[ ELECTRICAL SYMBOLS · REFERENCE ]
How to read an electrical schematic
An electrical schematic shows how a circuit works, not how it is built: every component as a symbol, every connection as a line, laid out so the logic can be followed. Control schematics are usually drawn as ladder diagrams.
- Shows
- How the circuit works: components, connections and logic
- Does not show
- Physical layout, cable routes or wire lengths. That is the wiring diagram
- States
- Contacts drawn in their normal state: de-energised, not pressed
- Ties together
- Coils and contacts by designator, wires by wire number, sheets by cross reference
Read it in this order
- 1. The title block and legend: the standard used, the symbol legend, voltage levels and the sheet numbering.
- 2. The supply: where power enters, the control transformer and its protection, and the two rails (L1 and L2, or +24 V and 0 V).
- 3. Each rung, left to right: on a ladder diagram every rung runs from one rail to the other, with the contacts first and the coil or load last.
- 4. The contacts: normally open or normally closed, and which coil or push button operates each one.
- 5. The coils: each relay coil and the list of its contacts, often with the rung or sheet numbers where they appear.
- 6. Wire numbers and terminals: each wire carries a number, and every field connection goes through a numbered terminal.
Example: a motor start-stop circuit
The classic rung: a normally closed stop button, a normally open start button, and the contactor coil M. Pressing start energises M; an auxiliary normally open contact of M, wired in parallel with the start button, holds the coil in after the button is released (the seal-in or holding contact). Pressing stop, or an overload trip opening its normally closed contact, drops the coil out.
Because the stop button is normally closed, a broken wire stops the motor instead of leaving it impossible to stop. That fail-safe logic is why stop circuits use NC contacts.
Schematic, wiring diagram, block diagram, one-line
| Drawing | What it shows |
|---|---|
| Schematic (circuit, elementary) diagram | How the circuit works, as symbols and lines |
| Ladder diagram | A control schematic drawn as rungs between two rails |
| Wiring (connection) diagram | How it is physically wired: devices, terminals and wires as built |
| Block diagram | Functions as boxes, for an overview |
| Single line diagram | A power system with one line per three-phase circuit. See the single line diagram guide |
Schematics in commissioning
Control circuits are proved against the schematics before they are energised: point-to-point wiring checks, insulation tests, then function tests of each rung and interlock, and loop checks for instrument signals. Each check is an inspection and test record against the tag.
Frequently asked questions
How do you read an electrical schematic?
Start with the legend and the supply, then follow each circuit from the supply to the load, noting each contact, what operates it and the coil or load it feeds. On ladder diagrams, read each rung left to right.
Are contacts on a schematic shown open or closed?
In their normal state: with the coil de-energised and nothing pressed or operated. A normally closed contact is drawn closed.
What is the difference between a schematic and a wiring diagram?
A schematic shows how the circuit works. A wiring diagram shows how it is physically wired, terminal by terminal.
What is a seal-in contact?
A normally open auxiliary contact of a relay or contactor, wired across the start button so the coil stays energised after the button is released.
- In production since 2014
- Thousands of daily users
- On MODEC FPSO projects since 2016
The symbols on this page were drawn by Deskely and are free to use. Standards are revised and every project issues its own legend sheets, which take precedence.