Basic Electrical Technology

Author: Chief Editor: Ye Qingwen
Publisher:
Publish Date: 2000-04-01
Features: Chapter 1 of this book introduces the names, specifications, and usage and maintenance of common electrical measuring instruments, tools, and protective equipment. Chapters 2 and 3 discuss the installation, laying, and usage and maintenance methods of indoor, overhead, and cable lines, as well as the types, selection, and connection methods of common conductors. It focuses on the production methods of 1kV and below cable terminal heads and intermediate joint boxes. Chapter 5 covers the types, applications, and starting control methods of AC asynchronous motors, as well as their operation, maintenance, installation, and repair methods. Chapters 4 and 6 discuss the performance, applications, and usage and maintenance of high- and low-voltage distribution equipment (including common relays), as well as the safe operation, electric shock first aid, and artificial resuscitation methods in substations. Chapter 7 introduces the polarity identification and parameter testing methods of transistors, as well as the application knowledge of transistor circuits and related welding skills. The book includes both theoretical knowledge and operational skills, emphasizing the improvement of operational skills and is highly practical. It is suitable for workers in this trade participating in national vocational skill certifications, vocational school graduates of this major preparing for exams, and self-study references for electricians.
Excerpt: Chapter 1 Training Requirements for Common Electrical Instruments, Tools, Climbing Equipment, and Protective Gear
Requirement: Be familiar with and understand the specifications and performance of common electrical measuring instruments, tools, climbing equipment, and protective gear, and be able to use and maintain them.
Section 1 Names, Specifications, Usage, and Maintenance of Common Electrical Measuring Instruments
Common electrical measuring instruments refer to indicating instruments, which are direct-reading instruments used to measure voltage, current, power, phase, and frequency. They directly convert the measured quantity into an angular displacement of the instrument's deflection and indicate the value of the measured quantity on the instrument's scale through an indicator, allowing the measured value to be read directly. Indicating instruments have a series of advantages, such as simple structure, stability, reliability, low cost, and ease of use and maintenance, and are still widely used in industrial and mining enterprises. Indicating instruments can not only directly measure electromagnetic quantities but also be paired with various sensors to measure non-electric quantities (such as temperature, pressure, flow rate).
I. Classification and Selection of Indicating Instruments
1. Classification of Indicating Instruments
(1) According to the structural form and working principle of electrical measurement indicating instruments, they are divided into magnetic-dynamic, electromagnetic, electrodynamic, induction, rectification, thermoelectric, and electronic systems, etc. Magnetic-dynamic instruments are widely used in the measurement of current and voltage in DC circuits due to their high sensitivity and accuracy, and are used to manufacture DC standard instruments. Electromagnetic instruments have a simple structure, strong overload capacity, good stability, and low cost, making them suitable for AC and DC dual-use instruments and are widely used in the measurement of current and voltage in AC circuits. Electrodynamic instruments can be used as AC and DC dual-use instruments, as well as high-accuracy current and voltage meters, and can also be used as power meters, frequency meters, and phase meters. Induction instruments are widely used in AC power meters. Electrostatic instruments can directly measure voltages of thousands of volts or even higher and are widely used in high-voltage measurements. They can also measure very low voltages and are suitable for AC and DC dual-use, but their main disadvantages are low accuracy, low sensitivity, dependence of readings on external electric fields, and non-uniform scales.
(2) According to the type of working current, instruments are divided into AC instruments, DC instruments, and AC-DC dual-use instruments.
(3) According to the accuracy grade of the instrument, measuring instruments are divided into seven grades: 0.1, 0.2, 0.5, 1.0, 1.5, 2.5, and 5.0. The accuracy of each grade reflects the basic error range of the instrument under specified conditions. For example, a 0.1-grade instrument has a basic error of ±0.1%, and a 5.0-grade instrument has a basic error of ±0.5%. Generally, 0.1-grade and 0.2-grade instruments are used as standard instruments, 0.5-grade to 1.5-grade instruments are used for laboratory measurements, and 1.5-grade to 5.0-grade instruments are used for engineering measurements. Due to the continuous development of the instrument manufacturing industry, indicating instruments with an accuracy of 0.05 grade have now appeared.
(4) According to the resistance of the instrument to the electric field or external magnetic field, they are divided into four grades: I, II, III, and IV. Grade I instruments allow their indication values to change by ±0.5% under the influence of external magnetic or electric fields; Grade II instruments allow a change of ±1%; Grade III instruments allow a change of ±2.5%; and Grade IV instruments allow a change of ±5%.
(5) According to the usage method of the instrument, measuring instruments are divided into installed and portable types. Installed instruments are also known as switchboard instruments, which are usually fixed on the panels of certain electrical devices or switchgear. Generally, they have larger errors and lower costs but stronger overload capacity. Portable instruments are usually made in a portable form and are generally used in laboratories or outdoors. These instruments generally have smaller errors and higher accuracy but higher costs.
(6) According to the usage conditions, measuring instruments are divided into five groups: A, A1, B, B1, and C. The working conditions and the hardest conditions of each group are shown in Table 1-1.