9.2 Electrical Drawings, Diagrams & BS EN 60617 Symbols
Key Takeaways
Electrotechnical drawings fulfill distinct operational roles: block diagrams portray high-level system functions, schematics detail circuit logic without physical scale, wiring diagrams specify terminal interconnections, and architectural layouts show scaled physical component locations.
Architectural installation plans are drawn to standard metric scales—predominantly 1:50 (1 cm = 0.5 m) and 1:100 (1 cm = 1.0 m)—allowing accurate containment routing and materials take-off.
Title blocks, revision tables, and alphanumeric column grids (letters and numbers) provide configuration control, ensuring electricians build from current, approved drawing revisions.
As-fitted (as-built) drawings record all on-site variations and actual cable routes at completion, and are normally included in the CDM Health and Safety File and the client's O&M manuals.
BS EN 60617 provides the harmonized graphical symbol standard used across the UK and Europe for accessories, lighting, distribution boards, and protective devices.
9.2 Electrical Drawings, Diagrams & BS EN 60617 Symbols
Technical drawings form the universal visual language of the electrotechnical industry. From initial design engineering and client tendering to site erection, containment routing, testing, and eventual maintenance, electricians must accurately interpret diverse drawing types. Misreading a drawing scale or confusing a graphical symbol can cause serious structural conflicts, material waste, or fatal circuit wiring errors.
Types of Electrical Drawings in Building Services
Electrotechnical projects rely on five fundamental categories of engineering drawings, each designed to communicate specific technical data:
| Drawing Type | Defining Characteristics & Contents | Physical Scale | Primary Functional Purpose |
|---|---|---|---|
| Block Diagram | High-level functional overview using simple geometric blocks and single interconnecting lines. Omits terminal markings, component internal circuitry, and cable routing details. | Not drawn to scale. | Illustrates the overall architecture and sequence of power distribution, sub-distribution, fire alarm networks, or security interlocks. |
| Schematic (Circuit) Diagram | Detailed diagram showing the electrical logic and operational sequence of a circuit. Uses standardized graphical symbols arranged logically by function from positive to negative (DC) or line to neutral (AC). | Not drawn to scale; positions do not match physical room layout. | Essential for understanding circuit operation, control sequence logic (e.g. motor DOL starters, two-way lighting), and systematic fault-finding. |
| Wiring (Interconnection) Diagram | Practical diagram showing actual point-to-point wiring, cable cores, conductor color codes, internal link wires, and terminal designations (e.g. L1, L2, N, COM, 1, 2, E). | Not to physical scale, but mirrors the internal arrangement of enclosures. | Guides assembly, panel wiring, consumer unit internal connections, and accessory termination on site. |
| Architectural Layout Plan (Site Installation Drawing) | Physical building floor plan drawn to an exact metric scale, overlaying civil features (walls, doors, windows) with electrotechnical symbols indicating equipment locations. | Drawn to an exact metric scale (typically 1:50 or 1:100). | Instructs electricians where to position sockets, switches, luminaires, consumer units, and containment runs during first-fix and second-fix. |
| As-Fitted (As-Built) Drawing | Updated final layout drawing revised after physical installation to record all site variations, rerouted conduits, structural offsets, and junction box coordinates. | Drawn to an exact metric scale. | Normally included in the client's CDM 2015 Health and Safety File and in the Operation & Maintenance (O&M) manuals. |
Note
Under BS 7671 Regulation 514.9.1, diagrams, charts, or tables must be provided for every installation, indicating the type and composition of circuits, the method of protection against indirect contact, and the identification of protective devices. Providing accurate as-fitted drawings satisfies this mandatory regulation.
Drawing Scales, Scaling Rules & Layout Navigation
Architectural site installation plans are drafted using standardized metric reduction scales to fit large buildings onto manageable sheets (A0, A1, A2, or A3).
┌─────────────────────────────────┐
│ Reading Standard Metric Scales │
└────────────────┬────────────────┘
│
┌─────────────────────────────┴─────────────────────────────┐
▼ ▼
1:50 Scale 1:100 Scale
• 1 cm on drawing = 50 cm on site • 1 cm on drawing = 100 cm on site
• 1 cm = 0.5 m • 1 cm = 1.0 m
• 20 mm on paper = 1.0 m physical • 10 mm on paper = 1.0 m physical
• Factor: Multiply paper mm by 50 • Factor: Multiply paper mm by 100
Metric Scale Rules
Electricians must use a dedicated metric scale rule (triangular or oval cross-section). Standard scales include:
- Scale 1:50: Commonly used for detailed room plans, domestic layouts, plant rooms, and risers. On this scale, 1 cm on paper = 50 cm (0.5 m) in reality. A wall measured as 80 mm on the drawing represents a physical distance of 80 mm × 50 = 4000 mm = 4.0 m.
- Scale 1:100: Standard for large commercial floor plans, multi-story office blocks, and industrial units. On this scale, 1 cm on paper = 100 cm (1.0 m) in reality. A measurement of 65 mm represents 65 mm × 100 = 6500 mm = 6.5 m.
- Scale 1:20 / 1:25: Used for complex assembly details, consumer unit enclosure internal layouts, and distribution board glanding plates.
- Scale 1:500 / 1:1250: Site location plans showing external underground cable trenches, substation compounds, DNO intake cables, and external street lighting.
Dimension Lines and Grid References
- Dimension Lines: Unbroken lines terminating in arrowheads, slashes, or dots, interrupted by numerical dimension values. Always prioritize stated numerical dimensions over scaled ruler measurements, as printing and paper expansion can distort paper scaling.
- Alphanumeric Grid System: Modern drawings feature structural column grid lines labelled with letters along one axis (A, B, C...) and numbers along the other (1, 2, 3...). An operative can quickly locate a lighting distribution panel at grid intersection "C4" without searching through the entire floor plan.
- Title Blocks & Revision Tables: Located in the bottom right-hand corner of the sheet. Displays the client name, project title, drawing number, scale, designer's initials, and critical revision history table (e.g. Rev A, Rev B, Rev C).
Warning
Always verify that you are working from the current, approved revision of a drawing before measuring cable lengths or cutting conduit. Installing work from a superseded revision (e.g. Rev A when Rev C has been issued) can result in demolished containment, rejected work, and unpaid rectification costs.
Standard Electrotechnical Graphical Symbols (BS EN 60617)
In the UK, electrotechnical graphical symbols comply with BS EN 60617 (IEC 60617), which superseded the legacy BS 3939 standard. Electricians must memorize these symbols for site plans and qualification exams.
1. Socket-Outlets
| Symbol Description | Graphical Representation Details | Typical Application |
|---|---|---|
| Unswitched Socket-Outlet | Semi-circle with a single straight radial line projecting outward. | Clock points, behind refrigeration appliances. |
| Switched Socket-Outlet | Semi-circle with a radial line carrying a single perpendicular flag or cross-bar. | General domestic single switched socket. |
| Twin Switched Socket-Outlet | Semi-circle with two radial lines, each carrying a perpendicular switch flag. | Standard double 13A switched socket. |
| Switched Socket with Pilot Lamp | Semi-circle with flagged radial line plus a filled small circle or adjacent pilot dot. | Workshops, computer suites, remote appliance control. |
| Floor-Mounted Socket | Standard socket symbol enclosed inside a rectangular outline. | Open-plan office commercial floor boxes. |
| Weatherproof Socket | Standard socket symbol with the letter W or IP56 adjacent. | Outdoor garden and exterior patio supplies. |
2. Lighting Switches & Controls
| Symbol Description | Graphical Representation Details | Operation & Circuit Function |
|---|---|---|
| One-Way Single-Pole Switch | Open circle with a single radial arm ending in an angled open flag (—o/). | Controls a lighting circuit from one single position. |
| Two-Way Switch | Open circle with two radial arms projecting in opposite directions (\o/). | Controls lighting from two distinct positions (e.g. top and bottom of stairs). |
| Intermediate Switch | Open circle with two intersecting diagonal crossing arms forming an "X" (Xo). | Inserted between two two-way switches for control from 3 or more locations. |
| Pull-Cord Ceiling Switch | Standard switch circle with an inverted small triangle or letter P attached. | Bathrooms, shower rooms, and damp locations. |
| Double-Pole Switch with Neon | Circle with two parallel switch bars and an integrated pilot indicator lamp. | Immersion heaters, waste disposal units, water heaters. |
3. Luminaires & Emergency Lighting
| Symbol Description | Graphical Representation Details | Installation Role |
|---|---|---|
| Ceiling Lighting Point | Simple "X" or cross (+ or ×) representing a generic luminaire position. | Pendant lamp holder, baton holder, downlight. |
| Linear (Fluorescent / LED) Luminaire | Elongated rectangle with an internal line or cross; length indicates lamp length. | Commercial offices, modular suspended ceilings. |
| Wall-Mounted Luminaire (Bulkhead) | Standard cross or semi-circle resting against a solid wall baseline (T shape). | Outdoor security lighting, stairwell bulkheads. |
| LED Luminaire | Luminaire symbol annotated LED (or as shown in the drawing key). | Downlights, LED panels and battens. |
| Self-Contained Emergency Luminaire | Luminaire symbol with one quadrant or half filled solid black. | Maintained or non-maintained escape route lighting. |
4. Distribution, Metering & Isolation
| Symbol Description | Graphical Representation Details | System Placement |
|---|---|---|
| Consumer Unit / Distribution Board | Rectangular box divided diagonally, half filled solid black, or cross-hatched. | Main electrical intake and sub-distribution positions. |
| Electricity Meter (kWh Meter) | Rectangular box inscribed with the letters kWh or M. | DNO intake position recording consumed energy. |
| Service Cutout (Head) | Rectangular box containing a fuse symbol (—[──]—) at the cable head. | DNO incoming termination point housing supply fuse. |
| Switch Disconnector (Isolator) | Two open contact circles linked by an open knife line with a mechanical T-bar. | Main isolation next to plant, HVAC, and motors. |
| Fuse | A small rectangle drawn across the conductor line. | Cut-outs, fused switches and fused connection units on circuit diagrams. |
| Circuit Breaker | A switch symbol with a small cross on the moving contact. | MCBs and RCBOs on single-line and circuit diagrams. |
5. Auxiliary, Protective & Heating Accessories
| Symbol Description | Graphical Representation Details | Installation Role |
|---|---|---|
| Cooker Control Unit | Rectangle with internal switch and socket symbols, or labelled CCU. | Kitchen cooker radial isolator with auxiliary socket. |
| Fused Connection Unit (FCU) | Rectangle enclosing a fuse rectangle ([──]), with switch line if switched. | Fixed appliances: boilers, hand dryers, towel rails. |
| Smoke Detector | Circle containing the letter S or an internal optical chamber graphic. | Automatic fire detection on escape routes. |
| Heat Detector | Circle containing the letter H or a thermometer graphic. | Kitchens and boiler rooms where smoke would false-alarm. |
| Room Thermostat | Circle or square containing the Greek letter theta (θ) or letter T. | Heating zone and underfloor heating control. |
| Bell Push & Sounder | Circle with small dot (push); semi-circle with two striking lines (buzzer/bell). | Doorbell entry circuits and local alarms. |
Tip
On the City & Guilds 2365 exam, pay special attention to the difference between switch symbols: a one-way switch has a single arm, a two-way switch has two divergent arms, and an intermediate switch has crossing arms forming an "X". In socket symbols, a single radial line denotes an unswitched socket, whereas perpendicular flags denote switched outlets.
Which category of electrical drawing is specifically drafted to show the detailed logical sequence of operation and internal component connections of a circuit, without being constrained by physical scale or component location?
Architectural layout installation drawing
Schematic (circuit) diagram
Site location plan
As-fitted cable route diagram
An electrician is reviewing an architectural lighting layout plan drawn to a scale of 1:50. Using an accurate rule, the distance measured between two luminaire positions on the drawing is 60 mm. What is the actual physical distance between these luminaires on site?
1.2 meters
1.5 meters
3.0 meters
6.0 meters
Under the Construction (Design and Management) Regulations 2015 (CDM) and BS 7671, what is the primary purpose of compiling as-fitted (as-built) drawings upon completion of an installation?
To permit the Distribution Network Operator to recalculate the national grid electricity tariff
To provide building control surveyors with proof of material purchase costs for local council taxation
To avoid having to issue BS 7671 electrical installation certificates to the client
To provide an accurate permanent record of actual installed cable routes and accessories for the client's Health and Safety File and O&M manual
Which electrotechnical graphical symbol to BS EN 60617 represents an intermediate lighting switch used in multi-point switching circuits?
An open circle with two intersecting diagonal crossing arms forming an 'X'
An open circle with a single radial arm terminating in a single angled flag
An open circle with two radial arms pointing in directly opposite directions
A solid filled square containing a perpendicular switch toggle line
Sections you finish are checked off in the contents.