Diagrams
20 Numbering System
Control circuits can range from the very simple to the extremely complex. Ladder diagrams show how a system works and are used for troubleshooting purposes. Wiring diagrams show where equipment is and how it is connected.
Being able to convert from one style of diagram to the other is an essential skill for electricians and people working with control circuits. To accomplish this, we use a simple numbering system.
The numbering system is a way of identifying and “naming” each electrically common point in a circuit. A wire is considered “electrically common” with another wire if they share a direct electrical connection with no switches or loads between them.
Consider the circuit below:
It is a simple three-wire circuit with single stop and start pushbuttons. Single-phase power is supplied from Lines 1 and 2 and a motor coil is the load.
Starting at Line 1, we label the first wire “1.” When we jump through the first device, (the normally closed stop button) we jump up to the next number. Wires 1 and 2 are not electrically common. Coming out of the stop button with wire number 2, we are going to two different places. The first is the normally open start pushbutton, the other is the normally open instantaneous contact. Since there are no loads or devices between the load side of the stop button, the line side of the start button, and the holding contact, these points are considered electrically common and can all share the same number. Jumping through these devices, wire number 3 is connected to the load side of both normally open devices and the motor starter itself, and so these three points are also electrically common.
Once the control circuit has been numbered, we can use that information to help complete wiring diagrams or determine the number of conductors needed in conduits for installation purposes.
In summary:
- L1 and L2 (or L1 and N) are used to designate the control circuit power.
- Assign number 1 to the first wire entering the control circuit.
- Trace through the schematic diagram increasing the numbers given to wires each time you pass through a device.
- Give the same number to all wires that are electrically common, spliced, or connected to the same terminal.
In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.
A diagram that shows how a circuit works logically and electrically. It uses symbols to identify components and interconnecting lines to display the electrical continuity of a circuit. It is often used for troubleshooting purposes. Also known as a ladder diagram.
A diagram shows how equipment is laid out and the connections between them. This type of diagram shows the physical relation of all devices in the system, the conductor terminations between these devices, and are commonly used in motor control installations. Also known as a connection diagram.
Used to convert between wiring and schematic diagrams, the numbering system is a method of identifying and labelling each electrically common point in a circuit. Two wires are considered electrically common if they share an electrical connection with no switches or loads between them, and so would be assigned the same number in the diagram.
Referring to two or more points in a circuit which have no loads or switches between them and have no potential difference between them.
A device for making or breaking the connection in an electric circuit.
A momentary contact device that has a built in spring to return the button to its normal position once release. Available with either normally-open, normally-closed or both sets of contacts.
A contact that under normal conditions has continuity through it. When the contact changes its state it interrupts the flow of current by opening its contacts. Can be associated with pushbuttons, pilot devices or magnetic contactors.
A contact that under normal conditions does not have continuity through it. When the contact changes its state it permits the flow of current by closing its contacts. Can be associated with pushbuttons, pilot devices or magnetic contactors.
A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide overload protection.
