A single-line diagram is a simplified representation of an electrical installation using a single line per circuit.
Imagine you have to design the electrical installation for a hospital or a 50-unit apartment building. If you had to draw all 3 wires (Phase, Neutral, Ground) for every outlet, every lamp, and every motor, the plan would be an illegible black smudge of thousands of lines.
To simplify this, we use the single-line diagram. And to prevent people from installing whatever they want, REBT is applied in Spain.
The REBT
In Spain, the main framework is the Low Voltage Electrotechnical Regulation (REBT), approved by Royal Decree 842/2002 along with its Complementary Technical Instructions (ITC-BT).
It is a mandatory regulatory provision within its scope. The regulation and its ITC-BT establish, among other things:
- The minimum cross-sections and protections for a dwelling’s interior circuits.
- The number of utilization points planned for each circuit.
- The identification of conductors.
- The verifications and documentation required based on the type of installation.
The fact that an installation works does not mean it is safe or compliant. Depending on its type and scope, it must be designed, executed, verified, and documented according to the REBT by the appropriate qualified professionals.
Objective of the REBT: Ensure the safety of people and the reliability of the supply. So your house doesn’t catch fire because you used a wire that’s too thin for the oven.
Friendly disclaimer
This course helps you understand installations. Of course, it does not replace the regulation or a qualified installer.
For real work, you must consult the current consolidated text and the relevant ITC-BT.
The single-line diagram
An electrical plan does not aim to represent exact physical reality (where the cable bends), but rather the connection logic.
In a single-line diagram, one line represents a circuit or a set of conductors. The associated marks and annotations indicate how many conductors there are, their cross-section, and other relevant characteristics.
Basic symbology
To read these diagrams, you need to know the basic symbols:
- The line: Represents the simplified circuit. It may have diagonal strokes or annotations to indicate the conductors.
- The X or Circle: A lamp or light point.
- The semicircle: A power socket (outlet).
- Protection symbols: Identify circuit breakers, residual current devices, fuses, and other devices. Their exact shape should be read according to the legend and standard used in the plan.
Anatomy of a household installation
Following the flow of energy from the street to your light bulb, these are the sections you will see in a standard single-line diagram:
Service Drop: The power company’s cable arriving at the building.
CGP (General Protection Box): Houses the protection elements for the general supply line, usually fuses.
LGA (General Supply Line): The thick cable running up the building.
Meters: Where your consumption is measured.
Individual Derivation: The cable running from the meter to your apartment’s door.
General Control and Protection Panel (CGMP): Houses the protections for the dwelling’s interior circuits.
Interior Circuits:
- C1: Lighting.
- C2: General power sockets.
- C3: Kitchen and Oven.
- C4: Washing machine, dishwasher, and electric water heater.
- C5: Bathroom power sockets and auxiliary kitchen outlets.
Why do we separate the circuits?
For a dwelling with basic electrification, ITC-BT-25 contemplates five minimum circuits. Enhanced electrification incorporates additional circuits. Why not just use one thick cable for the whole house?
- Safety: If there’s a short circuit in the oven, you don’t want to be left in the dark in the whole house. Only the oven’s circuit breaker trips.
- Sizing: The lighting wire can be thin (1.5mm²), but the oven wire must be thick (6mm²). If we mixed everything together, we would have to use thick (expensive) wire everywhere.
