Give a brief description of Galvanic Corrosion
When two metals of different nobility are immersed in an electrolyte (saltwater) the less noble metal starts to corrode. This is known as galvanic corrosion.
What THREE factors contribute to galvanic corrosion?
- Dissimilar metals used in conjunction with
- The conductive path
- Electrolyte presence.
What is the difference between an Anode and a Cathode?
In the galvanic series the most noble meal will be a cathode when the lesser noble metal will become an anode. When immersed in an electrolyte (seawater)
What is a simple definition of voltage?
Voltage is the pressure from an electrical circuit’s power source that pushes charged electrons (current) through a conducting loop
What is a simple definition of amperage?
It describes the amount of electrical charge that is flowing through a system or, alternately, the maximum amount of electrical current that a system is capable of handling safely.
What is a simple definition of hertz?
The hertz is the unit of frequency in the International System of Units, often described as being equivalent to one event per second.
What is a simple definition of kilowatts?
A watt is a power unit that measures how fast energy is consumed or generated. Think of it as a speedometer for energy consumption.
Kilowatts are the definition of 1000 watts of power.
What precautions should be taken when it is necessary to work on or near live equipment
- A second person, who should be competent in the treatment of electric shock should be continually in attendance.
- The working position to be adopted should be safe and secure to avoid accidental contact with the live parts. Insulated gloves should be worn where practicable
- Contact with the deck particularly if it’s wet should be avoided. Footwear may give inadequate insulation if it is damp or has metal studs or rivets. The use of a dry insulating mat at all times is recommended
- Contact with bare metal should be avoided. A hand-to-hand shock is especially dangerous. To minimise the risk of a second contact should the working hand accidentally touch a live part, one hand should be kept in a trouser pocket whenever practicable
- Wrist watches, metal identity bracelets and rings should be removed. They provide low resistance contacts with the skin. Metal fittings on clothing or footwear are also dangerous.
What are the procedures to follow before working on electrical equipment?
Before any work is done on electrical equipment, fuses should be removed or circuit breakers opened to ensure that all related circuits are dead.
If possible, switches and circuit breakers should be locked open or alternatively a “not to be closed” notice attached.
Where a fuse has been removed, it should be retained by the person working on the equipment until the job has been finished.
Check should be made that any interlocks or any other safety devices are operative.
Why do ships have an insulated distribution system rather than an earthed distribution system onboard?
An insulated system is one that is totally electrically insulated from earth (ships hull) whereas an earthed system has one pole or the neutral point connected to earth.
The priority requirement onboard ship is to maintain continuity of the electrical supply to equipment in the event of a single earth fault occurring. (insulated)
An earth fault occurring on one line of an insulated distribution system will not cause any protective equipment to operate and the system would continue to function normally.
An insulated distribution system requires TWO earth faults on TWO different lines to cause an earth fault current and operate the protective devices (breakers/fuses etc)
This is essential to the safety of the ship when operating in certain conditions
- Manoeuvring in port
- Loss of steering gear
What are the causes of earth leakage onboard ships?
- Breakdown of insulation
- Moisture
- Temperature
- Age deterioration of insulation
- Damage
- Poor practices and tools
In an insulated distribution system how do you know if you have an earth fault without the protective devices operating?
Regulations require that earth fault indicators are fitted to the main switchboard to indicate the presence of an earth fault on each insulated section of a distribution system.
Each lamp is connected between one phase and the common neutral point. Closing of a test switch connects the neutral point to earth. An earth on one phase will cause the lamp for for that phase to show a dull light or to go out, depending on the severity of the fault.
What is the procedure for connecting the shore supply
- A risk assessment and permit to work must be in place before any connections take place.
- The vessels connection box, the shore supply box and any other cables must be proven dead. The tester must be tested and all methods of making the system live must be isolated and locked.
- Adequate PPE should be provided such as rubber gloves, insulating boots, rubber mats and insulated tools.
- A competent person in charge and a defibrillator to hand.
- The supply must have the capacity to supply the ships load.
What other safety considerations should be observed whilst dealing with shore power supply?
- Ensure the shore supply is the same voltage, frequency and phase rotation (polarity) before connecting to the distribution system.
- On large vessels ensure that cables are clean and dry and have an insulation resistance of at least one megohm between phases
- The shore supply cables are in good condition
- Ensure that there is enough slack in the supply cables to allow for tidal range
- On larger vessels, an earth cable will be required foreward and aft
- On smaller vessels consider the use of a galvanic isolator to prevent stray current corrosion.
State the effects of the following when considering shore power supply.
- Incorrect rotation (polarity)
- Incorrect frequency
- Incorrect voltage
Incorrect rotation - Incorrect phase rotation (reverse polarity) will result in pumps, fans and motors to run backwards causing damage to their respective systems
Incorrect frequency - Incorrect frequency will cause pumps to run either too fast or too slow causing possible high temperatures, currents, low flows and poor performance
Incorrect voltage - Incorrect voltage could cause overheating and the possible burning out of motors. Protective devices may operate when they shouldn’t and not operate when they should.
Explain “single phasing” and the effects it might cause to the machinery.
Single phasing with reference to a 3 phase motor is when the current through one phase is lost, possibly from a fault condition like a blown fuse or a broken wire.
Effects of single phasing:
- The motor may make a buzzing sound
- The motor may be vibrating
- Incorrect pressures and flows will occur
- High temperatures will be present
- May cause protective devices to operate
- Thermal protective devices may operate due to overheating.
What protective devices can be used to protect against single phasing?
- Over current protection on all three phases
- inverse time over current relays
- Thermistors
How do you test earth bonding using a multimeter?
- The procedure for testing earth bonding on a motor would be to set the multimeter to resistance (ohms) placing one probe securely on the motor body making sure you have a good connection and placing the other probe to earth. The readings should not exceed 0.1 ohms.
How do you test insulation resistance using an insulation resistance tester on a 380v motor?
- Using an insulation resistance tester make sure the correct voltage is selected. In the case of a 380v motor then 500v should be selected. Place one probe on the motor body establishing you have a secure connection then place the other probe on one of the phases. When it is safe to do so press the “test” button. The resistance between each phase should not exceed 1 mega ohm 1M OHM. You should take 6 readings, 3 x phase to earth and 3 x phase to phase.
Explain the term “Synchronising” when referring to generators
Before two alternators can be connected electrically, they have to be synchronised. That is their electrical output waves must be in step, or in phase together. The process of achieving this state is known as “synchronising”
Explain the term “Parallel” when referring to generators
When the output of two (or more) alternators are electrically connected, they are said to be running in “parallel”
What would happen if the generators were to be synchronised “Out of phase”?
If two alternators are attempted to be synchronised out of phase, very large physical electromagnetic torques oppose each other and literally tries to pull the generator into synchronisation this may cause catastrophic damage to both generators, crankshafts, bearings, bedplates, rotors, stators and couplings.
What protects the generators from being synchronised out of phase?
- Check Synchroniser
Check synchroniser units are important safety devices, usually fitted to prevent faulty manual synchronising. This unit prevents the incoming circuit from closing unless the voltage, frequency and phase angle differences are within the pre-set limits.
Check synchronisers are often provided with an override switch for use in an emergency. This can lead to problems if the override is left activated after the emergency.
With regards to frequency when synchronising generators. Why is it better for the incoming machines frequency to be slightly higher than the busbar frequency?
What would happen if the frequency was lower than the busbar?
As it is virtually impossible to get the frequency of the incoming machine exactly the same as the busbar, the frequency of the incoming machine should be slightly higher than that of the busbar frequency. This is to ensure that when the main breaker is closed, the incoming machine will tend to take the load which will hold the breaker on the board.
If the frequency of the incoming machine was lower than the busbar then the incoming machine will tend to resist taking any load and the breaker may trip out on reverse power.
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Electrical Systems31
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