Megger Test Performance

Why Do We Use Megger Tester?

A Megger insulation resistance tester is especially useful when you have executed electrical work and it is preferable to be sure if there is any damage insulation or wiring mistake. The Megger company registered the name Megger in May 1903 and today offers a wide variety of IR testers. Failed or damaged insulation can cause a circuit breaker to trip or fuse to blow and may result in a severe electrical shock to people coming in contact with poorly insulated wire or a metal part touching it. Using a Megger to test insulation resistance helps reduce such dangers.

Megger Test Step 1 :

• De-energize the circuits that will be tested. If you are not sure which breakers or fuses control the circuits, switch off the main breaker.

• Put a note on the breaker for people to be noticed and lock the switch closed if possible

Megger Test Step 2 :

• Prepare a table in which you can record the insulation values that result from your tests.

a) For a 110 volt circuit, leave spaces for the test results of the insulation between the black wire and the white wire and the insulation between each wire and ground.

b) For a 220 volt circuit, leave spaces for the test results of the insulation between each of the three wires and the other two and between each of the three wires and ground.

c) For a piece of equipment such as a circuit breaker or an appliance, leave space for insulation values between each terminal and the other terminals and each terminal and ground

Megger Test Step 3 :

• Select 500 volts DC or 1000 volts DC as the test voltage on your Megger, depending on the model you are using. Check whether your model has an integrated voltage tester for live circuits. If it doesn’t, check the circuits to be tested with a voltage tester to make sure they are not live.

• Place the positive and negative probes of the Megger on the two conductors or terminals between which you are testing the insulation resistance.

• If you are testing insulation resistance to 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.

Continue with this testing procedure until you have values for all the spaces of your table.

Megger Test Step 4 :

• Examine the resistance values you have entered in your table. According to the National Electrical Code, all values should be over 25 megohm.

• If one of the values differs substantially from all the others, check your connections and repeat the tests.

• If a value is below 25 megohm, check the circuit for the cause of the poor insulation resistance value.

The procedure of insulation resistance test or megger test for HV-LV Transformer:

• First we will disconnect all the line and neutral terminals of the transformer.

• Megger leads are connected to LV and HV bushing studs to measure Insulation Resistance IR value in between the LV and HV windings. 

• Megger leads are connected to HV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the HV windings and earth. 

• Megger leads are connected to LV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the LV windings and earth.

The empirical relation as below gives the recommended minimum value for IR, its unit being mega ohm (MΩ). . The value measures gives us the idea about the insulation strength of the cable and whether it is deteriorated or not.

IRmin (in MΩ) = kV + 1

Where kV = rated service voltage in kV

There are instances when the measured IR is almost 10 to 100 times more than the IRmin found from above equation.

General measurement procedure consists of measuring IR between the three phases and also between individual phase and earth. IR is also measured for equipment body. The procedure varying from equipment to equipment. There are different voltage levels which are applied to cable based on their rating and size. For doing a Megger of a 33kV HT cable. The voltage level applied is of 5000V and the value of IR can be anywhere between 1GigaOhm to 200GigaOhm.

When we are using a multimeter, we are measuring a resistance, voltage, and current.  Based on this, I hope we are familiar with the term Insulation. This means that a current cannot pass or leak through a certain conducting wire when it is properly insulated or protected. These wires may be inside a building, appliances or an electric motor.

You are basically testing the resistance on a wire. For instance if you want to see if a motor is bad, you will “megger” it, testing each of the three phases of motor to ground and to each other to see if it is shorted to either ground or itself.

Working Principle of Megger

• Voltage for testing produced by hand operated megger by rotation of crank in case of hand operated type, a battery is used for electronic tester.

• 500 Volt DC is sufficient for performing test on equipment range up to 440 Volts.

• 1000 V to 5000 V is used for testing for high voltage electrical systems.

• circuit pointer Deflecting coil or current coil connected in series and allows flowing the electric current taken by the circuit being tested.

• The control coil also known as pressure coil is connected across the circuit.

• Current limiting resistor (CCR and PCR) connected in series with control and deflecting coil to protect damage in case of very low resistance in external circuit.

• In hand operated megger electromagnetic induction effect is used to produce the test voltage i.e. armature arranges to move in permanent magnetic field or vice versa.

• Where as in electronic type megger battery are used to produce the testing voltage.

• As the voltage increases in external circuit the deflection of pointer increases and deflection of pointer decreases with an increases of current.

• Hence, resultant torque is directly proportional to voltage and inversely proportional to current.

• When electrical circuit being tested is open, torque due to voltage coil will be maximum and pointer shows ‘infinity’ means no shorting throughout the circuit and has maximum resistance within the circuit under test.

• If there is short shows ‘zero’, which means ‘NO’ resistance within circuit being tested.

Where as in electronic type megger battery are used to produce the testing voltage.

• As the voltage increases in external circuit the deflection of pointer increases and deflection of pointer decreases with an increases of current.

• Hence, resultant torque is directly proportional to voltage and inversely proportional to current.

• When electrical circuit being tested is open, torque due to voltage coil will be maximum and pointer shows ‘infinity’ means no shorting throughout the circuit and has maximum resistance within the circuit under test.

• If there is short shows ‘zero’, which means ‘NO’ resistance within circuit being tested.

Types of Megger :

This can be separated into mainly two categories:

• Electronic Type (Battery Operated)

• Manual Type (Hand Operated)

Advantages of Electronic Type Megger :

• Level of accuracy is very high.

• IR value is digital type, easy to read.

• One person can operate very easily.

• Works perfectly even at very congested space.

• Very handy and safe to use.

Advantages of Hand Operated Megger :

• Still keeps important in such high-tech world as it’s an oldest method for IR value determination.

• No external source required to operate.

• Cheaper available in market.

But there are another types of megger which is motor operated type which does not use battery to produce voltage it requires external source to rotate an electrical motor which in turn rotates the generator of the megger.

Insulation Resistance or IR Test is carried out by maintenance engineers to ensure the healthiness of overall insulation system of an electrical power transformer. It reflects the presence or absence of harmful contamination, dirt, moisture and gross degradation. IR will be usually high (several hundred Mega ohms) for a dry insulation system. Maintenance engineers use this parameter as an index of dryness of the insulation system. 

This test is performed at or above rated voltage to determine if there are low resistance paths to ground or between winding to winding as a result of winding insulation deterioration. The test measurement values are affected by variables such as temperature, humidity, test voltage, and size of transformer. 

This test should be conducted before and after repair or when maintenance is performed. The test data should be recorded for future comparative purposes. The test values should be normalized to 20°C for comparison purposes. 

The general rule of thumb that is used for acceptable values for safe energisation is 1 MΩ per 1000 V of applied test voltage plus 1 MΩ.

Megger Safety Precautions :

While using a megger, you could be injured or damage equipment you are working on if the following MINIMUM safety precautions are not observed.

• Use megger on high-resistance measurements only, such as insulation measurements or to check two separate conductors on a cable.

• Never touch the test leads while the handle is being cranked.

• De-energize and discharge the circuit completely before connecting a megger.

• Disconnect the item being checked from other circuitry, if possible, before using a megger.

Benefits of Megger Testing :

• Proactive Equipment Condition Analysis

• Reduced Risk of Emergency Power System Failure

• Insured Availability

• Predictive Repairs

• Asset Management

• Predictive Equipment Life Expectancy

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