MIT515, MIT525, MIT1025, and MIT1525 insulation resistance testers
Measures up to 30 TΩ
Insulation resistance up to 30 TΩ at 15 kV, 20 TΩ at 10 kV, and 10 TΩ at 5 kV
Safety rated up to CAT IV
Up to 1000 V to 3000 m for the MIT1525, and CAT IV 600 V to 3000 m for the MIT515, MIT525, and MIT1025
Additional protection with dual-case design
A tough outer case to protect the tester, an inner fire retardant case, and IP65 rated when closed
Full set of diagnostic test modes
Including Polarisation Index (PI), Dielectric Absorption Ratio (DAR), Dielectric Discharge (DD), step voltage (SV), and ramp test
New PI PredictorTM
Obtain 10 minute PI values in typically half the time with the new patented PI predictor.
About the product
The MIT515, MIT525, MIT1025, and MIT1525 insulation resistance testers are compact, light 5 to 15 kV units for the diagnostic testing and maintenance of high voltage electrical equipment. They are ideal for original equipment manufacturers (OEMs) and industrial companies.
The MIT series has a full suite of test modes as well as on-board memory and the ability to stream data/download data to a PC/laptop. They also have rapid-charge batteries and operate from an AC source if the batteries are dead. Rapid charge batteries enable >60 minutes of testing after a 30-minute charge.
The MIT range includes:
- MIT515: 5 kV with IRT and DAR, but with no memory
- MIT525: 5 kV IRT with all test modes, including a ramp test plus advanced memory functions with recall to screen, RTC for time/date stamp of results, and USB drive to PC/PowerDB
- MIT1025: 10 kV IRT with all test modes, including a ramp test plus advanced memory functions with recall to screen, RTC for time/date stamp of results, and USB cable interface to PC/PowerDB
- MIT1525: 15 kV IRT with all test modes, including a ramp test plus advanced memory functions with recall to screen, RTC for time/date.
The MIT1525 is at the top of the range, and it performs insulation resistance tests up to 15 kV with a 30 TΩ maximum resistance and an accuracy of ±5 % from 1 MΩ up to 3 TΩ.
Safety rated to CAT IV, all these units are smaller and lighter than their predecessors, making them even easier to carry and store.
NEW
The patented PI predictor enables you to obtain 10 minute PI values in as little as 3 minutes! The new method starts to predict the final IR curve from 3 minutes into the test and as soon as the predictor is confident with the prediction, the test is stopped and the predicted value of the PI displayed. In most cases, this happens within 5 minutes, meaning the test time is typically halved!
Technical specifications
- Max resistance reading
- 30TΩ
- Power source
- Battery
- Power source
- Mains
FAQ / Frequently Asked Questions
The Polarisation Index is the ratio of insulation resistance at 1 minute to 10 minutes. It shows how the insulation is charging up and can determine whether it is clean and dry. For trending purposes, the PI value negates the effects of temperature compared to past results.
As the insulation value increases, the test current decreases and becomes harder to measure with the same level of accuracy.
If all you ever want to do is one-off type go/no-go testing, you’re correct to say that an instrument that tops out at a few GΩ is fine. But most people who carry out HV insulation testing are looking for more. Specifically, they want to be able to trend and compare results over time, as this provides a valuable warning of impending problems. Consider, for example, a piece of equipment that, over several years, has consistently had an insulation resistance of, say, 100 GΩ. However, the most recent test shows that this has fallen to 20 GΩ. Clearly, something has changed, and an investigation is in order. However, if you had carried out the tests with an insulation tester that reads “infinity” for all values above 10 GΩ, you would have noted no change, and no warning bells would have sounded!
The current rating is important, as an underpowered instrument will take a very long time to charge high capacitive test objects, such as long cables; it may also be unable to maintain the required test voltage when high levels of surface leakage are present. It is, however, necessary to be careful when comparing different instrument current ratings. An instrument with a 3 mA short-circuit capability that incorporates power regulation technology to ensure maximum power transfer into all load types will, for example, almost always be faster and more convenient to use than a 5 mA rated instrument that doesn’t use this technology.
The answer, at least in part, is in the question! An insulation resistance tester is designed to be used only on dead circuits, but that’s no guarantee that it won’t ever be accidentally connected to a live circuit. And if it is, an appropriate CAT rating is essential, especially as the environments in which HV insulation testers are most frequently used often have high supply transients. We recommend a CAT IV 600 V rating, and it’s imperative to be sure that this rating applies to all of the instrument’s terminals, including the guard terminal.
The answer to this question depends on the test set you are using. It is certainly challenging for instrument manufacturers to produce test sets that deliver good performance when the guard terminal is in use, not least because the guard terminal diverts a lot of current away from the measuring circuits. It is by no means unknown, for example, to have a surface leakage resistance of the order of 0.5 MΩ in a test sample with an insulation resistance of 100 MΩ. In other words, the guard terminal current is around 200 times greater than the current in the measuring circuit. This high level of guarded current can cause many problems in a poorly designed instrument, including greatly impaired accuracy. If you’ve got such an instrument, there’s not much you can do about it. However, if you’re buying a new instrument, the answer is simple. Insist that the manufacturer gives you meaningful data about measurement accuracy when the guard terminal is in use. The latest Megger units, for example, have a maximum error of 2 % when guarding 0.5 MΩ leakage with a 100 MΩ load.
Further reading and webinars
Related products
Troubleshooting
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.
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 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:
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 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).
User guides and documents
Software and firmware updates
FAQ / Frequently Asked Questions
The MIT525 and MIT1025 can record insulation temperature measured by an independent thermometer. If you do not wish to record temperature, do not change the default setting or reset it if it was previously set.
During insulation testing, we are often so preoccupied with the resistance of the actual insulator that we forget the resistance path on the outer surface of the insulation material. However, this resistance path is a part of our measurement and can dramatically affect our measurements. For example, when dirt or some other contaminant is present on a bushing's outer surface, the surface leakage current can be up to ten times that flowing through the insulation.The surface leakage presents as a resistance in parallel with the material's insulation resistance that we wish to isolate and measure. The instrument's measurement circuit can separate and ignore the surface leakage current when we use its guard terminal, performing a so-called three terminal test. Surface leakage mitigation is often necessary when high resistance values are expected, such as when testing high voltage components like insulators, bushings, and cables. These tend to have large surface areas exposed to contamination resulting in high surface leakage currents across them.