A line diagram is constructed to show the basic operation of the electrical control circuit and explain the process, in a logical order, of the electrical sequence of events.
This diagram does not show the actual wiring present in the system and may even eliminate actual connections not necessary for the understanding of the circuit’s operation.
As required by some classification societies, a line diagram generally depicts the following:
- The power source supply lines provided by the power circuit, generally running vertically.
- The control circuit, containing the controlling devices and the loads, generally running horizontally (light horizontal, and some vertical lines represent the control circuit).
- The relationship of the control devices to the loads they control.
- The ratings of machines, transformers, batteries, etc.
- All feeders connected to the main and emergency switchboards.
- Section and distribution boards
- Cable information such as insulation-type, size and current carrying capacity details of circuit breakers and fuses, etc.
Following a line diagram can be very involved. When it becomes necessary to understand the entire sequence of events in the operation of a particular component, foiling to interpret any of the controlling devices will circumvent any well-intended investigation.
The line diagram can be made easier to follow when the horizontal lines are numbered.
Many manufacturers number their diagrams so as to aid the engineer in troubleshooting.
If the manufacturer has not done this, it is advantageous to do this yourself.
Most loads have the same voltage requirement as the other electrical loads in the same circuit. In parallel-connected circuits, the voltage is constant across each branch circuit. Any loads in series must equal the applied voltage available in each branch of the line diagram ( ETbranc = Ebranch1 + E Ebranch2 )
Simple Design of Ship Line Diagram
The simple design of the line diagram is a graphic representation of operation, not the physical placement or the actual electrical connections. The line diagram needs to be consulted anytime a load is not energizing.
By identifying the component that is not functioning, you can then determine the control devices, switches, and protective devices that might have prevented a completed circuit to the component.
The interpretation of the line diagram starts with the concept of a node.
The node is an exceptionally important concept.
The schematic symbol represents the node as a solid dot indicating a connection of two or more wires.
Keep in mind that the difference in potential is available to many other circuits within this system through the same nodes.
Any time a positive node and a negative node have their different potentials joined through a load, the load can become energized, and that device should function.
Kirchhofif’s Current Law
Kirchhofif’s Current Law states that the algebraic sum of the currents entering and leaving a node is zero.
In other words, the sum of the currents entering a node must equal the sum of the currents leaving a node: I in = 1 out
As purposeless as it may sound at first, Kirchhoff’s description of the node holds a very important meaning to the understanding of the sequence of events in the electrical system.
Thus a node is an electrically conductive point in the diagram that does not consume power.
The size of this point is restricted only by open-circuit devices, such as open contacts and open switches, or the existence of a power-consuming component, such as a motor, resistor, light bulb, or solenoid.
Example line diagram of the ship’s distribution system
For example, a one-line diagram of the ship’s distribution system describes the power supply and its distribution to individual loads it is also known as a basic diagram.
The one-line diagram identifies the main feeder and branch circuits which will be explained later on in this chapter.
Major loads and controls are also identified. This provides an overview of the main electrical system. This information, although useful in certain applications.