What is a single line diagram?

A simplified diagram of a power system that shows each three-phase circuit as one line, with sources, transformers, switchgear, protective devices, busbars and main loads, plus their ratings. It is the master drawing for electrical design and operation.

A piping and instrumentation diagram shows the piping, equipment, valves, instruments and control loops of a process or HVAC system, with tags that identify each instrument and its function.

How do you read an instrument tag like PDT-201?

Under ISA 5.1, the first letters give the measured variable and any modifier (PD = pressure differential), the next letters give the function (T = transmitter) and the number is the loop. PDT-201 is the differential pressure transmitter in loop 201.

Are MEP symbols standardised?

Many are based on IEC 60617 and national standards, but consultants adapt them. Every drawing set has a legend, and it is the authority for that project.

MEP Symbols, SLD & P&ID Reader

Symbols, Legends, SLD & P&ID Reader

Drawings are the language of MEP design. Learn the symbols, read a legend, count from a layout, trace a single line diagram from any load back to its source, and decode the instrument tags on a P&ID.

Symbol library

Common MEP symbols. Filter by discipline and tap a symbol to see what it means and where you will meet it. Symbols vary: the drawing is always the final word.

Flashcards

Name the symbol. Fifteen cards, picked from the discipline you choose.

Read a layout

A small power and lighting layout with its legend. Answer the take-off questions the way you would when checking a drawing or a bill of quantities. Tap the plan to highlight a circuit.

Legend

Take-off questions

Single line diagram

A typical building LV system with a standby generator. Tap any element to learn what it is. Tap a load to trace its supply path. Switch to mains failure to see which loads the generator keeps alive.

EnergisedDe-energisedTraced path

Chilled water P&ID and tag decoder

A variable primary-secondary chilled water system. Tap any valve, pump or instrument. Then decode instrument tags with the decoder.

Tag decoder

Instrument tag

3D switch room and 4D mains failure

The SLD built as a switch room. Play the mains failure: the ATS waits, the generator starts, the essential section transfers, and the mains returns. Tap any equipment.

Drag to rotate · scroll or pinch to zoom · tap equipment

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Engineer's notes

Lessons from real design reviews.

Quiz

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