Reading drawings · Lesson 1 of 7

Types of electrical drawings

The drawing set for a machine or a panel can run to 80 pages. You will not read them all: each type of drawing answers one question. In this lesson you will learn the main types, tell them apart at a glance, and know which one to open when you are standing in front of a fault.

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1Like the drawings for a building

A building has an architectural drawing (where the rooms are), a plumbing drawing (how the pipes run) and an electrical drawing (where the sockets are). Same building, but each drawing shows you something different. A plant is exactly the same: the same panel has a drawing that tells you where the power comes from, a drawing that tells you how the machine works, and a drawing that tells you where each device is mounted.

The rule: before you open the drawing set, ask yourself "What do I want to know?". The question decides the drawing.

The question	The drawing
Where does the power come from and go to? At what ratings?	Single line diagram
How is the motor fed, phase by phase?	Power circuit
How does the machine start and stop?	Control circuit
Which terminal does this wire go to?	Wiring / Terminal diagram
Where is the device mounted in the panel?	Panel layout
Which tank is this sensor on, and what does it measure?	P&ID
What type of cable is it, how long, and where does it go?	Cable schedule

2A tour of the drawings

Tap each type and look at the drawing. You can pinch or press + to zoom and drag the drawing around. All the drawings here are for the same plant: pump -M1 is run from panel +CP1.

8 types of drawings

3Recognise them at a glance

Thick horizontal lines and numbers like kVA, kA and mm² ⇒ Single line.
3 lines side by side labelled L1, L2 and L3 at the top ⇒ power circuit.
Two lines L and N (or +24V and 0V) with coils and push buttons between them ⇒ control circuit.
Boxes shaped like the devices, with terminal numbers on them ⇒ Wiring diagram.
A strip of numbered squares with cables coming in ⇒ Terminal diagram.
Plate, ducts and DIN rails, drawn to scale ⇒ Panel layout.
Tanks, pipes and circles with letters like LT ⇒ P&ID.

Every page has a title block at the bottom right: the drawing name, page number, revision and date. In IEC drawings you will also find the full device reference written like this:

=A1 +CP1 -K1

Sign	Meaning	Example
=	The function, or part of the plant (Function)	=A1 filling line
+	The location (Location)	+CP1 control panel 1
-	The device itself (Product)	-K1 the contactor

Order of the drawing set: it usually goes like this: cover and index → legend page → single line → power circuits → control circuits → PLC inputs and outputs → terminals → panel layout → bill of materials (BOM) and cable schedule. Once you know this order you can find anything in a minute.

4Exercise: which drawing do I open?

You will read a situation from the site and choose the drawing you would open first.

Which drawing?

5From the field

Common mistakes:

An old drawing: the panel was modified and nobody updated the drawing. Check the revision number and the date, and ask for the as-built set.
Looking in the wrong drawing: searching for the coil wire on the single line. A single line has no control at all.
Skipping the legend page: every company has a few symbols of its own. The first two pages of the set will save you hours.

Safety: the drawing tells you what should be there, not what is actually there. Never trust that a circuit is dead because the drawing says so: isolate and lock off (LOTO) and prove for yourself that there is no voltage before you touch anything.

?Lesson quiz

Get at least 5 of 6 right to complete the lesson.

Symbols follow IEC 60617 and ANSI/NEMA (IEEE 315 / NFPA 79); device tags follow IEC 81346. Every company draws a little differently: the legend page at the front of the drawing set is the reference. Your progress is saved on this device only.
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