Megohm Test: A Measure of Insulation Strength

About the megohm test, every electrical system involves the flowing of current through conducting wires. The insulation of wires prevents electric current from flowing in any non-desired places. As a result, the current reaches its desired destination without any leakage. 

However, with time, some flaws develop in the insulation. Environmental factors like temperature, humidity, moisture, and dust particles degrade insulation resistance quality. As a result, the current manages to escape. This can damage your machines and electric circuits. Also, it can lead to fatal human injuries and deaths. 

So, how can you prevent this escape? Well, insulation testing is the solution.

What is a Megohm or Megger test?

This technique allows you to monitor and determine the electrical insulation integrity of any electric system, so that you can take preventive measures for any insulation degradation.

In this test, you use equipment named megohmmeter. And the insulation testers can easily determine the insulation condition on the wire, generators, and motor windings. 

This instrument allows the passing of high direct current (DC) voltage between one or other conductors. Since you are testing wire insulation integrity, no current should flow between the conductors. Thus, you expect a high resistance value of around 35-100meghoms.

The megohmmeter is mainly of two types:

• Hand operated- It has an inbuilt DC generator that works without any external power source. If you are using manual-type meggers, there will be a crank handle to generate the voltage. Additionally, you can control the voltage with the help of a clutch mechanism.
• Electronic megger: It is a compact device that uses a battery for operation.

Image: electronic megohmmeter

Construction of megger:

• Control coil and deflecting coil: Every megger has these two coils. And these two coils are at right angles to each other. However, they are parallel to the generator. The torque in these coils takes them in the opposite direction due to their reverse polarities.
• Permanent magnet: There is a permanent magnet with north and south poles. This magnet produces a magnetic effect.
• Pointer: The pointer is attached to the coils. The end of the pointer lies on a scale. The scale has a value ranging from “0” to infinity. The values are in Ohms.
• DC generator or battery: With this, you give the testing voltage to the circuit.
• Pressure coil and current coil: This is to prevent any excess current in case of low resistance from an external source.

Image: electric circuit of megohmmeter

Source: instrumentationtools.com 

Testing of megger:

To check whether your megger is working right, follow these steps.

First,do not connect anything across test terminals. It means it is an open circuit. Now when you operate the generator, the needle will move to infinity. 

Secondly, connect the test terminals, the pointer will deflect to zero. 

Working of megger:

For equipment ranging up to 440 Volts, you must use a test volt of 500VDC. On the other hand, for high voltage electrical systems, make use of 1000V-5000V of test voltage. 

First, connect one test terminal of Megger with the body of the electric equipment. Now, connect the other test terminal to one of the input terminals of that equipment. Finally, apply the DC test voltage. 

If there is no fault between the current-carrying conductor and body (which means good insulation strength), the needle will deflect towards very high resistance. On the other hand, if the insulation has leakage,  the needle will deflect to low or near-zero resistance.  

In a 3-phase machine, you can also connect the test terminals between two phases to check insulation health between them.

Insulation resistance test methods

You can do different insulation resistance tests with a megohmmeter.

Spot-reading test: 

You can use this method for testing house wiring or any electric panel with short capacitance. Temperature and humidity may affect the readings of this test. To perform this test, just connect your megger across the testing insulation. Now, apply the required voltage for a short period (approx. 60 seconds).

You must record the measurements on a regular basis. If there is a downward trend in resistance, consider it as a warning for future troubles. On the other hand, if the periodic readings are consistent, the insulation condition is good to go.

Image: insulation resistance with time in spot-resistance testing

Source: https://electrical-engineering-portal.com/

Time resistance method:

In this, you need not take the past tests to get conclusive information. This test is independent of temperature.  The test gives you a clear picture as it works on the absorption effect of insulation. Good insulation will have a better absorption effect as compared to moist ones. This is the reason; you can also call it an “Absorption test”.

To take the test, first, connect the megohm meter to the testing device for a short time. Secondly, take readings at regular intervals. Finally, note the difference in readings. 

Absorption current starts high and decreases gradually with the increase in the voltage. When a machine has healthy insulation, this trend continues. Thus, it shows a rising level of resistance. On the other hand, when insulation is poor, the graph flattens after an initial hike.

You can perform this test on transformers, bushings, cables, and motors. However, for such equipment, you need high insulation strength and consistent test voltage. Thus, you must take a line-operated megohmmeter for this testing. 

Image: curve showing dielectric absorption effect in time-resistance test

Source: https://electrical-engineering-portal.com/

Dielectric Absorption Ratio or Polarisation Index:

In time-resistance testing, you take readings at regular intervals. The ratio of 60 second reading to 30-second reading is a dielectric absorption ratio. On the other hand, the polarization index is the ratio of 10-minute reading and 1-minute reading.

With these values, you can determine insulation quality in a better way. 

For the dielectric absorption ratio, first, take a hand-operated device. Then, run the test for one minute. Finally, take readings at 60 seconds and 30 seconds. 

For the polarisation index, it is advisable to use a line-operated test instrument.  First, run the test for 10 minutes. Then, take readings at 1-minute and 10-minute intervals. Obtain a polarisation index and compare the results with the table given below.

Condition of insulation	Dielectric absorption ratio (60sec/30sec ratio)	Polarisation index (10minute/1 minute ratio)
Dangerous	—	Below 1.00
Poor	1.0-1.25	1.00-2.00
Good	1.4-1.6	2.00-4.00
Excellent	Above 1.6	Above 4.00

Note: Remember that these readings are relative. You can expect a change in them with the changing environmental conditions. For more accurate results, you need to take basic maintenance measures.

According to NETA/ANSI acceptance and maintenance standards, you must investigate a polarisation index value of less than one.

Preparing the Apparatus for Testing

Before you perform a megohm test, you must prepare the apparatus to avoid any errors in readings. Read this to-do list before performing the megger test.

Take out of service:

First of all, switch off the apparatus and open its switches. Now, disconnect it with any other equipment and circuits. This includes neutral and protective ground connections also.

Check what you want to include in the test:

When you connect any apparatus to the megger test, check what you need to test. Sometimes, extra equipment, circuits, and conductors attached with the equipment can lower the true insulation resistance readings.

Connecting and disconnecting linked equipment to apparatus is a difficult task. For this, it is better that you take one insulation reading with all the parts attached. Next, separate all the components. Now test each component separately. If you record this information, things will get easier in the future. For future tests, you need not separate the components unless very low readings are observed for the apparatus as a whole.

Discharge all the capacitance: 

Make sure that you discharge the capacitance of the apparatus. You must do it before and after the insulation strength test. The discharge period must be 4 times the time you gave voltage in the last test. Most of the megger instruments come with discharge circuits to discharge capacitance. If not, use a discharge stick to do this.

Avoid any current leakages:

When you shut down an apparatus, there can be some leakage through switches and fuse blocks. Such leakage can affect the real insulation resistance readings. Make sure you control this leakage through the guard terminal.

Apart from this, sometimes, there is current leakage into the apparatus from an energized line. This leakage results in inconsistent readings. You can detect this leakage easily. Just connect the apparatus to the megger but do not start it. Now, note the megger pointer. The deflection of the needle at this point will indicate leakage current. Never conduct a megger test on energized apparatus.

Connection schemes for testing insulation resistance of different electrical equipment:

You must know how to connect and disconnect a megger test set with any equipment. Here is the information regarding megger testing with different electrical instruments.

AC motors and starting equipment:

To test AC motors, connect one lead of Megger to the main switch of the motor and the other with the body. Next, keep the switch at the “ON” position and then apply the test voltage. To determine the exact point of insulation weakness, you must disconnect the component parts. Now, test these parts separately.

DC Generators/Motors:

To test the insulation strength of brushes, keep the brushes raised. Now apply the test voltage of Megger between any brush and body of the motor/generator. For testing insulation of armature, keep the brush in touch with the armature. 

Wiring installation:

For this, connect one lead of megger to earth. Now connect another lead with the wiring circuit at the distribution panel and apply the test voltage.

Appliances, Meters, Instruments, and other electrical apparatus:

In any appliance, you mainly check the current leak between the motor and exposed metal parts. So, first, disconnect the instrument from any power source. Place it on an insulating board. Now, connect the megger test set to check insulation resistance.

Power cables:

For cables, you perform a continuity test. Make sure that the current you send is within the capacity of the cable. First, connect the line terminal of the megohm meter with the conductor. Secondly, connect the guard terminal of the megger with insulation and ground the earth terminal. Finally, when the current passes through the cable, note the resistance. Maintain the contact for 30 seconds to 60 seconds.  

Image: megger test for cables

Source: https://www.elprocus.com/

Power transformers: You can conduct a megger test in a transformer to know if there is any flux leakage. For this, remove all the terminal connections. Connect the low voltage and high voltage bushing studs of the transformer with megger terminals. This gives you IR values between LV and HV. 

Now connect the megger leads with a high voltage bushing stud of the transformer and earth terminals. Note the reading between high voltage and ground. 

Now repeat the same procedure with low voltage and ground. Note the megohm readings. AC Generators: Connect one terminal of the megohm meter with earth and the other terminal with the input of the circuit breaker. Now, apply the test voltage.  It will show the winding insulation resistance.

Conclusion:

Wiring insulation is of utmost importance when it comes to the safety of electrical equipment. The routine maintenance of the electrical equipment is important. And megger testing is a good option for the same. To ward off any insulation leakage issue, you must use high-quality wire and cable assemblies. We can help you with the best quality cable assemblies and electrical harnesses. Contact us with your queries and we will be happy to serve you.