This provides the electrical engineer with an overview of the circuit or installation and is particularly useful for troubleshooting onboard. It is important to be familiar with the various ship electrical symbols.

Some of the commonly used ship electrical diagram symbols are.

Line or ladder diagrams are used to portray control symbols , and these are made up of two circuits: the control circuit and power circuit.

In this diagram lines represent ship electrical wires with control circuit wiring shown by lighter weight lines, power circuit wiring is shown by heavier weight lines. A small dot or node at the intersection of wires indicates a ship connection.

Contact symbols on ship electrical diagram

These are used to indicate open or closed paths of current flow In the ship power system. Contacts are shown as normally open (NO) or normally closed (NC). Relay contacts shown by this symbol require another device to actuate them, normally electrical coils.

The standard way to represent contact is to indicate the circuit condition that is produced when the actuating device is in the de-energised state (DIN 40900). For example, relay -K1 Is used as the actuating device for the motor starter. The contacts -K1/13-14; 23-24 are shown as normally open, indicating that the contacts are open when the relay coil is de-energised and a completed current path does not exist.

Normally open contact on ship electrical diagram

Ship circuit is first shown in the de-energised state. The contacts of relay -K1 are shown in their normal state (NO). When the -K1 relay becomes energised, contact -K1 /23-24 closes, completing the circuit for the contactor coil -K2. The motor starts as a result.

Normally closed contact on ship electrical diagram

Ship overload relay -F2 contact 95-96 is shown as normally closed (NC), this means the contact is closed when the relay is not tripped due to an overload. A complete ship electrical circuit exists as soon as the start button is pressed and so relay -K1 energises.
Input contacts are either normally open or closed, indicate the binary state of the condition:

On or Off (true or false)

Variations of simple control circuits are used to represent common logical conditions.

Ship diagram a) is called an’AND’operation circuit.

The circuit would be completed, ie the actuator -KM1 energises, If the Input conditions of-K1 and -K2 become true (both contacts are closed).

Ship diagram b) represents a circuit called an ‘OR’ operation circuit.

The circuit would be completed if either of the inputs -K3 or -K4 becomes true.

Ship diagram c) represents a circuit called a ‘NAND’ operation circuit.

The circuit would be completed If either or both of the input conditions remain false.

Ship diagram d) represents a circuit called a’NOR’ operation circuit.

The circuit would remain completed if neither of the two input statuses change.

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No special attention for equipment insulation required

–  Automatic detection and immediate isolation of ground faults

–  Ground fault current flows for a short period of time, restricting damage

–  Avoiding arcing ground over-voltages

–  Maintains phase voltages at a 
constant value to ground.

The main disadvantages low-impedance ship’s grounded systems are:

– Instant disconnection and loss of the service

– Fault currents can be large and can cause extensive damage and have the risk of explosion.

Most low-power, low-voltage systems in the range from 110-230V have a solid grounded neutral. This power is mostly supplied from a phase to neutral source like a transformer and is used to supply small power consumers and lighting. There are two basic types of distribution for solid or low impedance grounded systems:

1. 3-phase 4-wire with earthed with hull return
2. 3-phase 4-wire with neutral earthed without hull return (TN-S – system) for all voltages up to and including 500 V A.C.

The type without hull return (b) resembles installations commonly used on shore in houses and is used primarily in the accommodations of ships.

The additional advantage of such a system is that it will require the same skills for operation and maintenance as for onshore installations.

Labour legislation in various countries makes companies responsible for the safety of workers or crew on board of ships. Using this type of system would make it easier to comply as standards with respect to safety, training, operational authorisation, etc. would be the same.

Special consideration should be given to low-voltage supplies to for instance steering gear or pumps for essential services as these should not trip on a ground fault.

For these services it would probably be best to make a dedicated supply directly from the main power source. The diagram below shows the principle layout of a system with an ungrounded main power system but with a grounded low-voltage system.

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– Continuity of service on a ground fault

– Ground fault currents can be kept low

The main disadvantages of Insulated neutral ship’s system  are:


– High level of insulation may be necessary.

– High transient over – voltages may occur

– Grounded circuit detection may be difficult

In the latest edition of lEC 60092- 502 TANKERS both insulated and earthed distribution systems are permitted, however, systems with a hull return are not permitted. Return via the ship’s construction is only acceptable in limited systems, such as diesel-engine battery start systems, intrinsically safe systems and impressed-current cathodic protection systems, outside any hazardous area.

Most main electrical power systems on ships, in the range from 400V to 690V, will have an insulated neutral.

It is, however, important that a ground-fault is detected and cleared as quickly as possible. This is to avoid a large short-circuit current on a second ground-fault, which can be in excess of the 3 – phase fault current for which the equipment is rated, which can do damage beyond repair.

Hazardous areas will also have an insulated neutral power supply system, as the flashover from a faulted cable in a grounded system, which may cause an explosion, is too high.

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An overvoltage on an incandescent lamp produces a brighter and whiter light because the filament temperature is increased.

Its operating life is, however, drastically reduced.

A 50% increase over its rated voltage will reduce the lamp life by 50%.

Conversely ,a supply voltage reduction will increase the operating life of a GLS lamp but it produces a duller, reddish light.

Lamps rated at 240V are often used in ship’s lightning system operating at 220V.

This under-running should more than double the lamp life.

Similar effects on light output and operating life apply to-discharge lamps but if the supply-voltage is drastically reduced (below 50%) the arc discharge ceases and will not re-strike until the voltage is raised to nearly its normal value.

A fluorescent tube will begin to flicker noticeably as the voltage is reduced below its rated-value.

The normal sinusoidal a.c. voltage wave form causes discharge lamps to extinguish at the end of every half cycle  (every 10ms at 50Hz or every 6.7ms at 60Hz). Although this rapid light fluctuation is not detectable by the human eye, it can cause a stroboscopic effect where by rotating shafts in the vicinity of discharge lamps may appear stationary or rotating slowly could be a dangerous illusion to operators.

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Why high voltage system used on ships ?

The following advantages of high voltage system explain why it is preferred over low voltage system.

1. As we mentioned, higher power demand for heavy consumers on ships is the first reason to choose high voltage on ships. High power bow thruster electric motors, reefer containers in container ships, cargo cooling machineries in gas carriers, etc. are examples of such heavy power consumers.
2. High voltage machineries have much reduced size and weight compared to same power low voltage counterpart.
3. Reduced weight and space for machinery means increased space for cargo and more profit.
4. Using electric propulsion further reduces engine room size, again more cargo space and profit.
5. Ease of installation and reduced installation cost.
6. Conductor size is reduced due to low current flow in high voltage system, means reduced copper requirement and low cost.
7. In high voltage system, copper loss or I²R losses are much reduced when compared to low voltage system, as the current flow is less.
8. Overall estimated 1/3 rd reduction in cost compared to low voltage system.

What are the disadvantages of high voltage system on ships ?

1. Handling high voltage means high class insulation to be used on conductors. (Generally ‘F’ class and above)
2. Higher voltages means greater risk and hence require stringent safety procedures.
3. Skilled labour required for handling high voltage system.
4. Danger of arcing, chances of arc flash and arc blast.
5. Special switch gears are required to preventing arcing.

Why machinery working in high voltage system has reduced weight and size ?

Consider an electric motor consuming power of 500 kW

We have the power, P = √3 V I Cos ∅

In low voltage system, power, P = 500 x 1000 Watts, power factor, Cos ∅ = 0.8, Voltage, V = 440

P = √3 V I Cos ∅

Current, I = P / (√3 V Cos ∅)

I = 500000 /  (√3 x 440 x 0.8 )

I = 820 Ampere

Similarly, In high voltage system, power, P = 500 x 1000 Watts, power factor, Cos ∅ = 0.8, Voltage, V = 3.3 kV

P = √3 V I Cos ∅

Current, I = P / (√3 V Cos ∅)

I = 500000 /  (√3 x 3.3 x 1000 x 0.8 )

I = 109 Ampere

So for an electric motor, working in high voltage system draw very low current compared to that of low voltage system. As current carrying capacity of conductor reduces, size of the conductor can also be reduced much. This considerable reduction in conductor material result in reduced size of machinery and save space for installation.

Why copper loss and iron loss is less in high voltage system ?

From the above comparison on current flow between a high voltage system and low voltage system, it is clear that current draw with high voltage is much lesser. Hence copper loss or I²R losses and iron loss are considerably lesser.

What is meant by arcing, arc flash and arc blast in high voltage system ?

• Arcing is the production of unintentional electric arc during opening circuit breaker, isolator or contactor due to the discharge of electricity through the medium between the two contacts. (In fact arcing occurs during closing the breaker also).
• Arcing also occurs when heavy current flow to earth during an earth fault or short circuit fault due to insulation failure.
• During arcing temperatures at the arc terminals can go upto 20000 ºC or more, which is around 4 times the temperature of sun’s surface.
• The intense light formed at the point of arc is called as arc flash.
• Instant heating of air surrounding the arc occurs and conductors vaporises, resulting in formation of a high pressure wave. If the pressure wave is not released, it results in an explosion called arc blast.

What are the hazards of arc flash and arc blast ?

• Permanent damages to the electrical equipment.
• Irreversible damage to the human tissue and incurable burns due to very high temperatures.
• Arc flash produces intense UV light, resulting permanent or severe damages to the eye vision.
• Pressure wave from the arc blast compresses the eyes, resulting permanent or severe damages to the eye vision.
• Heavy noise (above 140 dB) may damage hearing ability, sudden pressure changes may rupture ear drums also.
• Arc blast explodes the equipment, ejecting parts with tremendous force and velocity. This may result in damages to personnel and property.
• Flammable materials present in the vicinity of arc may ignite, causing secondary fires.

What is the difference between short circuit and short circuit level ?

• A short circuit is a fault which occurs when the current in a system deviates its normal path and start flowing through an alternate path.
• Since the alternate path offers very low resistance, the current increases very much above the normal value.
• Short circuit level (SCL) is the maximum possible current that flows through a circuit during a short circuit fault.

What is the effect of short circuit fault in high voltage system ?

• High current flow during a short circuit fault result in increased temperatures, which damages insulation, produces high thermal and mechanical stresses in the system, may cause arcing, arc flash and arc blast.

What are the methods adopted to prevent ill effects of short circuit fault ?

• Protective relay installed in the system immediately trips and isolates the equipment during a short circuit fault within a short time. This prevents the effects of high current flow through the circuit.
• The generators, cables, equipment, switch gears, etc. associated with the system are designed to withstand the heavy current during short circuit fault for this short duration of time.

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Electronic Thermal Electromagnetic

Electronic OCIT (overcurrent inverse time) relays have largely superseded
electromagnetic types as they have no moving parts (except for their output trip relay) and their very reliable tripping characteristics can be closely matched to the motor circuit.
Such relay is are robust, smaller and lighter than the equivalent electromagnetic type.

The block diagram of the electronic OCIT relay shows that the current and time settings can be adjusted over a limited range to match the motor FLC and run-up time. A self-test of the OCR performance can usually be applied with a fixed setting of, typically, 6 x FLC and the tripping-time can be measured and compared against the manufacturers current/time characteristics.

Although electromagnetic devices with time delays can give adequate protection against large, sustained overloads to motors which are operated well below their maximum output and temperature, they have been found to be inadequate for continuous maximum rated (CMR) motors.

Most ship LV motors

are protected by less expensive thermal OCRs. Inverse-time thermal OCRs usually work with bi-metal strips.
The strips are heated by the motor current and bend depending on the temperature. If the motor takes an overload current, the strips operate a normally-closed (NC) contact which trips out the line contactor to stop the motor.

The minimum tripping current of such a device can be adjusted over a small range.

This adjustment alters the distance the strips have to bend before operating the trip contact.
For larger motors, the heaters do not carry the full motor current. They are supplied from current transformers (CTs) which proportionally step-down the motor current so that smaller heater components may be used to operate correctly, induction motors must be connected to a three phase a.c. supply.

Once started they may continue to run even if one of the three supply lines becomes disconnected. This is called single-phasing and can result in motor burn-out.

Single-phasing is usually caused when one of the three back-up fuses blows or if one of the contactor contacts is open-circuited.
The effect of single-phasing is to increase the current in the two remaining lines and cause the motor to become very noisy due to the uneven torque produced in the rotor.

An increase in line current due to single-phasing will be detected by the
Protective OCR.
The three thermal elements of an OCR are arranged in such a way that unequal heating of the bi-metal strips causes a differential movement which operates the OCR switch contacts to trip out the motor contactor.

For large HV machines a separate device, called a negative phase sequence (NPS) relay, is used to measure the amount of unbalance in the motor current.
For star connected motor windings the phase and line currents are equal to the line connected OCR is correctly sensing the winding current.
If the overcurrent setting is exceeded during a single-phase fault the motor will be tripped off.
The situation is not so simple with a delta connected motor. Normally the line current divides phasorally between two phases of the motor windings.
The phase current is just over half the line current.

When one of the lines becomes open circuited a balanced here phase condition no longer exists. Now the sets of line and phage currents are no longer balanced.

Look at the condition where the motor is at 50% of full load when single-phasing occurs: the line currents are 102 % of the full-load value but the current in winding C is 131% of its full-load value. The 102 % line current will probably not activate a line connected OCR and the motor remains connected.
However, the local overheating in winding C of the motor will quickly result in damage.

Motors can he protected against this condition by using a differential type relay which trips out with unbalanced currents.
In fact, most modern thermal OCRs for motors have this protection against single-phasing incorporated as a normal feature.

If single-phasing occurs when in operation on light load, the motor keeps on running unless the protection trips the contactor.
If the motor is stopped, it will not restart.
When the contactor is closed, the motor will take a large starting current but develop no rotating torque. The OCR is set to allow the starting current to flow long enough for the motor, under normal conditions, to run up to speed.
With no ventilation on the stationary motor, this time delay will result in rapid and severe overheating. Worse still, if the operator makes several attempts to restart the motor, it will burn out.
If a motor fails to start after two attempts, you must investigate the cause.
Undervoltage protection is necessary in a distribution system that supplies motors.
If there is a total voltage loss or black-out, all the motors must be disconnected from the supply.
This is to prevent all the motors restarting together which would result in a huge current surge, tripping out the generator again.
Motors must be restarted in a controlled sequence after a supply failure.

Undervoltage (UV) protection for LV motors is simply provided by the spring-loaded motor contactor because it will drop out when the supply voltage is lost.
For large HV motor the UV protection function will be covered by a relay separate from the OCR function or it may be part of a special motor relay which incorporates all of the necessary protection functions.
When the supply voltage becomes available, the motor will not restart until its contactor coil is energised.
This will usually require the operator to press the stop/reset button before initiating the start sequence.
For essential loads, the restart may be performed automatically by a sequence restart system.
This system ensures that essential service are restarted automatically on restoration of supply following a blackout. Timer relays in the starters of essential motor circuits are set to initiate start-up in a controlled sequence.

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Two main forms of speed change/control are available:

Pole-changing for induction motors to give two or more fixed speeds:
2-speed forced-draught fans
3-speed winches

Continuously variable speed control, e.g. smooth control of deck cranes, winches and electric ship propulsion using variable frequency.
Fixed set speeds can be obtained from a cage-rotor induction motor by using a dual wound stator winding, each winding being designed to create a different number of magnetic poles.
A 3 speed pole-change ship winch motor can be arranged by having two cage rotors mounted on the same drive shaft.

One stator winding (usually 2-pole) gives a low speed while the other is dual wound
to give medium speed (8-pole) and high speed (4-pole) outputs.

Speed control and drive direction are achieved by u set of switching and reversing contactors operated from the winch control pedestal.

Remember that to reverse the rotation of an induction motor it is necessary to switch over two of the supply lines to the stator winding.

An alternative method giving two fixed speeds in a 2:1 ratio from a cage-rotor induction motor is to use a single stator winding which has centre-tap connections available on each phase. This method uses a starter with a set of contactors to switch the phase windings into either single-star (low speed) or double-star (high speed).

Two of the supply lines are interchanged in the double-star connection – this is to maintain the same direction of rotation as in the low speed connection.

A continuously variable speed range of ship’s motor control involves more complication and expense than that required to obtain a couple of set speeds.
Various methods are available which include:

Electro-hydraulic drive.
Wound-rotor resistance control of induction motors
Ward-Leonard d.c. motor drive.
Variable-frequency induction or synchronous motor control.

The electro-hydraulic drive, often used for deck crane control, has a relatively simple electrical section. This is a constant single-speed induction motor supplied from a DOL or star-delta starter.
The motor runs continuously to maintain oil pressure to the variable-speed hydraulic motors.
A crude form of speed control is provided by the wound rotor induction motor.
The rotor has a 3-phasewinding (similar to its stator winding) which is connected to 3 slip rings mounted on the shaft.
An external 3-phase resistor bank is connected to brushes on the rotor slip rings. A set of contactors or a slide wiper (for small motors) varies the amount of resistance added to the rotor circuit.

Increasing the value of external resistance decreases the rotor speed.

Generally, the starters of wound-rotor motors are interlocked to allow start-up only when maximum rotor resistance is in circuit.
This has the benefits of reducing the starting current surge while providing a high starting torque.
The wound-rotor arrangement is more expensive than an equivalent cage-rotor machine. It requires more maintenance on account of the slip rings and the external resistor bank which may require special cooling facilities.

Where continuously variable speed has to be combined with high torque, smooth acceleration, including inching control and regenerative braking, it is necessary to consider the merits of a d.c. motor drive.
Speed and torque control of a d.c. motor is basically simple requiring the variation of armature voltage and field current.
The problem is: where does the necessary d.c. power supply come from on a ship with an a.c. electrical system?

A traditional method for lifts, cranes and winches is found in the Ward-Leonard drive. Here a constant speed induction motor drives a d.c. generator which in turn supplies one or more d.c. motors.
The generator output voltage is controlled by adjusting its small excitation current via the speed regulator. The d.c. motor speed is directly controlled by the generator voltage.

Obviously the motor-generator (M-G) set requires space and maintenance.
An alternative is to replace the rotary M-G set with a static electronic thyristor controller which is supplied with constant a.c. voltage but delivers a variable d.c. output voltage to the drive motor.

Although the Ward-Leonard scheme provides an excellent power drive, practical commutators are limited to about 750 V d.c. maximum which also limits the upper power range.
The commutators on the d.c. machines also demand an increased maintenance requirement.
To eliminate these problems means returning to the simplicity of the cage-rotor induction motor. However, the only way to achieve a continuously variable speed output by electrical control is to vary the supply frequency to the motor.
A static electronic transistor or thyristor (high power) controller can be used to generate such a variable frequency output to directly control the speed of the motor.

In an electronic variable speed drive (VSD), the fixed a.c. input is rectified and smoothed by a capacitor to a steady d.c. link voltage (about 600V d.c. from a 440V rms a.c. supply).
The d.c. voltage is then chopped into variable-width, but constant level, voltage pulses in the computer controlled inverter section using IGBTs (insulated gate bipolar transistors). This process is called pulse width modulation or PWM.
By varying the pulse widths and polarity of the d.c. voltage it is possible to generate an averaged sinusoidal a.c. output over a wide range of frequencies.
Due to the smoothing effect of the motor inductance, the motor currents appear to be approximately sinusoidal in shape.
By directing the currents in sequence into the three stator winding a reversible rotating magnetic field is produced at a frequency set by the PWM modulator.

VSDs, being digitally controlled/ can be easily networked to other computer devices e.g. programmable logic controllers (PLCs) for the overall control of a complex process.
A disadvantage of chopping large currents with such a drive creates harmonic voltage back into the power supply network.
A harmonic voltage waveform is a distorted sinusoidal wave shape.
Such harmonic voltage disturbances caused by current switching can interfere with other equipment connected to the power system.
progressive insulation breakdown due to high voltage spikes
flickering of the lighting
malfunction of low current devices such as electronic computers and instrumentation/control circuits

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The stator carries three separate insulated phase windings which are spaced 120 degree (electrical) apart and lying in slots cut into a laminated steel magnetic core.
This type of stator winding is similar to the construction used for an a.c. generator.

The ends of the stator windings are terminated in the stator terminal box where they are connected to the incoming cable from the three-phase a.c. power supply.

The rotor consists of copper or aluminium conductor bars which are connected together at their ends by short-circuiting rings to form a cage winding.
The conductor bars are set in a laminated steel magnetic core. The essential reliability of the induction motor comes from having this type of simple, robust rotor which usually has no insulation on the conductor bars and does not have any troublesome rotary contacts like brushes, commutator or slip rings.

The post Construction of ship’s electrical motors appeared first on Electro-technical Officer (ETO).

This means that the circuit-breaker contacts separate in air.

High voltage (HV) installations e.g. at 6.6 kV and 11 kV generally use the vacuum interrupter type or gas-filled (sulphur hexafluoride – SF6) breakers.

In a vacuum interrupter the contacts only need to be separated by a few millimetres as the insulation level of a vacuum is extremely high. The quality of the vacuum in the sealed interrupter chamber is checked by applying a short duration HV pulse (e.g. 10 kV for a 6.6 kV breaker) across the open contacts in the gas breaker the contacts separate in a special interrupter chamber containing SF6 gas typically at 500 kPa (5 bar) at 20C.
The operating mechanism for vacuum and SF6 breakers is similar to that employed for an ACB.

Various types of circuit breaker closing mechanism may be fitted:

Independent Manual Spring braker on ship

The spring charge is directly applied by manual depression of the closing handle. The last few centimetres of handle movement releases the spring to close the breaker. Closing speed is independent of the operator

Motor Driven Stored Charge Spring (most common type for marine applications) braker on ship

Closing springs are charged by a motor-gearbox unit. Spring recharging is automatic following closure of the breaker which is initiated by a push-button.
This may be a direct mechanical release of the charged spring, or more usually, it will be released electrically via a solenoid latch.

Manual Wound Stored Charge Spring braker on ship

This is similar to above method but with manually charged closing springs.

Solenoid braker on ship

The breaker is closed by a d.c. solenoid energised from the generator or bus-bars via a transformer/rectifier unit, contactor, push button and, sometimes, a timing relay.

WARNING: Circuit breakers store energy in their springs for:

Store-charge mechanisms in the closing springs.

Contact and kick-off springs.

Extreme care must be exercised when handling circuit breakers with the closing springs charged, or when the circuit breaker is in the ON position.

Isolated circuit-breakers racked out for maintenance should be left with the closing springs discharged and in the OFF position.

Circuit-breakers are held in the closed or ON position by u mechanical latch. The breaker is tripped by releasing this latch allowing the kick-off springs and contact pressure to force the contacts open.

Tripping can be initiated:

Manually a push button with mechanical linkage trips the latch.

Undervoltage trip coil or relay (trips when de-energised).

Overcurrent/short-circuit trip device or relay (trips when energised).

Solenoid trip coil – when energised by a remote push-button or relay (such as an electronic overcurrent relay).

Mechanical interlocks are fitted to main circuit breakers to prevent racking-out if still in the ON position. Care must be taken not to exert undue force the breaker will not move, otherwise damage may be caused to the interlocks and other mechanical parts.
Electrical interlock switches are connected into circuit-breaker control circuits to prevent incorrect sequence operation.
When a shore-supply breaker is closed onto a switchboard.
The ship’s generator breakers are usually interlocked OFF to prevent parallel running of a ship’s generator and the shore supply.

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