5 Reasons Why Your Business Needs insulation resistance tester for generators？

There are three commonly used methods for testing insulation resistance: spot reading test, time resistance test, and step voltage test. These three tests are used primarily to test insulation in motors, generators, cables, transformers, and other electrical apparatus. Additionally, there are two ratio test methods that are also helpful when testing rotating machinery, such as motors and pumps.

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To perform these tests, it is best to have a megohmmeter with a timed test function and the ability to select from a range of test voltages. It is also helpful to have a thermometer or similar temperature measurement device on hand. If the equipment temperature is below the dew point, a humidity measuring instrument will be necessary, especially when performing a spot test.

Safety

Before performing any insulation resistance test, be sure to observe the following safety measures as well as any additional guidelines specified in the documentation that comes with your test instrument, the local safety procedures of your organization, and the facility you are testing at. Insulation resistance testing involves the application of high DC voltages. Properly preparing the system under test and the instrument used to conduct the test are crucial to your safety and help prevent damage to your wiring and machinery.

1. Take the equipment under test out of service. Shut down the apparatus, open all switches, and de-energize the unit. Disconnect from all other equipment and circuits, including neutral and protective ground connections. Be sure to follow any applicable lockout/tagout (LOTO) procedures required during this step.
   
   
2. Perform a thorough inspection of the system. In general, the more equipment included in a test, the lower the resistance reading. Therefore, it is critical to inspect the system and understand exactly what you're including in the test. Make note of any equipment that might be damaged by high test voltages, and either adjust the test voltage accordingly or exclude these components from the test.
   
   
3. Discharge capacitive build-up before conducting an insulation resistance test. Note that modern megohmmeters automatically discharge this build-up when not running a test if the leads are connected to the device.
   
   
4. Check current leakage at switches and other connections.

When performing the test, restrict personnel access to the test site. Also, be sure to use personal protective gear such as gloves, safety glasses, and other protective equipment (where appropriate). After the test is complete, make sure the system under test is fully discharged. A minimum discharge time of four to five times the duration of the applied test voltage is recommended. As previously stated, some insulation resistance test instruments include a built-in feature to ensure a safe discharge after the test.

The testing process

Three electrical current components involved in an insulation test need to be understood when conducting a test.

To properly interpret test results, it's important to understand that the total current flowing through the insulation consists of these three components: capacitance charging current, absorption current, and conduction or leakage current.

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Capacitance charging current

• Capacitance charging current: relatively high upon start-up, drops exponentially within a few seconds to dozens of seconds. It is normally negligible when the reading is taken.
• Absorption current: decays at a decreasing rate. It may require up to a few minutes to reach zero depending on the insulation materials.
• Leakage current: constant over time.

For an effective test, results should be regularly recorded over a period of time and compared with earlier recorded values taken when the equipment was new and in good condition. The trend of the readings over a period of time will help identify the presence of anomalies. Insulation resistance values that are consistent over time indicate that the equipment's insulation properties are good. If the resistance values are decreasing, it indicates that potential issues can occur sometime in the future and more thorough preventive maintenance should be scheduled soon.

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 that 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 bythe temperature correction factor at the corresponding temperature.

What are the test methods for insulation resistance test?

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 3).

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