FM 55-509-1

CHAPTER 20

THE ELECTRICAL CIRCUIT

INTRODUCTION

The basic items found in the ship’s distributionhave been presented. Power-consumers, such asmotors and resistors, and those nonpower-consumingdevices, such as circuit breakers and switches, havebeen examined. Generators, through the distribu-tion system, provide power to the loads and switchesthat control or protect those loads. How these loadsare controlled and protected between the last light-ing or power panel will now be discussed.

WIRING SCHEMATICS

Diagrams are used to accurately portray theelectrical system. Over the years, many techniqueshave been used to simplify the diagram for the reader.These attempts often produced more questionsthan they answered. Symbols were not stan-dardized, and pictorial schematics showed the electri-cal system in various degrees of accuracy. Often theillustrator took for granted that his codes could beunderstood. In effect, there were no industry stan-dards. Although each diagram might be electricallyaccurate, it was not developed for uniform individualinterpretation. Today, as electrical systems becomemore complex, the electrical community has adoptedspecific standards to allow a more universal com-prehension of the electrical circuits they describe.Up-to-date industry standards have been presentedthroughout this text. However, you will still findmany variations due to physical constraints, cost, andthe broad time span encompassing our fleet.

BASIC DIAGRAM

Chapter 15 used a one-line diagram of theship’s distribution system in describing the powersupply and its distribution to individual loads. Theone-line diagram identified the main feeder andbranch circuits. Major loads and controls were alsoidentified. This provided abroad overall view of themain electrical system. This information, althoughuseful in certain applications, falls short of telling thecomplete story.

The circuit extending from the last overcurrentprotective device in the lighting or control panel iscalled a branch circuit. The branch circuit canthen be further divided into two more circuitswithin a motor controlling enclosure (motor con-troller). These circuits are called the power andcontrol circuits.

Power Circuit

The power circuit usually consists of heaviercables used to carry the higher currents necessary tooperate large components. Power circuits can bethree-phase, single-phase, or direct current. In themajority of cases, the power circuit will always carrythe highest current or voltage from the branch circuit.

Control Circuit

The control circuit is derived directly from thepower circuit. The control circuit provides power tothe timers, relays, and switches necessary to controlthe operating contacts of the main component in thepower circuit. The control circuit “controls” thenormally open contacts in the power circuit that turnon or turn off the main component.

The control circuit is almost always a single-phase derivative from a three-phase power circuit.The control circuit will almost always consist ofcables intended to carry less ampacity or low voltagesthan the power circuit.

The control circuit provides the logic behindthe operation of the main component in the powercircuit. The heavy vertical lines, L1 and L2, areconnected to the distribution system in an immediateand convenient manner. The control circuit consistsof an electrical load, the pilot light, and a controldevice (the float switch). Whenever the float switchrises and completes a circuit between L1 and L2, thepilot light will light.

The pilot light in Figure 20-1 could just as easilybe replaced with a relay. If the relay physically

20-1

FM 55-509-1

operated three normally closed contacts and thesecontacts were placed in the power supply lines ofa three-phase motor, then the motor operationwould indirectly be controlled by the float switch(Figure 20-2).

As long as the float switch was in the openposition (down), the E relay would not be energized.The contacts the E relay controlled would be closed,and the pump motor would run. When the float rosesufficiently to complete the control circuit, the Erelay would become energized. When the relay wasenergized, all its contacts would change position.This means that the three E contacts would open thepower circuit to the pump motor, and the motorwould stop. This three-phase circuit is controlledwith a simple single-phase circuit. The coil codeletter E is used to make a point E simply showspossession. All E contacts are controlled by the Ecoil. An E coil does not control an X contact or anyother contact not labeled E.

LINE DIAGRAM

The line diagram, or ladder diagram, is con-structed to show the basic operation of the electricalcontrol circuit and explain the process, in a logicalorder, of the electrical sequence of events. Thisdiagram does not show the actual wiring present inthe system and may even eliminate actual connec-tions not necessary for the understanding of thecircuit’s operation.

The line diagram shows specifically–

The power source supply lines provided bythe power circuit, represented in heavierblack lines generally running vertically.

The control circuit, containing the control-ling devices and the loads, represented bythin lines, generally running horizontally.

The relationship of the control devices tothe loads they control.

Figure 20-3 shows another line diagram. Theoperating coil and the pilot light represent theelectrical loads in this control circuit. The stop, start,auxiliary contacts, and overload contacts representthe controlling devices.

L1 and L2 are the power-supplying lines fromthe ship’s distribution system branch circuit. L1 andL2 provide the difference in potential (voltage)

20-2

FM 55-509-1

necessary to operate the control circuit components.The actual connection of L1 and L2 to the electricalsystem is often left out. It is, however, readily visiblewhen the actual circuit is inspected. Some of themore common connection points for L1 and L2 arethe magnetic motor starter terminals, disconnectswitch, or a small step-down transformer within thecontrol circuit enclosure.

Figure 20-4 shows the line diagram from theLCU 2000 emergency generator control circuit. Theline diagram is designed like a ladder. The heavyvertical lines represent the power supply. The verti-cal TB1 line represents the terminal that suppliespositive potential from the DC batteries, and theGRD vertical line represents the node of the negativebattery potential. The power circuit in this casereceives its power from the batteries, BT1 and BT2.

The light horizontal (and some vertical) linesrepresent the control circuit. The line diagram isdesigned mainly to show the operation of the controlcircuit and not the power circuit. In this case, thelargest load is the starter motor, incorporated in theline diagram. There is no reason to make a distinc-tion between a “power” circuit and a “control” circuitbecause there is no voltage charge.

Each of the circuits contain only one electricalload. This is because the electrical system is basedon parallel connections. Most loads have the samevoltage requirement as the other electrical loads inthe same circuit. In parallel-connected circuits, thevolt age is a constant across each branch circuit. Anyloads in series must equal the applied voltage avail-able in each branch of the line diagram (ETbranch= Elbranch + E2branch).

20-3

FM 55-509-1

20-4

FM 55-509-1

The simple design of the line diagram is agraphic representation of operation, not the physicalplacement or the actual electrical connections. Theline diagram needs to be consulted anytime a load isnot energizing. By identifying the component thatis not functioning, you can then determine thecontrol devices, switches, and protective devicesthat might have prevented a completed circuit tothe component.

Figure 20-4 identifies the starting motor andcontrol circuit. Check the legend in Table 20-1 forthe appropriate symbol or alphabetic/numericalcode.

The vertical power lines are supplied from thebatteries, BT1 and BT2. This identifies the source ofpower for the starter motor. Next, the starter motor,Bl, is identified.

NOTE: In the case of a starting motorand solenoid, there will always be twounusual parallel loads. This nature ofthe operation will be explained asrequired.

One circuit is completed directly from the bat-teries to the starter motor (Figure 20-5). The directbattery connection is a dashed line. A secondcircuit,a dotted line, provides additional control of thestarter motor.

As Figure 20-6 shows, when all contacts areclosed in the dotted and dashed circuits, a differencein potential exists across the starter motor arma-ture and the solenoids. This causes the starter tooperate.

20-5

FM 55-509-1

Wiring Diagram

Now that some components and controldevices have been identified on the line diagram, thewiring diagram must be consulted to locate the actualterminal connections and component locations. Fig-ure 20-7 shows the actual equipment instrumentpanel. The equipment shows a complex system ofwires and components, some of which you are seek-ing. The wiring diagram will simplify this search.

The wiring diagram shows the actual com-ponent location and the physical run of the wires. Italso shows some component parts. Figure 20-8shows the electrical interior of the starter motor andsolenoid.

Figure 20-9 shows the wiring diagram. Theright side door (rear inside) view is presented in thesame perspective as you would see if you were look-ing directly into the open panel. You see the inside

20-6

FM 55-509-1

of the open panel door, the back wall of the cabinet(inside view), and the bottom of the cabinet (insideview) in the wiring diagram in the same way as it ispresented on the equipment with the door open foryour inspection. The wiring diagram provides adetailed presentation of actual component anddevice, as well as terminal connections for theequipment. Ensure the equipment is not modifiedfrom the wiring diagram.

These views are separated by dashes which indi-cate the actual structure of the surrounding panel.The engine harness on the outside of the dashesmeans that these components are not located

within the control panel. These components arelocated elsewhere on the equipment. The itemsare relatively large and readily identifiable. Thestarter motor and batteries are identified here.

From the line diagram (Figure 20-5), we deter-mined the need to find the CB-15 circuit breaker; theK-12, K-14, and K-16 contacts; TB-1-1, TB-1-2, andB-1; and the GRD. These are all the components inthe starter motor control circuit. Look for the iden-tification markings on the wiring diagram. These aredotted lines. Notice how they are spread throughoutthe compartment. All the terminals are marked inthe same manner that they were marked on the linediagram.

The BT1 and BT2 batteries and the B-1 startingmotor from the line diagram are also identified withdased lines. Now testing and replacement can begin.The larger batteries and starter motor are easilylocated outside the control panel. The small con-trolling devices are located within the control panelexactly as they appear on the wiring diagram.

Additional Diagram Aids

Following a line diagram, such as Figure 20-4,can be very involved. When it becomes necessary tounderstand the entire sequence of events in theoperation of a particular component, failing to inter-pret any of the controlling devices will circumventany well-intentioned investigation. The line diagramcan be made easier to follow when the horizon-tal lines are numbered. Many manufacturershave already numbered their diagrams to aid theengineer in troubleshooting. If the manufacturer hasnot done this already, it is advantageous to do thisyourself.

CAUTION

Do not write over existing prints orpermanently mark the schematicsin controllers or other electricalcomponents. Instead, use a greasepencil or make a copy from a tech-nical manual. Maintain existingdiagrams in their original condi-tions and ensure they are alwayslegible. Note any modifications toa system in the logbook andprocure updated diagrams.

20-7

FM 55-508-1

20-8

FM 55-509-1

Figure 20-10 is a properly numbered line to bottom. The line numbers are always located ondiagram. The important horizontal lines are iden- the left side of the line diagram. Use a straight edgetified with a number, in numerical sequence from top to ensure accuracy.

20-9

FM 55-509-1

The right side of the line diagram has a numberon only those lines where a contactor, relay, orsolenoid actually operates contacts. The K-11 relay,for example, is located on line 1. The number to theright side of the line diagram indicates two things:

There is a component on this line thatcontrols another part of the circuit (theK-11 relay itself).

The location of the items being controlled.

The number 5, to the right of line 1, indicatesthat a set of normally open (NO) contacts exists online 5. If the number to the right of the line diagramwas underlined, such as the 17 at the bottom right ofthe diagram, then this would indicate that you arelooking for a contact that is normally closed (NC).

A diagram always illustrates contacts, switches,and devices in their de-energized position. They arepictured in the position they are in when the deviceis unaffected by an outside force.

The force that changes the position of contactscan come from any number of places. For example,the force can be the electromagnetic force from arelay coil becoming energized and physically movingan armature and changing the position of its contacts.The force can also be exerted from a finger, such asthe S-11 RUN/AUTO switch.

A normally open (NO) contact means that thecontact’s magnetic coil, for instance, has not yet beenenergized. Therefore, when the coil becomes ener-gized, the normally open contact closes, and a nor-mally closed contact would open.

BASIC CIRCUIT LOGIC

Electrical components are confined by theseries and parallel rules learned earlier. These rulesare essential in the understanding of the electricaldiagram. To place the series and parallel rules intoperspective, it is necessary to reexamine the linediagram. Every resistor, motor, coil, or indicatinglamp is designed to operate at a specific voltagevalue. If all these loads require 24 volts DC and theyare connected in parallel, then the voltage supply canproperly provide 24 volts to each device. If as few astwo 24-volt components were connected in series, the24-volt power supply could not provide enough volt-age to operate them properly. For this reason, loads

are generally restricted to one load per line. Eachcomponent is provided with access to a positivepotential and a negative potential. In alternatingcurrent, this is still true. AC provides alternatingdifferences in potential 120 times a second at 60hertz.

Control Device Locations

Components that consume power are alwaysconsidered electrical loads. Control devices arethose items that interrupt a circuit for specificreasons. Control devices should not consumepower. A push button, contact, and pressure switchare components that do not consume power becausethere is no resistance to the flow of current when theyare closed. When these devices are open, the cir-cuit is broken, and current cannot flow. It is in theengineer’s favor to locate all controlling devices inthe same branch circuit as the component he isinvestigating. It is easier to troubleshoot a systemwhen these components and their relationship to theload become identified. Control devices aregenerally located between L1 and the load. Thelocation is subject to the constraints of room andcost and thus may be placed elsewhere in the cir-cuit out of necessity.

Overload Placement

When overload protective devices are used incontrol circuits as a means of protecting motors fromoverload conditions, they will be located between thecontrol circuit load and L2. Figure 20-11 shows themagnetic motor starter coil and an overload. Theoverload de-energizes the control circuit when itopens. The is not to protect the control circuit, butrather the motor located in the power circuit notshown.

When the overload device is used to protect thecontrol circuit, such as a fuse or circuit breaker, thenit will be located in the power supply line before thecontrol circuit wiring (Figure 20-12).

STARTER MOTOR OPERATION OF THE LCU2000 EMERGENCY GENERATOR

To provide an insight into the function of acontrol circuit and the application of electri-cal schematics, the emergency diesel generatorstarting system for the 2000 series LCU will be

20-10

FM 55-509-1

addressed. This is a 24-volt DC system. All the rulesof electricity apply to this DC control circuit in thesame way as their relationship applies to the ACcontrol circuit. In the application of line diagramsand control circuits, there is basically no differencein determining the logical function of a circuit.If this was an AC line diagram, the first thing theengineer must do is to establish an imaginary direc-tion for current to flow. In other words, he will“magically” stop time with the AC in a perpetualstate of single direction current flow. In AC con-trol circuits (without semi-conductors), it does notmatter if he chooses his direction of current flowfrom L2 to L1 or from L1 to L2. The only thing thatmatters is consistency. Only in this manner can alogical sequence of events be discovered.

The lime diagram will be used to follow theprogress of the starting system sequence of events.The following discussion will be restricted to thestarter motor as closely as possible to eliminate con-fusion. Keep in mind that the difference in potentialis available to many other circuits within this systemthrough the same nodes. Any time a positive nodeand a negative node have their different potentialsjoined through a load, the load can become ener-gized, and that device should function.

The interpretation of the line diagram startswith the concept of a node. The node is an excep-tionally important concept. The schematic symbolrepresents the node as a solid dot indicating a con-nection of two or more wires (Figure 20-13).

Kirchhoff’s Current Law states that thealgebraic sum of the currents entering and leav-ing a node is zero. In other words, the sum of thecurrents entering a node must equal the sum of thecurrents leaving anode

I in= I out

As purposeless as it may sound at first,Kirchhoff’s description of the node holds a veryimportant meaning to the understanding of thesequence of events in the electrical system. Thefollowing definition of a node takes a few liberties. Anode is an electrically conductive point in thediagram that does not consume power. The size ofthis point is restricted only by opened circuit devices,such as open contacts and open switches, or theexistence of a power-consuming component, such asa motor, resistor, light bulb, or solenoid.

20-11

FM 55-509-1

bulb terminal connected anywhere on the dashed lineand the light bulb will light. In Figure 20-16, bothlight bulbs A and B will operate. In a parallel circuit,the node represents the same point as the connectionmade to the generator or battery terminal directly.

There are two nodes we are always concernedwith on the line diagram: the node of positive poten-tial and the node of negative potential. Whenever aload is connected between these two nodes, currentflows through the device, and it becomes energized.In Figure 20-14, the current entering the node at L2must equal the current leaving the node to the threeother electrical power-consuming devices (loads Rl,R2, and R3).

Figure 20-15 is a normal parallel circuit. Allthree loads, Rl, R2, and R3, have their polaritiesmarked. The positive node combines all the connect-ing wires between the positive terminal, Ll, and theelectrical load terminals of the same polarity. Theseare dotted lines. Another node combines all thenegative areas between the L2 terminal and theelectrical loads of the same polarity. These aredashed lines.

The dotted line node is the positive potential ofthe circuit; the dashed line node is the negativepotential of the circuit. Anytime an electrical loadis connected between a difference in potential, cur-rent will flow, and the component will be energized.

Starting Motor Circuit

Any electrical load connected between bothnodes at any place will energize. An additionallight bulb, for example, can have one bulb terminalconnected anywhere on the dotted line and the other

20-12

FM 55-509-1

This section presents the basic starting motorcircuit. The use of the emergency generator starterand charging circuit for the 2000 series LCU con-tains many additional variables. The automaticemergency starting functions, electronic governor,fuel module, and alternator circuit are also incor-porated in the following diagram. So the circuit canbe analyzed by the lime and wiring diagrams, thestarter motor will be started by the most directmethod possible keeping with the actual sequence ofevents in the process. Solid-state DC circuitry andelectronic governor control will not be addressed atthis time. For additional information and all pos-sible production updates, consult the applicabletechnical manual.

Developing the Node

Any device that does not consume power, suchas a closed set of contacts, a circuit breaker, or stoppush button (closed), becomes part of that node.Figure 20-17 shows the engine control line diagramnodes. The dotted lines indicate the positive node,and the dashed lines indicate the negative node.Anywhere a voltmeter is connected between thedotted and dashed lines, a reading from the powersource should be observed. This reading indicates adifference in potential. In this case, about 24 voltsDC should be noted from the batteries.

An open defines (establishes) a difference inpotential in the branch circuit of Figure 20-18. Thistakes precedence over any other item. If there is anopen to either side of a load, then current does notmove, and the difference in potential is establishedby the open. The node will extend through the loadto one of the open terminals. The same potential (inthis case, negative) will exist on each side of the load.If there is no difference in potential, then there is novoltage to be measured.

Second in priority is a power-consuming devicethat current actively moves through as shown inFigure 20-19. The voltage consumed, pushing currentthrough the load, defines the difference in potential.

Only when there is a completed circuit to theload does the difference in potential separate on eachside of the load. If there is an open to both sides ofa load, then the outer open terminals connecteddirectly to the power circuit define the furthest

reaches of the node. In Figure 20-20, neither nodeextends to or through the load.

Another power supply or capacitor may definea difference in potential in the branch. Care must beused when analyzing voltage readings.

If a difference in potential is not separated(defined) by any of the above mentioned componentsor devices, then the circuit is short-circuited.

A difference in potential is an imbalance ofnature’s atom. The negative electrons are at onenode, and the positive ions are at the other node.When an adequate path is completed between thetwo nodes, the electrons move (current flows) to thepositive terminal, energizing any electrical load theypass en route.

When a normally open switch closes, the nodeis extended as shown in Figure 20-21. Pressing andclosing the RUN/AUTO switch S-11 extends thepositive node to a load.

When a positive and negative node (the twodifferences in potential) are actually permitted toreach the load, the load becomes energized by theelectrons. The electrical load, in this case relay K-11,becomes energized. K-11 controls its normally opencontact online 5. The normally open contact labeledK-11 on line 5 now closes (Figure 20-22).

The dotted positive node has been extended toseveral circuits: the engine fault bypass (S-11), theengine fault indicator (DS-12), and the circuit tostarter relay K-12.

The positive node is temporarily extended tothe overspeed trip (S-3) and the starter relay K-12and through the CB-11 and CB-12 circuit breakers.This is temporary because these thermal circuitbreaker elements have a relatively high resistance tothem. Unless the oil pressure builds sufficiently toclose the oil pressure switch (S-1) and shunt thecurrent around the thermal elements, the circuitbreakers will open. This provides a limited period oftime for the generator to operate before the pressure(S-1) and temperature (S-2) switches activate andcontrol the relay K-12.

Figure 20-23 shows the relay K-12 energizing.K-12 has two NO contacts. NO K-12 contact closes

20-13

FM 55-509-1

20-14

FM 55-509-1

Figure 20-23 shows the relay K-12 energizing.K-12 has two NO contacts. NO K-12 contact closeson line 12 and extends the positive potential to thefollowing circuits:

M-11, the electronic oil pressure gauge.

M-12, the electronic water temperaturegauge.

M-13, the hour-meter gauge.

K-1, the fuel solenoid. This provides fuelto the diesel engine for starting.

The K-12 relay also has contacts it influencesonline 17. The NO K-12 contacts close and completethe following circuits:

A-1, the electric governor control.

VR-11 and CB-13, for current monitoring.

K-13, a 24-volt relay.

NOTE: K-13 energizes with the startingsystem long enough to bypass currentaround the thermal elements of CB-11and CB-12. After the diesel starts, theoil pressure switch closes, and K-13contacts are no longer needed.Moments later, relay K-13 de-energizes.

B-1, the starter motor solenoids.

When the difference in potential is extendedto the starting motor solenoids, the starter motorcontacts close, and the starter motor revolves(Figure 20-24).

20-15

FM 55-509-1

20-16

FM 55-509-1

2 0 - 1 7

FM 55-509-1

20-18

FM 55-509-1

STARTER MOTOR SOLENOID

The starter solenoid has two different coils.Both of these coils are needed to shift the starterpinion (Figure 20-25) into mesh with the flywheel andto close the solenoid contacts.

Pull-In Coil

The pull-in coil is pictured as the coil in thestarter B-1 with the vertical terminals in Figure 20-8.The pull-in coil is made of heavy copper conductors.This is necessary because the current that is going togo through the armature and series winding will alsogo through the pull-in coil. The armature, serieswinding, and pull-in coil are all heavy-gauge copperconductors of low resistance. The current draw by aslow-moving series motor is enormous.

The high current going through the pull-in coil,acting in conjunction with the hold-in coil (shown inFigure 20-8 with horizontal terminals), pulls the shift-ing fork and moves the pinion into position with theflywheel. If this extremely high current were to passthrough the pull-in coil for more than a moment, thepull-in coil would overheat and burn up. As theshifting fork is pulling the pinion into position withthe flywheel teeth, contacts S-1 in the starter motor(Figures 20-24 and 20-26) close and eliminate thepull-in coil from the circuit. Notice how both sides

of the pull-in coil have the same positive polarity (andtherefore no difference in polarity) in Figure 20-24.

The starter motor series field and armature arenow directly connected to the battery voltage, and thestarter armature rotates. Even though the pull-in coilis eliminated from the starting circuit, the S-1 con-tacts remain closed. This is because of the hold-incoil.

Hold-In Coil

The hold-in coil is a thin-diameter conductor.There are many turns of this conductor. A muchhigher resistance exists than existed in the pull-in coil.Together the pull-in and the hold-in coil were neces-sary to shift the pinion into position. Once the ironcore of the solenoid was positioned completelywithin the solenoid field, less magnetic force wasnecessary to retain it in position. The hold-in coilmaintains the S-1 contacts closed until the dieselstarts, and the circuit is de-energized.

Once the diesel starts, the alternator producespower and energizes coil K-14 (on line 22), or thevoltage regulator energizes coil K-16 (on line 18) andproves the generator is actively producing power(Figure 20-27). Contacts K-14 and K-16 on line 17open and disconnect the starter motor from the cir-cuit. Relay K-13 is also de-energized, and now the

20-19

FM 55-509-1

oil pressure switch (S-1) and the water temperature governor control (A-1) and the fuel solenoid (K-1)switch (S-2) monitor the safe operation of the genera- with the now closed contacts from the K-12 relay.tor prime mover by controlling the circuits to the

20-20

FM 55-509-1

20-21