Basic Electricity - Circuit Protection Devices and Switches

BASIC ELECTRICITY - Chapter 12

CIRCUIT PROTECTION DEVICES (Pages 12-29 to 12-34)

• Purpose: Protect aircraft electrical systems from damage and failure caused by excessive current.

Excessive Current

• Caused by a short circuit.

• Generates heat, which can melt insulation.

SHORT CIRCUIT

• Definition: Occurs when the positive side of a circuit comes into direct contact with the return side (negative side), resulting in:

  • Low resistance

  • High current flow

  • Overheating

• For example, a shorted lamp when measured has a voltage reading of zero (disconnect battery first).

TYPES OF CIRCUIT PROTECTION DEVICES

• Fuses

  • Protects from overcurrent.

  • Composed of a strip of metal that melts under excessive current, requiring replacement.

  • Rated in amperes (amps).

• Circuit Breakers

  • Used in place of fuses, breaking the circuit to stop current flow.

  • Can be reset after correcting the fault.

• Current Limiters

  • Functions similarly to fuses but can withstand overload for a short time.

  • Rated for 30 amps or greater; must be replaced.

  • Symbol is the same as a fuse.

• Thermal Protectors

  • Protects motors; opens the motor circuit when the temperature becomes excessive.

CLASSIFICATION OF CIRCUIT BREAKERS

• By operating principles:

  • Thermal Breakers: Operate based on heat.

  • Magnetic Breakers: Operate based on magnetic fields.

• By basic type:

  • Push/pull

  • Push to reset

  • Toggle

Aircraft Circuit Breakers (C/B)

• Characteristic: Trip-free type; once opened, the circuit remains open.

• It is impossible to hold the circuit breaker closed if a fault exists.

CONTROL DEVICES

• Define operation of electrical circuits, including:

  • Switches: Control current flow—start, stop, or change direction.

  • Relays: Remotely operate switches.

SWITCHES (Pages 12-31)

• Function: Control flow of electrons in electrical circuits.

  • Can start, stop, or change direction of current flow.

Definitions Related to Switches

• Pole: Movable blade or contactor; number of poles equals the number of circuits.

• Throw: Indicates the number of circuits completed through the switch with each pole.

• Positions: Number of states (ON or OFF) the switch can occupy.

Types of Switches

• Single Pole Single Throw (SPST): Simple ON/OFF.

• Single Pole Double Throw (SPDT): One input, splits to two outputs.

• Double Pole Single Throw (DPST): Controls two circuits simultaneously.

• Double Pole Double Throw (DPDT): Two circuits, each with two outputs.

NORMALLY OPEN (NO) & NORMALLY CLOSED (NC)

• NO: Switch stays open until closed.

• NC: Switch stays closed until opened.

SPECIALIZED SWITCHES

• Toggle Switch: Manually operated ON/OFF.

• Rocker Switch: Pressing one side turns ON/OFF.

• Rotary Selector Switch: Rotates to change circuits.

• Microswitch: Operated with minimal motion, commonly used in landing gear deployment.

SWITCH GUARDS (FAA, Page 12-34)

• Protect switches from unintended operation.

RELAYS (Page 12-34)

• Definition: An electrical switch operated from a remote location.

Purpose of Relays

• Operate circuits carrying large amounts of current from remote locations (e.g., aircraft starter).

• Heavy conductors are avoided.

Types of Relays

• Fixed core and movable core types.

SERIES DC CIRCUITS (Page 12-34)

• Definition: A circuit with only one path for electrons to flow.

• Characteristics:

  • If one lamp burns out, the entire circuit fails.

  • Total resistance is the sum of individual resistances: $R<em>t = R</em>1 + R<em>2 + R</em>3 + …$.

  • Current remains constant throughout: $I<em>t = I</em>1 = I<em>2 = I</em>3…$.

  • Voltage drop across each component used to push current through: $E<em>t = E</em>1 + E<em>2 + E</em>3…$.

Series Circuit Rules

1. $E<em>t = E</em>1 + E<em>2 + E</em>3…$

2. $I<em>t = I</em>1 = I<em>2 = I</em>3…$

3. $R<em>t = R</em>1 + R<em>2 + R</em>3…$

4. $P<em>t = P</em>1 + P<em>2 + P</em>3…$

Example Calculations for Series Circuits

• Example of calculating current (I) then power (P) using formulas.

VOLTAGE SOURCES IN SERIES (Page 12-36)

• A voltage source provides constant voltage.

• Two or more sources in series equal the algebraic sum.

KIRCHHOFF’S VOLTAGE LAW (Page 12-37)

• States that the algebraic sum of applied voltages and voltage drops around a closed circuit equals zero.

  • E.g., $ 45 - 10 - 20 - 15 = 0 $.

VOLTAGE DIVIDER (Page 12-38)

• Definition: A device obtaining multiple voltages from a single source.

• Implemented with resistors in series across the power source.

PARALLEL DC CIRCUITS (Pages 12-40)

• Primary Difference: More than one path for current flow; voltage remains constant.

• If one component fails, others remain functional; total resistance increases, total voltage remains the same, total current decreases.

Characteristics of Parallel Circuits

• Voltage is the same across each branch.

• Total resistance is less than the smallest resistor value.

• For equal resistors, total resistance is the resistor value divided by the number of resistors.

• For unequal resistors, total resistance uses the reciprocal of the sum of reciprocals.

KIRCHHOFF’S CURRENT LAW (Page 12-42)

• States that currents flowing into a junction equals those flowing out.

• For current dividers, total current equals the sum of individual branches: $I<em>t = I</em>1 + I<em>2 + I</em>3…$.

POWER IN PARALLEL CIRCUITS (Page 12-23)

• Power generated is calculated with: $P = I imes E$ and total power formula: $P<em>t = P</em>1 + P<em>2 + P</em>3…$.

PARALLEL CIRCUIT RULES

1. $E<em>t = E</em>1 = E<em>2 = E</em>3…$

2. $I<em>t = I</em>1 + I<em>2 + I</em>3…$

3. $R_t = ext{calculated as per previous instructions.}$

4. $P<em>t = P</em>1 + P<em>2 + P</em>3…$

EXAMPLES OF PARALLEL CIRCUIT CALCULATIONS

• Various calculations involving different resistors and voltages (examples included for practice).