The distribution system is an extension of the generator. All electrical loads are connected in parallel with the generator terminals through connection points (nodes) at the switchboards and distribution panels. Through proper design, large cables (feeders) provide power to bus bars inside the switchboards and each distribution panel (Figure 15-1). The use of a single feeder cable will eliminate dozens of individual parallel cables that would have otherwise been needed for the connection between the generator and each load.
The distribution system is also designed to protect the overall electrical environment from electrical component casualties. Circuit breakers and fuses are installed in switchboards and distribution panels to separate abnormally operating electrical apparatus from the rest of the system. Each circuit protective device, from the generator to the load, is set at decreasingly smaller ampacity increments. When all overcurrent and short circuit protective devices are properly selected and correctly adjusted, selective tripping can be provided. Selective tripping allows an abnormal circuit to be separated from the electrical distribution system very close to the fault.
The switchboard receives power directly from the generators. In turn, feeders extend from the switchboard circuit breakers to the power distribution panels, lighting distribution panels, or large motor circuits. Branch circuits leave the distribution panel circuit breakers and provide power to individual loads.
To understand the type of electrical system used by the Army's marine field, the term "grounding" must be understood. This is the most misunderstood term in electricity today. For the most part, Army vessels have an entirely ungrounded current-carrying system. However, every electrical device is grounded.
The term "ground" refers solely to physical connections between conductive materials and the earth. Whether or not this conducting connection carries current is irrelevant. To help understand this term, start with these definitions from the National Electrical Code, Article 100:
All electrical component enclosures, switchboards, motor housings, generator frames, and so forth must be grounded for safety.
The definitions of ground and grounded do not state whether or not the conductive material carries current. Engineers ground components for two reasons:
A ground can be either a current carrier under normal operating conditions or a current carrier for abnormal operating conditions. This depends on the application of the hull connection.
The "grounded" system of the automobile has current traveling through the
chassis, engine block, and all connected metal. The body of the car represents
the negative conductors of its electrical system. All circuits are completed
from the negative battery terminal through the chassis, the electrical loads,
positive wires, and back to the positive battery terminal. The ground symbol
identifies the point in the electrical circuit that connects to the metallic
structure. All the ground symbols in Figure
15-2 are connected like a node, as if the entire automobile frame was a
giant connector. This system is in fact insulated from earth by tires and is
not a grounded system.
Current-Carrying Grounds
The emergency generator used on the 2K series LCU is an example of a current-carrying grounded electrical system. The 24-volt DC battery starting and charging system is connected normally as a current-carrying ground. The main AC power production portion, however, retains the insulated conductors and does not use a current-carrying ground for normal operations. Grounded current-carrying electrical systems are avoided whenever possible on watercraft because of the galvanic corrosion and electrical shock hazards. In this situation, hazards and corrosion are minimized through strict adherence to Subchapter J, Subpart 111.05-11(a)(2), Title 46, Code of Federal Regulations.
The DC part of the emergency generator prime mover and power generation controls incorporate the metal structure as a current-carrying structural assembly of the generator. It is connected to the hull (grounded), and the automobile chassis is not. See Figure 20-4 for the diagram of this circuit.
Noncurrent-Carrying Grounds
The AC current-carrying conductors of the ship service generator are not designed to come in contact with the structural components of the equipment. Instead, only the structure and metallic enclosures are connected together and connected to the hull. All conductive equipment that surrounds electrical current-carrying conductors must be grounded; that is, connected to earth.
Figure 15-3 shows the generator housing physically connected to the prime mover's block. The prime mover may be directly bolted to the engine bed. The engine bed is connected to the hull and the hull to the water.
A break in this normal metal-to-metal contact, from the generator housing to the vessel hull, may come from the diesel generator set mountings. To eliminate excessive generator vibrations during normal operation, modern vessels do not have the prime mover rigidly connected to the engine bed. A resilient mount, not electrically conductive, acts as a shock absorber. A grounding strap or flexible shunt is attached from the generator housing to the engine bed. This connects the diesel generator set to the engine bed with a conductive path. In normal operation, this is not a current-carrying ground.
This type of ground is used for safety. Should an abnormal electrical situation develop within the generator, a low-resistance conductive path is provided for winding current to move to earth through to the vessel's hull. This is necessary to prevent current from using a human host as a conductor. Stray current will go to earth when provided a path to do so. The metallic connection between the generator and the engine bed must not have any resistance. If enough resistance is placed between electrical equipment and the vessel's hull, a condition can exist where the soldier will conduct current because he has less resistance to ground than the equipment.
A low-resistance path is necessary to allow current to flow to earth when an abnormal electrical condition exists. This means a clean, unpainted metal-to-metal contact surface. If vibration dampening is necessary, a grounding strap or flexible shunt is mandatory to complete the connection. Additional information is in the Code of Federal Regulations, Title 46, Subpart 111.05.
The electrical component housing that is properly grounded will not become a deadly device to a soldier under abnormal electrical conditions. The current will travel from the damaged generator windings, for example, to the generator housing. From the generator housing, the current will go through the shunt to the hull and earth. The current developed from the abnormal condition will bypass the soldier. The soldier still provides more resistance than the correctly grounded components in the system. Current will take the path of least resistance, around the soldier, through the metal-to-metal connections, to the mounting, to the hull and water, returning to earth.
To properly understand how these different grounds fit into the vessel's electrical system, the word "ground" must be understood as it was originally used. Decades ago, when DC tugboats were designed, the term "earth" was used instead of ground. Still later, the tugboats were called positive ground systems. (Foreign cars used to be termed positive earth systems). Over the years, many individuals and contractors mistook the term "ground" for a negative ground (as referred to on today's automobiles). Eventually all types of grounds were incorporated into the tugboats, and the true meaning of the term "ground" was lost. Both earth and ground have their original roots in terra firma. However, today the importance of the earth's electrical potential as a safety factor is realized.
Nikola Tesla, the father of alternating current, discovered the rotating magnetic field and developed the Tesla coil used in radio and television today. In 1900, he made one of his most important discoveries. He found that the earth could be used as a conductor and would respond to electrical vibrations. He called this terrestrial stationary waves. In one experiment, he lighted 200 lamps without wires from a distance of 25 miles.
The earth has a continuous electrical current that flows in, over, and around itself. This current seeks out equilibrium. In this way, it tries to establish a balance in its magnetic field. The potential of the earth is due to the transient electric currents that are electromagnetically induced within the earth itself. The electrical potential (voltage) of the earth has been measured. It is possible because of the conditions that form an electric cell-like device due to the electrochemical differences in local conditions.
In effect, the earth is its own low-voltage generator. The current produced is called telluric current. Telluric current, some believe, is caused by the motion of our sphere or the magma, the electrically conductive sphere itself, and the presence of the geomagnetic field. The earth is itself a generator and produces its own difference in potential.
Therefore, it is advantageous to maintain our very precise current-carrying electrical systems separate from the earth. When there is current flow between one electrical terminal and the hull, it is usually due to an unintentional ground, carrying current within our own electrical system.
The Institute of Electrical and Electronics Engineers (IEEE) Standard 45, Recommended Practice For Electrical Installations on Shipboard, recommends a dual voltage AC system of 450/120 volts AC ungrounded. Army ports, Naval ports, and Coast Guard facilities have 450 volts readily available for shore power hookups. The second voltage (120 volts) should be obtained through the use of transformers. This system is found in many Army watercraft.
An ungrounded current-carrying system means that all the current-carrying conductors are insulated from other conductors, other structures, and earth. Each electrical component has a complete circuit from the generator to itself and back to the generator through the use of these insulated wires. Each current-carrying conductor has an overcurrent protection device. This is because each of the circuits to the electrical component is a different potential than that of the hull (and the earth).
Even though Army vessels use the ungrounded current-carrying system, all electrical components must be grounded. The housings of all motors, generators, and motor controller housings must be directly connected to the hull. The switchboard housings, the distribution panels, the armor wrapping surrounding the electrical cables, and the electrical appliance casings must have a low-resistance connection to the hull. Any metal device that houses a current-carrying conductor must have its enclosure grounded. This direct connection allows the flow of current to the hull and not through the engineer in the event of an abnormal electrical condition.
The neutral is a ground system that uses an insulated current-carrying conductor and a direct wiring connection to the earth. Onshore facilities, for example, the receptacle outlet, has three conductor orifices: the neutral, a safety ground, and the phase conductor. The phase conductor is insulated throughout its circuit and retains the circuit breaker protection. The neutral conductor is insulated throughout the circuit, normally without a circuit breaker. Both the neutral and the phase conductor are current carriers under normal conditions. The ground wire is the safety circuit designed specifically to carry current during an abnormal condition to protect the operator.
The neutral, although an insulated current-carrying conductor, retains a single connection to earth. By connecting the neutral to earth, the earth's potential is locally stabilized as a neutral potential. This provides safety on shore because two of the three wires in every outlet receptacle will not create an electrical shock. The neutral and the ground are the same potential as earth; hence there is no difference in potential. There is no shock hazard between the neutral, the ground, and the earth. There remains only one "hot" wire even though the neutral is a current carrier.
A very real danger would exist on shore if the earth was a very good conductor and had the potential to encourage great current values to move between the phase and earth. Since the earth is not capable of this current encouragement, electrocution from accidental contact with the phase wire is reduced (although not eliminated).
Watercraft generators or transformers can use the center tap of a wye winding combination to derive a separate low voltage potential cheaply. The trouble with using a neutral current-carrying conductor as a potential stabilizing device is the connection to earth, directly through the hull. Army watercraft make limited use of this low voltage potential. Since the vast majority of conductors on board the craft are insulated current-carrying conductors, limited safety can be derived from such a current.
Danger exists when a neutral is connected to the hull because A, B, and C are always a difference in potential to the neutral. When the neutral is connected to the hull, the generator's center is extended as a node throughout the entire length of the ship incorporating all conductive materials connected to the hull. Unlike the earth's feeble ability to encourage current movement, the potential developed in the neutral connection will encourage any current from a difference in potential. Any accidental contact between any current-carrying conductor becomes potentially fatal. Unlike shore facilities that make extensive use of the neutral conductor to carry current, watercraft rarely employ such a circuit. All Army watercraft, with the exception of a few branch circuits on the LSVs, have a difference is potential between any current-carrying conductor (A, B, C, L1, L2, or L3) and the neutral point of a generator or transformer. There is no gain in safety by grounding a neutral conductor unless the neutral can limit the number of current-carrying conductors that maintain an aboveground voltage.
Army watercraft no longer use the center-tapped neutral from the generator. Only the LSV uses the neutral connection from the wye transformer to obtain this inexpensive low voltage value (Figure 15-4).
Figure 15-5 shows the optimum setup for vessels. It shows three single-phase transformers connected for three-phase power. Remember, it is the manner in which windings are connected, not the housing that encases them, that provides the desired voltage and current values. Three single-phase transformers can be wired in the same manner that the ship service armature windings were wired in Chapter 7 to produce a given three-phase output. The LSV using one three-phase transformer is wired identically.
Notice the neutral going to the hull from the secondary transformer side. Note further that there are no circuit protective devices in the neutral conductors.
As long as the neutral conductor, carrying the current to the electrical component, is the same potential as the earth (or hull) that a soldier is in contact with, then there will be no difference in potential between the soldier and the neutral conductor to cause injury. Without a difference in potential, there is no current flow. However, extra care must be exercised when working between a phase and the hull.
Never change the relationship of the circuit overcurrent protective device and the neutral conductor.
The neutral wire must not have an independent overcurrent protective device installed unless the device can open the neutral and all the associated phases in that circuit simultaneously.
If the neutral opened and the other power supply lead did not open, the motor would stop. There would no longer be a completed circuit to the motor. But there would be a difference in potential at the faulty component housing and a difference in potential in the hull the soldier contacts. If someone touches the faulty motor housing, current would take the only available path back to the earth -- through him and the hull.
This is of such importance that neutrals will be stressed again when your understanding of the distribution system is complete.
In addition to safety reasons, the generator is not connected electrically to the hull as a current-carrying conductor to prevent electrolysis. Electrolysis is the chemical process in which a substance loses or gains electrons. Recall the operation of a battery. The battery developed an EMF by chemical means. Oxidation, or rust, is a chemical process that reduces the number of electrons in a substance because the electrons migrate to other areas. Salt water and dissimilar metals connected together for vessel construction create a very large chemical reaction. Electrons lost by the oxidizing materials are taken up by other metals lower down the oxidation scale.
Some of these metals are called noble metals. Noble metals have an outstanding resistance to oxidation. Some noble metal oxidation rates in descending order are gold, silver, and platinum. When electrons leave a metal, oxidation reduces the material. Hull and piping would deteriorate rapidly if this condition were not properly addressed.
The steel hull is not one of the noble metals. However, steel is less easily oxidized than zinc. Zinc sacrificial anodes are attached to the hull of the vessel. Electrons leave the zinc for the steel hull during electrolysis. This provides a good degree of protection to the hull. For the zinc and steel to properly provide protection, the zinc anode should not be painted and retained at a size no smaller than 50 percent of the original.
Electrolysis in the hull cannot be completely eliminated, but it can be reduced. When stray currents are allowed to exist in the vessel's hull, electrolysis, called galvanic corrosion, is greatly increased. Years ago, the 1466 class LCU was incorrectly wired to the hull. The first indication of an extensive electrical problem was the hull's unusual deterioration. Electrically energizing the vessel's hull created a very large battery. The zinc anodes dissolved quickly, and the entire aft third of the hull bottom excessively deteriorated. After appropriate documentation was completed, the situation was corrected.
A vessel has an extensive electrical system designed to carry out more functions than can be found in a small community. Lighting, instrument panels, steering systems, stoves, and so forth are spread throughout the vessel. Every electrical item must be serviced regularly. Locating the electrical wires can be frustrating. In an emergency situation, restoring electrical power for the dewatering and fire pumps can mean the difference between life and death.
Chapter 10 discussed circuit breakers and fuses for the protection of electrical circuits. It briefly mentioned the parts making up the circuit. To understand the electrical distribution system, the different system components need to be examined in closer detail.
Distribution Center
The distribution center controls the ship service generated power and shore power. Automatic overcurrent protective devices connected to the bus monitor and protect the feeders and branch circuits.
Feeder
The feeder consists of the cables that extend from the main switchboard to the distribution panels. In some cases, the feeder may provide power directly to a large motor.
Bus Tie
The bus tie is the electrical connection between the main and emergency switchboard bus bars. It is not considered a feeder.
Emergency Switchboard
The emergency switchboard is a smaller version of the main distribution switchboard. Power is received either from the emergency generator or the main switchboard through a bus tie. This switchboard provides power to vital services, including the fire main, communications, emergency lighting, steering, and so forth. When the ship's main power is lost, emergency power is automatically provided by the emergency generators through the emergency switchboard by an automatic bus transfer (ABT) switch.
Motor Control Center (MCC)
The motor control center consolidates all the motor controlling equipment for all the major motors on board the vessel. Overcurrent and overload protection is provided to the motor and immediate circuitry.
Distribution Panel
The distribution panel is an enclosed metal panel that supplies power to components for a localized section of the vessel (Figure 15-6). Circuit breakers or fuses are installed to protect the branch circuits. The distribution panel has three bus bars (there is a fourth when the neutral is used). When the distribution panel is used to provide three-phase power to loads, then each bus bar is connected to one terminal of a three-pole circuit breaker (Figure 15-7). The other side of the circuit breaker is connected to the loads.
If the distribution panel is used to supply single-phase power to loads, then only two of the bus bars are used, and a two-pole circuit breaker is employed (Figure 15-8).
NOTE:The same two bus bars are NOT used to supply all the power to the loadsconnected in the single-phase distribution panel.
To keep the three-phase generator balanced with single-phase circuits, each phase (A-B, B-C, or C-A) is designed to carry a specific percentage of the load. The single-phase loads are equally divided between the three phases. Each distribution panel receives three-phase power. When single-phase loads are connected, the distribution panel divides up the three phases so that each single-phase circuit (A-B, B-C, or C-A) has one third of the total power available from that panel.
This electrical balance is continued by equally dividing the load at each panel throughout the vessel. All the other panels are divided in the same manner because in the event of a distribution panel casualty, the overall generator load will be decreased evenly across the phases. The generator will still be electrically balanced.
Power Distribution Panel
This panel is generally dedicated as a source of power for operating other components, such as the power supplied to the emergency batteries or to other distribution panels. More often, it can be distinguished as a panel used to distribute three-phase power to three-phase components.
Branch Circuits
These are the conductors that exist between the final overcurrent protective device in the distribution panel and the load; for example, motors, lights, and receptacle outlets.
Lighting Branch Circuits
These conductors supply power to lighting systems, bracket fans, small heating appliances, circuits, and motors 1/4 HP or less.
Appliance Branch Circuits
These conductors supply power to the outlets for the use of portable or nonfixed electrical apparatus.
Communication Branch Circuits
These conductors supply power to communication, audio, and visual signaling devices.
Cables
These are heavy-duty conductors used to carry current between a source and a load. Alternating current cables should contain all three current-carrying conductors in a single cable to cancel out heating caused by inductive effects.
Distribution Cables
These cables are used for the power distribution up to the rated voltage and ampacity of the cable. Low-voltage (600-volt) cables are generally found on Army watercraft. They are used for most electrical connections.
Control Cables
These are multiple parallel conductor cables used for --
Signal Cables
Signal cables of twisted pairs of conductor cables are used for signal transmission. Each twisted pair of conductors will have a shield to prevent interference.
Portable Cords
Portable cords are used for the temporary connection of portable appliances. They are not to be used for fixed wiring. They must conform with NAVSEA 0981-052-8090 [40].
Color coding allows the engineer/engineman to follow electrical circuits throughout a vessel without having to physically touch each cable. The color can be continuous throughout the length of the conductor, or the color noun-nomenclature, such as red, is printed every 24 inches on the insulation of the conductor.
The following are newer color designations for distribution cables:
Older wire designations are as follows:
Phase A: black.
Phase B: white.
Phase C: red.
2 wire DC: positive (+) black.
negative (-) white.
3 wire DC: positive (+) black.
negative (-) red.
Table 15-1 gives the newer color designations for control and signal cables.
Cable Tags
Embossed metal tags are used to identify cables throughout the vessel. The tags are located at the distribution panel and the component. Tags are also attached to the cables where penetration of the bulkhead is necessary. The tag code will start with the type of circuit it is. Army vessels use P to indicate power distribution panel cables, L for lighting distribution panel cables, and E for emergency distribution cables. If there is a number preceding that letter, this identifies the cable in the distribution panel. The next numbers indicate the voltage carried in that cable, such as 400 or 200. Any additional numbers above the whole hundred value are used to distinguish the differences between two like voltage distribution panels. For example, in the cable tag 3P-401 --
Wire Construction Details
With the addition of a set of prints, the engineer/engineman can locate all the necessary electrical components on his vessel. To make the most use out of the wiring construction details and to help distinguish between single and three-phase AC circuits, a basic understanding is needed. The wire construction detail is the code used for determining the current-carrying capacity, insulation material, shielding, application, and so forth. This information may be found in the IEEE Standard 45 manual. However, it is not necessary to understand everything about these intricate codes. There are some basic rules that can be easily applied. The following details are found on the distribution system prints:
This text will examine only the first and last character and the numbers. The first character indicates the number of individual conductors in the cable; 2 obviously means that there are two conductors in the cable. D means that there is a double conductor in the cable, or the same as 2. M stands for multiple conductors, and there may be several unused conductors for future electrical growth. T stands for a triple conductor, so there will be three conductors in the cable. If the last letter is an A or B, then the cable is armor-shielded. The last numbers indicate the size of the conductor in thousands of circular mils (KC MIL). Thus, 14 indicates that the area is 14,000 circular mils. The 100 indicates that there is a 100,000-circular mil area. It would be easy to identify the difference between the two cables. The TSGA-100 will be much larger than the 2SJ-14 cable.
This basic understanding is useful in locating the wire on the vessel after seeing it in the blueprints. Additionally, if there are two conductors in a cable, the cable cannot carry three-phase power. Wire size, application, and distribution panel location will help you locate components when troubleshooting.
Shipboard Electronic Equipment Wire Marking Systems
The following explanation is an example of the type of conductor marking used in shipboard electronic equipment. These conductors may be contained in cables within the equipment. Cables within equipment are usually numbered by the manufacturer. These numbers are found in the technical manual for the equipment. If the cables connect equipment between compartments on a ship, they will be marked by the shipboard cable numbering system previously described.
On the conductor lead, at the end near the point of connection to a terminal post, spaghetti sleeving is used as a marking material and insulator. The sleeve is marked with identifying numbers and letters and then slid over the conductor.
The marking on the sleeve identifies the conductor connections to and from by giving the following information (Figure 15-9):
These designations on the sleeve are separated by a dash. The order of the markings is such that the first set of numbers and letters reading from left to right is the designation corresponding to the terminal from which the conductor runs. Following this is the number to the terminal board to which the conductor runs. The third designation is the terminal to which the conductor runs.
For example, as shown in Figure 15-9, the conductor is attached to terminal 2A of terminal board 101 (terminal from 2A on the spaghetti sleeving). The next designation on the sleeving is 401, indicating it is going to terminal board 401. The last designation is 7B, indicating it is attached to terminal 7B of TB 401. The spaghetti marking on the other end of the conductor is read the same way. The conductor is going from terminal 7B on terminal board 401 "to" terminal 2A on terminal board 101.
It may be necessary to run conductors to units which have no terminal board numbers; for example, a junction box. In this case, an easily recognizable abbreviation may be used in place of the terminal board number on the spaghetti sleeving. The designation JB2 indicates the conductor is connected to junction box 2 (Figure 15-10).
In the same manner, a plug would be identified as P. This P would be substituted for the terminal board number marking on the sleeve. A complete description of shipboard electronic equipment wire marking is in NAVSEA Publication 0967 LP 1470010, Dictionary of Standard Terminal Designations for Electronic Equipment.
The wires and cables in the distribution system are expressed as a single line between the power supply source and the component. Instead of showing the actual number of conductors, only one line is illustrated. While the distribution wire diagram more closely resembles the actual cable runs, the complete circuits necessary for electrical operation are missing. The wire construction details become very informative now. The wire construction details state, rather than illustrate, the actual number of current paths in the component's circuit.
As an example, Figures 15-11 through 15-21 illustrate the power distribution system from an LCU.
Three-phase AC is developed in the generators. The AC is fed to the switchboard through a TSGA-30 cable (Figure 15-11). The TSGA-30 indicates three large 30,000-circular mil conductors.
Three-phase AC enters the distribution center's main switchboard through a three-pole circuit breaker (Figure 15-11 [a]). When this main circuit breaker is closed, power is provided to three bus bars inside the switchboard. Each bus bar (b) carries one phase of the generator's power. The bus bars are actually a mere connection point on this switchboard. The actual bus in Figure 15-11 has been expanded for clarity.
Current is available from the main distribution switchboard to the power distribution panel (P-400) through the three-pole circuit breaker (c) and TSGA-100 feeder (d).
A cable tag P-400 indicates that 450 volts are supplied to the power distribution panel number 400 (Figure 15-12). The feeder is protected by the circuit breaker in the main distribution switchboard (c). The TSGA-100 feeder is larger than the TSGA-30 generator cable because the TSGA-100 feeder must carry current from both generators when operating in parallel.
The power distribution panels do not have any circuit breakers for the entering feeder. The feeder is connected to three bus bars in the power distribution panel P-400. In this manner, current is available, through a common contact point, for all the three-phase branch cables and feeder cables. Current is connected to the cables through three-pole circuit breakers (e).
Where the cable goes can be determined by using the blueprints. In the same way, a defective motor can be identified, and then the source of its power (and circuit breaker) can be determined from the motor cable tag.
A three-phase 450-volt motor is connected to the feeder tagged P-408 (f). Refer to Figure 15-13.
After the cable passes through a disconnect switch (DS) (Figure 15-13 [g]) and enters a controller (C), the wire is branched into four directions.
P-408-C indicates the first of three control circuits used with this motor. Each circuit is tagged with the controllers number and wire number P-408. This is followed by an alphabetical designation P-408-C. This wire is also a MSCU-10. This indicates that there are multiple conductors as would be expected from a station that controls the raise, stop, and lower of a ramp motor (h), unarmored, and of limited diameter.
P-408-A 2SJ-14 indicates that this handcrank interlock limit switch is part of a two-wire, single-phase circuit in the same way the slack cable limit switch (P-408-B) is connected.
The motor uses the same size cable as the feeder that powers the controller. This is necessary because of the high current draw from the motor.
Skip the elementary branch circuit P-407, and refer to feeder P-409, Figure 15-12, at the power distribution panel. P-409 TSGU-9 indicates a cable going to power distribution panel P-409 in the air conditioning compartment (Figure 15-14). This feeder also carries three-phase current to three bus bars inside the P-409 power panel.
Power is provided to all the three-phase branch circuits through three-pole circuit breakers. Individual branch circuits are designated by the first number on the tag, such as 1P-409, 2P-409, and 3P-409. 4P-409 is a spare provided for future electrical growth. Note the location identification of the 3/4-HP motor vent (2-30-1) and the 7.5-KW heater (1-31-1) in the air conditioning compartment.
Now return to the P-400 power distribution panel and begin with feeder P-402. Feeder P-402 TSGU-14 is of special interest. This three-phase, 450-volt feeder is going to provide the 120-volt single-phase power necessary to operate the lighting and single-phase branch circuits. TSGU-14 enters three 10-kVA single-phase delta-delta connected step-down transformers (Figure 15-15). When three single-phase transformers are connected together delta-delta, this connection provides increased system reliability and the proper electrical three-phase relationship necessary to operate the lighting circuits.
The secondary side of the transformer bank is labeled L-100. This indicates that the cable will go to the lighting distribution panel using the TSGA-100 cable.
Notice that the secondary cable is larger in diameter than the primary cable (TSGU-14 versus TSGA-100). This is because a reduction in the voltage of a step-down transformer means an increase in the current on the secondary side of the transformers. With an increase in current, an increase in conductor size is necessary.
The L-100 TSGA-100 feeder enters the lighting distribution panel (L-100, Figure 15-16) and connects to three bus bars. Three-phase power is still maintained in the lighting distribution panel. Three-phase power is distributed to other lighting panels through the three-pole circuit breakers. Feeders L-101 and L-103 identify themselves as feeding three-phase current to lighting panels of like designations.
Tagged feeder L-101 TSGU-9 has three conductors entering lighting distribution panel L-101 (Figure 15-17). Each conductor connects to one of the three bus bars. Since lighting panel L-101 has only branch circuits distributing single-phase power to the loads, only two bus bars at a time are used. Figure 15-17 shows phases A and C (i) supplying current to the 1L-101 branch circuit. Branch 6L-101 is supplied by phases A and B (j). Branch 3L-101 is supplied with power from B and C (k). This keeps the load equally distributed across the distribution panel and the windings in the generator. Notice how all these branch circuits are two-conductor wires.
In this case, 1L-101/2SJ-14 and 2L-101/2SJ-14 supply the aft engine room lighting system. 3L-101/DSGU-14 provides power to the oil water separator motor and controls. The D in DSGU-14 indicates that there are only two wires. This means that the motor must be single phase.
Return to the lighting distribution panel L-100 in Figure 15-16. The L-102 feeder provides power to the isolation transformer bank. Again, the most effective way to use the benefits derived from a transformer bank results when three single-phase transformers are connected delta-delta. In this situation, the transformers do not step up or step down the voltage. The diameter of the conductor is the same on the primary side as it is on the secondary side of the transformer (Figure 15-18). The transformers are not used to increase or decrease the voltage. In this situation, the isolation transformers provide a nonmechanical electrical connection between the L-100 lighting panel and the L-102 lighting distribution panel.
The branch circuits in the L-102 distribution panel example are very prone to abuse. This vessel's upper and lower deck receptacles receive their power from this panel. Receptacles are available for use by personnel not necessarily proficient in the electrical crafts. All types of equipment can be connected to receptacles. This is not to say that all types of equipment should be connected here, merely that improper conditions are likely to present themselves. All transformers prevent catastrophic electrical system damage by opening up on their secondary side before the short circuit condition can be passed throughout the distribution system. The isolation transformer bank exists for this sole purpose of protecting the electrical environment. The isolation bank neither steps down nor steps up the voltage. If this nonmechanical electrical link were not provided for, a short circuit condition can result in an electrical casualty of the L-100 lighting panel and end all single-phase power from the panel.
Should an electrical casualty damage one of the transformers, the other two can be connected open-delta. The number of phases does not change. With the loss of one of the three transformers, a power reduction to approximately 58 percent is necessary to prevent the open-delta from becoming overloaded.
In another example, three single-phase transformers are used to step down the 450 volts to 120 volts and provide the same protection to the P-400 power panel
Return to lighting panel L-100 (Figure 15-16) and locate feeder L106/TSGA-23. This feeder goes to lighting panel L-106 in the pilot house (Figure 15-19). From here, a very important system can be traced out -- the emergency power supply.
From the lighting panel L-106, follow feeder 5L-106/DSGU-9 to the battery charger. From the battery charger to the loads, the cables will now be labeled as P for power. The batteries directly charged by the battery charger provide the power supply to the emergency power panel P-24. Notice how the circuit protective devices (fuses) are graphically represented in Figure 15-20. Each conductor has circuit protection.
Troubleshooting usually starts with the identification of a defective electrical component. The power supply will then be sought out. The tag on the wire of the component will indicate its source of power; for example, the lighting distribution panel L-101. This allows the engineer/engineman to work backwards from the component, isolating and de-energizing only the circuit needing service.
Personal involvement with new state-of-the-art electrical equipment and their appropriate manuals will complete your knowledge of the electrical system. This field is undergoing major changes that preclude this text from encompassing all aspects of the distribution system. The emergency power and lighting system should be one of the first systems you become comfortable with.
If the power should fail on board a vessel, lives and property are jeopardized. Many vessels regain control of their electrical systems through the use of an emergency generator and emergency switchboard. The generator is provided with its own starting system. When power is lost, the emergency generator must be able to automatically start and provide power to the emergency switchboard within a few moments.
Transferring power from ship to shore requires the momentary interruption of ship's power because most Army vessels are not equipped to cogenerate (or run in parallel with) shore power utility companies. Make provisions to ensure the emergency generator will not start during this momentary interruption.
The emergency generator power supply is made available to either the main distribution center or the emergency distribution switchboard through an automatic bus transfer (ABT) switch and the bus tie. Normally, the ship service generators supply power to the emergency switchboard from the main distribution center through a circuit breaker and automatic bus transfer switch. When power is lost, the ABT switch automatically connects the emergency generator to the emergency switchboard. The ABT simultaneously disconnects the main distribution switchboard from the emergency switchboard. This allows all the emergency power to be supplied to the vital services connected to the emergency switchboard. For unusual conditions, manual switches may allow the emergency generator to provide power to the main distribution center. Care must be taken not to overload the emergency generator.
The circuits connected to the emergency switchboard are determined by the design of the vessel. Some of these vital services are the --
Additional information may be obtained in the IEEE Standard 45, Section 36.
The operator monitors the power from the generators through the main power distribution switchboard. The marine engineer/engineman has the critical task of putting the generators on line. On line means that the generator is supplying power through the switchboard to the loads. If a generator is coming on line, this indicates that the generator is operating and waiting to furnish power to the switchboard.
The information below is provided as a general guide for the junior and senior marine engineman in the operation of the AC switchboard. In all cases, equipment should not be operated without first consulting the manufacturer's manuals and appropriate Army technical manuals. Look for additional information pertinent to the operation of the 2000-series LCU and logistics support vessel (LSV).
The following is based on the procedures necessary to operate and parallel the generic AC brushless generators. Every prime mover and generator manufacturer has its own specific needs and interrelated requirements for paralleling generators. Unlike many selective tasks you may have performed in the past, the act of paralleling generators depends on many outside influences.
Currently, the Army vessel inventory uses the Cummins, Detroit, and Caterpillar diesels as the prime mover to provide the motion necessary to produce an electromotive force. To become a good electrician, you must first be a good mechanic. Nowhere is it more evident than when generators need to be paralleled. In order to parallel generators, the prime movers must be in proper working order. The diesels need to be mechanically sound and properly tuned. The governors must be set properly. Before you ever consider major adjustments on the distribution switchboard, you must consult the operator and maintenance manual of the prime mover. If the prime movers do not operate with the expected speed characteristics, then there is no possible way for you to compensate for their inaccuracies at the switchboard.
Speed Droop
The efficient operation of a generator is not enough. Both generators must operate with the same efficient characteristics. Each generator must be tuned up individually, but their speed droop setting must be collectively consistent. The reason the speed droop is set is to establish a defined engine speed at no load and full load.
When a large electrical load is suddenly placed on the generators, the speed of their prime mover drops. This is because it is harder for the diesel to turn the generator rotor with the stronger magnetic field. The magnetic field in the rotor had to increase to compensate for the increase in electrical demand. How spontaneously the two generator prime movers react to this decrease in speed depends on the speed droop setting of the individual governors. The increase infield strength and armature current acts as a magnetic break, reducing diesel RPM.
Speed droop allows a decrease in diesel speed as the load is applied. In keeping with the elementary basics of this manual, the electronic speed and voltage controls of the new 2K LCUs will not be directly addressed. Although all functions presented are comparable between all generator sets, consultation of the specific manufacturer's manual is mandatory. General understanding of speed droop is best presented by the basic PSG governor function found on the 1600 series LCUs. Much of the following information is reprinted with permission of the Woodward Governor Company.
Speed droop is increased by moving the external knurled knob forward on the governor and decreased (toward zero droop) by moving the knurled knob toward the back of the governor. The droop setting must be made by trial and error because there is no calibration point (on these models). If it has not been previously set, the engineer must move the knob back and forth until he achieves the proper droop setting between full load and no load.
The procedure below is recommended by the original manufacturer's manual, incorporated within the initial TMs (Figure 15-21).
NOTE:A very accurate tachometer must be used to determine the speed drop. The same tachometer must be used for both generators. This will eliminate any calibration errors or erratic tachometer response.
The Detroit diesel manual for the speed droop adjustment on the 4/71, PSG governor is supplemented as follows:
If the speed droop is not accurately established between the two prime movers, balanced electrical distribution cannot be obtained.
Generator Voltage Droop
Voltage droop is the decrease in the normal generator voltage due to an increase in load current. The decrease in voltage from no load to full load is expressed as a percentage of the full-load voltage:
This is another adjustment that is necessary to properly parallel AC generators. The droop control should be set so that a given amount of reactive current applied separately to each generator will cause identical reductions in output voltage.
A large increase in current draw is followed by a slight decrease in voltage. This voltage drop needs to be compensated for accurately and consistently by both generators.
The procedure below is a general guidance to be used in accordance with appropriate technical and manufacturer's manuals. It is designed to clarify electrical procedures only.
Before adjusting for voltage droop, the droop control is initially set in the mid position. Shore power is secured. To adjust for voltage droop --
Repeat this procedure for the other generator, except do not open the generator circuit breaker after the last adjustment. Leave this generator on line. Then --
NOTE:The load should be motors. Motors provide the inductive reactance necessary for this setting.
Before trying to start the generator, ensure the following:
NOTE: Many generator electrical systems are not designed to be operated with the voltage regulator field open.
To place one generator on line --
The voltage setting of AC generators is the most critical setting for paralleling. The voltage of each generator must be set exactly the same. Since a vessel is subjected to extensive vibration and salt air corrosion, it is advisable to check the calibration of all the meters regularly.
The voltmeter should always reflect the rated voltage of the machine. In a perfect scenario, both generators will reflect the exact rated voltage. However, this is not often the case. Voltage settings are critical because any voltage difference between the generators will increase the reactance between the two generators. This will require the generator to supply extra current to overcome this wasted power. (Review Chapter 6 on inductive reactance.) This means increased heat in the electrical system and extra demand from the generators.
An expedient way to check the calibration of the voltmeter is as follows:
Now the number one voltmeter and the number two voltmeter are reading the same voltage source. If everything is correctly calibrated, then both meters will read the same voltage from the number one generator.
If both generator voltmeters do not display the same voltage reading, then there is an inconsistency. If you adjust each generator according to its own meter readings, then the voltage difference still exists. As an expedient measure, you can maintain the voltage difference as noted above, when each voltmeter is monitoring its own generator. Even though the meters are not identical, the voltages will be. Final paralleling voltage adjustment will compensate for any inconsistency.
In paralleling, it is more effective to have both the voltages slightly below or both voltages slightly above the rated voltage than for one generator to be only 1 volt above and the other generator at exactly the rated voltage. Differences in voltage increase electrical system reactance. The increased current and subsequent increase in heat decrease the life expectancy of components. In any application, when an inconsistency is found with the equipment, correct it immediately or refer it to a higher echelon of maintenance.
Use the following sequence to parallel two generators:
Once a generator is paralleled, its voltage and speed are determined by the bus. At the instant you throw the switch, the generator is connected to the bus, but it is not delivering power. It is said to be "floating" on the line. If you try to reduce the speed of the prime mover, the generator continues to run at rated speed. How is that possible? There is not enough mechanical power to drive the generator that fast. Therefore, the generator draws electrical power from the bus. It actually runs as a motor.
If you try to increase voltage by increasing DC excitation to the generator's field, terminal voltage will appear to remain the same. You have actually increased the generated voltage of one generator, but it does not show.
Why is it that the terminal voltage does not change? The extra generated voltage produces a reactive current flow into the bus. This current does not deliver any active (usable load) power. Only by increasing the prime mover speed will the generator accept its share of the load.
To shut down the generators --
Now you may continue to operate on one unit or continue to shut down the other generator as follows:
Remember, never perform any maintenance, servicing, or operating without first consulting the appropriate technical manuals.
Newer vessels designed for the Army will be using the electronic fuel control (EFC) and governing system. The following information is provided as introductory information only. In-depth information will be made available at the junior and senior marine engineer levels. Specific information is available from the vessel technical manuals and manufacturer's manuals.
The EFC system will be made up of three basic units (Figure 15-22):
The elimination of most throttle controls and linkages means that the system is less maintenance-intensive than current fuel control systems.
The MPU is an electromagnetic component mounted through the flywheel housing (Figure 15-23). The MPU is a permanent magnet with the circuit from the governor control tightly wrapped around it. The MPU comes in close proximity with the teeth of the flywheel. As the teeth from the flywheel move past the magnetic field from the MPU, the MPU's magnetic field becomes distorted. The motion of the distorted magnetic field induces an EMF into the governor control circuit wrapped around the magnet. As the flywheel teeth move further past the MPU, another portion of the MPU field becomes distorted in a like but opposite manner. This magnetic field motion in the MPU induces an EMF in the opposite direction as previous. The single-phase AC developed in the MPU is sent to the governor control. The cycles per second can be easily converted to revolutions per minute. This provides the governor with an indication of prime mover speed. The faster the flywheel turns, the faster the induced EMF frequency. In this manner, the governor control senses the changes in speed.
The governor control interprets these speed changes and, depending on the setting of the controls, provides an electrical signal to the actuator port.
The actuator is basically a solenoid valve that admits fuel to the diesel in a quantity determined by the signal from the governor control. The slower the speed of the prime mover, the more the governor control electrically opens the fuel port. The faster the prime mover speed, the smaller the quantity of fuel delivered through the actuator port.
The most important information for the junior and senior marine engineman concerns the batteries. The batteries provide the voltage and current necessary for operating the electronic fuel control system. This equipment is set up so that the generator prime mover cannot function without the batteries fully operational. Although provided with manual means to override this system, normal operation is prohibited. Emergency generators will not start automatically nor will the ship service generators continue to operate should the batteries fail to provide the necessary power for the EFC control circuits.