Modern microohmmeters "MIKO" for Measurement of transient resistances

Many various instruments (both of domestic and foreign manufacture) differing by the principle of operation, metrological characteristics, degree of automation, dimensions, weight and price, exist in the I&C market for measuring transient resistance. Among them, well-known and well-proven MIKO microhmmeters and milliohmmeters produced by SKB EP occupy their niche. The company does not stand still and continuously upgrades the existing assortment, as evidenced by the release of two new instruments: MIKO-10 and MIKO-21.

Both devices are designed for measuring the transient resistance of dismountable and non-dismountable electrical contact connections, as well as for measuring the transient resistance of the main contacts of high-voltage circuit breakers.

Specifications of MIKO-10 and MIKO-21 microhmmeters

SKB EP microhmmeters can measure the transient resistance of the contacts of column and dead tank circuit breakers in separate automated modes. To measure the resistance of the main contacts of dead tank circuit breakers equipped with built-in current transformers, MIKO devices have a special mode that takes into account the presence of a transient process of fixing the measuring current that occurs at the moment of current supply. When measuring the resistance of the contacts of column circuit breakers, the transient process does not occur, so the measurements are performed in a different, faster mode.

An important parameter when choosing a microhmmeter is the value of the measuring current. As well known, the transient resistance of oxidized contact connections depends on the current flowing through this connection. In this regard, the resistance measured at a low current may be overestimated, compared to the measurement results obtained at a high current. Currently, all manufacturers, including Russian ones, of high-voltage circuit breakers set the rated current at which the transient resistance should be measured in the range of 50 to 200A.

MIKO-21 is a mobile and well-protected (composite case) precision instrument (error no more than ±0.05%, eight-hour instability less than 0.005%), but at the price of a general-purpose industrial microhmmeter. High accuracy allows you to conduct laboratory tests and measurements, such as:

• measuring the temperature coefficient of resistance of stable resistors, shunts, and any metals;
• measuring the resistivity of metal samples;
• determining the length and weight of a coil of wire or cable without unwinding and weighing it;
• checking the correctness of cross section of the wire received from the supplier.

The methodology of these measurements can be obtained from the manufacturer of MIKO-21.

When measuring at a substation, the instrument is installed either near the circuit breaker or in the cradle of the lift. For the latter case, lightweight cables for all voltage classes are available. So, for 750 kV circuit breakers, the total length of two cables does not exceed 10 m, and the weight is less than 4 kg at a current of 200A.

MIKO-21 has 4 pre-programmed ways to start the measurement process: "Single" - press the "Start" button located on the front panel of the instrument; "Upon circuit closing" - the measurement process is started after an electrical contact occurs between the measured circuit and the current and potential contacts of the measuring cable, and information is displayed on the screen; "Periodic" - the measurement starts at pre-set time intervals. This mode can be used for product rejection; "Periodic circuit" - it is designed for automatic periodic start of measurement upon closing of the measuring circuit. 

The instrument contains an archive of rated values of high-voltage circuit breakers indicating the maximum and/or minimum permissible value of the transient resistance of contacts, as well as data sheets for rejected resistors indicating the permissible values of the upper and lower resistance thresholds. The built-in archive of rated values of electrical resistances allows the device to automatically detect and signal when the measurement result goes beyond the acceptable range.

MIKO-21 has a high-brightness color graphic display, and the instrument can be controlled (at the user's choice) either via a film keyboard or a touch screen display.

MIKO-10 is a portable and compact device, one of its main advantages is ergonomics. The microhmmeter is conveniently placed on the arm and fixed using straps, leaving both hands free, which avoids the use of long measuring cables and frees the hands when ascending or when connecting the measuring probes to the measured circuit. Also, the device can be easily removed from the arm and fixed to the belt, hung on your neck, or placed on any flat surface.

Due to the relatively small current (10A), MIKO-10 is mainly intended for checking contact connections with low level of oxidation. If the result of measuring the transient resistance does not exceed the rated value of the circuit breaker, then the contacts have low oxidation, and the result is correct. If the measured value is greater than the rated value, then you do not need to be in a hurry to reject the circuit breaker, but you should repeat the measurement with a microhmmeter designed for a current of at least 50A. In case of microhmmeters for amperage of 1-2A, this situation occurs much more often, since such currents are not able to significantly reduce the elevated resistance of the oxide film on contacts.

MIKO-10 includes 3 ways to start measurement: "Single" - measurement starts upon a user command; "Auto" - automatic start of the measurement when the measuring circuit is closed; and "Built-in CT" - for measuring the transient resistance of high-voltage circuit breakers with current transformers. The measurement is started at the user's command.

MIKO-10 is located on the arm

MIKO-10 is located on the belt

MIKO-10 is located on the neck

The instrument's scope of supply includes measuring cables with "alligator" clips or quickly installed clamps equipped with high-quality contacts made of beryllium bronze, as well as with "needle spring-loaded double prods" type clips. The latter allow you to quickly perform a variety of measurements on busbars, connections in pipelines, metal skins of aircrafts, etc. For heavily contaminated or painted surfaces, there is an option with prods that rotate when pressed.

Features of both instruments: communication with a PC via USB to organize and save the results on a computer, as well as generate measurement reports; automatic saving of measurement results in non-volatile memory of the instrument to analyze and record them into the measurement report; built-in battery power. Moreover, there is a function of compensation for external thermal EMF to improve the measurement accuracy.

If you are interested in instruments manufactured by SKB EP, and you want to learn more about MIKO-10 and MIKO-21 microhmmeters, please send your questions to the company's managers by phone 8 (812) 500-25-48 or by mail skb@skbep.com.