You have seen that good insulation has high resistance; poor insulation, relatively low resistance. The actual resistance value
s can be higher or lower, depending upon such factors as the temperature or moisture content of the insulation (resistance decreases in temperature or moisture). With a little record-keeping and common sense, however, you can get a good picture of the insulation condition from values that are only relative.
The Megger insulation tester such as a Megger MIT525 or MIT1025 is a small, portable instrument that gives you a direct reading of insulation resistance in ohms or megohms. For good insulation, the resistance usually reads in the megohm range. The Megger insulation tester is essentially a high-range resistance meter (ohmmeter) with a built-in direct-current generator. This meter is of special construction with both current and voltage coils, enabling true ohms to be read directly, independent of the actual voltage applied. This method is nondestructive; that is, it does not cause deterioration of the insulation.
The generator can be hand-cranked or line-operated to develop a high DC voltage which causes a small current through and over surfaces of the insulation being tested (Fig. 2). This current (usually at an applied voltage of 500 volts or more) is measured by the ohmmeter, which has an indicating scale. Fig. 3 shows a typical scale, which reads increasing resistance values from left up to infinity, or a resistance too high to be measured.
Figure 2–Typical Megger test instrument hook-up to measure insulation resistance.
How to Interpret Resistance Readings
As previously mentioned, insulation resistance readings should be considered relative. They can be quite different for one motor or machine tested three days in a row, yet not mean bad insulation. What really matters is the trend in readings over a time period, showing lessening resistance and warning of coming problems. Periodic testing is, therefore, your best approach to preventive maintenance of electrical equipment. Whether you test monthly, twice a year, or once a year depends upon the type, location, and importance of the equipment. For example, a small pump motor or a short control cable may be vital to a process in your plant.
Figure 3–Typical scale on the Megger insulation tester.
Experience is the best teacher in setting up the scheduled periods for your equipment. You should make these periodic tests in the same way each time. That is, with the same test connections and with the same test voltage applied for the same length of time. Also you should make tests at about the same temperature, or correct them to the same temperature. A record of the relative humidity near the equipment at the time of the test is also helpful in evaluating the reading and trend. Later sections cover temperature correction and humidity effects. Download the entire article on “The Complete Guide to Electrical Insulation Testing”
Megger 5-kV and 10-kV Insulation Resistance Testers – MIT525, MIT1025
- Industry best guard terminal accuracy
- Compact and lightweight for easy transport and use
- PI, DAR, DD, SV and ramp test
- Unique dual case design provides additional user protection
- Lithium-ion battery – extended capacity, rapid charge
- Advanced memory with time/date stamp
- CAT IV 600 V safety rating on all terminals
The new range of Megger insulation resistance testers consists of three models: two 5 kV units (MIT515 and MIT525) and a 10 kV unit (MIT1025). Resistance measurement is available up to 10 TΩ for the 5 kV models and 20 TΩ for the 10 kV model. The new instruments are smaller and lighter than previous models yet offer advanced features and rapid charge capability. A key productivity feature is the ability to take measurements when connected to line power/mains with a dead battery. Intelligent battery charging ensures the optimum charge rate as a function of battery level, resulting in minimum charge times.
The rugged, unique dual case design provides the ultimate protection for a portable instrument and a clip-on lead pouch ensures that leads remain with the instrument at all times. The case lid is removable for improved terminal access. IP rating is IP65 with the case closed preventing water/dust ingress. High reliability and safety are built in; all models are safety rated to CATIV 600 V and are double insulated.
Five preset voltage ranges are provided in insulation test mode, plus a user settable lock voltage range. Any selectable test voltage may be locked and restored via the selector switch, thereby increasing efficiency of commissioning and repetitive tests. Preconfigured diagnostic tests include Polarization Index (PI), Dielectric Absorption Ratio (DAR), dielectric discharge (DD), Step Voltage (SV) and ramp test.
The ramp function gradually increases voltage up to a selected level while graphing current vs. voltage (graph downloadable). Graphs can be compared to example curves in IEEE 95-2002 to reveal a variety of faults difficult to detect otherwise. Small defects can be easily detected without risking the sudden large voltage increments produced by a Step Voltage test. Monitoring the developing graph during test enables the operator to terminate prior to breakdown, thereby reducing the possibility of damage to already flawed insulation. These units are particularly informative on polyester, asphalt and epoxy-mica insulations. They can also test voltage suppression devices.
Simplicity of operation is achieved with two rotary switches and the large backlit display enables multiple results to be displayed simultaneously. Advanced memory storage includes time/date stamping of results, logging of data and recall of results to screen. A fully isolated USB device interface (type B) is used for safe transfer of data to Megger’s PowerDB asset management software.
Typical end users include:
- Electrical contractors
- Testing and service companies
- Wind farm and solar generation operators
- Power generation and distribution companies
- Industrial companies
- Rail companies