Why a shared vocabulary matters
Electrical design has its own language. The same word can mean slightly different things in a calculation, a drawing note and a manufacturer's data sheet, and a misread term is a reliable source of site queries. Knowing the vocabulary — not just recognising the words but knowing what each one controls — is what makes a drawing unambiguous.
This glossary groups the terms you meet most often by where they show up in a project, and gives a short working definition of each. It is written for the person who has to produce and read the documents, so the emphasis is on what each term means in practice rather than on a formal definition.
Model and drawing terms
- Single-line diagram — a schematic that shows the electrical system with one line per circuit, used to show protection, switching and the path from source to load.
- Schematic — a drawing that shows how a circuit works rather than where it is physically located, so components are arranged for clarity.
- Layout or plan — a drawing that shows the physical position of equipment, outlets, luminaires and routes in the building.
- Riser diagram — a vertical view that shows how supplies and distribution are stacked floor by floor.
- Legend — the key that defines every symbol used on a drawing, so two readers interpret the same symbol the same way.
- Detail drawing — an enlarged view of a junction, entry or fixing that would be unclear at the scale of the main plan.
- Block diagram — a simplified view that shows the main parts of a system and how they connect, without the detail of a full schematic.
Calculation and protection terms
- Design current (Ib) — the current the circuit is expected to carry in normal service, before protective device and cable are chosen.
- Cable current-carrying capacity (Iz) — the current a cable can carry continuously under stated installation conditions, after any derating.
- Voltage drop — the reduction in voltage along a circuit as current flows, expressed as a percentage of nominal voltage.
- Prospective fault current — the current that would flow if a short circuit occurred at a given point, which protective devices must be able to break.
- Loop impedance (Zs) — the impedance of the fault loop, used to confirm that a protective device will operate within the required time.
- Selectivity — the property that lets the device nearest a fault trip while upstream devices stay closed.
- Demand factor — the ratio of maximum demand to connected load, used to avoid sizing a supply for every load at once.
- Power factor — the ratio of real power to apparent power, which affects currents and therefore cable and device sizing.
Installation and containment terms
- Distribution board — an enclosure that divides a supply into final circuits, each with its own protective device.
- Switchboard or panel — a larger assembly, often the main point of control, that may include busbars, incoming and outgoing devices.
- Busbar — a rigid conductor that distributes current to several outgoing circuits inside a board or panel.
- Containment — the tray, conduit, trunking or duct that supports and protects cables along a route.
- Earthing system — the arrangement of the supply neutral and protective conductors, such as TN-S, TN-C-S or TT.
- Final circuit — the circuit from the last protective device to the equipment it supplies.
- Special location — an area with extra risk, such as a bathroom or pool zone, where additional protective measures apply.
Documentation and revision terms
- Cable schedule — a table listing every cable with its size, type, length, route and the circuits or boards it connects.
- Panel schedule — a table listing the circuits of a board, with device ratings and the loads they serve.
- Specification — the written requirements that define materials, standards and workmanship for the project.
- Material list — the counted list of equipment and bulk materials taken from the drawings, used for ordering and estimating.
- Revision — a controlled change to an issued document, marked by a revision identifier and recorded in the revision history.
- Issue status — the label that tells the reader whether a set is for review, tender or construction.
- As-built drawing — the record version that shows the installation as it was actually built, updated to include changes made on site.
Abbreviations you will see in notes
A short table of the abbreviations that appear most often in drawing notes and schedules.
| Abbreviation | Meaning |
|---|---|
| MCB | Miniature circuit breaker |
| MCCB | Moulded-case circuit breaker |
| RCD | Residual current device |
| RCBO | Residual current breaker with overcurrent protection |
| PE | Protective conductor (earth) |
| DB | Distribution board |
Abbreviations vary between countries and offices, so the legend of a project should always list the ones it uses rather than assuming that a reader shares the same shorthand.
Using the terms in a real model
In practice these terms are not separate ideas; they describe parts of one connected system. A single model that holds the drawings, the schedules and the calculations keeps the vocabulary consistent, so a term means the same thing on every document. GorkyCAD is a browser-based CAD system that works this way, with checks against IEC 60364 running on the same model that produces the panels and the cable schedule.
Seeing the terms in one place makes them easier to use correctly, and it also shows how they depend on each other: change the demand assumption and the cable size, the schedule and the material list all move with it. You can open the editor in your browser free of charge, with no card required.
Open the browser editor free — no card needed →
FAQ
What is the difference between a schematic and a layout?
A schematic shows how a circuit works and arranges components for clarity, while a layout shows where equipment and routes physically sit in the building.
What does Zs mean?
Zs is the earth-fault loop impedance. It is checked against the value needed for the protective device to disconnect within the required time.
Are IEC and national standards the same?
No. IEC standards give an international basis, and many countries adopt them with national amendments. Always design to the standard that applies where the project is built.
Why do drawings need a legend?
A legend fixes the meaning of every symbol, so that two readers — and the site team — interpret the same drawing the same way.