists, check carefully, pay attention to remove the sundries and dirt on the jaws, and then measure.
Since the clamp ammeter is in contact with the line under test, the clamp ammeter cannot measure the current of a bare conductor. 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 an insulating mat, and not touching other equipment to prevent short circuit or grounding.
When measuring
The first is to press the wrench tightly to open the jaws, put the wire to be tested into the center of the jaws, and then loosen the wrench to 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 treated to make the reading accurate. Also, do not clamp two wires at the same time. After reading, open the jaws, withdraw the measured wire, and set the gear to the highest current gear or OFF gear.
Secondly, it is necessary to select the appropriate range of the clamp ammeter according to the magnitude of the measured current. The selected range should be slightly larger than the value of the measured current. If it cannot be estimated, in order to prevent damage to the clamp ammeter, start measuring from the maximum range, and gradually change gears until the range is suitable. It is strictly forbidden to switch the gear position of the clamp ammeter during the measurement process. When changing gears, the measured wire should be withdrawn from the jaws before changing the gear position.
When measuring the current of less than 5 amps, in order to make the reading more accurate, when conditions permit, the measured current-carrying wire can be wound several times and then put into the jaw for measurement. At this time, the actual current value of the wire under test should be equal to the reading value of the meter divided by the number of wire coils put into the jaws.
When measuring, pay attention to keep a safe distance between each part of the body and the charged body. The safe distance of the low-voltage system is 0.1 to 0.3 meters. When measuring the current of each phase of the high-voltage cable, the distance between the cable ends should be more than 300mm, and the insulation should be good, and the measurement can only be carried out when it is considered convenient. When observing the timing of the watch, special attention should be paid to maintaining a safe distance between the head and the live part. The distance between any part of the human body and the live body should 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, the fusible fuses or busbars of each phase should be protected and isolated with insulating materials before measurement to avoid interphase short circuits. When one phase of the cable is grounded, it is strictly forbidden to measure to prevent the occurrence of ground breakdown explosion due to the low insulation level of the cable head and endanger personal safety.
after measurement
After the measurement is completed, if the clamp ammeter of the clamp ammeter is measured with a common magnetoelectric system clamp meter, the indicated value will be very different from the actual value to be measured, or even no indication. The reason is that the magnetoelectric system The head of the clamp meter is connected to the secondary coil of the transformer, and the voltage of the meter head is obtained from the secondary coil. According to the principle of electromagnetic induction, the mutual induction electromotive force is E2 = 4.44fWФm. It is not difficult to see from the publicity that the magnitude of the mutual induction 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 head will be much smaller than the voltage when measuring the same power frequency current (because this kind of meter head is based on AC 50Hz power frequency design). Sometimes the current is so small that the rectifying element in the meter head cannot even be turned on, so the clamp meter has no indication, or the indication value is very different from the actual value.
If the clamp meter of the electromagnetic system is selected, since the measuring mechanism has no secondary coil and rectifying element, the magnetic flux generated by the measured current passes through the meter head, magnetizes the static and moving iron pieces of the meter head, and deflects the pointer of the meter head, which is consistent with the measured meter. The frequency of the current does not matter, so the value of the rotor current can be correctly indicated.
(2) When using a clamp ammeter to measure a three-phase balanced load, the current indication value when two-phase wires are placed in the jaws is the same as the current indication value when one phase is placed. When using a clamp ammeter to measure a three-phase balanced load, there will be a strange phenomenon, that is, the indicated value when two-phase wires are placed in the jaws is the same as the indicated value when one-phase wire is put in, this is because the three-phase balanced load In the circuit, the current value of each phase is equal, and the following expression is used to express Iu=Iv=Iw. If one phase wire is placed in the jaw, the clamp meter indicates the current value of the phase, when two phase wires are placed in the jaw, the value indicated by the meter is actually the sum of the phasors of the two phase currents, According to the principle of phasor addition, I1+I3=-I2, so the indicated value is the same as when one phase is put in.
If the three phases are put into the clamp at the same time, when the three-phase load is balanced, I1+I2+I3=0, that is, the reading of the clamp ammeter is zero.
