Precautions before and after using the clamp watch
Before measurement
Firstly, the clamp type ammeter should be selected correctly based on the type and voltage level of the tested current, and the voltage of the tested circuit should be lower than the rated voltage of the clamp meter. When measuring the current of high-voltage lines, a high-voltage clamp ammeter that matches its voltage level should be selected. Low voltage clamp type ammeters can only measure the current in low-voltage systems and cannot measure the current in high-voltage systems.
Secondly, before use, it is necessary to check the appearance of the clamp type ammeter correctly. It is necessary 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 in the zero position, mechanical zero adjustment should be performed. The jaws of the clamp type ammeter should be tightly engaged. If the pointer shakes, the jaws can be reopened and closed again. If the shaking still exists, it should be carefully inspected, and attention should be paid to removing debris and dirt from the jaws before measurement.
Due to the need for clamp type ammeters to come into contact with the tested circuit, they cannot measure the current of bare conductors. When using a high-voltage clamp meter for measurement, it should be operated by two people. During measurement, insulated gloves should be worn, standing on an insulated pad, and other equipment should not be touched to prevent short circuits or grounding.
During measurement
Firstly, when using, the wrench should be tightened to open the jaws. Place the measured wire in the center of the jaws, then loosen the wrench and close the jaws tightly. If there is any noise on the joint surface of the pliers, it should be reopened and closed again. If there is still noise, the joint surface should be treated to ensure accurate readings. Additionally, it is not allowed to clamp two wires simultaneously. After reading, open the jaws, exit the measured wire, and set the gear to the highest current or OFF gear.
Secondly, the appropriate range of the clamp type ammeter should be selected based on the magnitude of the measured current. The selected range should be slightly larger than the measured current value. If it cannot be estimated, to prevent damage to the clamp ammeter, the measurement should start from the maximum range and gradually shift gears until the range is suitable. It is strictly prohibited to switch the gear of the clamp ammeter during the measurement process. When shifting gears, the measured wire should be removed from the clamp before changing the gear.
When measuring currents below 5 amperes, in order to make the reading more accurate, if conditions permit, the measured current carrying wire can be wound several times and placed in a clamp for measurement. At this point, the actual current value of the measured wire should be equal to the reading value of the instrument divided by the number of wire turns placed in the clamp.
When measuring, attention should be paid to maintaining a safe distance between all parts of the body and the charged body. The safe distance for low-voltage systems is 0.1-0.3 meters. When measuring the current of each phase of high-voltage cables, the distance between the cable heads should be at least 300 millimeters, and the insulation should be good. Only when the measurement is considered convenient can it be carried out. When observing the timing of the watch, special attention should be paid to maintaining a safe distance between the head and the charged part. The distance between any part of the human body and the charged part should not be less than the entire length of the clamp watch.
When measuring the current of low-voltage fusible fuses or horizontally arranged low-voltage busbars, each phase of fusible fuses or busbars should be protected and isolated with insulation materials before measurement to avoid causing phase to phase short circuits. When one phase of the cable is grounded, it is strictly prohibited to measure to prevent ground breakdown and explosion caused by low insulation level of the cable head, which may endanger personal safety.
After measurement
After the measurement is completed, the clamp type ammeter of the clamp type ammeter will have a significant difference between the indicated value and the actual value measured, or even no indication, if measured using a commonly used magnetoelectric clamp type ammeter. The reason is that the head of the magnetoelectric clamp type ammeter is connected to the secondary coil of the transformer, and the head voltage is obtained from the secondary coil. According to the principle of electromagnetic induction, the mutual inductance electromotive force is E2=4.44fW Ф m. It is not difficult to see from the public notice that the magnitude of the mutual inductance electromotive force is directly proportional to the frequency. When using this type of clamp meter to measure rotor current, due to the low frequency on the rotor, the voltage obtained on the meter head will be much smaller than when measuring the same power frequency current (because this type of meter head is designed for AC 50Hz power frequency). Sometimes the current is very small and cannot even make the rectifier element in the meter head conductive, so the clamp meter does not indicate or the indicated value differs significantly from the actual value.
If an electromagnetic clamp type meter is selected, the magnetic flux generated by the measured current passes through the meter head, magnetizing the static and dynamic iron plates of the meter head, and causing the pointer to deflect, which is not related to the frequency of the measured current. Therefore, the value of the rotor current can be correctly indicated.
(2) When measuring three-phase balanced load with a clamp shaped ammeter, the current indication value when placing two phase wires in the clamp is the same as the current indication value when placing one phase. When measuring three-phase balanced loads with a clamp type ammeter, a strange phenomenon occurs, that is, the indicated value when two phase wires are placed in the clamp is the same as the indicated value when one phase wire is placed. This is because in a three-phase balanced load circuit, the current value of each phase is equal, and the following formula is used to represent Iu=Iv=Iw. If a phase conductor is placed in the clamp, the clamp meter indicates the current value of that phase. When two phase conductors are placed in the clamp, the value indicated by the meter is actually the sum of the phase quantities of the two phase currents. According to the principle of adding phase quantities, I1+I3=- I2, so the indicated value is the same as when a phase is placed.
If the three phases are simultaneously placed in the jaws, when the three-phase load is balanced, I1+I2+I3=0, that is, the reading of the clamp ammeter is zero.
