How to read a single-line diagram
A single-line diagram draws the electrical switchboard with one line per circuit. You read it from top to bottom: the incoming supply, the main switch, the RCDs, each circuit's breaker and, at the bottom, what each circuit feeds, with the protection rating and the cable size.
The symbols you will find
Electrical diagrams use graphical symbols standardised by IEC 60617. The drawings in this guide are simplified to help you read; in a real design they carry more detail.
How to read a switchboard diagram, step by step
- Incoming supply. At the top are the grid connection, the meter and the main switch, which cuts off the whole installation.
- Busbar. The thick horizontal line distributes the energy to the protective devices below it.
- RCDs (residual current devices). Each RCD protects a group of circuits against earth leakage currents, which can flow through a person. Read the rating (40 A), the sensitivity (30 mA, the one used to protect people) and the type (AC, A, F or B), which tells you which forms of leakage current it reacts to.
- Circuit breakers. There is one per circuit. They protect the cable against overload and short circuit. Read the rating in amperes, the tripping curve (B, C or D), the breaking capacity in kA and the number of poles. Β«C16Β» means curve C, 16 A.
- Cables. Next to each outgoing line is the cable size in mmΒ² and often the cable designation. For example, Β«3G2.5Β» means three 2.5 mmΒ² conductors, one of them green-and-yellow, the earth.
- Circuits. At the bottom, each outgoing line has a code (C1, C2...) and what it feeds: lighting, sockets, appliances.
A single line can stand for several conductors (phase, neutral and earth). When the designer wants to show how many, they draw short oblique strokes across the line, one per conductor, or write the number next to it.
The rule that ties it together: the breaker protects the cable. The breaker rating must be higher than the current the circuit draws and must not exceed the current the cable can carry.
Typical values in a home
| Circuit | Cable size | Breaker |
|---|---|---|
| Lighting | 1.5 mmΒ² | 10 A |
| General-purpose sockets | 2.5 mmΒ² | 16 A |
| Hob and oven | 4 or 6 mmΒ² | 25 or 32 A, depending on the power |
These are reference values for installations in Portugal, governed by the technical rules for low-voltage installations (RTIEBT, Ordinance no. 949-A/2006). The design confirms each circuit from its power, its length and the way the cable is installed.
Lighting examples
On the single-line diagram, lighting appears only as a circuit with its breaker. How each light is switched is shown on the floor plans and wiring diagrams. These are the three most common arrangements:
- One-way switch: one control point. The switch always sits on the phase, so a light that is off carries no voltage.
- Two-way switching: two two-way switches joined by two conductors, the strappers. Each switch changes the path of the current, so either one turns the light on or off. In the drawing, both are set upwards: the current flows on the upper strapper and the light is on. If either one moves down, they are no longer on the same strapper and the light goes off. It is the arrangement used on stairs and corridors.
- With an intermediate switch: for three or more points, add one intermediate switch between the two two-way switches for each extra point. The intermediate switch has two positions: straight, where each strapper carries on ahead, and crossed, where the two strappers swap. In the drawing, the current leaves two-way switch 1 on the upper strapper, goes straight through the intermediate switch and reaches two-way switch 2, which is set upwards: the light is on. If someone presses the intermediate switch, it goes to crossed and the current moves to the lower strapper; since two-way switch 2 is set upwards, the circuit opens and the light goes off. Any of the three switches, when it changes position, swaps the strapper the current flows on, so each of them turns the light on or off.
Power and control: a direct-on-line motor starter
In building services, such as pumps, fans and HVAC compressors, the diagram is split in two. The power circuit carries the energy to the motor. The control circuit decides when the motor runs and uses very little power. The direct-on-line starter is the classic example.
What each part does
- Q1, magnetic circuit breaker: protects against short circuits.
- KM1, contactor: a remotely controlled switch. Its main contacts (1-2, 3-4 and 5-6) connect the motor when the coil A1-A2 is energised.
- F2, thermal overload relay: measures the motor current and, on sustained overload, opens its NC contact 95-96 in the control circuit. It is set to the motor's rated current. It does not protect against short circuits; that is Q1's job.
- S1, start button (NO) and S2, stop button (NC).
- KM1 13-14: the contactor's NO auxiliary contact, in parallel with S1, which provides the hold-in.
The sequence
- With Q1 on, press S1. The KM1 coil is energised and the contactor closes: the motor starts.
- At the same time contact 13-14 closes. When S1 is released, current flows through 13-14 and the coil stays energised.
- Pressing S2 opens the control circuit: the coil is de-energised, the contactor opens and the motor stops.
- If the motor is overloaded, F2 opens 95-96 and the same happens. To restart, S1 must be pressed again.
Terminal numbers are standardised. On auxiliary contacts, a last digit pair 3-4 marks an NO contact and 1-2 an NC contact; 95-96 and 97-98 are the NC and NO contacts of the overload relay.
What is worth checking on a switchboard
- Breakers rated above what the circuit's cable can carry.
- Circuits with no residual current protection.
- A single RCD for the whole house: a leak in one appliance switches everything off.
- Circuits with no label on the board, or a diagram that no longer matches what is installed.
This guide teaches you to read diagrams, not to work on switchboards. Any work on an electrical installation must be done by a qualified electrician, with the installation switched off.
Frequently asked questions
What is the difference between a single-line and a multi-line diagram?
A single-line diagram draws each circuit with one line and shows how the switchboard is organised. A multi-line diagram draws each conductor separately and is used to make the connections.
What is an RCD for, if there are already circuit breakers?
The breaker protects the cable against overload and short circuit. The RCD detects current leaking to earth, which can flow through a person, and switches the circuit off.
What does C16 mean on a circuit breaker?
Tripping curve C and a 16 A rating. The curve tells you how quickly the breaker reacts to current peaks.
Who prepares the single-line diagram?
It is part of the electrical installation design, prepared by a qualified technician. Afazeres Curiosos prepares electrical designs for houses and other buildings.
How do I request a proposal or contact you?
You can request a quote or an online consultation in the Client Room, write to geral@afazerescuriosos.pt, call +351 21 145 1077, message us on WhatsApp or book a preliminary call to frame your request.
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