Measurement features of the DC electrical resistance in switching devices components

8 february 2019

One of the main ways to control the technical condition of components of high-voltage circuit breakers, disconnectors, separators, short-circuiters and current leads is to check the electrical resistance to direct current. Table 1 shows the requirements of RD 34.45-51.300-97 (Scope and requirements for electrical equipment testing), which regulate the typical electrical resistance depending on the inspected object.

Measured object	Typical resistance	Features related to measurements
Resistance of the main circuit	1 to 1200 μΩ	The test current should be between 50 and 200 A. In addition, measurements must take into account the presence of a built-in current transformer in the dead tank circuit breakers.
Current lead contacts resistance	1 to 200 μΩ	The test current must not exceed 1/3 of the rated current.
Resistance of shunt resistors of arc extinguish devices	10 to 20 kΩ	When the measured resistance increases, the test current decreases proportionally. In this case, it becomes commensurable with the interference currents created by industrial-frequency electromagnetic fields emitted by various substation equipment. In this regard, the measuring instrument must have measures to reduce the impact of these interferences on the measurement result.
Resistance of ohmic voltage dividers	10 to 20 kΩ
Resistance of pre-insertion resistors	200 to 1200 Ω
Resistance of windings of control electromagnets	0.3 to 50 Ω	When measuring the electrical resistance of various windings, there are additional requirements for the stability of the test current and protection of the instrument against self-induction EMF that occurs when measurement ends.
Resistance of secondary windings of current transformer of dead tank circuit breakers	up to 10 Ω
Resistance of secondary windings of measuring voltage transformers	up to 100 kΩ

Let's look at some of the features associated with measuring these objects in more detail:

1. According to the contact theory, measurement of the electrical resistance of a solid section and a section with a contact connection has its own specifics. For example, the resistance of a section with a contact connection depends on the presence of oxides, oil and other films, the size of microroughness between the contacts, the compression force of the contacting surfaces, and the measuring current. Since the purpose of contact resistance monitoring is to assess the technical condition of the contacts and their associated components (for example, the condition of the springs that ensure contact compression), the measurement must be performed at a certain test current to obtain reproducible results.

In foreign regulatory documents, the test current for monitoring the contact resistance is regulated in IEC 56 (current not less than 50A) and ANSI C37/09 (current not less than 100A), similar requirements are contained in the Russian GOST 8008-75. In addition, for modern vacuum and SF6 circuit breakers of both foreign and Russian manufacture, the test current is specified in the corresponding operating manual.

2. Most high-voltage circuit breakers have built-in current transformers in their design, which imposes another requirement on the measuring instrument that must take into account the presence of a contact process that occurs at the moment of current supply, in order to eliminate additional error.

3. The resistances of contact connections and contacts of high-voltage circuit breakers can also be measured at alternating current, while the electrical resistance of the control electromagnet windings is measured only at constant and stable current. If the current is not stable, an additional error occurs, which is associated with the emergence of a voltage moving in the forward or opposite direction with respect to the test current (depends on the sign of the derivative of the test current).

In addition, the measuring instrument must be protected against self-induction EMF that occurs when the current circuit is broken, for example, when measurements are completed.

4. When measuring the resistance of the secondary windings of current transformers, there are no problems associated with the value of the test current. It is worth paying attention to the measurement of the electrical resistance of the secondary windings of voltage measuring transformers, since there have been cases of their damage by test current. In order to avoid such problems, it is recommended to perform the measurement using a milliohmmeter with relatively small output power and test current.

Thus, to cover a number of tasks related to the measurement of electrical resistance, the company must have instruments with the following properties:

Microohmmeter — output current from 50 to 200 A;
Milliohmmeter — highly stable test current and protection from self-induction EMF;
Ohmmeter (kiloohmmeter) — measurements in conditions of extreme electromagnetic interference with a frequency of 50 Hz.

The versatile MIKO-2.3 meets the stipulated requirements, weighs only 2.7 kg, and operates in four modes: microohmmeter (current up to 1000A), milliohmmeter, kiloohmmeter, and thermometer, which allows you to cover all the tasks of measuring resistance in electrical equipment. Each of the four modes is activated automatically when the corresponding input cable from the instrument set is connected.

In all the types of electrical equipment described above, the use of micromillikiloohmmeter MIKO-2.3 is cost-effective. The instrument allows you not only to measure all the above-listed parameters, but also will cost and weigh significantly less than a set of diagnostic tools.

In addition to versatile equipment, SKB EP produces highly specialized instruments: microohmmeters (MIKO-1, MIKO-10, and MIKO-21) and milliohmmeters (MIKO-7M(A), MIKO-8M(A), and MIKO-9A). You can learn more about these instruments on the company's website.