Line Diagrams

  • The method of expressing the language of control symbols is a line diagram, also referred to as a ladder diagram.
  • Line diagrams are made up of two circuits, the control circuit and the power circuit.
  • Electrical wires in a line diagram are represented by lines.
  • Control-circuit wiring is represented by a lighter-weight line and power-circuit wiring is represented by a heavier-weight line.
  • A small dot or node at the intersection of two or more wires indicates an electrical connection.

  • Line diagrams show the functional relationship of components and devices in an electrical circuit, not the physical relationship.
  • For example, the following illustration shows the physical relationship of a pilot light and a pushbutton.

The functional relationship can be shown pictorially with the following illustration.

Reading a Line Diagram

  • This functional relationship is shown symbolically with a line diagram.

  • Line diagrams are read from left to right.
  • Depressing the pushbutton would allow current to flow from L1 through the pushbutton, illuminating the pilot light, to L2.
  • Releasing the pushbutton stops current flow turning the pilot light off.

Power Circuit and Control Circuit

  • The power circuit, indicated by the heavier-weight line, is what actually distributes power from the source to the connected load (motor).

  • The control circuit, indicated by the lighter-weight line, is used to “ control ” the distribution of power.

Connecting Loads and Control Devices

  • Control circuits are made up of control loads and control devices.
  • The control load is an electrical device that uses electrical power.
  • Pilot lights, relays, and contactors are examples of control loads.
  • Control devices are used to activate the control load. Pushbuttons and switches are examples of control devices.
  • The following illustration shows the proper connection of a pilot light (load) with a pushbutton (control device).

  • The power lines are drawn vertically and marked L1 and L2. In this example the voltage potential between L1 and L2 is 120 VAC.
  • The pilot light selected must be rated for 120 VAC. When the pushbutton is depressed, the full 120 volt potential is applied to the pilot light.

Connecting the Load to L2

  • Only one control load should be placed in any one circuit line between L1 and L2.
  • One side of the control load is connected to L2 either directly or, in some instances, through overload relay contacts.
  • In the following example a pilot light is directly connected to L2 on one circuit line.

  • A contactor coil is indirectly connected through a set of overload contacts (OL) to L2 on a second circuit line. This is a parallel connection.
  • Depressing the pushbutton would apply 120 VAC to the pilot light and the “ M ” contactor.
  • Control loads are generally not connected in series.
  • The following illustration shows two ways to connect a control load.

  • In one instance the control loads are improperly connected in series.
  • When the pushbutton is depressed, the voltage across L1 and L2 is divided across both loads, the result being that neither load will receive the full 120 volts necessary for proper operation.
  • If one load fails in this configuration, the entire circuit is rendered useless.
  • In the other instance the loads are properly connected in parallel.
  • In this circuit there is only one load for each line between L1 and L2.
  • The full 120 volts will appear across each load when the pushbutton is depressed.
  • If one load fails in this configuration, the other load will continue to operate normally.

Connecting Control Devices

  • Control devices are connected between L1 and the load.
  • The control device can be connected in series or parallel, depending on the desired results.
  • In the following illustration, the pushbuttons are connected in parallel.

  • Depressing either pushbutton will allow current to flow from L1, through the depressed pushbutton, through the pilot light, to L2.
  • In the following illustration, two pushbuttons are connected in series.

  • Both pushbuttons must be depressed in order to allow current to flow from L1 through the load to L2.

Line Numbering

  • Numbering each line makes it easier to understand more complex line diagrams.
  • In the following illustration, line 1 connects pushbutton 1 to pilot light 1.

  • Line 2 connects pushbutton 2 to pilot light 1. Line 3 connects switch 1 to pilot light 2 and the “ M ” contactor on line 4.

1.Bunty B. Bommera
2.Dakshata U. Kamble

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