Electrical Schematics & Circuit Tracing

In this live stream, Bryan walks through electrical diagrams and teaches his apprentices how to use schematics for circuit tracing. Ladder schematics are the most common type of diagram you will see; those and connection diagrams are what we mostly use in circuit tracing.

There are five main types of diagrams: ladder (schematic), point-to-point (connection), pictorial, shop drawings, and as-built diagrams.

Ladder schematics are basic and include common symbols to indicate pressure switches, contacts, power supplies, loads, etc., that illustrate how equipment is wired. They are NOT accurate visual representations of the components. Ladder diagrams connect circuits from one side to another (such as L1 to L2, hot to common, etc.). In the HVAC industry, we usually work with parallel circuits, which make up the majority of the ladder diagrams we see. Series circuits may occasionally appear, but they are much less prevalent than parallel circuits. (Safeties are series of switches; they are NOT series circuits.)

Point-to-point or connection diagrams are becoming increasingly common. They do not illustrate clear sides of the circuit, but they show you each conductor and where those conductors go. You can trace the diagrams relatively easily, so you can tell which components are connected. You can also tell which components are optional and learn more about the physical features of the equipment; you cannot do either of those effectively with most ladder diagrams.

Pictorial diagrams illustrate circuits with visual representations of their components instead of mere lines and symbols. (Motors look like motors, not series of lines and circles.) They are uncommon because they are difficult to draw. However, pictorial diagrams illustrate the connections and the real components very well and are quite easy to work with.

As-built drawings illustrate the equipment as it is built. Other schematics, especially point-to-point diagrams, illustrate optional components. Crankcase heaters are common optional components that you would see on a point-to-point diagram but not an as-built one. Therefore, component identification is a major skill for reading all types of schematics.

Bryan also covers some common symbols on diagrams. One common symbol is a bell (pressure switch). Pressure switches remain closed until a pressure change opens them. Most safeties are single-pole, single-throw. Contacts show up as gaps on schematic diagrams. A normally open contact will be a mere gap, and a normally closed switch has a slash through the gap. Little circles usually represent relay coils, but squiggly lines may also represent relay coils. A capacitor is represented by a gap with one straight line and one curved line on each side of the gap. Arrows indicate time delay circuits.

When it comes to learning schematics, a good exercise is to take apart scrap units and rewire them from scratch with help from the schematic. Learning schematics takes time, patience, and lots of repetition with various equipment.

Remember, always read the manual, check the notes, and refer to the legend!

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