Support for BM5200 5 kV insulation resistance tester
This symbol indicates that the instrument has experienced an overcurrent causing the fuse to blow. The fuse is in the battery compartment; you can replace it with the spare fuse.
A broken standoff between the display and control board will cause zero voltage output on all ranges. Such a break is usually caused by rough handling. You will need to send the instrument for repair in such a case.
Interpreting test results
Insulation resistance readings should be considered relative. They can be quite different for one motor or machine tested three days in a row, yet it does not mean bad insulation. What matters is the trend in readings over a longer period, showing lessening resistance and warning of coming problems. Periodic testing is, therefore, your best approach to preventive maintenance of electrical equipment, using record cards or software to trend the results over time.
Whether you test monthly, twice a year, or annually depends upon the equipment's type, location, and importance. For example, a small pump motor or a short control cable may be vital to a process in your plant. Experience is the best teacher in setting up the scheduled periods for your equipment.
We recommend making these periodic tests in the same way each time. That is, with the same test connections and test voltage applied for the same length of time. Additionally, we recommend performing tests at about the same temperature or correcting them to the same reference temperature. A record of the relative humidity near the equipment during the test is also helpful in evaluating the reading and trend.
In summary, here are some general observations about how you can interpret periodic insulation resistance tests and what you should do with the result:
|Condition||What to do|
|Fair to high values and well maintained||No cause for concern|
|Fair to high values but showing a constant tendency towards lower values||Locate and remedy the cause and check the downward trend|
|Low but well-maintained values||Condition is probably acceptable, but you should investigate the cause of low values|
|So low as to be unsafe||Clean, dry out, or otherwise recondition the insulation to acceptable values before placing equipment back in service (test wet equipment after drying out)|
|Fair or high values, previously well-maintained but showing a sudden decrease||Make tests at frequent intervals until you locate and remedy the cause of low values; or until the values have become steady at a lower level but safe for operation|
The resistance of insulating materials decreases markedly with an increase in temperature. However, we’ve seen that tests by the time-resistance and step-voltage methods are relatively independent of temperature effects, giving relative values.
To make reliable comparisons between readings, you should correct the measurements to a base temperature, such as 20 °C, or take all your readings at approximately the same temperature.
A good rule of thumb is: For every 10 °C increase in temperature, halve the resistance; or, for every 10 °C decrease, double the resistance.
Each type of insulating material will have a distinct degree of resistance change with temperature. Factors have been developed, however, to simplify the correction of resistance values. Please refer to the document linked below to find such factors for rotating equipment, transformers, and cable (Section: Effect of Temperature on Insulation Resistance).
User guides and documents
The timed IR test is a test that automatically terminates after a user-adjustable time (t). You can select “SETUP” on the range switch and adjust timer (t) using the up and down arrow buttons to set the desired time, followed by a single press of the PI-t (left arrow) button. Default time (t) is set at one minute because IR1min is frequently referred to in international standards.
Polarisation index (PI) test performs a test that calculates the ratio of insulation resistance at ten minutes, IR10 min, to insulation resistance at one minute, IR1 min. This test provides a simple indication of insulation polarisation when subjected to high voltage DC. A high PI value indicates a high degree of insulation polarisation and, therefore, good insulation condition. Generally, the PI value should be two or above.
Polarisation occurs at different rates ranging from minutes to several hours, which led the IEEE to create a ratiometric PI test. The IEEE standard 43-2000, “Recommended Practice for Testing Insulation Resistance of Rotating Machinery,” limits the use of PI test on winding systems to those with IR1 min being less than 5000 MΩ.
For basic insulation resistance tests with little possibility of surface leakage affecting the measurement, it is not necessary to use the guard terminal, i.e., if the insulator is clean and there are unlikely to be any adverse current paths. However, in cable testing, for example, there may be surface leakage paths across the insulation between the bare cable and the external sheathing due to moisture or dirt. To obtain an accurate measurement, particularly at high testing voltages, a bare wire may be bound tightly around the insulation and connected via the third test lead to the guard terminal ‘G.’