The use of clamp ammeters
Before measurement
Firstly, the clamp ammeter should be selected correctly based on the type and voltage level of the measured current. The voltage of the tested circuit should be lower than the rated voltage of the clamp ammeter. When measuring the current of high-voltage lines, a high-voltage clamp ammeter that matches its voltage level should be selected. Low voltage clamp 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 correctly check the appearance of the clamp ammeter. 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 zeroing should be performed. The jaws of the clamp ammeter should be tightly engaged. If the pointer shakes, the jaws can be opened and closed again. If the shaking still exists, careful inspection should be carried out to remove debris and dirt from the jaws before measurement.
Due to the contact between the clamp ammeter and the measured circuit, the clamp ammeter cannot measure the current of a bare conductor. When using a high-voltage clamp meter for measurement, it should be operated by two people. When measuring, 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 tested 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 opened and closed again. If there is still noise, the joint surface should be treated to make the reading accurate. 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 change gears until the range is appropriate. 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 amps, 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 tested 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 cable heads should be at least 300 millimeters, and the insulation should be good. It can only be carried out when it is considered convenient for measurement. 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 the fusible fuse or busbar 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, measurement is strictly prohibited 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 magneto electric clamp type ammeter. The reason for this is that the head of the magneto electric clamp type ammeter is connected to the secondary coil of the transformer, and the voltage on the head 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 lower 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 rectifying 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 meter is used, due to the lack of secondary coils and rectifier components in the measuring mechanism, the magnetic flux generated by the measured current passes through the meter head, magnetizing the static and dynamic iron plates of the meter head, causing the pointer of the meter head to deflect, which is not related to the frequency of the measured current, so it can accurately indicate the value of the rotor current.
