Electrical Symbols and Line Diagram

7/28/2019 Electrical Symbols and Line Diagram

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Electrical Symbols and LineDiagrams

Chapter 3

Material taken from Chapter 3 of Electric Motor Controls, G. Rockis, 2001

One-Line Diagrams

One-line diagram a diagram that usessingle lines and graphic symbols toindicate the path and components of anelectrical circuit.

One-line diagrams are used when

information about a circuit is requiredbut detail of the actual wire connectionsand operation of the circuit are not.


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Line DiagramsA line (ladder) diagram is a diagram that

shows the logic of an electrical circuit orsystem using standard symbols.

A line diagram is used to show therelationship between circuits and theircomponents but not the actual locationof the components.

Line diagrams provide a fast, easyunderstanding of the connections anduse of components.

Electric Motor Controls, G. Rockis, 2001


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Wiring Diagrams

Wiring (connection) diagram adiagram that shows the connection ofan installation or its component devicesor parts.

Wiring diagrams show, as closely as

possible, the actual location of eachcomponent in a circuit, including thecontrol circuit and the power circuit.


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Manual Control Circuits

Manual control circuit any circuit thatrequires a person to initiate an action for thecircuit to operate.

A line diagram may be used to illustrate amanual control circuit of a pushbuttoncontrolling a pilot light.

A line diagram may be used to illustrate thecontrol and protection of a 1 motor using amanual starter with overload protection.


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Automatic Control CircuitsAutomatically-controlled devices have

replaced many jobs that were onceperformed manually.

As a part of automation, control circuitsare designed to replace manual devices.

Electric Motor Controls,

G. Rockis, 2001


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Magnetic Control CircuitsAlthough manual controls are compact

and sometimes less expensive thanmagnetic controls, industrial andcommercial installations often requirethat electrical control equipment belocated in one area while the loaddevice is located in another.

Solenoids, contactors, and magneticmotor starters are used for remotecontrol of devices.

Solenoids

A solenoid is anelectrical devicethat convertselectrical energyinto a linear

mechanical force.


G. Rockis, 2001


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Contactors

Contactor a control device that uses asmall control current to energize or de-energize the load connected to it.

A contactor does not include overloadprotection.

A contactor has a frame, plunger, andcoil like a solenoid.


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Contactors The action of the plunger, however, is

directed to close (or open) sets ofcontacts.

The closing of the contacts allowselectrical devices to be controlled fromremote locations.



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Magnetic Motor Starters

A magnetic motor starter is anelectrically-operated switch (contactor)that includes motor overload protection.

Magnetic motor starters are identical tocontactors except that they have

overloads attached to them.


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Magnetic Motor Starters The overloads have heaters or

electronic overloads (located in thepower circuit) which sense excessivecurrent flow to the motor.

The heaters open the NC overloadcontacts (located in the control circuit)

when the overload becomes dangerousto the motor.



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Logic Applied to LineDiagrams

Chapter 4

Material taken from Chapter 4 of Electric Motor Controls,

G. Rockis, 2001

Basic Rules

The electrical industry has established auniversal set of symbols and rules onhow line diagrams (circuits) are laidout.


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One Load Per Line No more than one load should be

placed in any one circuit line betweenL1 and L2.

A pilot light can be connected into acircuit with single-pole switch.



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One Load Per Line Two loads must not be connected in series on

one line of a line diagram.

If the two loads are connected in series, thenthe voltage between L1 and L2 must divideacross both loads when S1 is closed.

The result is that neither device receives theentire 120 V necessary for proper operation.

One Load Per Line

Loads must beconnected inparallel when morethan one load mustbe connected in theline diagram.

This circuit has twolines, one for thepilot light and onefor the solenoid.



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Load ConnectionsA load is the electrical device in the line

diagram that uses the electrical powerfrom L1 to L2.

Control relay coils, solenoids, and pilotlights are loads that are connecteddirectly or indirectly to L2.



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Load Connections Magnetic motor starter coils are

connected to L2 indirectly throughnormally closed overload contacts.

Anywhere from 1 to 3 NC overloadcontacts are shown between thestarter and L2 in all line diagrams.

To avoid confusion, it is common

practice to draw one set of NCoverload contacts and mark thesecontacts all overloads (OLs).



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Control Device Connections Control devices are connected between

L1 and the operating coil (or load).

Operating coils of contactors andstarters are activated by control devicessuch as pushbuttons, limit switches,and pressure switches.

Figure 4-5. Control devices are connected between L1 and

the operating coil.



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Control Device Connections Each line includes at least one control

device.

The operating coil is ON all the time ifno control device is included in a line.

A circuit may contain as many controldevices as is required to make the

operating coil function as specified.



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Line Number Reference Each line in a line

diagram should benumbered startingwith the top lineand reading down.


Numerical Cross-ReferenceSystems

Numerical cross-reference systems arerequired to trace the action of a circuitin complex line diagrams.

Common rules help to quickly simplifythe operation of complex circuits.


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NO Contacts Relays, contactors, and magnetic motor

starters normally have more than one set ofauxiliary contacts.

These contacts may appear at severaldifferent locations in the line diagram.

Numerical cross-reference systems quicklyidentify the location and type of contactscontrolled by a given device.

A numerical cross-reference system consistsof numbers in parenthesis to the right of theline diagram.


G. Rockis, 2001


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NC Contacts In addition to NO contacts, there are

also NC contacts in a circuit.

To differentiate between NO and NC,NC contacts are indicated as a numberwhich is underlined.


G. Rockis, 2001


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Wire-Reference Numbers Each wire in a control circuit is

assigned a reference point (number)on a line diagram to keep track of thedifferent wires that connect thecomponents in the circuit.

Each reference point is assigned areference number.

Reference numbers are normallyassigned from the top left to thebottom right.


G. Rockis, 2001


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Manufacturers Terminal

Numbers Manufacturers of electrical relays, timers,

counters, etc., include numbers on the terminalconnection points.

These terminal numbers are used to identifyand separate the different component parts(coil, NC contacts, etc) included on theindividual pieces of equipment.

Manufacturers terminal numbers are oftenadded to a line diagram after the specificequipment to be used in the control circuit isidentified.


G. Rockis, 2001


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Signals, Decisions, and ActionA circuit must respond as designed,

without any changes.

To accomplish this consistency, allcontrol circuits are composed of threebasic sections: the signals, thedecisions, and the action sections.



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SignalsA signal starts or stops the flow of

current by closing or opening thecontrol devices contacts.

Current is allowed to flow through thecontrol device if the contacts areclosed.

Current is not allowed to flow throughthe control device if the contacts areopened.

Signals

Pushbuttons, limit switches, flowswitches, foot switches, temperatureswitches, and pressure switches may beused as the signal section of a controlcircuit.


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SignalsAll signals depend on some condition

that must take place. This condition canbe manual, mechanical, or automatic.

A manual condition is any input into thecircuit by a person. Foot switches andpushbuttons are control devices that

respond to a manual condition.

Signals

A mechanical condition is any input intothe circuit by a mechanically movingpart.

A limit switch is a control device thatresponds to a mechanical condition.


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SignalsAn automatic condition is any input

which responds automatically tochanges in a system.

Flow switches, temperature switches,and pressure switches respond toautomatic conditions.

Decisions

The decision section of a circuitdetermines what work is to be doneand in what order the work is to occur.

The decision section of a circuit adds,subtracts, sorts, selects, and redirects

the signals from the control devices tothe load.


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Decisions The way the control devices are

connected into the circuit gives thecircuit logic.

The basic logic functions are AND, OR,NOT, NOR, and NAND logic.

The decision section of the circuitaccepts informational input (signals),makes logical decisions based on the

way the control devices are connectedinto the circuit, and provides theoutput signal that controls the load.

Action

Once a signal is generated and thedecision has been made within a circuit,some action (work) should result.

In most cases it is the operating coil inthe circuit which is responsible for

initiating the action.


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Action This action is direct when devices such

as motors, lights, and heating elementsare turned ON as a direct result of thesignal and the decision.

This action is indirect when the coils insolenoids, magnetic starters, and relays

are energized.

Logic Functions Control devices such as pushbuttons,

limit switches, and pressure switchesare connected into a circuit so that thecircuit can function in a predeterminedmanner.

All control circuits are basic logicfunctions.

Logic functions are common to allareas of industry.


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AND Logic AND logic is used in industry when two

pushbuttons are connected in series tocontrol a solenoid.


OR Logic OR logic is used in industry when a

pushbutton and a temperature switch areconnected in parallel.



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AND/OR Logic Combination The decision section of any circuit may

contain one or more logic functions.


NOT Logic

NOT logic has an output if the control signalis OFF.



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NOR Logic NOR logic is an extension of NOT logic in that two

or more NC contacts in series are used to control aload.

In this circuit, additional operator safety is providedby adding several emergency stop pushbuttons(NOT logic) to the control circuit.


NAND Logic NAND logic is an extension of NOT logic in

which two or more NC contacts areconnected in parallel to control a load.



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Memory In industrial control circuits, it is more common

to find pushbuttons with return spring contacts(momentary contacts) than those withmechanically stay held in one position(maintained contacts).

Auxiliary contacts are added to give circuitswith pushbuttons memory.


Start/Stop Stations Controlling Magnetic

Starters



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Two Magnetic Starters Operated by TwoStart/Stop Stations with Common

Emergency Stop


Start/Stop Station Controlling Two or

More Magnetic Starters



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Pressure Switch with Pilot Light

Indicating Activation

Pilot lights are manufactured in avariety of colors, shapes, and sizes tomeet the needs of industry.

The illumination of these lights signalsan operator that any one of a sequenceof events may be taking place.



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Start/Stop Station with Pilot LightIndicating NO Device Activation

Pilot lights may be used to show when anoperation is stopped as well as when it isstarted.


Pushbutton Sequence Control Conveyor systems often require one

conveyor system to feed boxes or othermaterials onto another conveyor system.

A circuit is needed to prevent the pileupof material on the second conveyor if thesecond conveyor is stopped.

A sequence control circuit does not letthe first conveyor operate unless thesecond conveyor has started and isrunning.


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Electrical Symbols and Line Diagram