Precautions for using electrical instruments

Precautions for using common electrical instruments

1. Use of clamp meter
A clamp meter is an instrument used to measure the current of an operating electrical circuit. It can measure current without power failure.
1. Structure and principle of clamp meter
A clamp meter is essentially composed of a current transformer, a clamp wrench and a rectifier-type magnetoelectric system with reaction force instrument.
2. Use of clamp meter
(1) Mechanical zeroing is required before measurement
(2) Select the appropriate range, first select the large range, then the small range or estimate the value on the nameplate.
(3) When using the minimum range for measurement, if the reading is not obvious, you can wind the measured wire a few turns. The number of turns should be based on the number of turns in the center of the jaws. The reading = indicated value × range / full deviation × number of turns
(4) When measuring, the measured wire should be placed in the center of the jaws and the jaws should be closed tightly to reduce errors.
(5) After the measurement, the conversion switch should be placed at the maximum range.
3. Precautions for clamp meter
(1) The voltage of the measured circuit should be lower than the rated voltage of the clamp meter.
(2) When measuring the current of a high-voltage line, wear insulating gloves, insulating shoes, and stand on an insulating mat.
(3) The jaws must be tightly closed and the range cannot be changed while the current is on.

2. Use of megohmmeter
When the megohmmeter is working, it generates high voltage, and the object to be measured is electrical equipment, so it must be used correctly, otherwise it will cause personal or equipment accidents. Before use, the following preparations must be made first:
1. Before measuring, the power supply of the measured equipment must be cut off and short-circuited to the ground for discharge. The equipment must not be allowed to be measured with power on to ensure the safety of people and equipment.
2. For equipment that may induce high voltage electricity, this possibility must be eliminated before measurement.
3. The surface of the measured object must be clean to reduce contact resistance and ensure the correctness of the measurement results.
4. Before measurement, check whether the megohmmeter is in normal working condition, mainly check its "0" and "∞" points. That is, shake the handle to make the motor reach the rated speed. The megohmmeter should point to the "0" position when short-circuited and the "∞" position when open-circuited.
5. When using the megohmmeter, it should be placed in a stable and firm place, away from large external current conductors and external magnetic fields.
After completing the above preparations, you can start measuring. When measuring, you must also pay attention to the correct wiring of the megohmmeter, otherwise it will cause unnecessary errors or even mistakes.
The megohmmeter has three terminals: one is "L", which is the line terminal, one is "E", which is the ground terminal, and the other is "G", which is the shield terminal (also called the protection ring). Generally, the insulation resistance to be measured is connected between the "L" and "E" terminals, but when the surface leakage of the insulator to be measured is serious, the shield ring of the object to be measured or the part that does not need to be measured must be connected to the "G" terminal. In this way, the leakage current flows directly back to the negative terminal of the generator through the shield terminal "G" to form a loop, and no longer flows through the measuring mechanism (moving coil) of the megohmmeter. In this way, the influence of surface leakage current is fundamentally eliminated. It should be noted that when measuring the insulation resistance between the cable core and the outer surface, the shield terminal "G" must be connected properly, because when the air humidity is high or the cable insulation surface is not clean, the surface leakage current will be very large. In order to prevent the influence of leakage on the internal insulation measurement of the measured object, a metal shielding ring is generally added to the cable surface and connected to the "G" terminal of the megohmmeter.
When using a megohmmeter to measure the insulation resistance of electrical equipment, it is important to note that the "L" and "E" terminals cannot be connected in reverse. The correct connection method is: the "L" line terminal is connected to the conductor of the measured equipment, the "E" ground terminal is connected to the grounded equipment shell, and the "G" shield terminal is connected to the insulation part of the measured equipment. If "L" and "E" are connected in reverse, the leakage current flowing through the insulator and the surface will be collected to the ground through the shell, and then flow into the measuring coil through the ground through "L", so that "G" loses its shielding effect and brings a large error to the measurement. In addition, because the degree of insulation between the internal lead of the "E" terminal and the casing is lower than the degree of insulation between the "L" terminal and the casing, when the megohmmeter is placed on the ground and used, when the correct wiring method is used, the insulation resistance of the "E" terminal to the instrument casing and the casing to the ground is equivalent to a short circuit, which will not cause errors. However, when "L" and "E" are connected reversely, the insulation resistance of "E" to the ground is connected in parallel with the insulation resistance being measured, which makes the measurement result smaller and brings larger errors to the measurement.

3. Precautions for using AC millivoltmeter 1. Short-circuit zero adjustment before measurement. Turn on the power switch, clamp the red and black clips of the test line (also called open-circuit cable) together, turn the range knob to the 1mv range, and the pointer should point to zero (some millivoltmeters can be zeroed through the zero adjustment potentiometer on the panel. If there is no zero adjustment potentiometer on the panel, the internal zero adjustment potentiometer has been adjusted). If the pointer does not point to zero, check whether the test line is broken or in poor contact, and replace the test line. 2. The sensitivity of the AC millivoltmeter is high. After turning on the power, the pointer will deflect due to the interference signal (induction signal) at a lower range, which is called the self-starting phenomenon. Therefore, when not testing the signal, the range knob should be turned to a higher range to prevent the pointer from bending. 3. When the AC millivoltmeter is connected to the circuit under test, its ground terminal (black clip) should always be connected to the ground of the circuit (become a common ground) to prevent interference. 4. When adjusting the signal, the range knob should be turned to a larger range first, and then gradually reduced after the signal is changed. 5. The scale of the AC millivoltmeter dial is divided into two scales: 0-1 and 0-3. The range knob switches the range into one range (1mv, 10mv, 0.1v...) and three ranges (3mv, 30mv, 0.3v...). For any range that is one, read the data directly on the 0-1 scale line, and for any range that is three, read the data directly on the 0-3 scale line. The unit is the unit of the range, and no conversion is required. 6. Before use, check whether the range knob is consistent with the range mark. If it is misaligned, reading errors will occur. 7. The AC millivoltmeter can only be used to measure the effective value of sinusoidal AC signals. If non-sinusoidal AC signals are measured, they must be converted. 8. Note: The AC voltage range of the multimeter cannot be used to replace the AC millivoltmeter to measure AC voltage (the multimeter has a low internal resistance and is used to measure power frequency voltages of about 50Hz).