The Megger test or insulation resistance test is a method of testing making use of an insulation tester resistance meter that will help to verify the condition of electrical insulation. Megger tester is being used since 1889, popularity raised during 1920s, since long back devise is same in its uses and purpose of testing, few real improvement appeared in recent years with its design and quality of tester. Now high quality option are available which are easy to use and quite safe.
Megger Testing is a spot insulation test which uses an applied DC voltage, either 250Vdc, 500Vdc 1,000Vdc (low voltage equipment) or <600V, 2,500Vdc and 5,000Vdc (high voltage equipment) to measure insulation resistance in either kΩ, MΩ or GΩ. The measured resistance is intended to indicate the condition of the insulation or dielectric between two conductive parts, where the higher the resistance, the better the condition of the insulation. Ideally, the insulation resistance would be infinite, but as no insulators are perfect, leakage currents through the dielectric will ensure that a finite (though high) resistance value is measured.
Why Megger Testing is Done?
Insulation resistance quality of an electrical system degrades with time, environment condition i.e. temperature, humidity, moisture and dust particles. It also get impacted negatively due to the presence of electrical and mechanical stress, so it’s become very necessary to check the IR (Insulation resistance) of equipment at a constant regular interval to avoid any measure fatal or electrical shock.
Another scenario is if your home has just experienced a fire and you need to start the rebuilding process. Your Insurance Company tells you that local jurisdiction, or the Insurance Company themselves, require a “Megger Test” to verify the integrity of the wiring system in your home. When it comes to megger testing Malcots is the way to go.
Megger testing does not cause any damage, making it a good option when someone does not want to put holes in walls to test electrical insulation for any problems or issues. The testing device only goes between 500 and 1,000 volts, which is relatively low. Due to the low voltage, some punctures in insulation go undetected. It generally provides information about the leakage current and whether insulation areas have excessive dirt or moisture as well as the amount of moisture, deterioration and winding faults.
What is Done During Megger Testing?
We can test your circuits for the existing connections or melted fault areas that may have occurred during a fire event. These results are then analysed and specific circuits can be isolated and replaced to insure that there are no further problems on your affected circuits. If you have had a fire talk to your Adjuster and see if Megger Testing is required. It is normally covered by Insurance since the last thing they want to do is pay another claim a month after you are able to reclaim your domicile.
Malcots has the equipment on hand and the expertise to perform your Megger Testing and get those results logged and filed with your insurance carrier as well as the Local Building Department. We are here to help you make sure your existing wiring is safe, and of course, to install new wiring as needed as well. We are here for all of your electrical needs.
How Megger Testing is Performed?
A multimeter is used as an insulation tester in some conditions and mostly a continuity test only is performed. But for detecting and testing for a leakage current during a normal or overloaded condition, a special instrument known as the Insulation tester is used.
We measure electrical leakage in wire and results are very reliable as we shall be passing electric current through device while we are testing. We verify the electrical insulation level of any device such as motor, cable, generator winding, or general electrical installation. This is a very poplar test being carried out since very long back. Not necessary it shows us exact area of electrical puncture but shows the amount of leakage current and level of moisture within electrical equipment/winding/system.
Following equation is used for calculation:
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.
The procedure of insulation resistance test or megger test are given below:
- 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.
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.
- 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 circuit pointer shows ‘zero’, which means ‘NO’ resistance within circuit being tested.
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
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.