Single-line diagram symbols: the essentials and the pitfalls

The symbols for the single-line diagram and position plan of a domestic installation are set by Book 1 of the RGIE: chapter 2.13 refers to table 2.23, a non-exhaustive list. Where an item is not listed, subsection 3.1.2.1 allows any other clearly identifiable symbol defined in the legend. Here are the families to know, described in words, and the mix-ups to avoid.

Protective devices: RCDs and circuit breakers

The single-line diagram shows the type and characteristics of the residual current devices and of the overcurrent protective devices (subsection 3.1.2.2). Two families not to confuse:

• The RCD watches for leakage currents to earth and protects people. Table 2.23 gives as an example a device annotated ΔIn=300mA, type A, In=40A: sensitivity, type and rated current. The role of each sensitivity is detailed in our article on 300 mA and 30 mA RCDs.
• The circuit breaker trips on overload or short circuit. Table 2.23 gives as an example a 20 A miniature circuit breaker, curve C, alongside the 16 A type gG fuse. The numbering of the points on each circuit starts from this device (subsection 3.1.2.1).

For the automatic circuit breaker, table 2.23 specifies that letters written next to the symbol give its mode of operation: M for the overcurrent release, O for the undervoltage release, Δ for the residual current sensitivity. A protection symbol with no annotation does not give the characteristics required by subsection 3.1.2.2.

Sockets: single, multiple, earthed

Table 2.23 distinguishes several socket-outlets. Three points to watch:

• Single or multiple socket-outlet. The table has a dedicated symbol for the multiple socket-outlet (shown for three). For counting, subsection 5.3.5.2 limits a new domestic installation to eight single or multiple socket-outlets per final circuit. Where the circuit also supplies fixed appliances, a fixed appliance or a set of fixed appliances controlled by a common switching device is treated as a socket-outlet.
• With earth contact. The table has a symbol for the socket-outlet with a protective conductor contact. For a new domestic installation, subsection 5.3.5.2 provides that all socket-outlets, except SELV ones, have an earth contact connected to the protective conductor, unless supplied through an individual isolating transformer. On a survey of an existing installation, record faithfully what is in place: for the existing parts of domestic installations, part 8 of Book 1 provides derogations from these two rules, including more than eight socket-outlets per final circuit provided the power of the connected fixed and stationary appliances does not exceed the power the wiring can carry (sections 8.2.1 and 8.2.2) and, for parts carried out before 1 October 1981, socket-outlets without an earth contact where the wiring has no protective conductor and those socket-outlets are protected by a high or very high sensitivity RCD (section 8.2.1). How this applies to a specific installation is for the accredited body to decide.
• Special socket-outlets. The table also covers the splash-proof, watertight or hermetic socket-outlet, the child-protected socket-outlet, the socket-outlet with a two-pole switch and the socket-outlet with an isolating transformer, for example for a shaver. Use these symbols rather than a free-text note.

Lighting and its controls

This is one of the fullest families in table 2.23. The symbols to know:

• The lighting point and the wall-mounted lighting outlet.
• The switch: general symbol, then two-pole, three-pole, with pilot lamp, dimmer.
• The single-pole two-circuit switch (double switching): it switches two circuits separately from one place.
• The two-way switch: it switches a circuit from two different places.
• The intermediate switch: combined with two two-way switches at the ends, it switches a circuit from any number of places.
• The push button, alongside the separate symbols for the timer and the impulse relay.

The table’s descriptions are enough to tell two-way and intermediate switches apart: two control points, two two-way switches; from three onwards, intermediate switches between the two two-way switches at the ends. The two-circuit switch, for its part, controls two circuits from the same place. On the position plan, each switch carries the letter of its circuit and the order number of the point it controls (subsection 3.1.2.1).

Fixed appliances and dedicated circuits

The single-line diagram shows fixed machines and appliances (subsection 3.1.2.2), and table 2.23 has dedicated symbols for many of them: electric cooker, hob, oven, microwave oven, washing machine, tumble dryer, dishwasher, fridge, freezer, water heater, heating appliance, motor, fan, bell, electric door strike, charging point for electric road vehicles. Show the appliance supplied (a washing machine, for example), without removing the socket-outlet if there is one: subsection 4.2.4.3 provides that the circuits of washing machines, tumble dryers and dishwashers are connected behind a high or very high sensitivity RCD.

For a fixed household electrical appliance with no dedicated symbol in the table, use the general symbol for a fixed household electrical appliance from table 2.23; where no symbol is listed, a clearly identifiable symbol defined in the legend (subsection 3.1.2.1). On a survey of an existing installation, establish what each unlabelled feed supplies before drawing it.

Consistent referencing

Draw the same item the same way on every page. Each lighting point, socket-outlet and control unit carries the same reference on the diagram and on the position plan: circuit letter, point order number (subsection 3.1.2.1). The compliance inspection report certifies that the installation has been carried out in accordance with the single-line diagrams and position plans (subsection 6.4.6.4): check that every item listed in subsection 3.1.2.3 appears on both documents. The full expectations are in our RGIE requirements for the single-line diagram.

In Amperio, the single-line diagram draws on more than 240 standardised symbols, and the diagram and the plan are built together with the same referencing. Every symbol remains editable.

This article is informative. It replaces neither the RGIE (Book 1) nor the advice of your inspection body.