Contents Page
WHAT IS “Good” insulation?
What Makes Insulation Go Bad?
How Insulation Resistance is Measured
How to Interpret Resistance Readings
Good Insulation Resistance?
As you know good insulation has high resistance and poor insulation relatively low resistance. The actual resistance values can be higher or lower, depending upon such factors as the temperature or moisture content of the insulation (resistance decreases in temperature or moisture).
Good Insulation Resistance?
As you know good insulation has high resistance and poor insulation relatively low resistance. The actual resistance values can be higher or lower, depending upon such factors as the temperature or moisture content of the insulation (resistance decreases in temperature or moisture).
Remember that good insulation has high resistance; poor insulation, relatively low resistance. The actual resistance values can be higher or lower, depending upon such factors as the temperature or moisture content of the insulation (resistance decreases in temperature or moisture).
With a little record-keeping and common sense, however, you can get a good picture of the insulation condition from values that are only relative.
The Megger insulation tester is a small, portable instrument that gives you a direct reading of insulation resistance in ohms or megohms. For good insulation, the resistance usually reads in the megohm range.
The Megger insulation tester is essentially a high-range resistance meter (ohmmeter) with a built-in direct-current generator. This meter is of special construction with both current and voltage coils, enabling true ohms to be read directly, independent of the actual voltage applied.
The generator can be hand-cranked or line-operated to develop a high DC voltage which causes a small current through and over surfaces of the insulation being tested (Fig. 2). This current (usually at an applied voltage of 500 volts or more) is measured by the ohmmeter, which has an indicating scale.
Fig. 3 shows a typical scale, which reads increasing resistance values from left up to infinity, or a resistance too high to be measured.
What is “Good” Insulation?
Every electric wire in your plant – whether it’s in a motor, generator, cable, switch, transformer, etc. – is carefully covered with some form of electrical insulation. The wire itself is usually copper or aluminum, which is known to be a good conductor of the electric current that powers your equipment. The insulation must be just the opposite from a conductor: it should resist current and keep the current in its path along the conductor.
To understand insulation testing you really don’t need to go into the mathematics of electricity, but one simple equation – ohm’s law – can be very helpful in appreciating many aspects. even if you’ve been exposed to this law before, it may be a good idea to review it in the light of insulation testing.
Purpose of megger test
The purpose of insulation around a conductor is much like that of a pipe carrying water , and ohm’s law of electricity can be more easily understood by a comparison with water flow. In Figure 1 we show this comparison. Pressure on water from a pump causes flow along the pipe (Fig. 1a). If the pipe were to spring a leak, you’d waste water and lose some water pressure. With electricity, voltage is like the pump pressure, causing electricity to flow along the copper wire (Fig. 1b).
As in a water pipe, there is some resistance to flow, but it is much less along the wire than it is through the insulation.
Common sense tells us that the more voltage we have, the more current there’ll be. Also, the lower the resistance of the wire, the more current for the same voltage. Actually, this is ohm’s law, which is expressed this way in equation form:
e = I x R
where,
e = voltage in volts
I = current in amperes
R = resistance in ohms
Note, however, that no insulation is perfect (that is, has infinite resistance) so some electricity does flow along the insulation or through it to ground. Such a current may only be a millionth of an ampere (one microampere) but it is the basis of insulation testing equipment. note also that a higher voltage tends to cause more current through the insulation.
This small amount of current would not, of course, harm good insulation but would be a problem if the insulation has deteriorated. now, to sum up our answer to the question “what is ‘good’ insulation?”
We have seen that, essentially, “good” means a relatively high resistance to current. Used to describe an insulation material, “good” would also mean “the ability to keep a high resistance.” So, a suitable way of measuring resistance can tell you how “good” the insulation is. Also, if you take measurements at regular periods, you can check trends toward its deterioration (more on this later).
What Makes Insulation Go Bad?
When your plant electrical system and equipment are new, the electrical insulation should be in top notch shape. Furthermore, manufacturers of wire, cable, motors, and so on have continually improved their insulations for services in industry. nevertheless, even today, insulation is subject to many effects which can cause it to fail – mechanical damage, vibration, excessive heat or cold, dirt, oil, corrosive vapors, moisture from processes, or just the humidity on a muggy day.
In various degrees, these enemies of insulation are at work as time goes on – combined with the electrical stresses that exist. As pin holes or cracks develop, moisture and foreign matter penetrate the surfaces of the insulation, providing a low resistance path for leakage current.
Once started, the different enemies tend to aid each other, permitting excessive current through the insulation. Sometimes the drop in insulation resistance is sudden, as when equipment is flooded. Usually, however, it drops gradually, giving plenty of warning, if checked periodically. Such checks permit planned reconditioning before service failure.
If there are no checks, a motor with poor insulation, for example, may not only be dangerous to touch when voltage is applied, but also be subject to burn out. What was good insulation has become a partial conductor.
The Complete Guide to Electrical Insulation Testing is a comprehensive resource designed to provide in-depth knowledge about the principles, techniques, and importance of insulation testing in electrical systems. Electrical insulation testing is critical for ensuring the reliability and safety of electrical equipment and systems, as insulation degrades over time due to factors like environmental conditions, electrical stress, and mechanical wear.
Purpose of Electrical Insulation Testing
- Ensure Safety: Prevent electrical shocks and system failures caused by insulation breakdown.
- Detect Faults Early: Identify issues such as moisture ingress, contamination, and aging before they cause critical failures.
- Maintain Reliability: Ensure the proper functioning of equipment by verifying insulation integrity.
- Prevent Downtime: Minimize costly outages and improve operational efficiency.
Key Concepts in Insulation Testing
Insulation Resistance (IR):
- Measurement of the resistance offered by the insulation to the flow of electric current.
- High resistance indicates good insulation, while low resistance suggests deterioration or defects.
Dielectric Strength:
- The maximum voltage that an insulating material can withstand without breakdown.
Polarization Index (PI):
- The ratio of insulation resistance measured after 10 minutes to that measured after 1 minute at a constant voltage.
- Used to assess insulation aging and contamination.
Types of Electrical Insulation Testing
Spot Reading Test:
- A quick test that measures insulation resistance at a specific moment.
- Useful for routine maintenance checks.
Step Voltage Test:
- Insulation is tested by applying voltages in incremental steps to assess its ability to handle different stress levels.
Time Resistance Test:
- Measures how insulation resistance changes over time under constant voltage.
- Helps in detecting moisture and contamination.
Dielectric Absorption Test:
- Analyzes how insulation absorbs charge over time, providing insights into its health and performance.
High-Potential (Hi-Pot) Testing:
- Applies a voltage much higher than the normal operating voltage to test the insulation's ability to withstand extreme conditions.
Equipment for Insulation Testing
- Megohmmeters (Insulation Resistance Testers):
- Used for measuring insulation resistance in megohms.
- Available in analog and digital versions.
- Hipot Testers:
- Used for high-voltage tests to verify dielectric strength.
- Multimeters:
- For basic insulation resistance measurements in smaller applications.
Factors Affecting Insulation Resistance
- Temperature: Insulation resistance decreases with an increase in temperature.
- Humidity: High moisture content reduces insulation resistance.
- Contamination: Dirt, oil, and corrosive elements can degrade insulation.
- Aging: Over time, materials lose their dielectric properties due to thermal and mechanical stresses.
Steps in Conducting Insulation Testing
- Preparation:
- De-energize the equipment and ensure it is disconnected from the circuit.
- Clean the surface to remove contaminants.
- Test Procedure:
- Connect the insulation tester to the equipment following the manufacturer's guidelines.
- Apply the test voltage and record the insulation resistance.
- Interpretation of Results:
- Compare readings against equipment standards or historical data to determine insulation condition.
Interpreting Results
- High Resistance (e.g., above 1 GΩ): Indicates healthy insulation.
- Moderate Resistance: May suggest minor contamination or aging.
- Low Resistance (e.g., below 1 MΩ): Indicates possible failure due to moisture, damage, or deterioration.
Maintenance and Best Practices
- Regular Testing: Conduct routine insulation testing to monitor trends over time.
- Trend Analysis: Track insulation resistance values to predict potential failures.
- Proper Storage: Store insulation materials in dry, temperature-controlled environments to extend their lifespan.
- Use Correct Voltage: Apply the appropriate test voltage based on the equipment's specifications.
Applications of Insulation Testing
- Power Transformers: Ensure insulation between windings and the core.
- Motors and Generators: Test insulation in windings to prevent breakdowns.
- Cables: Verify insulation health before and after installation.
- Switchgear and Circuit Breakers: Assess dielectric properties to ensure safety.
Benefits of Insulation Testing
- Reduces the risk of electrical accidents and fire hazards.
- Increases equipment life by detecting and addressing insulation issues early.
- Ensures compliance with safety and industry standards.
- Enhances system reliability and operational efficiency.
By following the guidance in The Complete Guide to Electrical Insulation Testing, engineers and technicians can establish robust maintenance practices, ensuring the long-term safety and performance of electrical systems.
Resource: Megger handbook
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