• contact resistance low
• Keep insulation resistance high and
• Lubricate correctly and maintain a uniform air gap
• Ensure both the interior and exterior are always clean and dry

Provided these requirements are met, an induction motor should give trouble- free service during its long life.

The most common cause of induction motor failure is failure of stator insulation due to dampness is a major problem with marine motors.

Open ventilated motors are most at risk, particularly when they are not used for long periods.

Anti-condensation heaters should be regularly checked to see that they are actually working and keeping the motor dry.

For all motors, cleanliness is next to godliness.

A regular cleaning routine is required to remove harmful deposits of dust, dirt, grease and oil from both inside and outside the motor. The cleaning of the external surface is especially important for totally enclosed motors which run continuously.

The heat generated in these motors is removed through the external surface. A thick laver of dust will reduce the heat dissipation and result in very high temperatures. Internal dust and dirt in open ventilated motors must be regularly removed by blowing or extraction and ventilation screens and ducts cleared out.

If motors are to be blown out, the air used must be absolutely dry and the pressure should not be more than 1.75bar.

If the pressure is higher than this it forces the dust into the winding insulation rather than removing it.

When blowing out a motor remember to cover up other machines in the area to protect them from flying dust. Suction cleaning is better than blowing out.

How often should a marine motor be cleaned?

Basically this will be determined by the local conditions and the type of ventilation. Only the external surfaces of totally enclosed motors will require regular cleaning. But both the outside and inside of open ventilated motors will require routine attention.

The inside of a totally enclosed motor can be cleaned if the motor has been dismantled for bearing replacement.

Motors in areas where considerable amounts of air-borne dust are expected, hatch-cover motors are an example, will obviously require more frequent cleaning.

Contamination by oil and grease from motor bearings is often a cause of insulation failure.

The insulation should be cleaned by brushing or spraying with one of the many proprietary brands of cleaning fluid which are available. Badly contaminated motors may require total immersion of the stator windings in cleaning fluid.

Broken or missing bearing covers must be repaired or replaced to prevent grease escaping.

When a motor has been dismantled for cleaning and overhaul it should be thoroughly inspected. In this way, faults can be detected before they evolve into a major breakdown.

The post Maintenance of Marine Electric Motors appeared first on Electro-technical Officer (ETO).

Some protective functions may be grouped together within a single relay case.

Settings for level and time-delay must be periodically checked by injecting currents and/or voltages directly into the relay (usually via a special multi-pole socket adjacent to the relay and internally wired to it).

OCIT (Over Current Inverse Time) protection of ship generator

The Over Current Inverse Time relay function monitors general balanced overloading and has current / time settings determined by the overall protective discrimination scheme
Typical setting ranges for current (I) and time (t) are:

I>:0.7-2.In, (In : normal or rated generator current)

and t: 1-10s

OC(INST.) – “Instantaneous” trip protection of ship generator

“Instantaneous” trip to protect against extremely high overcurrent caused by a short-circuit fault.
Typical setting ranges are:

I >>: 2 – 10 In, and t: 0.1 – 1s

NPS (Negative Phase Sequence relay) protection of ship generator

A Negative Phase Sequence relay determines the amount of unbalance in the stator currents which is an indirect measure of the generator stator and rotor temperature.

A relatively small degree of unbalance causes a significantly increased temperature rise so the NPS current setting is low at around 0.2.In.

DIFF (Differential Measurement) protection of ship generator

This is a differential measurement of current at each end of a stator phase winding.

This comparison of current is to detect an internal fault in the stator windings which may be caused by partially short-circuited coil turns and/or earth faults.

Current settings for this very serious fault are very low e.g. about 0.1.In.

EL (Earth Leakage) protection of ship generator

An Earth Leakage relay (sometimes called Zero Phase Sequence) detects an earth fault current returning back through the earthed neutral connection.

In a ship’s HV generator system the earth fault current is limited by a high impedance NER (neutral earthing resistor) or earthing transformer so the pick-up current setting is very low, e.g. 1-5 A with a time delay of 0.1- 0.5 s.

UV/OV (Under Voltage and Over Voltage) protection

Under Voltage and Over Voltage functions are monitored by these relays with
settings of around 0.8 Un and 1.2 Un respectively (Un : rated voltage) with time delays of about 2s.
An overvoltage function may not be required in many protection schemes.

UF/OF (Under and Over Frequency) protection of ship generator

Under and Over Frequency settings are typically 58 Hz and 62 Hz for a 60 Hz system.

LO (Lock Out) protection of ship generator

This is the master Lock Out or trip / hand-reset relay responsible for tripping the generator circuit breaker.
Its action is instantaneous when triggered by protective relay.
It can also be used to trip the generator prime-mover and initiate generator field suppression together with the signalling of an alarm.

ARP (Reverse Power Protection) of ship generator

Generators intended to operate in parallel must have reverse power protection (RP).
A reverse power relay monitors the direction of power flowing between the generator and the load.
If a prime-mover failure occurred the generator would act as a motor.

The reverse power relay detects this fault and acts to trip the generator circuit-breaker.

The pick-up power level setting and time-delay setting are adjustable and are pre-set to suit the prime-mover.
If the prime mover is a turbine, very little power is absorbed when motoring and a reverse-power pick-up setting of 2-3 % is usual.
If the prime mover is a diesel then a setting range of 5-15% is usually adopted. A time delay range of about 0.5 – 3 s is usual.

The RP (Reverse Power) relay operation is easily checked during a generator changeover.

The outgoing generator is gradually throttled down so that it motors causing the reverse power relay to trip its generator circuit-breaker.

The post Main protection and safety system of ship generator appeared first on Electro-technical Officer (ETO).

Keep in mind an essential list when working with electrical equipment.

Need to do on ship

• Get to know the ship’s electrical system and equipment. Study the ship’s diagrams to pinpoint the location of switches and protection devices supplying distribution boards and essential items of equipment.
• Write down this information in a note book.
• Become familiar with the normal indications on switchboard instruments so that abnormal operation can be quickly detected.
• Operate equipment according to the manufacturer’s recommendations.
• Maintain equipment according to the manufacturer’s recommendations or the ship owner’s maintenance procedures.
• Ensure that all guards, covers and doors are securely fitted and that all bolts and fixings are in place and tight.
• Inform the Officer of the Watch before shutting down equipment for maintenance.
• Switch off and lock-off supplies, remove fuses, and display warning notices before removing covers of equipment for maintenance.
• Confirm that circuits are DEAD (by using an approved voltage tester) before touching conductors and terminals.

Do not

• Touch live conductors under any pretext.
• Touch rotating parts.
• Leave live conductors or rotating parts exposed.
• Overload equipment.
• Neglect or abuse equipment.

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When converting a ship’s wiring diagram into a circuit diagram certain basic rules and conventions should be followed.

• Every sequence should be drawn from left to right and from top to bottom (where possible)

• Each stage should be in order of occurrence from left to right.

• All contacts and components which are in series should be drawn in a straight line (where possible) with the component they control.
• All contacts and components which are in parallel should be drawn side by side and at the same level to emphasise their parallel function.

• All major components operating at bus-bar voltage should be drawn at the same level (or aligned horizontally) to help identify the required components quickly.
• All contacts should be shown open or closed in their normal or de-energised condition.

There are other conventions but these cover the main points of good systematic diagrams on ship electrical system. Block, system, circuit and wiring diagrams are the main types in general use for electrical work on vessel. Other types of diagram are sometimes used to give information for which the basic types are unsuitable (e.g. a pictorial view of a component).

Need to study the ship’s electrical diagrams to gain an understanding of equipment operation prior to carrying out maintenance or fault finding.

Diagrams on ship should be regarded as an essential tool when carrying out work on ship’s electrical equipment.

The post How to find fault on ship wiring diagram? appeared first on Electro-technical Officer (ETO).

The electrical power system on board ship is designed to provide a secure supply to all loads with adequate built-in protection for the equipment and operating personnel.

The general scheme of a ship’s electrical power system is common to nearly all ships.

The main a.c. generators (sometimes called alternators) produce the electrical power. It is supplied to the main switchboard and then distributed to the various auxiliary services comprising the electrical load. An emergency generator and emergency switchboard maintain supplies in the event of a main power failure

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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.

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 ship cable tag 3P-401

3 indicates the third wire in distribution panel P-401.

P indicates that this cable acts as a power-supplying cable or that it provides three-phase AC to its loads.

An L would indicate a lighting or single-phase AC supply panel cable.

4 with two additional digits means that there is 400 to 499 volts in that cable..

If there was only one additional digit, then the voltage in the cable would be what is actually printed. For example, 24 indicates 24 volts.

01 distinguishes the cable’s source.

This identifies the difference between the P-401 
distribution panel and the P-400 distribution panel.

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To understand the electrical distribution system, the different system components need to be examined in closer detail.

Distribution Center on ship

The distribution center controls the ship service generated power and shore power. Automatic overcurrent protective devices connected to the bus monitor and pro

tect the feeders and branch circuits.

Ship’s 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 on ship’s system

The bus tie is the electrical connection between the main and emergency switchboard bus bars. It is not considered a feeder.

Emergency Switchboard on ship

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) on ship

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 on ship

The distribution panel is an enclosed metal panel that supplies power to components for a localized section of the vessel. 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.

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.

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.

Ship’s 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 on ship

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 on ship

These conductors supply power to lighting systems, bracket fans, small heating appliances, circuits, and motors 1/4 HP or less.

Appliance Branch Circuits on ship

These conductors supply power to the outlets for the use of portable or nonfixed electrical apparatus.

Communication Branch Circuits on ship

These conductors supply power to communication, audio, and visual signaling devices.

Ship’s 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 on ship

These cables are used for the power distribution up to the rated voltage and ampacity of the cable. Low-voltage (600-volt). They are used for most electrical connections.

Ship’s Control Cables

These are multiple parallel conductor cables used for:

• Control circuits where an electrical signal energizes a magnetic control device to physically open or close the main contacts of a motor.
• The control cable does not carry the main motor operating current, but only the current used in energizing the coil of the magnetic control device.
• Indicating circuits in meters and other audio and visual indicating apparatus.
• Communication, electronic, and other similar circuits.
• 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.

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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 post What is ground on vessel’s electrical system appeared first on Electro-technical Officer (ETO).

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. 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.

The post Introduction in marine electrical system appeared first on Electro-technical Officer (ETO).

While not covering phase-to-phase faults this arrangement does have the advantage that individual phases of the generator do not have to be brought out at the neutral end.

It is usual to back-up restricted earth fault protection with an unrestricted earth fault relay energized from a single current transformer in the generator neutral.

This relay will respond to earth faults anywhere on the svstem and therefore it should be of the time delay type with current and time settings selected to give discrimination with feeder earth fault protection When neutral earthing resistors are employed an additional relay providing standby earth fault protection may also be used.

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