Power & Distribution Flashcards
(102 cards)
1
Q
What is the purpose of the Health and Safety at Work Act 1974?
A
Places duties on employers, employees, suppliers and manufactures of goods and building owners to ensure that the way that they carry out their activities does not place anyone in harms way
2
Q
When did the Electricity at Work Regulations 1989 become law?
A
April 1st 1990
3
Q
Explain reg 4
A
Quality of systems construction, suitability, quality and maintenance and use of systems
4
Q
Explain reg 5
A
Ensure that design specs are not exceeded in use
5
Q
Explain reg 8
A
Requirements for earthing equipment and circuits
6
Q
Explain reg 11
A
Excess current protection in electrical circuits
7
Q
Explain reg 13
A
Precaution for working on dead equipment
8
Q
Explain reg 14
A
When and when not to work on or near live conductors
9
Q
Explain reg 15
A
Requirements for adequate working space, access and lighting
10
Q
What is the definition of electric shock?
A
The involuntary reaction of the human body caused by conducting an electric current
11
Q
What are the causes of electric shock and explain them?
A
Equipment failure - casing become live and lethal
Human failure - individual touches an exposed live terminal
Combination of both - human failure causes equipment failure
12
Q
What BRs are useful for electrical safety?
A
BR 2000(20)
BR 167
13
Q
State and explain the 5 key HV personnel?
A
Authorising engineer - usually MEO
Authorised person - normally officer or senior rate, endorsed by AE
Authorised person in control - has specific control of the HV systems
Competent person - has sufficient technical knowledge or experience to allow them to avoid danger
HV aware - everyone else
14
Q
State the 5 pieces of HV documentation?
A
Permit to work
Sanction for test
Limitation of access
Switching schedule
Isolation certificate
15
Q
What do the first and second numbers of an IP number mean?
A
First - level of protections of persons and resistance to solid objects
Second - level of protection against the ingress of water
16
Q
What are the 7 causes of accidents?
A
Pre occupation, ignorance, lack of skill, over familiarity, tiredness and fatigue, physical hazards, horseplay
17
Q
What are the voltage performance requirements of an AC generator?
A
Rated voltage 450 V
Variation of all balanced loads less than 1%
Steady state voltage at all loads to be within 1% of 450 V
Transient voltage variation max of 15% going on and 7.5% going off
Power factor 0.4 going on 0.8 going off
Time of recovery to within 1% of final voltage = 1 second
Harmonic content of line voltage at no load = no more than 2% of nominal
18
Q
What are the frequency performance requirements of an AC generator?
A
Nominal frequency 60 Hz
Frequency within 57-63 Hz
Steady state frequency from no load to full load = 3.5-4%
Transient frequency variation for 25% load changes = +/- 2.5% for mechanical governors and +/- 1.5% for electric governors
Time of recovery to within 1% for mechanical governors and 0.2% for electric governors = 2 seconds
19
Q
What is the power factor of an AC generator?
A
0.8 lagging
20
Q
What is the frequency equation?
A
F=NP/60
21
Q
Explain the operation of a brushless generator?
A
Prime mover rotates all on shaft
Belt driven PMG supplies 115v 1600Hz to AVR
AVR supplies variable DC supply to AC exciter stator
AC EMF induced in exciter rotor
AC converted to DC by rotating rectifier assembly and supplied to main generator rotor
EMF induced in main generator stator (450 V AC)
Sample fed to AVR to regulate the excitation voltage
22
Q
What are the advantages of a brushless generator?
A
Smaller in size
Less maintenance
No rubbing parts
23
Q
What is the disadvantage of a brushless generator?
A
Slower in response to load changes
24
Q
Explain the signs of a diode open circuit?
A
Generator output voltage will fall and be compensated for by AVR
Extra strain caused on remaining diodes, leading to further breakdown
Generator should be offloaded within 30 mins
25
Explain the signs of a diode short circuit?
Generator output voltage will collapse due to loss of exciter field current
Overload of the AC exciter, causing damage to diodes
Supply breaker will open due to under voltage to protect distribution system
Additional high currents will lead to overheating
26
What daily checks should be carried out on a generator?
Bearing and casing not overheated
Generator air temp within prescribed limits
No signs of leaking of the air cooler
No undue noise or vibration
No apparent defects at the PMG drive belt
No diode failure warning at the detector unit
27
What monthly checks should be carried out on a generator?
Carry out VA and record in log
Condition monitor all bearings
Check for water in generator
Clean ventilation grills and emergency cooling flaps
28
What is the purpose of the AVR?
To maintain a constant voltage to within +/- 1% of the nominal voltage.
29
What parameters must the AVR meet?
Under conditions of rapidly changing load, the AVR must restore terminal voltage to within +/-1% of set voltage within 1 second. Momentary voltage change must not exceed 15% when load going on and 7.5% when load going off
30
What are the components of the Mk 121 AVR unit and what do they do?
Mk 17 filter unit - smooths out supply and incorporates Mk 168 power unit
Mk 1 EC excitation contactor - connects supply to the MOSFETS
Mk 28 CU output control unit - provides firing pulses for MOSFETS
Mk 169P output power amplifier - 4 MOSFETS array for excitation field current control
Mk 41 MU monitoring unit - to ensure all parameters are within tolerance
Mk 1 PN protection unit - providing system shutdown facility should fault conditions arise
Mk 1 UF interface unit - provides various voltage and frequency signals for modules within the AVR
Mk 168P low voltage power supply - provides stabilised and regulated 24 V DC and +/-15 V DC for all low voltage applications within the AVR
Mk 1 CN compensation unit - compares actual volts against desired volts to produce error output
31
What are the conditions of a steady load?
Generator voltage within +/- 1% of nominal output
32
What are the conditions of a transient load?
Maximum voltage variation should be +/- 15% for load increases and +/- 7.5% for load decreases, the AVR should restore this to within +/- 1% within 1 second
33
When are generator trials necessary?
The prime mover or governor is changed or has been through a major overhaul
The AVR or exciter is changed or has been through a major overhaul
The generator rotor or stator is changed or has been through a major overhaul
After a major modification to switchboard wiring
34
What are the requirements for generator tests?
Load barge or dockside load test facility
High speed pen recorder, laptop computer or new data acquisition recorder
The generator is isolated from the distribution system
Trials form D346(F)
35
What are fingerprint trials?
A simplified form of generator trials, carried out by ships staff
36
When are fingerprint trials carried out?
On completion of load trials for baseline data
When performance of prime mover, generator or AVR is suspect
37
What form is needed for fingerprint trials?
D346(F)
38
What are the 6 functions of breakers?
Over current protection
Under voltage protection
Reverse current protection
Fault discrimination
Safely open and close circuit
Clear maximum short circuit fault current safely
39
List the main switchboard breakers and what they do?
Supply breaker - connects/disconnects the generator output to the switchboard
Interconnector breaker - links fwd and aft switchboards
Busbar linking breaker - links switchboards together at busbars
Feeder breaker - provides power to EDCs/EDPs
Shore supply breaker - connects shore supply box on upper deck to switchboard busbars
40
What 4 trips are on an air circuit breaker?
Under voltage
Over current
Anti shock
Position interlock
41
What 3 types of over current trip are there?
Long time delay
Short time delay
Instantaneous
42
How can air circuit breakers be operated?
Manually and automatically
43
Explain the manual operation of an air circuit breaker?
Depress spring charging handle 11 times
No resistance when fully charged
White indicator flag will change to yellow
Pressing I button will close breaker
Action of breaker closing will charge opening springs
Once closed, safety devices will monitor their parameters and trip as required
Pressing O button will open the breaker
44
Explain the automatic operation of an air circuit breaker?
Spring charging is done by 220 V AC single phase motor
Once power is supplied on either side of the ACB, motor will continue to run as long as LMS 1 is closed
Motor takes approx 20 seconds to charge springs
White indication flag turns to yellow
Breaker can now be closed from local or remote
Action of breaker closing will charge the opening springs
Once closed, safety devices will monitor parameters and trip as needed
Breaker can now be opened from local or remote
45
What is a MCCB and what sizes do they come in?
Moulded case circuit breaker
Mechanically operated air circuit breaker
100 amp and 250 amp
46
State and explain the safety devices on a MCCB?
Thermal magnetic trip unit - two dissimilar metals, react to heat from overload and high current
Manual trip - push to trip button, tripper bar rotates up and right, allowing trip mechanism to unlock and contacts to open
Electromagnetic trip - current through blow apart contacts creates opposing magnetic fields, short circuit = increased opposing forces, current flows through conductor that passes close to trip unit, increased force unlatches trip unit
Current limiting fuses - fuses will blow when instantaneous trip occurs to prevent breaker being remade before clearing fault
Residual current devices - supply phase and neutral connected to winding of current transformer, insulation fault = unbalanced currents, a flux is induced and current will flow into secondary winding, activating the trip
47
State the 4 kinds of change over switches?
Hand change over switch
Automatic change over switch
Remote operated change over switch
Fast acting change over switch
48
Why can we not have all ACOSs on ships?
Weight
Cost
Throw over
49
What are the requirements for an ACOS?
Two sources of supply must never be paralleled through the switch
When both N and A supplies are within tolerance, N should be selected automatically
When N falls below nominal value, A should be automatically selected
There is a suitable delay before this occurs to prevent the switch operating on voltage transients
When N is restored to its nominal value, it is selected automatically
There must be a delay between disconnection of one supple and connection of the load to another, allowing the magnetic field in the motor to die away
50
What equipment is supplied by ACOS?
MDFL pumps
Steering motors
Chilled water pumps
Weapons systems
51
What equipment is supplied by HCOS?
Fire pumps
Portable pump sockets
Ventilation systems
52
What equipment is supplied by FACOS?
Radar electronic support measures
Two internal communication network nodes
Combat management systems
Principle anti air missile systems
53
What are the maintenance routines for ACOS?
Insulation resistance checks
Earth bonding inspection and testing
Internal cleaning
Tightness of connections
Inspections of relays and contactors (timing checks)
Timing and functional tests
Inspections of relays and contactors
Drop off and pick up limit tests
54
What are the two types of thyristor converters on a 23?
750 V DC 2250 amp for armature circuit
270 V DC 42.5 amp for field circuit
55
What is caused by harmonic currents/pollution?
Voltage distortion
Zero crossing noise
Unbalanced line currents
Overheating of machinery
56
What is the purpose of HV harmonic filters and how are they cooled?
Reduce heating effects
Provide power factor correction
Reactor conductors cooled by MIDEL
57
What is the purpose of LV harmonic filters and how are they cooled?
Provide power factor correction
Provide power that meets requirements for harmonic distortion in LV system
Cooled by chilled water
58
What is the purpose of the NER?
Limit individual alternator fault currents to 10 amps for a single earth
59
What is the purpose of the DBR?
Provide voltage control and a path for regenerated power produced when going from ahead to astern
60
What is the output of the EMGEN?
450 V 60 Hz 3 phase 250 kW
61
How can the EMGEN be started?
Battery or hydraulically
62
What does the EMGEN supply on a 23?
2/5 HPSW pumps
GT power cool pumps
5 smoke clearance fans
Emergency lighting
Sickbay
Portable pump sockets
Steering gear
63
What does the EMGEN supply on a 45?
No 3 HPAC
N supply to stbd steering motor
A supply to port steering motor
Emergency damage control cable
10 emergency distribution boxes
5 spare
64
What are the 3 modes of operation of the EMGEN?
Manual
Automatic
Test
65
With regards to IP codes, what does M and W mean?
M - moving or rotating machinery
W - weather conditions
66
What are the two system characteristics for a 23?
600 V 1.3 MW 61.2 Hz 0.684 lagging
450 V 1.1 MW 60 Hz 0.8 lagging
67
What are the minimum precautions to be taken when working on live equipment?
Should only be carried out if the MEO is satisfied that there is no alternative
The person doing the work must be properly trained and briefed
A safety number must be present who is familiar with shock procedures, resus methods, can remove a person from electric shock and knows where the isolation is
Avoid all direct contact with any circuit or component that may be energised
Ensure no direct contact is made with any earthed metal
Use insulated tools
Take care that tools and test equipment do not cause a short circuit
Wear electrical insulated gloves that have been inspected prior to use
Wear insulated footwear
Stand on a rubber mat
Do not work on live equipment while there is violent movement of the ship
68
What are the precautions to be taken when working on dead equipment?
Always adhere to safety procedures and precautions
Isolate equipment using correct LOTO procedure
Check for dead (phase to phase, phase to earth)
Voltage tester must be tested on a known source before use
Ensure capacitors are discharged and shorted
Ensure earths are properly connected
Use the correct tools to ensure no injury or damage to equipment occurs
69
What is the definition of a dangerous compartment and give examples?
Any compartment which has a fire or explosion hazard.
Aircraft hanger/vehicle deck
Lox/gox plants
Certain workshops
Fuel stowages
Paint/inflam stores
70
What are the main reasons for choosing electrical propulsion?
Economy and environment
Flexibility of layout
Load diversity between ships service load and propulsion
Ease of control and reduced manning
Low noise and vibration
71
What is the definition of a converted supply?
A supply derived from the main 450V 60Hz 3 phase supply, which requires a change in frequency and/or voltage
72
What PPE is to be worn when working with batteries?
PVC apron
PVC gauntlets
Rubber knee boots
Rubber goggles
Full face visor
73
What is the principle operation of cathodic protection?
To maintain the ships hull at a negative potential when measured with a silver reference electrode. The zone of cathodic protection is -750mV to -850mV. The hull potential value is supplied to a comparator amplifier which compares it to an internal reference potential set at -800mV
74
What are the two types of fires and their characteristics?
Fast burning - flaming phase very soon after ignition
Slow burning - may not flame at all but likely to be very smoky
75
What are the five types of fire detector heads?
Analog, optical, ionisation, heat, infared
76
What are the advantages of an AC induction motor?
Smaller in size than a DC motor
Simple construction
Easy to maintain
No electrical connection between stator and rotor
77
What are the disadvantages of an AC induction motor?
High starting current
High fuse rating
Dedicated starters or variable speed drives are required for speed control
78
What actions are to be taken in the event of a main supply failure?
Check that supply has actually failed (not a blown fuse or tripped MCCB)
If alternate supply is available, then switch over to this and inform SCC/HQ1 of supply failure and switch
Where no alternate supply is available, report loss of supply to SCC/HQ1, section may require a cable run to the effected equipment, no cables to be run without permission from DCO
79
As I/C of an electrical repair team, what are the principle tasks when closing up for action?
Organise teams
Delegate authority
Prioritise tasks
Set up communication
Test all equipment
Train team members
80
Explain the operation of a DOL starter?
With the indicator switch closed, power is available to transformer T1 and to C1.1, C1.2, C1.3.
Power is supplied to the supply available relay which is energised, closing contact SAR and illuminating the supply available light.
Power is also available to the top of the local start switch and the base of all relays.
When the start button is pressed, the circuit is completed to the tops of relays CON 1/4 and CSR.
Relay CON 1/4 energizes closing C1.1, C1.2, C1.3 and C1.4 causing the motor running light to illuminate.
Relay CSR also energizes closing contact CSR 1.1 allowing the motor to continue running after the start button is released.
81
What is the equation for speed control?
N=60f/P
82
What are the typical converted supplies?
24 V DC - TRUs, batteries, RTRs
50 V DC - TRUs, batteries, RTRs
115 V 400 Hz - SFCs
200 V 200 Hz - aircraft supplies
83
What are the safety devices for the static frequency changer?
The output voltage exceeds 126.5 V (10% over voltage)
The power module temperature exceeds 120 degrees
The load current exceeds 10.2 amps (130%)
84
What are the safety precautions when servicing batteries?
No smoking or naked lights and display notices
Ensure that ventilation is running and ventilation cowls are in position
Wear full PPE at all times
Do not allow batteries to gas excessively on charge
Ensure that eye wash bottles are full and in date
Ensure that temperature limits are not exceeded
Use insulated tools to avoid touching live terminals
Keep all electrical connections tight
Fresh supply of running water
85
What are the ways we monitor the cathodic system?
Daily - total anode current, individual anode current, output voltage, ship speed, reference electrode potential
Monthly - hull potential readings using portable reference electrode placed 8 ft below waterline
86
Explain the operation of an AC induction motor?
A 440 V 3 phase supply is fed to the windings of the stator. This produces a rotating magnetic field with two poles. This field sweeps past conductors at synchronous speed. This induces EMFs in the conductors of the rotor. The EMF causes a magnetic field to be set up which interacts with the magnetic field of the stator. This interaction of the two fields causes a torque to be applied to the rotor, which will then move in the direction of the magnetic field. As the rotor picks up speed, the rotating field cuts the conductors less, causing the induced EMF and current to decrease, meaning the rotor will run at a speed slightly less than synchronous.
87
What is the equation for fuse size?
NRCx8/1.66
88
What precautions must be taken when running emergency cables?
Wear rubber gloves
Run cables from service to supply
Lash all cables overhead or as clear as possible from walkways
Use through bulkhead terminals, permanent risers, deck tubes, do not pass cables through doors or hatches
Keep cables off the deck and out of water, carry them do not drag
Equipment being supplied by emergency cables should have danger notices at all live points
Check all connections are tight, cables secures and cable bights clear of obstruction
Where portable equipment is being used, ensure that earth connections are made
Carry out final check of emergency run before connecting to a live source
If cables are being run to a live through bulkhead terminal, do not connect to this point until the rest of the run is complete and has been checked
89
What are the electrical installation requirements for dangerous compartments?
Cable runs - not to be run through dangerous compartments unless unavoidable, at least 5 feet off the deck
Light fittings - approved, flameproof type, all switches must be double pole and mounted outside the compartment
Motors and control gear - prohibited, if unavoidable must be in flameproof enclosure with armoured cable
Portable lighting - must be of low voltage, must be of approved type
90
How is cooling achieved for a generator?
Closed air circuit water cooler. Shaft mounted impeller that circulates internal air across a cooler tube stack.
91
How is emergency cooling achieved for a generator?
By opening emergency cooling flaps to allow a free flow of air from the compartment to pass over the generator.
92
What precautions are to be taken when operating a HCOS?
Only operate in an emergency
Never operate on load
Ensure remote start facility is inhibited during the changeover sequence
Ensure motor is stopped before operation
If motor cannot be sighted, pause for 10 seconds to allow rotating field to dissipate
93
When will the SFC trip?
Output voltage exceeds 126.5 V (10% over voltage)
The power module temperature exceeds 120 degrees
The current exceeds 10.2 amps (130%)
94
What are the three methods of corrosion protection?
A correctly applied paint scheme
Impressed current cathodic protection
Sacrificial anodes
95
What is the zone of cathodic protection?
-750 mV to -850 mV
96
What is the result of an over protected hull?
Risk of alkali attack
Blistering of paint
Danger of hydrogen embrittlement
97
What is the result of an under protected hull?
Freely corroding/intense corrosion
98
What are the 4 types of maintenance?
Preventative
Scheduled
Corrective
Condition monitoring
99
Explain the operation of the SFC?
Frequency changes step up and refine the frequency from the ships 60Hz supply to 400Hz needed to control steering gear stabilisers and aircraft services
The transformer takes the 440V 60Hz input and transforms this supply to an appropriate sine wave voltage
The rectifier takes the transformed AC waveform and rectifies it to a raw DC voltage
The smoothing circuit, consisting of capacitors and inductors, produces a smoother, more acceptable direct voltage
The inverter changes DC to AC
The filter 'cleans' the waveform and produces a sine wave
100
What must be carried out on the completion of maintenance?
Remove LOTO and isolations
Ensure equipment is tested and functional
Inform user that equipment is available
Complete appropriate documentation
101
What are common maintenance tasks?
Earth bonding
Insulation measurements
Voltage and current measurements
Tightness of connections
Thermo-graphic surveys
Functional tests
Cleaning
Lubrication
Vibration analysis
102
What are common faults?
Earth faults
Short circuit
Open circuit
Component failure
