Modern instruments for accurate measurement of contact resistances

When operating and repairing electrical equipment, it is necessary to measure the resistance of electrical circuits:

• contact resistances of switching devices and contact connections of current leads;
• bolted and brazed joints;
• contacts of automatic circuit breakers;
• rail connections;
• resistances of wheel pairs of cars, etc.

For measuring resistances, there are many various instruments (both of Russian and foreign manufacture) that differ in their operating principle, metrological characteristics, degree of automation, weight and size, and price. The main requirements for microohmmeters are to provide a fairly high current through the contact resistance and its stability. 

The oxide film and non-metallic inclusions of contacts cause a nonlinear dependence of the contact resistance on the flowing current. Therefore, the most reliable measurements will be made when the current of the microohmmeter approaches the working current of the contacts. However, microohmmeter with a current of up to 10 A give overestimated resistance readings, which can lead to unjustified rejection of contacts. 

Doubts about this are reflected in the IEC 56 standard, which regulates the minimum permissible value of the test current — 50A. The instability of the microohmmeter current causes an additional error in measuring the resistance, which is proportional to the change in the circuit inductance and current during the measurement. 

The state of the contacts of high-voltage (HV) circuit breakers is determined by measuring the DC resistance of the poles of circuit breakers. This parameter for each pole must not exceed the normalized value given in the technical documentation for the corresponding equipment.

Most HV circuit breakers have built-in current transformers. This leads to another important requirement for a microohmmeter: not every such instrument is suitable for measuring the contact resistance in a circuit with a current transformer. It is worth noting that the test current of the microohmmeter must flow through the circuit stably and for a sufficient time, and the instrument itself must work steadily under powerful electromagnetic fields. 

HV circuit breakers have a variety of designs. To make measurements on high-power circuit breakers, it is necessary to ascend using a stairs or a lift. To reduce the number of ascents and descents, as well as to make a measurement in one ascent, it is desirable to have a light microohmmeter, as well as cables that are convenient to connect and of sufficient length. In addition, when ascending to the circuit breaker, your hands should be free for safety reasons, so it is desirable to have a microohmmeter that can be hung on your shoulder. This convenience, in turn, imposes another requirement on the microohmmeter — power autonomy, i.e. a mains-powered instrument cannot be used.

When choosing the most convenient microohmmeter, you should pay attention to the following:

• current not less than 50 A;
• sufficient duration of current flow;
• high current stability;
• error no more than 1 %;
• autonomy (built-in battery);
• small overall dimensions and weight;
• a set of cables for various applications. 

The specified requirements are fully met by the MIKO-1 industrial microohmmeter specially designed for the conditions of high-power interference of industrial frequency and providing their effective suppression (at least a million times for frequencies of 49.7–50.3 Hz). In accordance with the requirements of GOST 52931-08, MIKO-1 is protected against electromagnetic fields with a strength of up to 400 A/m by shielding, as well as using special microcircuit. In this regard, the readings of the instrument remain stable even in the conditions of a 500 kV substation.

Integrated battery of the microohmmeter provides its autonomy and portability and has a capacity sufficient to perform up to 100 measurements. The battery is charged from an internal charger powered by a DC or AC mains.

The low weight of MIKO-1 (3.6 kg) and a special bag ensure its convenient relocation, and also allow you to ascend to any circuit breaker together with the instrument. The operating current is 50 A for the measured resistance up to 2,000 μΩ, and 5 A for resistances in the range of 2000÷20,000 μΩ. A stable generator that sets the operating current eliminates the measurement error caused by the inductance of the resistance being measured. The accuracy of the instrument is ±0.5 %.

To make connection to the object more convenient, three types of cables with lengths of 3.5 and 7.5 m and with different tips are specially designed:

• "crocodile" (the current and potential wires are structurally combined in one clip);
• needle-type on potential leads and "crocodile" on current leads;
• spring loaded contacts.

Unlike analogues in the same price category, MIKO-1 stabilizes the current during measurement, which minimizes additional error; has a small overall size and weight, autonomous power supply; is resistant to electromagnetic fields; reliable; easy to operate and maintain. At the same time, the included cables are convenient to use and connect to the object, and the user's participation in the measurement process is minimal.