Electrical Symbols: Summary of Electrical Schematics

The language of the Electrical diagrams are universal. From designing a residential electrical system to commissioning an industrial plant, electrical symbols allow us to accurately and clearly represent how the various components of a circuit are connected and function. Understanding them is not just a technical matter—it is an essential skill for any professional in the electrical industry.
In Spain, this symbolism is governed by specific regulations that ensure all parties involved—installers, engineers, and maintenance technicians—interpret the diagrams in the same way. This guide covers the main groups of symbols that appear in electrical diagrams, ranging from the most basic elements to energy measurement and conversion devices.

The UNE-EN 60617 Standard and Its Relationship to Electrical Schematics

The UNE-EN 60617 standard is the reference standard in Spain and the European Union for the graphical representation of components in schematic diagrams and single-line diagrams. Adapted from the international standard IEC 60617, it establishes a standardized set of symbols and their meanings, applicable to both low-voltage installations and high-voltage systems and industrial automation.
Its practical importance is enormous: it ensures that a technician in Bilbao and an engineer in Seville will interpret the same installation diagram in exactly the same way, without any ambiguity. Thanks to this standard, electrical diagrams become universally readable technical documents throughout Europe.
In practice, these regulations apply to the design of electrical systems, automation plans, control diagrams, and all technical documentation related to the sector. Understanding the fundamentals of these regulations is therefore essential for any professional who works with electrical diagrams.

Conductors, Passive Components, and Basic Elements

Object (Outline of an Object)

In standardized diagrams, the outline of an object—usually represented by a dashed rectangle—visually delimits a functional unit within the diagram. It indicates that the elements enclosed within that outline are part of the same device or piece of equipment, making it easier to read the diagram in complex installations.

Screen and Shielding

The shielding symbol represents the physical protection that surrounds a conductor or set of conductors to prevent electromagnetic interference. In practical terms, a cable for residential electrical installations does not always require shielding, but in industrial environments or installations with sensitive equipment, shielding is a critical element. The specific symbol indicates whether the shielding is grounded, which is important for the safety of the installation. The 6mm² cable with reinforced insulation is a common example in more demanding circuits, where conductor protection is essential.

Wires and Connections

Conductors are represented by lines. A solid line between two points indicates a single-pole cable; multiple parallel lines or a line with an associated number represent a three-phase cable or a two-pole cable. Where conductors cross without a connection point, there is no electrical connection between them, while a black dot at the intersection indicates an actual electrical connection.
The ground connection deserves special mention: its symbol—three horizontal lines of decreasing size—appears in virtually all installation diagrams. There are also distinct symbols for ground, protective ground, and functional ground, each with different implications for the installation design. The service drop cable is usually represented with specific indications of cross-sectional area and insulation type next to the conductor symbol.

Connections and Terminals

Terminals and terminal blocks are essential components in both residential and industrial installations. Their symbol—a small circle or rectangle over the conductor—indicates an accessible and removable connection point. In electrical panel diagrams, terminal blocks are numbered to uniquely identify each connection.

Types of Currents

The symbol for alternating current (~) and the symbol for direct current (—) are commonly displayed next to the symbols for power sources or equipment to indicate the type of current they use. Three-phase current is represented by three lines or by the specific symbol of three out-of-phase waves, which is particularly relevant for motors and industrial equipment.

Polarity, Ground, and Chassis

Polarity is indicated by the (+) and (−) symbols next to the terminals of components that require it, such as polarized capacitors or DC power supplies. The distinction between protective ground, earth, and functional ground is essential in the design of safe electrical installations.

Sockets, Pins, and Connectors

Sockets and plugs are represented by specific symbols that indicate the number of poles, the arrangement of the contacts, and whether they include a ground contact. Multi-pole sockets for industrial use have more complex representations that reflect their actual configuration.

Lighting

Light fixtures, lamps, and lighting devices have their own symbols that indicate the type of light source—incandescent, fluorescent, or LED—and how they are connected. In residential wiring diagrams, the symbol for a light fixture is one of the most common.

Resistors, Capacitors, and Inductors

Power Switching Devices, Relays, and Drives

Switches

Switches are the most common switching devices in any electrical installation. In standardized symbols, a single-pole switch is represented by a diagonal line crossing the conductor; a double-pole switch is represented by two parallel lines. The staircase switch—or two-way switch—has its own symbol, distinct from that of a simple switch, precisely because its function is different: it allows a single light fixture to be controlled from two or more locations. The three-way switch appears in installations with three or more control points.

Device Actuators

Actuators are the components that cause a change of state in a device: they can be a coil, a pushbutton, an electrical signal, or a mechanical actuator. Their correct use in schematics is essential for understanding electrical command and control circuits.

Relays

Three-phase relays and contactors are key components in the automation and protection of industrial facilities. Their symbol clearly distinguishes between the operating coil and the associated contacts, which can be normally open or normally closed. Measurement and protection relays provide specific functions for monitoring electrical parameters.

Control Contacts

Control contacts—normally open (NO) and normally closed (NC)—are the components that transmit commands in control circuits. Understanding them correctly is essential for diagnosing malfunctions and designing automation systems.

Power Components

Power components include fuses, circuit breakers, and ground-fault circuit interrupters. Each has its own standardized symbol, which reflects its operating principle and its location in the installation diagram.

Measuring and Indicating Instruments

Indicating instruments and recording instruments are typically represented as circles with a letter or symbol inside that identifies the measured quantity: A for ammeter, V for voltmeter, W for wattmeter. Visual and audible signals—indicator lights, sirens, and bells—complete this group.

Measurement and Protection Relays

Measurement relays monitor parameters such as voltage, current, and frequency, and activate when these values fall outside the established ranges. They are key components in industrial facilities and electrical distribution systems.

Recording Devices

Unlike indicators, recording devices store the changes in a quantity over time. The oscilloscope—or oscillograph, in standard terminology—is the most representative example.

Indicator Devices

They display the value of an electrical quantity in real time. Correctly interpreting them in schematics allows you to quickly identify the monitoring points in a system.

Visual and Audible Signaling

Indicator lights, warning lights, and audible alarms play an essential role in control panels: they inform the operator of the system's status immediately and without the need for additional instruments.

Energy Production, Transformation, and Conversion

This group includes symbols for devices that generate, transform, or convert electrical energy: from a battery to a three-phase motor or a distribution transformer.

Power Supplies and Batteries

Power sources and storage devices—such as batteries and generators—are the points of origin for energy in a circuit. Their symbols indicate polarity and, in the case of batteries, the number of cells.

Rotary Machines and Motors

Motors, along with transformers, are the most common components in industrial facilities. The motor symbol includes information about the type—induction, synchronous, or DC—and its connection configuration.

Transformers

Transformers allow the voltage to be adjusted to meet the needs of each section of the system. Their symbol represents the magnetic coupling between the primary and secondary windings.

Converters and Rectifiers

Rectifiers convert alternating current to direct current; inverters do the opposite. Converters perform both functions and are increasingly common in photovoltaic solar power installations and uninterruptible power supply systems.