Electrical Systems

Chapter 34: Fundamentals of Electricity

Technical General for Aviators — Capt. Pankaj Pahil

34.1 Basic Electrical Concepts

An electric current is the flow of electrons through a conductor. The force that causes this
flow is known as the Electro Motive Force (EMF).
 Voltage (V or E): The measure of electrical pressure or EMF. It represents the potential
difference between two points. Voltage is measured in Volts with a voltmeter, which is
connected in parallel across the circuit.
 Current (I): The rate of flow of electrons. It is measured in Amperes (Amps) with an
ammeter, which is connected in series within the circuit.
 Resistance (R): The opposition to the flow of current. It is measured in Ohms (Ω).
Resistance of a conductor is affected by its material, length (longer = more resistance), cross-
sectional area (thicker = less resistance), and temperature.
 Ohm's Law: The fundamental relationship between voltage, current, and resistance in a DC
circuit.
Voltage (V) = Current (I) × Resistance (R)
 Power (P): The rate at which electrical work is done. It is measured in Watts (W).
Power (P) = Voltage (V) × Current (I)

These devices are designed to automatically interrupt the flow of current in a circuit if the
current exceeds a safe level, preventing damage and fire.
 Fuses: A fuse contains a thin wire with a low melting point. If the current gets too high, the
wire melts and breaks the circuit. A blown fuse must be replaced.
Current Limiters: A type of slow-blow fuse used in high-power circuits (like generator
utputs) that can handle a temporary overload without blowing.
 Circuit Breakers (CBs): An automatic switch that "trips" (opens) when it detects an
vercurrent. Unlike a fuse, a circuit breaker can be manually reset after it has cooled down.
Most aircraft CBs are of the trip-free type, meaning they cannot be held closed against a
fault current.

A capacitor is a device that stores electrical energy in an electric field. It consists of two
conductive plates separated by an insulating material called a dielectric.
 Functions:
1. Stores an electrical charge.
2. Appears to pass Alternating Current (AC).
3. Blocks the flow of Direct Current (DC) once charged.
 Capacitance: The ability of a capacitor to store a charge, measured in Farads (F).

A battery converts chemical energy into electrical energy. It is comprised of one or more
cells.
 Primary Cells: Cannot be recharged (e.g., standard flashlight batteries).
 Secondary Cells: Can be recharged by passing a current through them in the reverse
direction (e.g., aircraft and car batteries).
 Capacity: A battery's ability to supply current over time, measured in Ampere-hours (Ah).
An 80% charge is typically required before flight.
 Lead-Acid Batteries: Commonly used on light aircraft. They consist of lead peroxide
positive plates and spongy lead negative plates immersed in an electrolyte of sulphuric acid
and water. The battery is fully charged when the electrolyte's specific gravity is around 1.30.
 Nickel-Cadmium (NiCd) Batteries: Used on larger aircraft. They are lighter and perform
better at low temperatures than lead-acid batteries but are more susceptible to a condition
called thermal runaway, where the battery overheats during charging.