Electrical and electronic symbols are standardized graphical representations used in circuit diagrams and schematics. Understanding these symbols is essential for marine engineers, electrical engineers, ETOs, and technicians.
Electrical symbols provide a universal language for electrical engineering.
Without standard symbols:
Benefits
Electrical symbols represent the function of a component rather than its physical appearance. When reading a circuit diagram, follow these simple steps:
Step 1: Identify the Power Source
Look for where the electrical power starts, such as:
Step 2: Follow the Wires
Trace the electrical path by identifying:
Step 3: Identify Control Devices
These control the flow of electricity:
Step 4: Locate Protection Devices
These protect the circuit from faults:
Step 5: Find the Load
The load is the device that uses electrical power, such as:
Tip: Always read a circuit from the power source to the load—never start from the middle.
Symbol | Component | Meaning | |
Straight Line | Electrical Wire | Conducts electric current between components |  |
Connected Junction | Connected Wires | Wires are electrically connected |  |
Crossing Lines | Unconnected Wires | Wires cross but are not connected |  |
2. Switch and Relay Symbols
Component | Meaning | |
SPST Switch | Single Pole Single Throw switch; ON/OFF control |  |
SPDT Switch | Selects between two circuits |  |
Push Button (NO) | Normally Open; circuit closes when pressed |  |
Push Button (NC) | Normally Closed; circuit opens when pressed |  |
DIP Switch | Used for device configuration |  |
Relay | Electromagnetic switch used for remote operation |   |
Jumper | Temporary electrical connection |  |
Solder Bridge | Permanent connection made by solder |  |
3. Ground Symbols
Component | Meaning | |
Earth Ground | Safety grounding connected to earth |  |
Chassis Ground | Connected to equipment body/frame |  |
Common Ground | Reference point for circuit voltages |  |
4. Resistor Symbols
Component | Meaning | |
Resistor | Limits current flow |  |
Potentiometer | Adjustable resistor with 3 terminals |  |
Rheostat | Variable resistor with 2 terminals |  |
Trimmer Resistor | Preset adjustable resistor |  |
Thermistor | Resistance changes with temperature |  |
LDR (Photoresistor) | Resistance changes with light intensity |  |
5. Capacitor Symbols
Component | Meaning | |
Capacitor | Stores electrical energy |   |
Polarized Capacitor | Electrolytic capacitor with fixed polarity |  |
Variable Capacitor | Adjustable capacitance |  |
6. Inductor Symbols
Component | Meaning | |
Inductor | Stores energy in magnetic field |   |
Iron Core Inductor | Uses iron core for higher inductance |  |
Variable Inductor | Adjustable inductance |  |
7. Power Supply Symbols
Component | Meaning | |
Voltage Source | Provides constant voltage |  |
Current Source | Provides constant current |  |
AC Source | Produces alternating voltage |  |
Battery Cell | Single DC source |  |
Battery | Multiple cells connected together |  |
Generator | Converts mechanical energy to electrical energy |  |
Controlled Source | Output depends on another circuit parameter |  |
8. Measuring Instrument Symbols
Component | Meaning | |
Voltmeter | Measures voltage (connected in parallel) |  |
Ammeter | Measures current (connected in series) |  |
Ohmmeter | Measures resistance |  |
Wattmeter | Measures power |  |
9. Lamp Symbols
Component | Meaning | |
Lamp/Bulb | Produces light when energized |    |
Applications:
10. Diode Symbols
Component | Meaning | |
Diode | Allows current in one direction |  |
Zener Diode | Voltage regulation |  |
Schottky Diode | Low forward voltage drop |  |
Varactor Diode | Variable capacitance diode |  |
Tunnel Diode | High-speed switching |  |
LED | Emits light |  |
Photodiode | Produces current when exposed to light |  |
11. Transistor Symbols
Component | Meaning | |
NPN Transistor | Current flows when base is positive |  |
PNP Transistor | Current flows when base is negative |  |
Darlington Pair | High current gain transistor arrangement |  |
JFET (N/P) | Junction Field Effect Transistor |  |
NMOS | N-channel MOSFET |  |
PMOS | P-channel MOSFET |  |
12. Miscellaneous Symbols
Component | Meaning | |
Motor | Converts electrical energy into mechanical energy |  |
Transformer | Changes AC voltage levels |  |
Fuse | Protects against overcurrent |  |
Buzzer | Produces audible alarm |  |
Electric Bell | Rings when energized |  |
Speaker | Converts electrical signals into sound |  |
Microphone | Converts sound into electrical signals |  |
Bus | Group of wires carrying data/power |  |
Optocoupler | Provides electrical isolation |  |
Operational Amplifier | Amplifies analog signals |  |
Crystal Oscillator | Generates precise frequency |  |
ADC | Converts analog signals to digital |  |
DAC | Converts digital signals to analog |  |
13. Antenna Symbols
Component | Meaning | |
Antenna | Transmits and receives radio waves |   |
Dipole Antenna | Simple two-element antenna |  |
Used in:
14. Logic Gate Symbols
Gate | Function | |
NOT | Output is opposite of input |  |
AND | Output HIGH when all inputs are HIGH |  |
NAND | Opposite of AND |  |
OR | Output HIGH when any input is HIGH |  |
NOR | Opposite of OR |  |
XOR | Output HIGH when inputs are different |  |
D Flip-Flop | Stores one bit of data |  |
Multiplexer (MUX) | Selects one input from many |   |
Demultiplexer (DEMUX) | Routes one input to selected output |  |
The most commonly encountered symbols onboard ships are:
Electrical symbols provide a standardized way to represent electrical components in circuit diagrams. They make schematics easier to read, simplify troubleshooting, improve communication between engineers, and reduce the risk of installation or maintenance errors.
The best way to learn electrical symbols is to start with the most commonly used components, such as wires, switches, resistors, capacitors, motors, transformers, and fuses. Regularly reading circuit diagrams and practicing with real electrical schematics will help you recognize symbols more easily.
Marine engineers and Electro-Technical Officers (ETOs) frequently work with symbols for motors, generators, transformers, circuit breakers, relays, contactors, fuses, batteries, earth ground, push buttons, indicator lamps, solenoid coils, measuring instruments, and three-phase AC supplies.
A schematic diagram shows how an electrical circuit functions using standardized symbols, making it ideal for understanding circuit operation and troubleshooting. A wiring diagram, on the other hand, shows the actual physical connections and wire routing, making it useful for installation, maintenance, and repair.
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