Electricity is fundamental to all industrial control and installation work in Kenya. Understanding basic electrical quantities and principles is essential for safely designing, installing, and maintaining electrical systems in factories, processing plants, and automated facilities. Mastery of SI units, conductors, and insulators allows professionals to make accurate calculations, select appropriate materials, and adhere to national and international standards in their daily tasks.

2.1 Basic SI Units Electrical

Accurate measurement and calculation in electrical work rely on the correct use of SI units. The three most important electrical quantities are voltage (measured in volts), current (amperes), and resistance (ohms). Industrial control technicians frequently use these units to size cables, specify circuit protection, and troubleshoot faults.

2.1.1 Overview of SI Units

The International System of Units (SI) provides the standard units for measuring all electrical quantities. The main electrical SI units are:

• Voltage (\(V\)): The potential difference or electrical pressure.
• Current (\(A\)): The flow of electric charge per second.
• Resistance (\(\Omega\)): The opposition to current flow.
• Power (\(W\)): The rate at which electrical energy is consumed or produced.
• Energy (\(J\) or \(kWh\)): The total work done by electricity.

Governing Formulas

• Ohm’s Law:$$ V = I \times R $$
• Electrical Power:$$ P = V \times I $$
• Electrical Energy:$$ E = P \times t $$

Worked Examples

Example 1:
A technician measures a current of \(4\,A\) flowing through a resistor of \(10\,\Omega\). Find the voltage across the resistor.

Given: \(I = 4\,A\), \(R = 10\,\Omega\)

$$ V = I \times R $$$$ V = 4 \times 10 $$$$ V = 40 $$ Answer: \(40\,V\)

Example 2:
A control panel in a Nairobi factory is supplied with \(230\,V\) and draws a current of \(2.5\,A\). Calculate the power consumed by the panel.

Given: \(V = 230\,V\), \(I = 2.5\,A\)

$$ P = V \times I $$$$ P = 230 \times 2.5 $$$$ P = 575 $$ Answer: \(575\,W\)

Example 3:
A relay coil has a resistance of \(200\,\Omega\) and is connected to a \(24\,V\) supply. What is the current through the coil?

Given: \(R = 200\,\Omega\), \(V = 24\,V\)

$$ I = \frac{V}{R} $$$$ I = \frac{24}{200} $$$$ I = 0.12 $$ Answer: \(0.12\,A\)

Example 4:
A motor runs at \(1.5\,kW\) for \(3\) hours in a county government workshop. How much energy is consumed in kilowatt-hours (\(kWh\))?

Given: \(P = 1.5\,kW\), \(t = 3\,h\)

$$ E = P \times t $$$$ E = 1.5 \times 3 $$$$ E = 4.5 $$ Answer: \(4.5\,kWh\)

Example 5:
A lighting circuit is protected by a \(6\,A\) breaker and operates at \(240\,V\). What is the maximum power the circuit can supply?

Given: \(I = 6\,A\), \(V = 240\,V\)

$$ P = V \times I $$$$ P = 240 \times 6 $$$$ P = 1440 $$ Answer: \(1440\,W\)

Chapter Summary

This chapter introduced the fundamental SI units used in electrical engineering, focusing on the measurement of power in watts, current in amperes, resistance in ohms, and voltage in volts. Learners explored the differences between conductors and insulators, examining the characteristics of metals and non-metals and their practical applications in electrical circuits. Key electrical quantities such as charge, force, work, and power were defined, with step-by-step calculations demonstrating how to determine these values in real engineering contexts. Ohm's Law was presented as a core principle, with detailed examples illustrating its use in analyzing and solving basic circuit problems. The chapter also covered essential measurement techniques, emphasizing the proper use and calibration of instruments like multimeters and ammeters. By mastering these principles and calculation methods, students are equipped with the foundational knowledge required for safe and effective work in electrical and electronic systems.