Everyone in a plant works from the same drawings. The P&ID shows how the process is plumbed and instrumented, and the electrical schematic shows how the panel is wired. Newcomers freeze at the wall of circles and lines, while experienced techs read them like a sentence. In this lesson you click apart a real instrument tag, learn the symbols, and trace a circuit, so that a drawing becomes a sequence you can follow from one end to the other.
🌊 P&ID (Piping & Instrumentation Diagram)
The process map: every vessel, pump, valve and instrument, and the pipes connecting them. It answers "what flows where, and what watches it?" The instrument tags on a P&ID are the same tags you'll address in the PLC.
⚡ Electrical schematic
The wiring map: power rails, contacts, coils, sensors and terminals, usually drawn as ladder rungs. It answers "how does the panel actually connect?" This is what you trace when a circuit doesn't work.
Every instrument on a P&ID carries an ISA-5.1 tag, a short code that tells you exactly what it does. It looks cryptic until you split it. Click each part of the tag below to decode it, then try the others.
The pattern: [variable][function]-[loop number]. So FT-101 = Flow Transmitter, loop 101. LIC-205 = Level Indicating Controller, loop 205. Same loop number = same control loop, so FT-101, FIC-101 and FV-101 all belong together.
| Letter | 1st position (variable) | Later position (function) |
|---|---|---|
| F | Flow | – |
| L | Level | – |
| P | Pressure | – |
| T | Temperature | Transmitter (if later) |
| I | Current | Indicator |
| C | User's choice | Controller |
| T | – | Transmitter |
| V | Vibration | Valve |
| S | Speed | Switch |
| A | Analysis | Alarm |
🧠 Read it left to right: first letter = what's measured, the rest = what the device does. PIC = Pressure Indicating Controller. LSH = Level Switch High. TT = Temperature Transmitter. The bubble's lines tell you where it lives (field vs control room).
You don't need all 300 symbols. These dozen cover most of what you'll meet:
⭕ The instrument bubble tells you location: a plain circle = field-mounted; a circle with a solid line = main control room (on the HMI); a dashed line = behind a panel. A circle inside a square often means a PLC/shared-control function.
An electrical schematic is read like a book: left rail is power, right rail is the return, and each horizontal rung is one circuit. To debug, you trace from the left rail through every contact to the coil on the right. The first open contact is your culprit.
📖 Read rung by rung. Each rung starts at the left power rail and ends at a coil/load on the right. Contacts in series = AND; parallel = OR.
🔢 Cross-references. A number next to a contact points to the rung where its coil lives, which is how you jump between pages to follow a signal.
🔍 Debug by tracing. Coil not energising? Walk left-to-right with a meter; the first contact that's open when it should be closed is the fault.