How to use and precautions for common electrical measuring instruments

Common measuring instruments used by electricians include megohmmeters, multimeters and clamp-on ammeters. If these instruments are not used correctly or are slightly negligent during measurement, the meter may be burned out or the measured component may be damaged, or even endanger personal safety. Therefore, it is very important to master the correct use of common electrical measuring instruments.

1 Megohmmeter

The megohmmeter, also known as the megohmmeter, is used to test the insulation condition of circuits or electrical equipment. The usage and precautions are as follows:

(1) First, select a megohmmeter that is suitable for the voltage level of the measured component. For circuits or electrical equipment of 500V and below, a 500V or 1000V megohmmeter should be used. For circuits or electrical equipment above 500V, a 1000V or 2500V megohmmeter should be used.

(2) When using a megohmmeter to test the insulation of high-voltage equipment, two people should be involved.

(3) Before measuring, the power supply of the measured circuit or electrical equipment must be completely disconnected, that is, it is not allowed to measure the insulation resistance under power. And it is necessary to make sure that no one is working on the circuit or electrical equipment before measuring.

(4) The megohmmeter must use an insulated wire, and it is not advisable to use a double-stranded insulated wire. The end of the megohmmeter should have an insulating sheath; the line terminal "L" of the megohmmeter should be connected to the measured phase of the equipment, the ground terminal "E" should be connected to the equipment housing and the non-measured phase of the equipment, and the shield terminal "G" should be connected to the protective ring or the cable insulation sheath to reduce the error caused by the insulation surface leakage current to the measurement.

(5) The megohmmeter should be calibrated on an open circuit before measurement. When the "L" and "E" ends of the megohmmeter are unloaded, the pointer of the megohmmeter should point to "∞" when the "L" and "E" ends of the megohmmeter are shaken; when the "L" and "E" ends of the megohmmeter are short-circuited, the pointer of the megohmmeter should point to "0". This indicates that the megohmmeter functions well and can be used.

(6) Before testing, the circuit or electrical equipment under test must be grounded and discharged. When testing the circuit, permission must be obtained from the other party before proceeding.

(7) When measuring, the speed of shaking the handle of the megohmmeter should be uniform at 120r/min; after maintaining a stable speed for 1 minute, take the reading to avoid the influence of the absorption current.

(8) Do not touch two wires with both hands at the same time during the test.

(9) After the test, remove the wires first and then stop shaking the megohmmeter. This is to prevent the electrical equipment from charging the megohmmeter back and causing damage to the megohmmeter.

(10) During thunderstorms, it is strictly forbidden to test the insulation of the circuit.

2 Multimeter

A multimeter is a comprehensive instrument that can measure AC or DC voltage and current, as well as the resistance of components, general parameters of transistors, and gain of amplifiers. Therefore, the wiring of the multimeter conversion switch is relatively complicated and its use must be mastered.

(1) Before using the multimeter, calibrate the mechanical zero and electrical zero. If you want to measure current or voltage, you should first adjust the mechanical zero of the meter pointer; if you want to measure resistance, you should first adjust the electrical zero of the meter pointer to prevent the battery voltage in the meter from dropping and causing measurement errors.

(2) Before measuring, be sure to select the gear, that is, voltage gear, current gear or resistance gear, and also select the right range. The initial selection should be from large to small to avoid damaging the pointer. It is forbidden to switch the range under power. The principle of range selection is "U and I in the upper half, R in the middle is more accurate", that is, when measuring voltage and current, the pointer should be above 1/2 of the scale, and when measuring resistance, the pointer should be in the middle of the scale to be accurate.

(3) Pay attention to the polarity of the test leads when measuring DC. When measuring high voltage, the red and black test leads should be inserted

into the "2500V" and "-" jacks, the multimeter should be placed on an insulating bracket, and then the test leads should be touched to the conductor under test with an insulating tool.

(4) When measuring transistors or integrated components, the R×1 and R×10k ranges should not be used.

(5) During live measurement, care should be taken to prevent short circuits and electric shock accidents.

(6) When not in use, the switch should not be stopped at the ohm range to prevent the battery from discharging when the test leads are short-circuited.

3 Clamp ammeters

Clamp ammeters are divided into high and low voltage types, which are used to directly measure the current in the circuit without disconnecting the circuit. Its usage is as follows:

(1) When using a high-voltage clamp meter, pay attention to the voltage level of the clamp meter. It is strictly forbidden to use a low-voltage clamp meter to measure the current of a high-voltage circuit. When measuring with a high-voltage clamp meter, two people should operate it. Non-duty personnel should also fill out a second work ticket. When measuring, they should wear insulating gloves and stand on insulating pads. They should not touch other equipment to prevent short circuit or grounding.

(2) When observing the meter timing, pay special attention to keeping 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.

(3) When measuring on a high-voltage circuit, it is forbidden to use a wire to connect another meter from the clamp meter. When measuring the current of each phase of a high-voltage cable, the distance between the cable head wires should be more than 300mm and the insulation should be good. The measurement can only be carried out when it is considered convenient.

(4) When measuring the current of a low-voltage fusible fuse or a horizontally arranged low-voltage busbar, the fusible fuses or busbars of each phase should be protected and isolated with insulating materials before measurement to avoid causing a phase-to-phase short circuit.

(5) When one phase of the cable is grounded, measurement is strictly prohibited. This is to prevent the cable head from breaking through the ground and exploding due to low insulation level, which could endanger personal safety.

(6) After measuring with a clamp ammeter, pull the switch to the maximum range to avoid accidental overcurrent the next time you use it; it should also be stored in a dry room. (end)