What is a MeggerTest for Cable and Transformer

Megger test is also called as Insulation Resistance testing (IRT) or Portable Appliance Testing (PAT). PAT testing is specific to the UK, Australia, and some parts of Europe, where appliance testing is done in public areas like hotels, homes, hospitals, shops for testing electrical equipment to protect from damage. The main concept of the insulation test is completely based on Insulation testing. Megger testing comes from import Mega-Ohm, which is the measurement of results to look for an insulation test. Also, the company Megger provides test equipment for insulation testing; testing is performed on some electrical wire or electrical equipment.

What is the Need for Megger Test?

All electrical systems used in various areas like industries, hospitals, homes, automobiles, etc, are interconnected using electrical wires. One must ensure that the connections are made appropriately using electrical wires for good insulation in order to protect from any internal or external damage of electrical systems.

To check whether connections are made perfectly, we use an electrical instrument named megger. In an insulation testing, we send a test voltage down through an electrical system, to check if there is any leakage of current which is gone to be passed through the insulated wiring of all the appliances of the machine. To do that, we send a higher voltage than standard in most cases to pressure test the wires and to see how well they behave.

A good analogy in day to day life is plumbing in our home and water, where the plumber would check if there is any water leakage while bolding the house. For example, if we consider a water pipe with a pressure gauge on it and the pump at the end. The pipe is closed on the other side.

Case(i): A plumber wants to show that it’s not leaking, he will apply mini pressure to the piping. The piping might be fine, which means no leakage.

Case(ii): If he applies larger pressure, in order to know the defects. If there is a small hole in the pipe and water that is flowing inside the pipe, there will be some leakage. This means there is a loss of water, and there is a deflection of the pointer in pressure gauge, based on this he can detect and fix the problem. In the same way, if we do a pressure test in the electrical system, we apply high voltage & a pressure gauge is used to measure the resistance where we follow Ohm’s law.

What is Good Insulation?

According to ohm’s law

      V = IR       …… (1)

V =Voltage; I = Flowing Current; R= Resistance.

      R =V/I     …… (2)

If we consider voltage “V” to be constant, and “I” varies then “R” varies.

Similar to an example of plumbing in wire

Where insulation = pipe carrying water

Case (i): If we have a little current flow while applying a constant voltage, then the resistance will drop.

Case (ii): If we have no current flow, resistance will be high. Alternatively, when we do pressure testing in electrical systems, a higher value of resistance has to be maintained. Which is a condition for good insulation?

What is Megger Testing?

Megger is an electrical instrument that is used for testing insulation resistance and machine windings to safeguard all electrical equipment from major damage.

Megger Test Procedure

To measure electrical leakage in wire, we pass the current through devices then; we verify the electrical insulation level in any device such as a motor, cable, and transformer. The result of this can be measured in terms of Mega ohm.

Working of Megger

• For electrical systems whose voltage is high, it requires 1000V to 5000V for testing.
• Connection of deflecting coil must be in series, so that the current that flows, test the circuit.
• This circuit is connected with a PC coil across it.
• To protect the circuit, two resistors (Current Coil Resistor& Pressure Coil Resistor) are connected in series and also
• using two coils like a control coil &deflecting coil.
• In a manually operated megger, a test voltage is generated by the EMI effect.
• When the voltage increases, then the deflection pointer shows to infinity. Similarly, if current increases deflection
• the pointer shows zero.

Therefore

Torque α Voltage,     …..( 3 )

Torque α 1/Current.    ….( 4 )

If a short circuit, the pointer indicates 0 reading.

Megger Test for Cables

Insulation resistance test for cable using megger is a continuity test, where the power of the circuit is off.

For example, if the cable is of the capacity of 5Amps, we can send current less than or equal to 5Amps and not more than that. If we send more than 5Amps this may cause the failure of the cable. Hence we do an insulation resistance test, to know how much resistance it can withhold. Insulation resistance is always measured in Mega Ohms. The device used to measure IR is known as Megger.

An application of this cable is power systems, where we do the IR test for proper maintenance of the system. So that we can know IR value for better performance.

Construction

Megger is a DC generator. It consists of three terminals

• Line terminal,
• Guard terminal,
• and the earth terminal.

In the above circuit, the guard is connected on top of an insulator, the line terminal is connected to the conductor, which is to be tested, and the earth pin is grounded.

Higher Resistance = Higher Insulation = No current flow.

Steps

• Connect the circuit as described above.
• Press the test button on megger, the megger will generate current.
• This current flows through cable, the resistance in scale is noted which ranges between 35 to 100 Mega Ohms.
• Note to maintain this contact for 30 to 60 seconds.
• Acceptable IR for electrical cable = 1 Mega Ohm for 1000 V.

Result

If the range shown is between 35 to 100 Mega Ohm’s, it means it’s a good insulator.

Megger Test for Transformer

A transformer is an electrical device, which is based on the principle of mutual induction. IR test is performed to ensure that there is no flux leakage in the transformer.

Working Procedure

Following are the steps for insulation testing the Transformer,

• Step 1: Remove all the terminal connections.
• Step 2: Connection between two terminals of megger and LV- low voltage and HV – high voltage bushing studs of the transformer are made. So that we can note down the IR values ranges between LV – HV.
• Step 3: Connection between megger leads to transformers high voltage bushing stud and transformers earth terminal are made. IR is measured between transformer windings of Hight Voltage -ground.
• Step 4: When leads of megger are connected to transformers Low Voltage bushing stud and transformers earth terminal. IR is measured between Low Voltage windings – ground.

Result

• IR values are noted down at every 10 sec, 15 seconds, and 1minute.
• When the applied voltage increases, the insulation resistance value also increases.
• The coefficient of Absorption is given as 1 min value / 15sec.
• Polarization of index is 10 min value / 1 min value

Safety Measures while Doing Megger Test

• Use megger for higher resistance
• While the device is on testing mode, do not touch the leads.
• Make sure that the electrical system is disconnected before the megger is connected.

Advantages of Megger Test

The benefits of megger instrument for testing applications are

• Emergency failure in the power system can be reduced
• Repairs can be predicted in advance
• Predictivity leads to more life extension of the electrical system, which is being tested.

Here, we have described why do we do insulation resistance or megger testing for electrical systems, and also we have seen insulation resistance or megger testing procedure and result done on cable and transformer, along with precautions and advantages. Here is a question Why don’t we use a multimeter for testing the electrical system, instead of megger?