Sonel MIC-10 and MIC-30 insulation resistance testers

Insulation resistance testers were among the first products offered by SONEL S.A. The products introduced the most recently are two state-of-the art models: MIC-10, a simpler one, and MIC-30, which is provided with additional functions. These testers, which are intended for tests using 1,000 V test voltage, are to replace the MIC-3 and MIC-1000 which have been in production for several years.

Insulation resistance tests are performed in order to check the actual condition of insulation of electric systems and appliances. This is of key importance to the safety of use and to proper functioning of electrical equipment. Proper condition of insulation is a guarantee of protection against direct touch.

When it comes to testing the condition of insulation, regularity is of key importance. Such tests are an integral element of control and test works and, as such, make it possible to detect any deterioration in the condition of electrical systems. The five principal factors affecting degradation of electrical systems are: electrical and mechanical damage, aggressive chemicals, thermal exposure, and environmental pollution. During normal operation of electrical systems and appliances, the very impact of those factors lead to aging of insulation.

The new stricter safety standards and regulations require changes in the design of testing equipment. This has resulted in the development of a series of SONEL housings of different shape, ergonomic features, and colors. However, what is more important is the changes in the design, which resulted in a new, much higher IP rating of the meters and much higher test categories.

The MIC-10 and MIC-30 insulation resistance testers are intended for direct measurement of resistance of motors, power equipment, conductor insulation in electrical systems, and, thanks to the possibility to select low test voltages, in telecommunication networks. Also, the testers enable measuring direct current and alternate current voltages as well as the capacity of objects. All the test inputs are provided with protective systems. This gives the user the guarantee that incidental power grid voltages (up to 750 V) will not damage the tester. The testers are compliant with the PN-EN 61557 standard.

The most important characteristics of the MIC-10 and MIC-30 testers are:

insulation resistance measurement: MIC-10 – up to 10 GΩ; MIC-30 – up to 100 GΩ;
test voltage selection: 50, 100, 250, 500, and 1000 V; also, MIC-30 enables voltage adjustment in the range of 50-1000 V, with a 10 V increase;
measurements using the two- and the three-wire method;
the MIC-30 tester enables performance of tests using an adapter put in a power socket (automatic measurement of all measurement combinations) and identification of leakage current; this is the first insulation resistance tester in the market that enables performing tests in such a manner;
automatic discharge of the capacity of a tested object upon completion of the insulation resistance tests;
direct measurement or one or two absorption coefficients (MIC-30);
storage of pre-set values of voltage and time T1, T2, and T3 (MIC-30);
measurement of direct and alternating current voltage;
measurement of capacity of the tested object;
acoustic determination of five-second intervals enabling the measurement of time characteristics during insulation resistance tests;
measurement of continuity of protective connections and equipotential bonding with a ≥200 mA (in compliance with the PN-EN 61557-4 standard), with self-calibration of the test leads;
measurement of resistance with low current with acoustic and optical signaling;
resistance to interferences;
protection rating of the housing according to the PN-EN 60529 standard: IP67;
testing category according to the PN-EN 61010-1 standard: IV 600 V (III 1000 V),
possible use in a broad range of temperatures: from -10 oC to +50 oC;
memory comprising 10 banks, 99 cells each, and communication with computers using a radio interface; cells contain complete test results (also in the case of power socket tests using power network adapters);
indication of battery status;
automatic switch-off when not in use (AUTO-OFF).

The use of the device is very simple, if not intuitive, which is particularly important in the event many tests are performed. All test modes and voltages are selected, similar to other SONEL testers, using a selector switch.

The theory of insulation resistance testing

Insulation resistance is the total of volume resistance, which depends on the type of the insulation material, and surface resistance, which depends on the cleanliness of the surface. Its value depends on a number of factors, to include temperature and voltage.
The factors that have the greatest impact on the process of measuring the parameters that characterize insulation resistance are: humidity, temperature, test voltage, test time, and cleanliness of the surface of the insulating material.

The insulation leak current is a small resistance current which has to components, i.e. current flowing across the insulation material and current flowing on the surface of the insulation material. The current increases quickly to reach its constant value and remains the same for the specific test voltage. An increase in the value of the current may lead to damage in the future. The leak current should be measured when the insulation capacity has been charged and when absorption has stopped. The absorption current, which initially has a significant value, decreases to zero after a certain time (which is longer than the time for the capacity current).

Measurement of insulation resistance

Measurement of insulation resistance consists in supplying a constant test voltage U at the terminals of the tested object and in determining the current flowing in the test circuit. Based on the measured value, the processor installed in the meter calculates the value of insulation resistance. The test voltage is generated by a programmable, highly-efficient and highly-stable converter, even in the event of wide dynamics of resistance-capacity loads.
Fig. 4. Measurement of insulation resistance.
    The testers are provided with a protection device that prevents performing the tests if voltage is detected in the object in question. The MIC-30 tester has an integrated clock which enables performance of tests during a time that has been pre-set by the user.
During an insulation resistance test, the value of resistance initially increases to reach a certain value and then stabilizes. This is due to physical and structural changes taking place in the insulation materials and caused by the electric field and the flow of the current. Insulated metal parts become capacitors – initially, capacity current of significant values flows through them. After some time, the current is reduced to zero and the speed of its reduction depends on the capacity of the tested object. Large objects with significant capacity, such as motors, take more time to charge. The charge collected in the tested object is a significant threat to people and animals; therefore, after the test has been completed, it must absolutely be discharged. The MIC-10 and MIC-30 testers have systems enabling automatic discharge of the tested objects after the tests are completed; the discharge is indicated by an appropriate message shown on the tester’s display.

   Very often insulation resistance tests are performed in difficult environments. In order to eliminate the impact of surface resistance in motors, power supply equipment, etc., the three- cable test method is used. When measuring the insulation resistance of, for example, a coaxial (antenna) cable between one conductor and the cable’s shield, the impact of surface resistance is eliminated by connecting the metal foil wrapped on the insulation of the tested conductor with the appropriate tester measurement input. Thanks to this, any currents that could flow on the surface of the cable and interfere with the test results will hit the foil shield and will become eliminated by the tester. A similar measure must be applied when testing the insulation of motors between two conductors. In such a case, the conductors that are not being tested must be connected with the additional input in the tester.

Measurement of direct and alternating current voltage

Due to a hazard to the users, the voltage is measured automatically during the insulation resistance test. When voltage is detected in the tested object, the value of the voltage is displayed and continuation of the test is prevented.

Equipment, accessories

As a standard, the testers are equipped with a set of test leads, alligator clips, and two test probes, a battery package, as well as, in the case of the MIC-30 tester, a shielded cable and an OR-1 radio receiver for communication with a computer. The complete set is sold in a convenient and pleasant looking case. As a standard, every instrument in the set is provided with a calibration certificate, a warranty card, and a user's instruction. The MIC-30 tester is provided with software for reading data from computer memory; it can also be provided with the SONEL PE computer program for preparation of test documentation.
Their modern casing, thought-through design, ergonomy of use, excellent metrological parameters, and good prices make the MIC-10 and MIC-30 testers very interesting tools for insulation tests.