An example schematic drawing I created to show some standard symbols

I like to learn by seeing examples. To try and show how a typical schematic diagram might look, along with some standard design symbols, I put together these drawings using Microsoft Powerpoint . . . .

Marking up and creating new schematic diagrams are integral parts of doing any mod package design. A sample schematic and block diagram are shown in Figures 1 and 2. They highlight some of the more typical elements. These are simplified circuits meant to convey the basic concepts covered in this and a previous post. Click the images to load larger (more readable) versions of them.

valve control schematic diagram

Figure 1 – A simplified valve-control schematic diagram.

valve control circuit block diagram

Figure 2 – The accompanying block diagram for the schematic shown in Figure 1.

A couple of things are worth pointing out in these pictures. The schematic diagram of Figure 1 shows a simplified 120-volt valve control circuit. The valve itself is not shown for simplicity, only one of the relays that operates it. There are three 2-conductor cables that make up that fictional circuit, and their names are 1ABC234X, 1ABC234Y, and 1ABC234Z. That’s inferred from the scheme shown on the block diagram (Figure 2).

Panel 1 houses the control relay (labeled 42R because it reverses the valve into the closed position) and the circuit breaker. A control hand switch (labeled HS) and red indicator light are housed in Panel 2, and the valve itself has a limit switch used to drive the indication circuit.


Two of the conductor names are “L” and “N”, because “L” connects to the hot leg and “N” connects to the neutral. The “A” and “B” conductors make up the interconnection links between various control elements, and there is a spare conductor (SP) that isn’t used anywhere in the circuit.

One reason the conductors are labeled is to alert the technician who may be doing work inside the panel what voltage levels he is dealing with. If he has to manipulate a wire labeled “L” or “X” or “H” he will automatically know that he is dealing with a direct connection to the power source.

If he is manipulating wires labeled “A” or “B”, as in the above circuit, he knows he is working with a wire that is connected to an element like a fuse, a contact, a relay, or a light. Wire labels are also great accounting tools. They make it easier to troubleshoot circuits and keep track of installation progress.


The devices shown on schematics often have numbers next to them, just like the control relay shown in Figure 1. Those numbers are defined by ANSI and IEEE. The numbers describe the function performed by that device. Here is a short list of some of the most common numbers and their function:

27 – undervoltage relay
42 – running device, usually appended with an “F” or “R” to denote forward and reverse
49 – thermal relay
52 – 120 VAC breaker
62 – time-delay relay
72 – 125 VDC breaker
74 – alarm relay

Click this link to read a good resource that lists them all. It’s a helpful document published by ABB.