Module 3: Electrical Fundamentals

Module 3: Electrical Fundamentals – Study Material

1. Module Overview

Module 3 of the EASA Part-66 syllabus provides the foundational knowledge of electrical theory required for aircraft maintenance certifying staff. This module covers the behaviour of DC and AC circuits, the characteristics of essential components (resistors, capacitors, inductors, transformers), and the principles of power generation and distribution within aircraft systems. A thorough understanding of these fundamentals is critical for safe troubleshooting, testing, and certification of electrical work on modern aeroplanes, which increasingly rely on complex electrical and electronic systems.

The module is structured to build from basic atomic theory and units of measurement, through DC circuit analysis, to the more complex concepts of AC theory, magnetism, and finally, the practical application of these principles in aircraft batteries and measurement instruments. The knowledge levels range from a general overview (Level 1) to a detailed theoretical understanding (Level 3) required for complex fault diagnosis and system analysis.

2. Key Concepts Explained in Detail

2.1 Basic Electrical Units and Ohm's Law

The foundation of all electrical work is the relationship between the fundamental units: Voltage (V), Current (I), and Resistance (R). These are defined by the International System of Units (SI).

Ohm's Law is the fundamental relationship governing these three quantities:

> V = I × R

This law is used to calculate any one of the three values if the other two are known. For example, a navigation light bulb rated at 28 V and drawing 2 A has a filament resistance of R = V / I = 28 V / 2 A = 14 Ω.

Electrical Power (P) is the rate at which electrical energy is converted into another form of energy (e.g., heat, light, or mechanical motion). It is calculated using:

> P = V × I

Using Ohm's Law, this formula can be rearranged to P = I² × R or P = V² / R. For instance, a 28 V DC motor drawing 10 A consumes P = 28 V × 10 A = 240 W of electrical power.

2.2 DC Circuits and Circuit Protection

A Direct Current (DC) circuit is one where current flows in a single, constant direction. Aircraft systems commonly use 28 V DC for many services, supplied by batteries and generator-driven rectifiers.

Circuit Protection: A short circuit occurs when a live conductor makes direct contact with the return path (e.g., the aircraft structure/ground), bypassing the intended load. This creates a path of almost zero resistance, causing a dangerously high current to flow. This can lead to overheating, fire, and damage to wiring and components. Protective devices like fuses and circuit breakers are designed to detect this excessive current and interrupt the circuit. A chafed wire touching the airframe is a classic example of a short circuit.

Voltage Drop: In a practical circuit, the wiring itself has resistance. When current flows, a voltage drop (V = I × R) occurs across this resistance. If connections are poor or wiring is undersized, the resistance increases, leading to an excessive voltage drop. This results in a lower-than-expected voltage at the load. For example, a dim cockpit light measuring 20 V instead of 28 V is a classic symptom of high resistance in the supply wiring or a poor connection.

2.3 Capacitance and Capacitors

A capacitor is a passive component that stores electrical energy in an electric field. It consists of two conductive plates separated by an insulating material called a dielectric. Its ability to store charge is called capacitance (C), measured in Farads (F). Aircraft systems commonly use capacitors in microfarads (µF) or picofarads (pF).

> C_total = C1 + C2 + C3 + ...

For example, a 20 µF and a 30 µF capacitor in parallel yield a total of 50 µF.

> 1/C_total = 1/C1 + 1/C2 + ...

Capacitive Reactance (Xc): In an AC circuit, a capacitor offers opposition to the flow of current, known as capacitive reactance. This opposition is frequency-dependent and is calculated using:

> Xc = 1 / (2πfC)

Where:

For a 50 µF capacitor connected to a 115 V, 400 Hz supply, the reactance is:

Xc = 1 / (2 × π × 400 Hz × 50 × 10⁻⁶ F) ≈ 7.96 Ω.

2.4 AC Circuits and Power

Alternating Current (AC) is an electric current that periodically reverses direction. Aircraft AC systems typically operate at 115 V (phase-to-neutral) and 400 Hz. The higher frequency allows for smaller and lighter transformers and motors.

Three-Phase Systems: Aircraft often use three-phase AC power for heavy loads like pumps and generators. In a balanced star (wye) connection, the relationship between the phase voltage (V_ph, measured between a phase and neutral) and the line voltage (V_L, measured between any two phases) is:

> V_L = √3 × V_ph

For a system with a 115 V phase voltage, the line-to-line voltage is V_L = 1.732 × 115 V ≈ 199.2 V.

Power in AC Circuits: In AC circuits, the voltage and current may not be in phase. The phase difference is denoted by the angle φ (phi). This leads to three types of power:

The relationship between these three types of power is described by the Power Triangle:

> S² = P² + Q²

The Power Factor (PF) is the ratio of true power to apparent power: PF = P / S = cos(φ). A purely resistive circuit has a PF of 1 (φ = 0°). An inductive circuit has a lagging PF (current lags voltage), while a capacitive circuit has a leading PF (current leads voltage).

For example, if a circuit has a phase angle of 60° and a true power of 500 W, the apparent power is S = P / cos(φ) = 500 W / 0.5 = 1000 VA. Similarly, if the apparent power is 200 VA and the true power is 173.2 W, the reactive power is Q = √(S² - P²) = √(200² - 173.2²) ≈ 100 VAR.

2.5 Transformers

A transformer is a static device that transfers electrical energy between two or more circuits through electromagnetic induction. It consists of a primary winding and a secondary winding wound around a common magnetic core. Transformers can only operate on AC.

The relationship between the voltages and the number of turns in the primary (Np) and secondary (Ns) windings is:

> V_s / V_p = N_s / N_p

This is known as the turns ratio. A transformer with more turns on the secondary than the primary is a step-up transformer (increases voltage). A transformer with fewer turns on the secondary is a step-down transformer (decreases voltage).

For example, a transformer with 200 turns on the primary and 400 turns on the secondary, connected to a 115 V supply, will produce a secondary voltage of:

V_s = V_p × (N_s / N_p) = 115 V × (400 / 200) = 230 V.

Conversely, a transformer with 1000 turns on the primary and 250 turns on the secondary will step down the voltage to V_s = 115 V × (250 / 1000) = 28.75 V.

2.6 Batteries and Internal Resistance

Aircraft batteries, typically 24 V lead-acid types, are critical for starting engines and providing backup power. A battery is not a perfect voltage source; it has an internal resistance (r) due to the resistance of the electrolyte and plates.

> V = E - (I × r)

> V = E + (I × r)

For example, a 24 V battery with an internal resistance of 0.05 Ω being charged with 10 A will have a terminal voltage of V = 24 V + (10 A × 0.05 Ω) = 24.5 V.

Battery Condition: A fully charged 24 V lead-acid battery has an open-circuit (no load) terminal voltage of approximately 25.2 to 27.6 V. This is due to the electrochemical potential of the cells (about 2.1–2.3 V per cell). A reading of 27.5 V with the generator off indicates a fully charged battery, not a fault.

2.7 Temperature Coefficient of Resistance

The resistance of a conductor changes with temperature. For most metals (like copper), resistance increases as temperature increases. This is quantified by the temperature coefficient of resistance (α).

The resistance at a new temperature (R_t) can be calculated using:

> R_t = R_0 [1 + α (t - t_0)]

Where:

For example, a sensor coil with a resistance of 120 Ω at 20 °C, made of copper (α = 0.004/°C), will have a resistance of R_t = 120 Ω × [1 + 0.004 × (30 °C - 20 °C)] = 120 Ω × 1.04 = 124.8 Ω at 30 °C.

2.8 Electrical Measurement and Instruments

Accurate measurement is essential for troubleshooting. The correct use of a Digital Multimeter (DMM) is a key skill.

3. Important Formulas and Regulations

3.1 Key Formulas Summary

3.2 Regulatory References

This study material is aligned with the requirements of:

4. Common Relationships Between Concepts

5. Typical Exam Focus Points

Based on the syllabus and typical exam questions, candidates should focus on the following areas: