Module 3: Electrical Fundamentals — EASA Part-66 Category B1.4 Study Material

Overview

Module 3 of the EASA Part-66 syllabus (Appendix I to Regulation (EU) No 1321/2014, Annex III) provides the foundational knowledge of electrical theory and practice required for certifying staff working on aircraft. For Category B1.4 (helicopters with turbine engines), this module covers the fundamental principles of electricity, from basic atomic theory through to complex AC generation and semiconductor devices. The module is structured across 14 sub-topics (3.1 through 3.14), each addressing a specific area of electrical fundamentals.

The knowledge levels for this module range from Level 1 (overview) for basic concepts to Level 3 (detailed theory) for critical areas such as DC generators, AC generators, transformers, and semiconductor devices. This study material synthesises the core knowledge required for the B1.4 licence, with particular emphasis on helicopter-specific applications such as 28 V DC systems, 400 Hz AC generation, starter-generators, and battery systems.

3.1 Electron Theory and Electrostatics

Atomic Structure and Charge

All matter consists of atoms containing three primary particles:

Protons — positively charged, located in the nucleus

Neutrons — electrically neutral, located in the nucleus

Electrons — negatively charged, orbiting the nucleus in shells

The fundamental unit of charge is the coulomb (C), where one coulomb equals approximately 6.24 × 10¹⁸ electrons. The charge on a single electron is approximately −1.602 × 10⁻¹⁹ C.

Conductors, Insulators and Semiconductors

Materials are classified by their ability to conduct electrical current:

Material Type	Examples	Characteristics
Conductors	Copper, aluminium, silver	1–3 valence electrons; free electrons available for current flow
Insulators	Rubber, glass, mica	5–8 valence electrons; tightly bound electrons
Semiconductors	Silicon, germanium	4 valence electrons; conductivity can be controlled

Silicon is the most commonly used semiconductor material for power diodes in aircraft alternators due to its high temperature tolerance and low leakage current.

Electrostatic Principles

Like charges repel; unlike charges attract

Electric field — the region around a charged body where a force is exerted on other charges

Capacitance — the ability to store charge in an electric field, measured in farads (F)

The relationship between charge (Q), capacitance (C) and voltage (V) is:

Q = C × V

_t = R₁ + R₂ + R₃ R_t = 4 + 6 + 2 = 12 Ω Current: I = V / R_t = 12V / 12Ω I = 1 A Voltage drops (V = I × R): V₁ = 1×4 = 4V | V₂ = 1×6 = 6V | V₃ = 1×2 = 2V KVL: V_supply = V₁ + V₂ + V₃ = 12V ✓ PARALLEL CIRCUIT 12V R₁=6Ω R₂=12Ω R₃=4Ω I₁=2A I₂=1A I₃=3A I_total = I₁ + I₂ + I₃ = 6A Conductance method: 1/R_t = 1/R₁ + 1/R₂ + 1/R₃ 1/R_t = 1/6 + 1/12 + 1/4 = 0.5 R_t = 2 Ω Voltage: Same across each branch = 12V I₁ = 12/6 = 2A | I₂ = 12/12 = 1A | I₃ = 12/4 = 3A KCL: I_total = I₁ + I₂ + I₃ = 6A ✓ OHM'S LAW — V = I × R V I × R I × R V = I × R I = V / R R = V / I Where: V = voltage (V), I = current (A), R = resistance (Ω) Aircraft Example (28V DC): Solenoid valve coil: R = 14 Ω I = V / R = 28V / 14Ω = 2A Power: P = V × I = 28V × 2A = 56W Power Formulas: P = V × I P = I² × R P = V² / R SERIES vs PARALLEL — KEY DIFFERENCES Property Series Circuit Parallel Circuit Current (I) Same through all components Divides between branches (KCL) Voltage (V) Divides across components (KVL) Same across each branch Resistance (R) R_t = R₁ + R₂ + R₃ (increases) 1/R_t = 1/R₁ + 1/R₂ + 1/R₃ (decreases) Open Circuit Effect Entire circuit stops (no current) Only that branch stops; others continue EASA Part-66 Module 3 — Electrical Fundamentals • B1.4 Helicopter Maintenance

3.2 Electrical Terminology and DC Circuits

Fundamental Units

Quantity	SI Unit	Symbol
Voltage	Volt	V
Current	Ampere	A
Resistance	Ohm	Ω
Conductance	Siemens	S
Power	Watt	W
Energy	Joule	J
Inductance	Henry	H
Capacitance	Farad	F

Ohm's Law

The fundamental relationship between voltage, current and resistance:

V = I × R

Where:

V = voltage in volts (V)

I = current in amperes (A)

R = resistance in ohms (Ω)

Example calculation: A solenoid-operated valve in a 28 V DC system has a coil resistance of 14 Ω. The current drawn is:

I = V / R = 28 V / 14 Ω = 2 A

Electrical Power

Power is the rate at which electrical energy is converted to another form. In DC circuits:

P = V × I

Alternative forms using Ohm's law substitution:

P = I² × R

P = V² / R

Example calculation: A 28 V DC system with a load drawing 10 A consumes:

P = 28 V × 10 A = 280 W

Example calculation: A load with 24 V across it and 12 Ω resistance dissipates:

P = V² / R = 24² / 12 = 576 / 12 = 48 W

Resistance and Conductors

The resistance of a conductor depends on:

Length (L) — resistance is directly proportional to length

Cross-sectional area (A) — resistance is inversely proportional to area

Material resistivity (ρ) — a material property

R = ρ × L / A

Example: If a wire has a resistance of 0.5 Ω and its length is doubled (with the same cross-sectional area and material), the new resistance becomes 1.0 Ω.

A busbar with a resistance of 0.001 Ω indicates excellent conductivity, as expected for a power distribution component.

Internal Resistance and EMF

Every voltage source has internal resistance. The relationship between generated EMF (E), terminal voltage (V), load current (I) and internal resistance (r) is:

E = V + (I × r)

Example: A 28 V DC generator with 0.01 Ω internal resistance delivering 150 A at its terminals has a generated EMF of:

E = 28 V + (150 A × 0.01 Ω) = 28 + 1.5 = 29.5 V

Example: A 24 V battery with 0.02 Ω internal resistance delivering 200 A to a starter motor experiences an internal voltage drop of:

V_drop = 200 A × 0.02 Ω = 4 V

This internal drop reduces the terminal voltage during high-current draws, which is critical for starter motor operation.

Kirchhoff's Laws

Kirchhoff's Current Law (KCL): The sum of currents entering a junction equals the sum of currents leaving that junction.

Kirchhoff's Voltage Law (KVL): The algebraic sum of all voltage drops and rises around any closed loop equals zero.

3.3 Electrical Test Equipment and Measurements

Multimeters

A multimeter combines several measurement functions:

Voltmeter — measures voltage; connected in parallel across the component

Ammeter — measures current; connected in series with the circuit

Ohmmeter — measures resistance; must only be used on de-energised circuits

Critical safety note: An ammeter must never be connected in parallel — this creates a short circuit. An ohmmeter must never be used on a live circuit.

Clamp-on Ammeters

For measuring current in a live circuit without breaking it, a clamp-on ammeter (current clamp) is used. It measures the magnetic field around the conductor and displays the corresponding current. This is the correct method for troubleshooting live circuits where circuit interruption is not permitted.

Megohmmeter (Megger)

The megohmmeter measures insulation resistance in megohms (MΩ). Key considerations:

Test voltage selection: The megger test voltage must be appropriate for the system voltage. For a 28 V DC system, a 500 V megger is not recommended — the high test voltage can stress and damage insulation. Typically, a 50 V or 100 V megger is specified for 28 V systems.

Minimum acceptable values: For aircraft wiring, a minimum insulation resistance of 1 MΩ is generally acceptable, though many operators require higher values.

Interpretation: A reading of 2 MΩ on a 28 V system is above the 1 MΩ minimum and indicates acceptable insulation, provided the correct test voltage was used.

Frequency Meters

A frequency meter is used to measure the frequency of an AC signal. This is essential when checking generator output, typically 400 Hz in aircraft systems.

Continuity Testing

A continuity test using the ohmmeter function of a multimeter should show:

Good wire: Near-zero resistance (complete path)

Open circuit: Infinite resistance (no path)

Phase Rotation Meters

A phase rotation meter verifies the phase sequence of a three-phase system. When connected with leads in the order A-B-C, a clockwise rotation indicates the sequence is correct. Incorrect phase sequence can cause three-phase motors to rotate in the wrong direction.

3.4 Batteries

Lead-Acid Batteries

Lead-acid batteries are commonly used in helicopter applications. Key characteristics:

Nominal cell voltage: 2.0 V per cell

Fully charged open-circuit voltage: 2.1 V per cell (25.2 V for a 24 V battery)

Electrolyte: Sulphuric acid (H₂SO₄) diluted in water

Specific Gravity Measurement:

The state of charge (SOC) of a lead-acid battery is determined by measuring the specific gravity (SG) of the electrolyte using a hydrometer:

State of Charge	Specific Gravity (at 25°C)
Fully charged	1.280
50% charged	~1.200
Discharged	1.120–1.150

Example calculation: A battery with fully charged SG of 1.28 and discharged SG of 1.12, measured at 1.20:

SOC = (SG_measured − SG_discharged) / (SG_charged − SG_discharged) × 100%

SOC = (1.20 − 1.12) / (1.28 − 1.12) = 0.08 / 0.16 = 50%

Voltage-Based SOC Assessment:

A fully charged 24 V lead-acid battery has an open-circuit terminal voltage of approximately 25.2 V. This is the healthy no-load condition. The nominal voltage of 24 V is the rated value, while 28 V is the system bus voltage during charging.

Battery Capacity and State of Charge

Battery capacity is measured in ampere-hours (Ah). The state of charge after a discharge event is calculated as:

SOC = (Capacity − Discharge) / Capacity × 100%

Example: A 25 Ah battery supplies 400 A for 30 seconds during an engine start:

Discharge = 400 A × (30 / 3600) h = 400 × 0.00833 = 3.33 Ah

SOC = (25 − 3.33) / 25 × 100% = 21.67 / 25 × 100% = 86.7%

Battery Charging in Parallel Systems

When a battery is connected in parallel with a generator or ground power unit (GPU):

The battery is charged when the generator voltage exceeds the battery's terminal voltage

The charging current is limited by the battery's internal resistance

A fully charged 24 V battery (25.2 V) connected to a 28.5 V bus will receive a charging current

The battery acts as a filter in the DC system, smoothing voltage ripple

3.5 DC Generators and Motors

DC Generator Principles

A DC generator converts mechanical energy into electrical energy through electromagnetic induction. The output voltage is proportional to:

Field current

Rotational speed

Number of armature conductors

Generated EMF: E = V + (I × R_internal)

Voltage Regulation:

Voltage regulation is defined as:

% Regulation = (V_no-load − V_full-load) / V_full-load × 100%

Example: A generator rated at 28 V, 100 A. At 80 A load, the voltage is 30 V. With the regulator set for 28 V at rated load:

% Regulation = (30 − 28) / 28 × 100% = 2/28 × 100% = 7.14%

DC Generator Types

Type	Field Connection	Characteristics	Applications
Series-wound	Field in series with armature	High starting torque; poor speed regulation	Starter motors
Shunt-wound	Field in parallel with armature	Good speed regulation; low starting torque	Constant-speed applications
Compound-wound	Both series and shunt fields	High starting torque + stable speed	Starter-generators

Starter-Generators

Helicopter piston engines commonly use compound-wound machines for starter-generator duty:

Starting mode: The series field provides high starting torque to crank the engine

Generating mode: The shunt field provides stable voltage regulation

The solenoid (contactor) in the starter circuit uses a small control current to close heavy-duty contacts, carrying the high starter current (typically 200–400 A).

Generator Control and Protection

Voltage Regulator:

Senses system voltage

Adjusts generator field current to maintain constant output voltage (typically 28 V)

Compensates for variations in engine speed and load

Failure mode: If the regulator fails with maximum field current, the output voltage will rise above the regulated value, potentially causing overvoltage damage to the electrical system.

Reverse Current Cut-out (RCC) Relay:

Opens the circuit between the generator and battery when generator voltage drops below battery voltage

Prevents the battery from powering the generator as a motor

Essential for parallel operation of generator and battery on the same bus

3.6 AC Theory

AC Waveform Fundamentals

Frequency (f): The number of complete cycles per second, measured in hertz (Hz). Aircraft systems typically use 400 Hz.

Period (T): The time for one complete cycle:

T = 1 / f

Example: For a 400 Hz system:

T = 1 / 400 = 0.0025 s = 2.5 ms

Three-Phase Systems

In a three-phase system:

Three voltages are separated by 120 electrical degrees

The phase sequence (A-B-C) determines the direction of rotation for three-phase motors

Star (Wye) Connection:

V_line = √3 × V_phase

Example: With a phase voltage of 115 V AC:

V_line = 1.732 × 115 = 199.2 V ≈ 200 V AC

AC Generator Frequency

The frequency of an AC generator is determined by:

f = (P × N) / 120

Where:

f = frequency in Hz

P = number of poles

N = rotational speed in RPM

Example: A 4-pole alternator producing 400 Hz:

N = (f × 120) / P = (400 × 120) / 4 = 48,000 / 4 = 12,000 RPM

3.7 Transformers

Transformer Principles

A transformer transfers electrical energy between circuits through mutual inductance. For an ideal transformer:

V_p / V_s = N_p / N_s

Where:

V_p = primary voltage

V_s = secondary voltage

N_p = primary turns

N_s = secondary turns

Example 1: A transformer with a turns ratio of 10:1 (primary to secondary), primary voltage 115 V AC:

V_s = V_p × (N_s / N_p) = 115 × (1/10) = 11.5 V AC

Example 2: A transformer with 1000 primary turns and 250 secondary turns, primary voltage 115 V AC:

V_s = 115 × (250 / 1000) = 115 × 0.25 = 28.75 V AC

Transformer Applications in Aircraft

Step-down transformers reduce 115 V AC to lower voltages for avionics

Step-up transformers increase voltage for ignition systems

Isolation transformers provide electrical separation between circuits

3.8 Capacitance and Inductance

Capacitors

A capacitor stores energy in an electric field between its plates. Key properties:

Capacitance is measured in farads (F) — typically microfarads (µF) in aircraft applications

Capacitors are used for filtering, timing circuits, and power supply smoothing

Series Connection:

1/C_total = 1/C₁ + 1/C₂

Example: Two 100 µF capacitors in series:

C_total = 1 / (1/100 + 1/100) = 1 / (0.02) = 50 µF

Parallel Connection:

C_total = C₁ + C₂

Inductors

An inductor stores energy in a magnetic field. Inductance is measured in henries (H).

Flyback Diodes:

When an inductive load (such as a relay coil) is switched off, the collapsing magnetic field induces a high-voltage spike. A flyback diode connected in parallel with the inductive load provides a path for the current when the switch opens, suppressing the voltage spike and protecting switching components.

3.9 Semiconductor Devices

Diode Fundamentals

A diode is a semiconductor device that allows current to flow in one direction only:

Forward bias: Anode positive relative to cathode — current flows

Reverse bias: Cathode positive relative to anode — current blocked

Silicon is the standard material for power diodes in aircraft alternators due to its high temperature tolerance and low leakage current.

Zener Diodes

A Zener diode is designed to operate in reverse breakdown and maintains a nearly constant voltage. This makes it ideal for:

Voltage regulation circuits

Reference voltage sources

Schematic symbol: Similar to a regular diode but with a bent cathode line (like an 'S' shape).

Semiconductor Applications

Rectifiers: Convert AC to DC in alternators

Voltage regulators: Maintain constant output voltage

Protection circuits: Suppress voltage spikes

3.10 DC Motors

Motor Principles

A DC motor converts electrical energy into mechanical energy. The interaction between the magnetic field and armature current produces torque.

Series-Wound Motors

Characteristics:

Field winding in series with armature

Field flux proportional to armature current

Very high starting torque

Poor speed regulation under varying load

Application: Starter motors — at standstill, the inrush current is high, producing very high starting torque essential for cranking piston engines.

Shunt-Wound Motors

Characteristics:

Field winding in parallel with armature

Good speed regulation

Low starting torque

Application: Constant-speed applications where load variations are expected.

Compound-Wound Motors

Characteristics:

Both series and shunt field windings

High starting torque (series component)

Stable speed regulation (shunt component)

Application: Starter-generators in helicopter piston engines.

3.11 AC Generators (Alternators)

Three-Phase AC Generation

Aircraft alternators typically produce three-phase AC power at 400 Hz and 115 V AC phase voltage (200 V line-to-line in star connection).

Phase Relationships

In a three-phase system, the three phases are separated by 120 electrical degrees. The phase sequence (A-B-C) is critical for:

Three-phase motor rotation direction

Synchronisation of parallel generators

Correct connection of loads

Magneto Ignition Systems

A magneto uses a rotating permanent magnet to induce an AC voltage in the primary coil, which is then stepped up by a transformer to produce the ignition spark. The capacitor (condenser) in the magneto circuit absorbs energy when the breaker points open, preventing arcing and allowing the primary current to drop quickly, inducing a high voltage in the secondary coil.

3.12 Wiring and Protection Devices

Circuit Protection

Circuit breakers are protective devices that trip on excessive current, preventing damage to wiring and equipment. Key principles:

A circuit breaker rated at 5 A trips when current exceeds 5 A

The wire rating must be above the breaker rating so the breaker protects the wire

An overload is the most likely cause of a breaker trip

Electrical Bonding

Bonding ensures electrical continuity between metallic components for:

Lightning protection

Static discharge

Return current paths

Acceptable resistance values:

Primary structural bond (e.g., engine to airframe): maximum 0.1 Ω (100 milliohms)

A reading of 1.5 Ω indicates a poor bond requiring corrective action

Wiring Practices

Twisted shielded pairs are used in helicopter audio and sensor wiring to:

Reduce electromagnetic interference (EMI)

Minimise crosstalk between adjacent circuits

Protect low-level signals in avionics

Insulation Resistance Testing

Minimum acceptable insulation resistance for aircraft wiring: 1 MΩ

Test voltage must be appropriate for the system voltage

For 28 V systems, use a 50 V or 100 V megger, not a 500 V megger

3.13 Test Instruments and Troubleshooting

Systematic Troubleshooting

When a component is not operating:

284.Verify power supply: Measure voltage at the component terminals

285.Check wiring: Continuity test from source to component

286.Check component: If correct voltage is present but the component does not operate, the fault is within the component itself

Measurement Techniques

Measurement	Instrument	Connection Method
Voltage	Voltmeter	Parallel across component
Current	Ammeter	Series in circuit
Current (live)	Clamp-on ammeter	Around conductor
Resistance	Ohmmeter	De-energised circuit
Frequency	Frequency meter	Parallel across source
Insulation resistance	Megohmmeter	Between conductor and earth
Specific gravity	Hydrometer	Electrolyte sample

Common Relationships Between Concepts

Ohm's Law and Power Triangle

The relationship between voltage, current, resistance and power forms the foundation of all circuit analysis:

V = I × R and P = V × I

These combine to give:

P = I² × R

P = V² / R

Generator and Battery Parallel Operation

In a helicopter DC system:

Generator voltage > battery voltage → battery charges

Generator voltage < battery voltage → battery discharges (RCC opens)

Battery acts as a filter, smoothing voltage ripple

Transformer and Generator Relationships

Transformer: V_p / V_s = N_p / N_s

Generator frequency: f = (P × N) / 120

Three-phase star: V_line = √3 × V_phase

Battery State of Charge Indicators

Specific gravity: Directly proportional to SOC

Open-circuit voltage: Higher than nominal when fully charged

Ampere-hour accounting: SOC = (Capacity − Discharge) / Capacity

Typical Exam Focus Points

For the EASA Part-66 Module 3 examination at B1.4 level, candidates should focus on:

313.Ohm's law and power calculations — be able to calculate current, voltage, resistance, and power in DC circuits (Level 3)

314.Battery characteristics — specific gravity interpretation, voltage-based SOC assessment, internal resistance effects, ampere-hour calculations (Level 3)

315.DC generator principles — EMF calculation, voltage regulation, field current control, reverse current protection (Level 3)

316.Starter-generator operation — compound-wound machine characteristics, series field for starting torque, solenoid operation (Level 3)

317.Three-phase AC systems — phase relationships (120°), star/delta connections, line-to-phase voltage relationships, frequency calculation (Level 3)

318.Transformer calculations — turns ratio, voltage transformation for step-up and step-down applications (Level 3)

319.Semiconductor devices — Zener diode operation, flyback diodes, silicon as the standard material (Level 2)

320.Test equipment usage — correct instrument selection, connection methods, megger voltage ratings, clamp-on ammeter application (Level 2)

321.Wiring and protection — circuit breaker operation, bonding resistance limits, insulation resistance minimums, twisted shielded pairs (Level 2)

322.Troubleshooting logic — systematic approach to fault diagnosis, voltage presence at component terminals (Level 2)

323.Unit conversions — SI units, period/frequency relationships, ampere-hour calculations (Level 2)

324.Capacitor combinations — series and parallel connections, total capacitance calculations (Level 2)

Regulatory References

This study material aligns with:

Regulation (EU) No 1321/2014, Annex III (Part-66)

Appendix I — Basic Knowledge Syllabus, Module 3: Electrical Fundamentals

AMC/GM to Part-66 — Acceptable Means of Compliance and Guidance Material

Aircraft Maintenance Manuals (AMM) — specific procedures and limits for individual aircraft types

The knowledge levels for Module 3 topics range from Level 1 (overview) to Level 3 (detailed theory), with the most critical topics for B1.4 certifying staff being DC generators, AC generators, transformers, batteries, and semiconductor devices.

Module 3: Electrical Fundamentals