How significant is insulation resistance testing? Since 80% of electrical maintenance and testing involves evaluating insulation integrity, the answer is "very important." Identify potential electrical issues to reduce unpredictable, premature equipment repair and replacement costs.
Preventive maintenance is a predetermined task performed based on a schedule and its objective is to keep equipment in good condition to avoid breakdowns. With properly scheduled monitoring and data collection, this testing can be very useful in analysing and predicting the current and future behaviour of equipment. Early problem detection helps avoid major repairs, resulting in cost savings when compared to a run-to-failure maintenance practice. Preventive maintenance has the added benefit of pre-planning for necessary parts and resources.
This article describes the insulation resistance testing method commonly used for preventive maintenance activities. Insulation resistance testing is commonly performed as part of electrical testing in a preventive maintenance program for rotating machines, cables, switches, transformers, and electrical machinery where insulating integrity is needed. Insulation resistance testing in the preventive maintenance program helps identify potential electrical issues to reduce unpredictable, premature equipment repair and replacement costs.
Insulation resistance is used to verify the integrity of the insulation material, whether cable insulation or motor/generator winding insulation. Insulation resistance testing is carried out by applying a constant voltage to the equipment under test while measuring the flowing current. High DC voltage is used, causing a small current to flow through the insulator surface. The total current consists of three components.
What is insulation resistance testing?
Basically, you're applying a voltage (specifically a highly regulated, stabilized DC voltage) across a dielectric, measuring the amount of current flowing through that dielectric, and then calculating (using Ohm's Law) a resistance measurement. Let's clarify our use of the term "current." We're talking about leakage current. The resistance measurement is in megaohms. You use this resistance measurement to evaluate insulation integrity.
Current flow through a dielectric may seem somewhat contradictory, but remember, no electrical insulation is perfect. So, some current will flow.
What's the purpose of insulation resistance testing? You can use it as:
- A quality control measure at the time a piece of electrical equipment is produced;
- An installation requirement to help ensure specifications are met and to verify proper hook- up.
- A periodic preventive maintenance task; and
- A troubleshooting tool.
How do you perform an insulation resistance test?
Generally, you connect two leads (positive and negative) across an insulation barrier. A third lead, which connects to a guard terminal, may or may not be available with your tester. If it is, you may or may not have to use it. This guard terminal acts as a shunt to remove the connecting element from the measurement. In other words, it allows you to be selective in evaluating certain specific components in a large piece of electrical equipment.
Obviously, it's a good idea to have a basic familiarity with the item you're testing. Basically, you should know what is supposed to be insulated from what. The equipment you're testing will determine how you hook up your megohmmeter.
After you make your connections, you apply the test voltage for 1 min. (This is a standard industry parameter that allows you to make relatively accurate comparisons of readings from past tests done by other technicians.)
During this interval, the resistance reading should drop or remain relatively steady. Larger insulation systems will show a steady decrease; smaller systems will remain steady because the capacitive and absorption currents drop to zero faster than on larger systems. After 1 min, you should read and record the resistance value.
When performing insulation resistance testing, you must maintain consistency. Why? Because electrical insulation will exhibit dynamic behaviour during the course of your test; whether the dielectric is "good" or "bad." To evaluate a number of test results on the same piece of equipment, you have to conduct the test the same way and under the relatively same environmental parameters, each and every time.
Your resistance measurement readings will also change with time. This is because electrical insulation materials exhibit capacitance and will charge during the course of the test. This can be somewhat frustrating to a novice. However, it becomes a useful tool for a seasoned technician.
As you gain more skills, you'll become familiar with this behaviour and be able to make maximum use of it in evaluating your test results. This is one factor that generates the continued popularity of analog testers.
What affects insulation resistance readings?
Insulation resistance is temperature-sensitive. When temperature increases, insulation resistance decreases, and vice versa. A common rule of thumb is insulation resistance changes by a factor of two for each 10 DegC change. So, to compare new readings with previous ones, you'll have to correct your readings to some base temperature. For example, suppose you measured 100 megaohms with an insulation temperature of 30 degrees C. A corrected measurement at 20 Degrees C would be 200 megaohms (100 megaohms times two).
Also, "acceptable" values of insulation resistance depend upon the equipment you're testing. Historically, many field electricians use the somewhat arbitrary standard of 1 megaohm per kV. The Inter National Electrical Testing Association (NETA) specification Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems provides much more realistic and useful values.
Remember, compare your test readings with others taken on similar equipment. Then, investigate any values below the NETS standard minimums or sudden departures from previous values.
Factors that affect the insulation resistance
The factors that commonly affect the insulation resistance are:
- Surface condition - For example, oil or carbon dust on the equipment’s surface can lower the insulation resistance.
- Moisture - If the equipment’s surface temperature is at, or below, the dew point of the ambient air, a film of moisture forms on its surface would, lowering the equipment’s resistance value.
- Temperature - The insulation resistance value may vary inversely with the change of the temperature. Its influence on readings can be mitigated by performing preventive maintenance testing at the same temperature each time. If the temperature cannot be controlled, normalizing to a base temperature such as 40 °C is recommended. This is commonly done using the estimation rule, “Every 10 °C increase in temperature halves the insulation resistance, while a 10 °C reduction doubles the resistance”. As different materials may have different degrees of resistance change due to temperature, for more precise temperature correction, some may adopt a temperature correction factor; the measurement reading should be multiplied by the temperature correction factor at the corresponding temperature.
What are the test methods for insulation resistance tests?
There are three types of tests for measuring insulation resistance, and each test applies its own methodology that focuses on a specific insulating property of the devices being tested. Users need to choose the one that best fits the test requirements.
Spot test - This test is suitable for a device with a small or negligible capacitance effect, e.g. short wiring run.
A test voltage is applied for a short interval until a stable reading is achieved, or for a fixed period of time, normally 60 seconds or less. The reading is collected at the end of the test. For the historical record, a chart is plotted based on the history of the readings. Observation of the trend is taken over a period of time, normally over years or months(see figure 1).
This test is normally performed for Go/NoGo testing or historical records. Temperature and humidity variations may affect the readings and have to be compensated for if necessary.
Time-resistance test - This test is suitable for the predictive and preventive maintenance of rotating machines.
Successive readings are taken at a specific time, typically every few minutes, and differences in readings compared. Good insulation will show a continual increase in the resistance value. If the reading is stagnant and it does not increase as expected, the insulation may be weak and attention may be needed. Moist and contaminated insulation may lower resistance readings since they will increase the leakage current during testing. The temperature influence on this test is negligible as long as there is no significant temperature change in the device under test.
The polarization index (PI) and dielectric absorption ratio (DAR) are commonly used to quantify the time-resistance test result.
Step voltage test - This test is particularly useful when the rated voltage of the equipment is higher than the available test voltage generated by the insulation resistance tester.
Different voltage levels are applied in steps to the device under test. The recommended ratio of the test voltage is 1:5. The test at each step is the same length, usually 60 seconds, and goes from low to high. This test is normally used at test voltages lower than the rated voltage of the equipment. The rapid increase of the test voltage level creates additional stress on the insulation and causes the weak point to fail, subsequently leading to a lower resistance value.
Safety consideration
As insulation resistance testing involves high DC voltage application, the following safety precautions should be taken: - Make sure that the device under test is discharged.
- Conduct the test at the de-energized condition to ensure that no test voltage other than that from the insulation resistance tester is applied.
- Restrict personal access when high voltage testing is being conducted.
- Use of personal protective equipment (e.g. protective gloves) where applicable.
- Ensure suitable test leads are used and that they are in good condition. Using unsuitable test leads not only contributes to errors in readings, but they may also be hazardous.
After the test, make sure the device is fully discharged. This can be done by shorting the terminal with a suitable resistor. A minimum discharge time of four times the applied voltage duration is recommended. Some insulation resistance testers may have the built in self discharge circuit to ensure a safe discharge after the test. Testers with this feature ensure devices are safely discharged after every test.
Planning for a maintenance program
When planning for a maintenance program, equipment that needs maintenance needs to be identified, and priorities set accordingly. A motor or machine that supports the whole line should be a high priority. The frequency of checks to be conducted should also be defined. The frequency can be varied from unit to unit depending on the criticalness of the unit in the environment. Past history will be a good guide for determining when the next maintenance activities will be needed.
The maintenance record should cover the following:
- Date of the test
- Test voltage and current
- Test time
- Insulation resistance value
- The temperature of winding/equipment
- Identification of the equipment/device under test
- Parts or equipment that were included in the test
- Relative humidity.
As with every preventive maintenance program, record keeping and plotting of consecutive readings can identify trends and enable you to predict and plan for the next action.
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