In many industrial systems, the repair process involves using ladder diagrams for troubleshooting. Unfortunately, these drawings frustrate many people in the field. However, if you understand a few fundamentals, then ladder drawings make sense and become easy to use.

Four fundamentals include:

  1. It's called a ladder diagram because it really does resemble a ladder. Ladders have two vertical rails that are connected by horizontal rungs.
  2. The drawing will show you two factors: power and control. If you're looking at a 480V drive motor for a conveyor system, then everything in the chain of 480V to the motor is “power,” including the breaker, overloads, and starter. The control part not only includes solenoids and control relays, but also signal lights and annunciators.
  3. When you look at the drawing, the flow of power or control is from left to right. Input is on the left; output or result is on the right. The flow also goes from top to bottom, unless otherwise noted.
  4. Usually, the diagram deptcts the system in its unpowered state (the power switch is shown open). As you apply power (close the switch), you follow state changes across to the right rail and see, for example, which coils should pick up to change the state of which contacts. If you measure something different with your DMM at a given point, then you have located a problem (for example, a coil that didn't pick up).