What Is a Megohm?

Megger ^[1] (megger) is a measuring instrument commonly used by electricians. It is mainly used to check the insulation resistance of electrical equipment, household appliances or electrical lines to earth and between phases to ensure that these equipment, electrical appliances and circuits work in normal conditions. To avoid accidents such as electric shock casualties and equipment damage.

The scale of a megohmmeter is in megohms (M). The megohmmeter is composed of medium and large scale integrated circuits. This watch has high output power, high short-circuit current value, and multiple output voltage levels (each model has four voltage levels).

Megohmmeters are commonly used and indispensable meters for power, post and telecommunications, communications, electromechanical installation and maintenance, and industrial enterprise sectors that use electricity as industrial power or energy. It is suitable for measuring the resistance value of various insulating materials and the insulation resistance of transformers, motors, cables and electrical equipment, etc. ^[1]

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Basic parameters
Test voltage	DC 25, 50, 100, 250, 500, 1000V
Measurement range	25 50V: 2M 200M, 3 ranges 100 250V: 2M 2000M, 4 ranges 500 1000V: 2M 4000M, 4 ranges
Accuracy	± 2% rdg. ± 5dgt. (25 100V test voltage: 0 20.00M) (250V test voltage, 0 100.0M) (500 1000V test voltage, 0 999M) ± 5% rdg. (Test voltage of 25 50V: 19.0 200.0M) (Test voltage of 100V: 19.0 2000M) (Test voltage of 250: 100.1 2000M) (Test voltage of 500 1000V: 1000 4000M)
Response time	Fast: less than 0.7 seconds, slow: less than 1.5 seconds (in manual range)
Sampling Rate	Fast: 10 times / second, Slow: 1 time / second
Features	Comparison function: judge pass or fail Test timing function: 0.5 99 seconds Delay timing function: 0.1 99 seconds
power supply	100 240V AC, (50 / 60Hz)
Volume and weight	215 width × 61 height × 213 thickness mm, 1.1kg
annex	Power cord (1)

Option

Remote control box or test line (cannot be used alone, you need to use test tools or probes when measuring)

9185 test lead 85 cm long

9294 test probe (included) about 1.2m long

9299 switch probe 80 cm (31.5 in) long

printer

9442 Printer

9443-02 AC Power Adapter for 9442, EU

9443-03 AC power converter for USA, for 9442 printers

9444 connecting cable (for 9442 printer)

1196 recording paper (25m, 10 rolls / box, for 9442)

PC communicator

9637 cable 9pin-9pin, 1.8m long

9638 cable 9pin-25pin, 1.8m long

Other options

* 9094 1.5m signal output line

The digital megohmmeter is composed of medium and large scale integrated circuits. This watch has large output power, high short-circuit current value, and multiple output voltage levels (there are four voltage levels). The working principle is that the DC high voltage generated by the internal battery as a power source through DC / DC conversion is output from the E pole to the L pole through the test product, thereby generating a current from E to L pole, which is completed by the divider after I / V conversion The calculation directly displays the measured insulation resistance value on the LCD.

1. High output power, strong load capacity and strong anti-interference ability.

2. The case of this watch is composed of high-strength aluminum alloy. Equipotential protection ring and fourth-order active low-pass filter are set inside the machine, which can effectively shield the external power frequency and strong electromagnetic fields. For capacitive test objects, the output short-circuit current is greater than 1.6mA, which makes it easy to quickly increase the test voltage to the rated value of the output voltage. For the low resistance measurement, the voltage drop does not affect the test accuracy due to the proportional method design.

3. No manual work is required. It is powered by batteries and the range can be switched automatically. The clear panel operation and LCD display make the measurement very convenient and fast.

4. The output short-circuit current of this meter can be directly measured, and no load measurement is needed for estimation.

It is stipulated that the voltage level of the megohmmeter should be higher than the insulation voltage level of the measured object. Therefore, when measuring the insulation resistance of equipment or lines with a rated voltage below 500V, a 500V or 1000V megohmmeter can be selected. When measuring the insulation resistance of equipment or lines with a rated voltage above 500V, a 1000 ~ 2500V megohmmeter should be used. For insulators, 2500 ~ 5000V megohmmeter should be used. In general, when measuring the insulation resistance of low-voltage electrical equipment, a megohmmeter with a range of 0 ~ 200M can be selected.

1. Before the measurement, the power of the device under test must be cut off and short-circuited to ground and discharged. Under no circumstances should the equipment be energized for measurements to ensure the safety of persons and equipment. For equipment that may induce high voltage, this possibility must be eliminated before measurements can be taken.

2. The surface of the test object should be clean. Reduce the contact resistance to ensure the accuracy of the measurement results.

3. Before the measurement, the megohmmeter should be subjected to an open circuit and short circuit test to check whether the megohmmeter is good. That is, before the megohmmeter is connected to the test object. Shake the handle to make the generator reach the rated speed (120r / min), and observe whether the pointer points to the "" position of the scale. Short the terminals "line (L) and ground (E)", and slowly shake the handle to see if the pointer is at the "0" position of the ruler. If the pointer cannot point to the position of the finger, it indicates that the megohmmeter is faulty. It should be used after maintenance.

4. The megohmmeter should be placed in a stable and stable place, and away from large external current conductors and external magnetic fields.

5. The wiring must be correct. There are generally three terminals on the megohmmeter, where L is connected to the conductor part of the measured object and the ground, and E is connected to the case or the ground of the measured object. G is connected to the shield of the test object or the part that does not need to be measured. When measuring insulation resistance, generally only the "L" and "E" terminals are used. However, when measuring the cable's insulation resistance to ground or the leakage current of the device under test is serious, it is necessary to use the "G" terminal and connect the "G" terminal to the shield or the casing. After the line is connected, you can turn the joystick clockwise. The speed of shaking should be slow and fast. When the speed reaches about 120 revolutions per minute (ZC-25 type), keep rotating at a constant speed. Take a reading after 1 minute, and take a reading while shaking. Do not stop reading.

6. Set the megohmmeter to the horizontal position during the shake test. Do not short circuit the end buttons when the handle is turned. Shake the handle from slow to fast. If the pointer is zero, it means that the insulation under test may have a short circuit. At this time, you cannot continue to shake the handle. In order to prevent the coil inside the meter from heating and damaging.

7. The reading is completed. Discharge the device under test. Discharge method is to take the ground wire used in the measurement off the megohmmeter and short-circuit the device under test (not megohmmeter discharge).

Before measurement, first cut off the power of the device under test, and connect the conductive part of the device to the ground, and discharge it fully to ensure safety. Electrical equipment that has been measured with a digital megohmmeter must also be grounded in time before re-measurement. Before the measurement, check whether the digital megohmmeter is intact, that is, before the digital megohmmeter is not connected to the measured object, turn on the power switch to detect the battery condition of the digital megohmmeter. If the digital megohmmeter battery is under voltage, replace the battery in time Otherwise, measurement data is not advisable. Insert the test lead into the terminal wire (L) and ground (E), select the test voltage, disconnect the test lead, press the test button, and observe whether the number shows infinite. Short the terminal line (L) and ground (E) and press the test button to see if 0 is displayed. If the LCD screen does not display "0", it indicates that the digital megohmmeter is faulty and should be used after maintenance. Must be properly wired. Digital megohmmeters generally have three terminals labeled L (line), E (ground) and G (shield). Among them, L is connected to the conductive part of the measured object and the ground, E is connected to the shell or the ground of the measured object, and G is connected to the shield of the measured object or the part that does not need to be measured. Terminal G is used to shield the surface current. For example, when measuring the insulation resistance of a cable, the leakage current on the surface of the insulating material will make the measurement result inaccurate, especially in the case of high humidity and the cable insulation surface is not clean, which will cause a large measurement error. In order to avoid the influence of surface current, add a metal shield ring on the surface of the test object, and connect it to the "shield" terminal of the digital megohmmeter. In this way, the surface leakage current IB starts from the positive pole of the generator and flows back to the negative pole of the generator through the terminal G to form a circuit. IB no longer passes the megohmmeter's measuring mechanism, so the effect of surface leakage current is fundamentally eliminated. Do not use double-stranded insulated wires or stranded wires to connect the wires between the terminal and the device under test. They should be connected separately with single-stranded wires to avoid errors due to poor insulation of the stranded wires. In order to obtain correct measurement results, the surface of the device under test should be wiped clean with a clean cloth or cotton yarn. Measure the insulation resistance of devices with large capacitance. Do not disconnect the megohmmeter immediately after reading. Otherwise, the capacitor that has been charged will discharge the megohmmeter and may burn the megohm meter. After reading, the test wire should be disconnected first, and then the test should be stopped. Do not touch the conductive part of the equipment under test until the megohmmeter and the measured object are fully discharged. When measuring the insulation resistance of the equipment, the temperature, humidity, and related conditions of the test object should also be recorded in order to analyze the measurement results.

1. It is forbidden to measure the insulation resistance during lightning or near high-voltage equipment. It can only be measured when the equipment is not energized and there is no induced electricity.

2. During the shake test, no one can work on the device under test.

3. Megohmmeter wires cannot be twisted together, they must be separated.

4. Before the megohmmeter stops rotating or before the equipment under test is not discharged. Never touch it with your hands. When disassembling, do not touch the metal part of the lead.

5. At the end of the measurement. Discharge for large-capacitance devices.

6. The measuring flexible wires leading out of the megohmmeter terminals should have good insulation, and an appropriate distance should be maintained between the two wires and between the wires and the ground to avoid affecting the measurement accuracy.

7. To prevent leakage resistance on the surface of the device under test, when using a megohmmeter. The middle layer of the device under test (such as the inner insulation between the cable shell cores) should be connected to the protective ring.

8. Regularly check its accuracy.

Megohmmeter Calibration Procedure

1. Range of 10 3 to 10 12 impedance bridge with 1% accuracy. Relative Humidity Hygrometer High accuracy Thermometer.

2. Open the cover and be careful not to damage the two wires on the circuit board that connect the power switch.

3. Find the three Calibration Pots on the bottom right of the circuit board.

4. Make the watch at least 1/2 hour under this environmental condition, and then start the test after self-balancing.

5. Use one end of the ACL-800 meter to connect the crocodile clip at one end and the banana plug at the other end.

6. Insert the 3.5mm long plug into the socket of the watch.

7. Connect the two ends of the resistor with alligator clips.

8. Three calibration regulators. The top one is for humidity measurement, the middle one is impedance, and the bottom one is temperature adjustment with small screwdriver. Adjust clockwise for increasing value, counterclockwise for decreasing value.

9. Press the power switch and compare the values of "Temperature", "Humidity" and "Resistance" at the same time.

10. Release the power switch and slowly adjust the corresponding correction regulator.

11. Press the power switch again and observe the LCD display.

12. If you need to recalibrate, you can press the power switch and adjust the corrector again.

13. Close the cover and tighten the four fixing screws.

14. Press the power switch to determine if the meter is working properly.

Megohmmeter resistance measurement

This measurement method is used to measure the surface resistance between one point on the surface of the object and another ground point on the surface. The measurement method complies with the EOS / ESD S4.1 measurement standard.

A. Insert one end of the two wires into the two 3.5 mm jacks of the watch, and then connect one of them to the crocodile clip and the other to a 5-pound disk probe.

B. Connect the crocodile clip to a known ground point, and place the disc probe on the surface of the object to be measured according to the measurement requirements.

C. Press the measurement button until the resistance (in ohms), relative humidity, and temperature values are displayed on the display. The measurement results conform to EIA, EOS / ESD, ANSI, IEC-93, CECC, and ASTM measurement standards. For high-impedance materials In order to ensure the high-precision measurement results during the measurement, care should be taken not to overlap the two leads, and do not touch the probe, the lead and the measured object by hand.