# Reading Electrical Schematics and P&IDs: An Engineer's Guide > Every automation engineer's working life revolves around two drawing types: electrical schematics , which show how the control system is wired, and P&IDs… - Canonical URL: https://wiki.aspotomasyon.com/article/reading-electrical-schematics - Publisher: ASP OTOMASYON A.Ş. (ASP OTOMASYON Wiki) - Category: Technical Guides - Published: 2026-08-11 · Updated: 2026-09-13 - Reading time: 4 min · Language: English Every automation engineer's working life revolves around two drawing types: **electrical schematics**, which show how the control system is wired, and **P&IDs** (Piping and Instrumentation Diagrams), which show how the process is instrumented. Reading them fluently is the difference between working from evidence and working from guesswork — a technician who can read the drawing set troubleshoots in minutes what others probe for hours. ![A commissioning evidence chain: every acceptance result should lead back to a requirement and identify the configuration that was actually tested.](https://wiki.aspotomasyon.com/assets/diagrams/technical-guides-context.svg) *A commissioning evidence chain: every acceptance result should lead back to a requirement and identify the configuration that was actually tested.* ## Electrical Schematics: Conventions Industrial electrical drawings (IEC 61082 / ANSI style) follow conventions that make them readable: - **Reference designations** — each component carries a class letter and number: `K1` contactor, `Q1` circuit breaker, `F1` fuse, `S1` pushbutton, `M1` motor, `X1` terminal block. The same designation appears consistently across drawings. - **Cross-references** — a contact shown in a different drawing lists the location (e.g., `K1` coil on sheet 3, contacts on sheets 5–7); locating a coil and its contacts is the most common reading task. - **Sheet structure** — power circuits and control circuits are often on separate sheets; the control circuit is usually drawn with the load line at the bottom (IEC) or along the side (ANSI). - **Wire numbers** — each wire has a unique number; the same number appears at both ends and at every junction, enabling continuity tracing from the drawing alone. - **Component states** — contacts are drawn in their *de-energized* or normal state (what the component does with no power/no process), with a note for spring-return or process-actuated devices. Reading order: start with the power distribution (incoming supply, main disconnect, breakers), then the power circuits (motors, heaters), then the control circuits (relays, PLC I/O), then the field wiring (terminal to device). ## From Schematic to Panel and Field The schematic describes the panel; the terminal diagram maps it to the real world: - **Terminal diagrams** — show each terminal strip ( `X1` , `X2` …) with wire numbers and destinations (cable to field, cable to another panel). The terminal number plus cable number is the address of every field point. - **Cable schedules** — list every cable: number, type, cores, from/to, and spare cores. The spare-core policy (how many spares on an instrument cable) is an engineering decision visible here. - **I/O lists** — the bridge between electrical and automation: every PLC/DCS channel with its tag, signal type, terminal, and cable. The I/O list is the contract the schematic and the PLC program both serve. ## P&IDs: The Process Map A P&ID (per ISA-5.1 / ISO 10628) shows the process equipment and every instrument, valve, and control function connected to it — without wire-level detail. Reading essentials: - **Instrument bubbles** — circles with a tag: the first letters are the function (PT pressure transmitter, TT temperature transmitter, FT flow transmitter, LV level valve, PIC pressure indicating controller), the suffix is the loop number. A letter per position describes the full function (e.g., `PIC` = pressure indicating control). - **Line symbols** — the symbol inside the bubble shows where the instrument is: field-mounted (plain), panel-mounted (line across), or software/DCS function (dashed). A transmitter (no line through) vs a controller (line) changes where the logic lives. - **Control loops** — a line connecting transmitter bubble, controller bubble, and final element (valve) shows the loop topology: simple feedback, cascade (two bubbles with an interconnecting line), or ratio (dashed lines for computed signals). - **Valve symbols** — the actuator symbol (handwheel, diaphragm, motor) and the failure mode arrow ( `FC` fail closed, `FO` fail open, `FL` fail last) tell you the safety behavior when air/power is lost — critical information that never appears on electrical drawings. - **Instrumentation letters** — `I` indicator, `R` recorder, `A` alarm, `S` switch, `Y` relay/convertor: the bubble `PIT` (pressure indicating transmitter) describes both display and transmission. ## Checking the Drawing Set Drawings lie — they age, get revised incompletely, and inherit as-built drift. A field check method: pick one loop, verify it end to end — P&ID (instruments and function), loop diagram (wiring and power), I/O list (channel and terminal), schematic (actual terminals), and PLC tag (address and scaling) — and record every mismatch. Plants that do this regularly find their drawings become trustworthy again, which pays off in every later project. ## Summary Electrical schematics answer "how is it wired," P&IDs answer "how is it instrumented." Read schematics by reference designation, cross-reference, and wire number; read P&IDs by instrument bubble, function letters, and failure-mode symbols; and always reconcile both against the I/O list and loop diagrams. The drawing set is the plant's shared memory — the engineer who reads it fluently operates with evidence, not assumptions. ## Apply this to your project - Agree the test boundary and site authorization, especially where outputs or communications can affect operating equipment. - Record expected and actual values, equipment identifiers, software revision, test conditions, and exceptions. - Retest changed behavior and archive the final configuration with the test record and recovery instructions. [Follow an ASP project guide](https://wiki.aspotomasyon.com/guides) to connect the fundamentals with implementation and acceptance. ![Engineering review sequence: Prepare → Execute → Resolve → Release. Keep the recorded evidence with the accepted configuration.](https://wiki.aspotomasyon.com/assets/diagrams/technical-guides-review.svg) *Engineering review sequence: Prepare → Execute → Resolve → Release. Keep the recorded evidence with the accepted configuration.* ## Primary references and further reading Use the original specifications and product documentation for implementation details. The examples in this guide are illustrative engineering scenarios, not published project results. - [NIST SP 800-82 Rev. 3 — OT configuration and operational considerations](https://csrc.nist.gov/pubs/sp/800/82/r3/final) - [ISA — Automation standards and committees](https://www.isa.org/standards-and-publications/isa-standards/isa-standards-committees) --- Cite the canonical article: https://wiki.aspotomasyon.com/article/reading-electrical-schematics (Reading Electrical Schematics and P&IDs: An Engineer's Guide, ASP OTOMASYON, updated 2026-09-13).