MIT515, MIT525, MIT1025, and MIT1525 insulation resistance testers

Unfortunately, lithium-ion batteries eventually wear out and can no longer accommodate a charge. This event is a common and, sooner or later, inevitable issue, but fortunately it is easily corrected. Replacement batteries are available from Megger, and you can quickly change one following the instructions in the User Guide.

Do a visual inspection of the unit, and don’t overlook the lead set. It is understandable to focus on the instrument and take the lead set for granted, but the leads are commonly knocked about from handling more than the instrument. In particular, the strain relief at the end of the lead becomes damaged - its absence is a strong indication that the lead set soon needs to be replaced. Damaged leads tend to affect the most negligible leakage currents first, so the instrument may not be able to indicate measurement into the tera-ohm (TΩ) range. This symptom means that the lead set should be repaired or replaced.

These are control and measurement boards post error codes. These appear on the display as “E” followed by a 1- or 2-digit number. The User Guide gives brief definitions. These are not user-adjustable. They indicate component failures or calibration resets that a Megger repair technician or authorised repair centre must perform.

Rough handling or bouncing around in a truck can cause this plastic insert to break. At this point, the display is merely hanging onto the top panel with no support. The display may still work for a time, but erratic performance will steadily increase. Contact your local Megger repair technician or authorised repair centre to repair the display.

This symptom indicates that the power supply transformer has broken off the power supply board, usually due to rough handling and/or dropping. The transformer, being relatively heavy, will come loose from its mountings. This breakage interrupts or terminates power to the circuitry, resulting in a ‘dead’ instrument. Contact your local Megger repair technician or authorised repair centre.

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 SW 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:

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 to halve the resistance for every 10 °C increase in temperature 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 "Stitch In Time" to find such factors for rotating equipment, transformers, and cables (Section: Effect of Temperature on Insulation Resistance).