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What Is the Insulation Resistance Meter and How to Test It?

Author: Apogeeweb
Date: 28 Nov 2018
 6442

Warm hints: This article contains about 3000 words and reading time is about 15 min.

Introduction

The insulation resistance meter is a special instrument for measuring the maximum resistance value, insulation resistance, absorption ratio and polarization index. Its scale unit is megger, which has a high voltage power supply. The insulation performance of electrical products is one of the important signs to evaluate its insulation, which is reflected by the insulation resistance.

 

Catalog

Introduction

Ⅰ Brief Introduction of Insulated Resistor Meter

Ⅱ Structure and Composition of Insulation Resistance Meter

  2.1 DC High Voltage Generator

  2.2 Measurement Circuit

Ⅲ Preparations for the Use of Insulated Resistor Meters

Ⅳ How to Use Megger and Its Requirements

Ⅴ Selection Requirements and Pre-use Inspection of Megger

Ⅵ Wiring and Measurement

Ⅶ FAQ

 


Brief Introduction of Insulated Resistor Meter

The insulation resistance meter is also called megger, shaker, and tramegger. The insulation resistance meter is mainly composed of three parts. The first is a DC high voltage generator to generate a constant current high voltage. The second is the measurement loop. The third is the display.

 

The insulation resistance meter is a special instrument for measuring the maximum resistance value, insulation resistance, absorption ratio and polarization index. Its scale unit is megaohm, which itself has a high voltage power supply. The insulation performance of electrical products is one of the important signs to evaluate its insulation, which is reflected by the insulation resistance. We determine the insulation resistance of the product, which refers to the insulation resistance between the live part and the exposed non-charged metal part (outer case).

Isulation resistance meter

According to different products, apply high-current high-voltage, such as 100V, 250V, 500V, 1000V, etc., to specify a minimum Insulation resistance value. Some standards stipulate that the insulation resistance is not less than 1MΩ per kV voltage. In the household appliance product standard, only the thermal insulation resistance is usually specified, and the insulation resistance value under normal conditions is not specified. The insulation resistance value under normal conditions is determined by the enterprise standard.

 

If the normal insulation resistance value is low, there may be some hidden danger or damage in the insulation structure. If the insulation resistance of the motor winding to the outer casing is low, it may be caused by damage to the insulation of the winding during the weld. When using an appliance, the circuit generates an overvoltage due to sudden power-on or power-off or other reasons, causing breakdown at the insulation damage, resulting in safety or life-threatening to the human body.

 

 


Structure and Composition of Insulation Resistance Meter

2.1 DC High Voltage Generator

Measuring the insulation resistance must apply a high voltage at the measuring end. This high voltage value is specified in the national standard of the insulation resistance meter as 50V, 100V, 250V, 500V, 1000V, 2500V, 5000V...

 

There are generally three methods for generating DC high voltage. The first type of hand-cranked generator. At present, about 80% of the megger produced in China is using this method (the source of the name of the shaker). The second is to boost the DC voltage through the mains transformer. The method used in general utility meters. The third is a method of generating a DC high voltage using a transistor oscillating or dedicated pulse width modulation circuit, a general battery type and a commercial type insulation resistance meter.

5.jpg

 


2.2 Measurement Circuit

In the shake table (Megger) mentioned above, the combination of the measurement loop and the display section is one. It has a flow ratio meter head. The head has two coils with an angle of 60° (left and right). One coil is at both ends of the voltage and the other coil is in the measuring loop. middle. The deflection angle of the head pointer is determined by the current ratio in the two coils. Different deflection angles represent different resistance values. The smaller the measured resistance value, the larger the coil current in the measurement loop, and the greater the angle of deflection of the pointer.

 

Another method is to use a linear ammeter as the measurement and display. The flow ratio used in the front of the meter is non-uniform due to the magnetic field in the coil. When the pointer is at infinity, the current coil is just where the magnetic flux density is the strongest. Therefore, although the measured resistance is large, the current coil current flows. Rarely, the deflection angle of the coil will be large at this time. When the measured resistance is small or 0, the current flowing through the current coil is large, and the coil has been deflected to a place where the magnetic flux density is small, and the deflection angle caused by the coil is not large. This achieves a nonlinear correction.

 

The resistance display of a typical megger header needs to span several orders of magnitude. However, when the linear current meter is directly connected to the measuring circuit, the scales at the high resistance value are all squeezed together and cannot be resolved. In order to achieve nonlinear correction, nonlinear components must be added to the measuring circuit. Thereby, a shunting effect is achieved at a small resistance value. At high resistance, no shunting occurs, resulting in resistance values that are orders of magnitude.

 

With the development of electronic technology and computer technology, digital meters have gradually replaced pointer meters. The digital measurement technology of insulation resistance has also been developed. Among them, the voltage ratio meter circuit is one of the better measurement circuits. The voltage ratio meter circuit is composed of a voltage bridge and a measurement bridge. The signals output by these two bridges are directly converted into digital value display by A/D conversion and then processed by the single-chip microcomputer.

 

Isulation resistance meter-measurement circuit

 


Preparations for the Use of Insulated Resistor Meters

When the megger is working, it generates high voltage itself, and the measuring object is electrical equipment, so it must be used correctly, otherwise, it will cause personal or equipment accidents. Before using, you must first make the following preparations:

(1) Before the measurement, the power of the device under test must be cut off, and the ground should be short-circuited. The device must not be energized for measurement to ensure the safety of the person and equipment.

 

(2) For equipment that may induce a high voltage, the possibility must be eliminated before measurement can be made.

 

(3) The surface of the object to be tested should be cleaned to reduce the contact resistance and ensure the correctness of the measurement results.

 

(4) Before the measurement, check whether the megger is in normal working condition, mainly check the two points of “0” and “∞”. That is, the handle is shaken to make the motor reach the rated speed. The megger should be in the “0” position when it is short-circuited and “∞” when it is open.

 

(5) The megger should be placed in a stable and stable place and away from large external current conductors and external magnetic fields.

Isulation resistance meter-the use

After the above preparations are made, the measurement can be carried out. When measuring, pay attention to the correct wiring of the megger, otherwise, it will cause unnecessary errors or even errors.

 

There are three terminals of the megger: one is "L", that is, the line end, one "E" is the ground end, and another "G" is the shield end (also called the protection ring). Generally, the insulation resistances to be tested are connected. Between the "L" and "E" ends, but when the surface of the insulator to be tested is seriously leaky, the shield ring of the test object or the portion not to be measured must be connected to the "G" end. Thus, the leakage current flows directly back to the negative end of the generator via the shield end "G" to form a loop, rather than flowing through the megger measuring mechanism (moving coil). This fundamentally eliminates the influence of surface leakage current.

 

In particular, 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 because the air humidity is high or the cable is insulated. When the surface is not clean, the leakage current on the surface will be very large. In order to prevent the measured object from being affected by leakage, its internal insulation measurement is generally added to the outer surface of the cable with a metal shielding ring and the megger of the megger end connected.

 

When using the megger to measure the insulation resistance of electrical equipment, be sure to note that the "L" and "E" ends cannot be reversed. The correct connection is: the "L" line end button is connected to the conductor of the device under test, "E" The grounding device is grounded, and the "G" shield terminates the insulated part of the device under test. If the "L" and "E" are reversed, the leakage current flowing through the insulator and the surface is collected to the ground through the outer casing, and the ground flows through the "L" into the measuring coil, so that the "G" loses the shielding effect and gives the measuring tape. There is a big error.

 

In addition, because the inner lead of the "E" end is insulated from the outer casing by a degree of insulation lower than the "L" end and the outer casing when the megger is placed on the ground when the correct wiring is used, the "E" end is opposite to the outer casing of the instrument. The insulation resistance of the shell to the ground is equivalent to a short circuit and does not cause an error. When the "L" and the "E" are reversed, the insulation resistance of the "E" to the ground is in parallel with the measured insulation resistance, and the measurement result is biased. Small, causing large errors in the measurement.

 

It can be seen that in order to accurately measure the insulation resistance of electrical equipment, etc., the megger must be used correctly, otherwise, the accuracy and reliability of the measurement will be lost.

4.jpg

 


How to Use Megger and Its Requirements

1, The megger must be placed horizontally in a stable and stable place to avoid measurement errors caused by jitter and tilt when shaking.

 

2, The wiring must be correct, and the megger has three wiring posts, "E" (ground), "L" (line) and "G" (protective ring or shield terminal). The function of the guard ring is to eliminate the leakage between the "L" and "E" terminal blocks on the surface of the case and the leakage of the surface of the tested insulation.

 

When measuring the insulation resistance of the electrical equipment to the ground, the "L" is connected to the part to be tested with a single wire, and the "E" is connected to the equipment casing by a single wire; if the insulation resistance between the two windings in the electrical equipment is measured, "L" and "E" are connected to the terminals of the two windings respectively; when measuring the insulation resistance of the cable, in order to eliminate the error caused by surface leakage, the "L" terminal, the "E" connector, the "G" terminal and an insulating layer between the outer casings.

Isulation resistance meter-how to use

The connecting line of "L", "E", "G" and the object to be tested must be a single wire, which is well insulated and must not be twisted. The surface must not be in contact with the object to be measured.

 

3, The rotation speed of the rocking handle should be uniform, generally specified as 120 rpm, which is allowed to vary by ±20%, and should not exceed ±25%. Usually shake for a minute, and wait until the pointer stabilizes and then read. If there is a capacitor in the circuit under test, first keep shaking for a period of time, let the megger charge the capacitor, and then read the pointer after the indicator is stable. After the measurement, remove the wiring and stop shaking. If the pointer is found to be zero during the measurement, stop shaking the handle immediately.

 

4, After the measurement, we should discharge the device entirely, otherwise, it’s easy to cause accidents.

 

5, It is forbidden to measure the insulation resistance on equipment with high voltage conductors at or near lightning. It can only be measured if the device is not powered and cannot be charged by other power sources.

 

6, Before the megger stops rotating, do not touch the measuring part of the equipment or the megger wiring pile by hand. It is also not possible to directly touch the bare part of the lead when disconnecting the wire.

 

7, The megger should be regularly verified. The calibration method is to directly measure the standard resistance with a certain value and check whether the measurement error is within the allowable range.

 


Selection Requirements and Pre-use Inspection of Megger

(1) The voltage level of the meter should be selected according to the rated voltage at the time of operation of the electrical component under test. When measuring the insulation resistance of a thermal element embedded in the winding and other heating elements, a 250V insulation resistance meter should be used.

 

(2) Before use, check whether the watch and its lead wire are normal. Short-circuit the two lead wires, shake the meter or turn on the meter power switch to enter the measurement state. The pointer of the meter is deflected to 0 or the digital indicator value is 0. Then the two lead wires are disconnected for measurement. The indication value is ∞, then Description is normal.

 

7.png

 


Ⅵ Wiring and Measurement

(1) When measuring general electrical appliances such as motors, the L end of the meter is connected to the component under test (such as the winding), and the E end is connected to the casing; when measuring the cable, in addition to the above provisions, the G end of the meter should be The sheath of the cable under test is connected. When using a hand-operated megger, the speed of the hand should be around 120r/min, and shake it until the indicated value is stable.

 

(2) After the measurement, the conductor is placed between the device under test (for example, the winding) and the casing, and then the lead wire is removed. Direct disconnection has the potential to be stored by a charged shock.

 

3.jpg

 


Ⅶ FAQ

1. What is the minimum acceptable value of insulation resistance?

Insulation resistance should be approximately one megohm for every 1,000 volts of operating voltage, with a minimum value of one megohm. For example, a motor rated at 2,400 volts should have a minimum insulation resistance of 2.4 megohms.

 

2. How do you calculate insulation resistance?

We should all be familiar with Ohm's law. If we apply a voltage across a resistor and then measured the consequential current flow, we can then use the formula R=U/I, (where U=Voltage, I=Current and R=Resistance) to calculate the resistance of the insulation.

 

3. Can a Megger damaged insulation?

Using a megger meter is not destructive. When insulation has been damaged, a megger tester is used in determining the cause by testing. The test is typically conducted at rates that are low as compared to the insulation. The insulation tests may range from as low as 40 V dc to 10 kV.

 

4. How do you test Megger Insulation Resistance?

If you are testing insulation resistance to the ground, place the positive probe on the ground wire or the grounded metal junction box and the negative probe on the conductor or terminal. Energize the Megger for 1 minute. Read the value of the resistance at the end of the minute test and note it in your table.

 

5. Why does insulation resistance increase with time?

As the voltage builds up, the absorption level in the insulation decreases. This gradual change reflects the storage of potential energy in and along with the insulation. Incidentally, absorption current is an important part of the time resistance method of insulation testing.

 

6. What is an insulation resistance meter?

Portable insulation resistance testers and megohmmeters are designed to help prevent hazards such as electric shock and short-circuits caused when the insulation in electrical devices, parts, and equipment used in industrial plants, buildings, and other settings degrades over long periods of use.

 

7. How does an insulation resistance meter work?

Insulation testers use a high voltage, low current DC charge to measure the resistance within wires and motor windings to identify current leakage and faulty or damaged insulation, which can lead to arc faults, blown circuits, and risk of electrical shock or fire.

 

8. What is a good insulation resistance value?

Insulation resistance should be approximately one megohm for every 1,000 volts of operating voltage, with a minimum value of one megohm. For example, a motor rated at 2,400 volts should have a minimum insulation resistance of 2.4 megohms.

 

9. What is the lowest acceptable insulation resistance for a cable?

1 MΩ

Perform the insulation resistance test between conductors at a reduced test voltage of 250 V DC. However, where this option is used, the minimum acceptable insulation resistance remains 1 MΩ.

 

10. What is the minimum insulation resistance of a motor?

It is recommended that the insulation resistance of the motor should at least be 1 MΩ1), and for the voltage to ground of 200 V, I0r should be 200 μA or below.

 

 


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