I. Structural and Functional Characteristics of Insulation Resistance Meters
A hand-cranked insulation resistance meter consists of a dial, pointer, terminals (E for ground, L for phase), nameplate, manual crank, instruction manual, red test leads, and black test leads. A digital insulation resistance meter consists of a digital display, test lead connection jacks, backlight switch, time setting button, measurement knob, and range adjustment switch. The image below shows the actual appearance of an insulation resistance meter.
II. Insulation Resistance Meter Usage Guidelines
When using an insulation resistance meter to test the insulation resistance value of an indoor power supply circuit, first loosen the L line terminal, then connect the U-shaped interface of the red test lead to the connection terminal (L), then tighten the L line terminal, then loosen the E ground terminal, connect the U-shaped interface of the black test lead to the connection terminal, and tighten the E ground terminal, as shown in the figure below.
Before using an insulation resistance meter for measurement, an open-circuit and short-circuit test should be performed to check if it is functioning properly. Separate the red and black test clips and crank the lever clockwise; the insulation resistance meter pointer should indicate "infinity." Then short-circuit the red and black test clips and crank the lever clockwise; the insulation resistance meter pointer should indicate "zero." This indicates that the insulation resistance meter is functioning properly. Be careful not to crank too fast, as shown in the diagram below.
When testing the insulation resistance value, connect the black test lead of the insulation resistance meter to the outer casing of the device under test, and the red test lead to the part to be tested. Turn the crank clockwise as shown in the figure below, and you can judge whether the insulation performance of the device under test is good by the dial reading.
Insulation resistance meter definition
An insulation resistance meter, also known as a megohmmeter, is a specialized instrument used to measure maximum resistance, insulation resistance, absorption ratio, and polarization index. Its unit of measurement is megohms, and it has its own high-voltage power supply. The insulation performance of electrical products is one of the important indicators for evaluating their insulation quality, and it is reflected through insulation resistance.
Insulation resistance meters are available in several rated voltages, such as 250, 500, 1000, and 2500V, and in several measuring ranges, such as 500, 1000, and 2000Mn.
Classification of Insulation Resistance Meters
(1) Hand-cranked megohmmeter: Test voltages range from 100V to 2500V, with an upper range of 2500MΩ, and it is widely used. However, it is labor-intensive to operate, has low measurement accuracy (affected by hand-cranking speed, scale nonlinearity, and tilt angle), low output current, and weak backflash resistance, making it unsuitable for measuring large equipment such as transformers. However, due to its low price, it has not been replaced and still has a certain market share.
2) Digital Insulation Resistance Meters: With the advent of digital integrated circuits in the measuring circuit, hand-cranked megohmmeters have been replaced by digital insulation resistance meters. The development of microcontrollers has made digital megohmmeters even more intelligent, completing timing, calculation, and storage simultaneously. Test voltages range from 5000V to 10000V, and even 15000V. They can directly read the absorption ratio and polarization index, with a measurement upper limit exceeding 100TΩ. They feature a self-discharge circuit and strong backflash protection, making them widely used in power systems.
Insulation resistance meter structure and composition (1) DC high voltage generator
Insulation resistance measurement requires the application of a high voltage to the measuring terminals. The national standard for insulation resistance meters specifies this high voltage value as 50V, 100V, 250V, 500V, 1000V, 2500V, 5000V…
There are generally three methods for generating DC high voltage. The first is a hand-cranked generator. Currently, about 80% of megohmmeters produced in my country use this method (hence the name "cranked megohmmeter"). The second is to step up the voltage using a mains transformer and then rectify it to obtain DC high voltage. This is the method generally used in mains-powered megohmmeters. The third is to use a transistor oscillator or a dedicated pulse width modulation circuit to generate DC high voltage. This is the method generally used in battery-powered and mains-powered insulation resistance meters.
(2) Measurement circuit
In the megohmmeter mentioned earlier, the measurement circuit and display section are integrated. This is accomplished by a current ratio meter head, which consists of two coils with an included angle of approximately 60°. One coil is connected in parallel across the voltage terminals, while the other is connected in series in the measurement circuit. The deflection angle of the meter pointer depends on the current ratio in the two coils. Different deflection angles represent different resistance values. The smaller the measured resistance, the larger the current in the coil connected in series in the measurement circuit, and thus the larger the pointer deflection angle. Another method is to use a linear ammeter for both measurement and display. In the current ratio meter head used earlier, because the magnetic field in the coil is non-uniform, when the pointer is at infinity, the current coil is exactly where the magnetic flux density is strongest. Therefore, even if the measured resistance is large, the current flowing through the current coil is small, resulting in a large deflection angle. When the measured resistance is small or zero, the current flowing through the current coil is large, and the coil has deflected to a region of lower magnetic flux density, resulting in a smaller deflection angle. This achieves non-linear correction. A typical megohmmeter displays resistance values spanning several orders of magnitude. However, this doesn't work when a linear ammeter is directly connected in series in the measurement circuit. At high resistance values, the scale readings become crowded together and indistinguishable. To achieve non-linear correction, a non-linear component must be added to the measurement circuit. This component creates a current shunt at low resistance values and prevents current shunt at high resistance values, thus allowing the resistance display to span several orders of magnitude. With the development of electronic and computer technologies, digital meters are gradually replacing analog instruments.
Digital measurement technology for insulation resistance has also been developed, with the voltage ratio meter circuit being one of the better measurement circuits. The voltage ratio meter circuit consists of a voltage bridge circuit and a measurement bridge circuit. The signals output from these two bridge circuits are converted into digital values by A/D conversion and then processed by a microcontroller for direct digital value display.
Instructions for using an insulation resistance meter
Insulation resistance meters generate high voltage during operation, and since the object being measured is electrical equipment, they must be used correctly; otherwise, personal injury or equipment damage may occur. Before use, the following preparations must be made:
(1) Before measurement, the power supply to the device under test must be disconnected and short-circuited to ground. Measurement is never allowed while the device is energized to ensure the safety of personnel and equipment. (2) For devices that may induce high voltage, this possibility must be eliminated before measurement can be performed. (3) The surface of the object under test must be clean to reduce contact resistance and ensure the accuracy of the measurement results.
(4) Before measurement, check whether the megohmmeter is in normal working condition, mainly by checking the 0 and ∞ points. That is, turn the handle to make the motor reach the rated speed. The megohmmeter should indicate the 0 position when short-circuited and the ∞ position when open-circuited.
(5) When using a megohmmeter, it should be placed on a stable and secure surface, away from large external current conductors and external magnetic fields. After completing the above preparations, you can start the measurement. During the measurement, pay attention to the correct wiring of the megohmmeter, otherwise it will cause unnecessary errors or even mistakes.
A megohmmeter has three terminals: L (line terminal), E (ground terminal), and G (shield terminal, also called the guard ring). Generally, the insulation resistance being measured is connected between L and E. However, when there is significant leakage current on the surface of the insulation being measured, the guard ring or the part not being measured must be connected to the G terminal. This allows the leakage current to flow directly back to the negative terminal of the generator via the G terminal, forming a loop instead of flowing through the megohmmeter's measuring mechanism (moving coil). This fundamentally eliminates the influence of surface leakage current. It is particularly important to ensure the G terminal is properly connected when measuring the insulation resistance between the cable core and its outer surface. This is because when the air humidity is high or the cable insulation surface is not clean, the surface leakage current will be substantial. To prevent leakage current from affecting the internal insulation measurement, a metal guard ring is typically added to the cable's outer surface and connected to the G terminal of the megohmmeter.
When using a megohmmeter to measure the insulation resistance of electrical equipment, it is crucial to ensure that the L and E terminals are not reversed. The correct connection is as follows: connect the L terminal to the conductor of the equipment under test, connect the E terminal to the equipment casing grounded to the ground, and connect the G shield terminal to the insulating part of the equipment under test. If L and E are reversed, leakage current flowing through the insulation and its surface will collect at ground level, then flow through L into the measuring coil, causing G to lose its shielding effect and introducing significant measurement errors. Furthermore, because the insulation between the internal lead of the E terminal and the casing is lower than that between the L terminal and the casing, when the megohmmeter is placed on the ground and the correct wiring method is used, the insulation resistance between the E terminal and the instrument casing, and between the casing and ground, is equivalent to a short circuit, causing no error. However, when L and E are reversed, the insulation resistance between E and ground is connected in parallel with the insulation resistance being measured, resulting in a lower measurement result and a larger measurement error.
Therefore, in order to accurately measure the insulation resistance of electrical equipment, the megohmmeter must be used correctly; otherwise, the accuracy and reliability of the measurement will be lost.
Instructions and requirements for using an insulation resistance meter
1. Before measurement, the megohmmeter should be kept horizontal. Hold the meter body with your left hand and turn the megohmmeter handle with your right hand at a speed of about 120 r/min. The pointer should point to infinity (∞). Otherwise, it indicates that the megohmmeter is faulty.
2. Before measurement, the power supply to the electrical appliance and circuit under test should be disconnected, and the relevant components should be temporarily grounded to ensure the safety of personnel and the megohmmeter and the accuracy of the measurement results.
3. Correct wiring is essential during measurement. The megohmmeter has three terminals (L, E, G). When measuring the circuit resistance to ground, connect terminal L to the exposed conductor of the circuit, and terminal E to the grounding wire or metal casing. When measuring the insulation resistance of a circuit, connect the beginning and end of the circuit to L and E respectively. When measuring the insulation resistance of a cable, to prevent leakage current from the cable surface from affecting measurement accuracy, the cable's shield should be connected to terminal G.
4. The insulation of the measuring cord leading out of the megohmmeter terminals should be good, and an appropriate distance should be maintained between the two cords and between the cord and ground to avoid affecting the measurement accuracy.
5. When shaking the megohmmeter, do not touch the terminals of the megohmmeter or the circuit being tested with your hands to prevent electric shock. 6. After shaking the megohmmeter, do not short-circuit the terminals to avoid damage.
