Technical Drawing, Schematics and Documentation
This domain tests whether you can read and produce the documents an electrician actually works from: single-line diagrams, circuit schematics, wiring diagrams, control ladder diagrams, cable schedules and bills of material. Questions are usually interpretive - identify a component from its symbol and reference designation, trace how a circuit operates, or spot what is missing from a title block or schedule.
Core rules
- Single-line diagram shows the power path (source, switchgear, transformer, busbar, outgoing ways) without control detail; the schematic shows function; the wiring diagram shows physical terminals and conductor numbers.
- Reference designations follow IEC 81346 style prefixes: −Q for switchgear, −F for protective devices, −K for relays and contactors, −M for motors, −S for control switches, −H for indicators, −T for transformers.
- Coil and contact cross-referencing: a contactor coil at one location carries the page/line of its contacts, and each contact carries the coil's location.
- Contacts are drawn in the unoperated, de-energised state; a normally closed contact is shown closed until the device operates.
- The title block records drawing title, project, drawing number, revision, scale, date, drawn/checked/approved; revisions must be listed so an older print cannot be used for construction.
- Cable schedules give cable tag, type and size, from/to, length and core identification; the bill of material lists every item with a tag matching the drawings.
Worked example
Reading a direct-on-line starter ladder: pressing −S2 (NO start) completes the path from the live rail through −S1 (NC stop) and the overload contact −F2 (NC) to coil −K1, which energises and closes its main contacts for motor −M1. The hold-in contact −K1 (13-14) wired in parallel with −S2 keeps the coil supplied when the start button is released. If −F2 trips, its 95-96 contact opens and drops −K1 out, so the motor cannot restart until the overload is reset - and the drawing must show this contact in series with the coil, not in the main circuit.
Traps that cost marks
- Reading an unoperated NC contact as if it were open in service.
- Confusing a wiring diagram's terminal numbers with the schematic's wire numbers.
- Failing to cross-reference a coil to every contact of the same device, missing a hidden interlock.
- Ignoring the revision column and working from a superseded drawing.
- Treating the single-line diagram as if it showed control interlocks or protection settings.