September 2012
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Ladder Diagrams vs. Point-to-point Wiring Diagrams

The basics of electrical design and construction

Steven R Calabrese

Steven R. Calabrese
Control Engineering Corp.

Contributing Editor


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Building upon the Back to Basics series (Part 1, Part 2, Part 3) that took us through the hot summer months (hot being an understatement!), I present a piece here that harkens back to the days that I used to teach controls in-house to my mechanical contracting colleagues.

Ladder diagrams and point-to-point wiring diagrams are, how should I say, the same yet different. Both are representations of an electrical/electronic system. Both are used to understand how an electrical system is supposed to be wired to operate. Yet the differences between the two merit some thoughtful insight and discussion, and is what this column seeks to achieve.

Ladder Diagrams

A ladder diagram represents, in schematic form, the logical flow of electrical current. The form is traditionally used to design (from scratch) an electrical control system. Named so because of its resemblance to a ladder (really?), the typical diagram consists of “rails and rungs”. The rails are power, be it 24-volt, 120-volt, or whatever, the left rung is considered the “hot” and the right rung is the “common” or “neutral”.

The rungs are where the logic takes shape. “ladder logic” or “relay logic” are terms conventionally used to describe how logical implementation of switches and relay contacts are depicted in a ladder diagram. Starting from the left, on any given rung of the ladder, a symbol for a switching device will be shown connected to the hot rail. This device can be a simple manual switch, a relay contact (either normally open or normally closed), or perhaps a temperature or pressure actuated switch or some other automatic control device. Moving from left to right on the rung, we may encounter additional switching devices in series or even in parallel, depending upon what’s needed in terms of logic. Finally, at the very right and tied to the common rail, is the “load”. This is an electrical device that can be switched on and off, and in its energized state performs a specific function. The load can be a fan or pump motor (or motor starter), a valve or damper actuator, or a relay whose contacts can serve in the formation of additional relay logic within the same ladder diagram.

Moving down the rails, from rung to rung, we find more ladder logic, more switching on the left, and more loads on the right. A ladder diagram can be as simple as a single rung - think of a 120-volt fan being controlled by a wall switch – or it can consist of dozens and dozens of rungs. Open up the controls compartment of a piece of packaged rooftop equipment, look on the inside of the door for the schematics, and you’ll see what I mean!

A ladder diagram is not a “physical” representation of a wired system. It’s created as the first step in designing an electrical control system. With the ladder diagram generated and the logic verified, a point-to-point wiring diagram can be produced, which brings us to…

Point-to-point Wiring Diagrams

This is where the rubber meets the road. Point-to-points (heretofore referred as PTPs), represent the actual physical wiring of an electrical system. As such, a PTP will show devices as how they’re physically connected to with wires, showing terminal designations of used and even unused terminals. For instance, if a relay has two sets of contacts and only one is being used in the circuit, a PTP will often show even the terminals of the unused set of contacts as well (for added realism!).

So the PTP shows actual wiring, as compared to the ladder diagram, which is more conceptual in nature. The PTP may go so far as to show wire colors and numbers, and may even depict the wires in true color! Of course that doesn’t do you any good unless you’re printing to a color printer or viewing the pdf from a laptop.

So the PTP is considered to be the actual “wiring diagram” for the system/equipment. The PTP is an extension of the ladder diagram, meaning that typically the design process starts with generating the logic and thus the ladder diagram, from which the designer creates the PTP. Back when I was designing electrical control systems, I would pencil draw the PTP from scratch, using the ladder diagram as my reference. I would then CAD the PTP, print it out, and check or highlight each wire against the ladder diagram, to be absolutely certain that my wiring diagram matched the ladder diagram.

Utilizing Both Ladder Diagrams and PTPs

Reliable Controls Ladder diagrams and point-to-point diagrams are both extremely useful in understanding, troubleshooting, and repairing electrical systems and equipment. Used together, they can shed light on both the conceptual design and intended operation, and the physical construction and actual wiring. Having one without the other will put you at a disadvantage, at least in terms of being able to thoroughly understand the system and how it's intended to function.

Summarizing the content presented herein, let’s list in bullet point form the features of each type of diagram:

Ladder Diagrams

PTP Diagrams


Tip of the Month: Next time you come across an equipment submittal with wiring diagrams, see if you can determine whether the diagrams are conceptual, or actual. Hint: if they’re reprinted from a brochure or O&M manual, they’re most likely generic diagrams and not very specific to any given piece of equipment. If they’re on a title block with a date and a specific project name, you can be confident that they’re an accurate depiction of the wiring for the system or equipment that it’s referencing. Careful though…I’ve been duped more than a few times in this scenario!
 


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