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Файл:English for marine electro-technical officers. Supplementary book. Учебное пособие
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1.5. Multimeters
Routine electrical test work involves measuring current,
voltage and resistance i.e. Amps, Volts and Ohms. This is most
conveniently done using a multimeter with all the necessary
functions and ranges. The instrument may be the traditional
switched-range analogue type (pointer and scale) or the more
common digital type with auto-ranging and numerical display.
1.6. Live-line testers
When equipment is to be inspected for maintenance, it
is important that supplies be switched OFF and locked OFF.
The equipment must then be PROVED to be dead to eliminate
the danger of electric shock. A live-line (or voltage) tester is
a simple device to check only whether or not a voltage exists at
terminals.
Live-line testers, up to 500 V, are of various types.
Some light up (e.g. screwdriver type with a neon indicator),
some make a noise, others operate LED's or mechanical indicators (flags) to indicate the approximate value of voltage.
It is important that voltage testers themselves be
PROVED to operate correctly before use. This can be conveniently carried out at the electrical workshop test panel.
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Home-made test lamps should not be used as they can
be dangerous because protective equipment, e.g. fuses and
ringer guards, are not fitted.
Answer the questions:
1. What are the main tests for?
2. Where should the resistance be measured?
3. Is an insulation tester a high reading resistance meter,
using a high test voltage?
4. Test voltages of 1 000 V and 5 000 V are used for
high voltage (HV) systems on board, aren’t they?
5. Does an insulation tester normally incorporate a low
voltage continuity test facility?
6. What is the continuity tester used for?
7. What is important when electrical equipment is to be
inspected for maintenance?
8. What is the function of a live-line tester?
9. Are there various types of live-line testers?
10. Why is it dangerous to use home-made test lamps?
Words and expressions (1.7, 1.8)
to be updated
должен быть обновлен
apparent
очевидный
23
to be pin-pointed
быть указанным,
определенным
imminent
неизбежный, предстоящий
essential
важный, значительный
muddler
путаница
perception
восприятие, понимание
spatial
пространственный
persistence
постоянство
background
основа
underpinning
поддержка
remedy
устранение
face rap
маска для лица
consequential
последовательный
to rectify
выпрямлять
assumption
предположение, допущение
1.7. General electrical maintenance
All equipment is subject to wear and tear, eventually
reaching the end of its useful life when it must be replaced. As
equipment nears the end of its safe working life its condition
can deteriorate to such an extent as to be a danger to personnel
and other plant. The purpose of maintenance, therefore, is to
extend the useful life by repair and / or replacement of defective parts and to maintain it in a safe and serviceable condition.
24
The marine environment is particularly arduous for
electrical equipment due to the damp, salt-laden atmosphere,
extremes of temperature and constant vibration. Shipboard
equipment is in particular need of correct maintenance.
The continuous operation of equipment on board ship
demands high efficiency and optimum economy in order to
help keep operational costs to a minimum to maintain financial
competitiveness.
Nearly all equipment needs maintenance.
An efficient maintenance engineer must get to know the
power system and its equipment. The ship's drawings and circuit diagrams must be checked and updated to relate them to
the actual equipment. Electrical services and equipment must
be kept under continuous observation so that normal healthy
operating conditions become known, and abnormal operation
becomes quickly apparent. Faults can then be pin-pointed and
corrected before a breakdown occurs.
Maintenance can be classified as:
- breakdown maintenance;
- planned maintenance;
- condition monitoring.
Breakdown maintenance (corrective maintenance) is
when equipment is left untouched until a breakdown occurs. At
this time the equipment is repaired or replaced and any other
specified maintenance procedure carried out.
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Planned maintenance (preventive maintenance) is when
equipment is regularly inspected and maintained according to
a fixed timetable and set of procedures specifying the actual
work to be done to prevent equipment failure.
Condition monitoring (another form of preventive
maintenance) is when equipment is regularly monitored and
tested. When monitoring indicates that a breakdown is imminent, the equipment is repaired or replaced and any other specified maintenance procedures are carried out. Regular insulation
testing and vibration testing are two forms of condition monitoring.
1.8. Fault finding
Generally, fault finding is not an easy task.
It is essential to have a good understanding of the operation of the particular equipment and general insight into
some of the diagnostic skills used to solve the problem.
Here is a list of the general techniques used:
Planning
A good fault-finder has a mentally planned strategy.
The evidence is carefully considered before deciding what action to take. In contrast, the “muddler” acts on impulse.
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A good diagnostician will use most of the following
mental abilities:
- memory;
- logical thinking;
- perception;
- spatialhhechanical ability;
- social skills;
- persistence.
Background (underpinning) knowledge Together with
the mental abilities above, knowledge and experience are essential. This is wide ranging and includes knowledge of components, methods and systems together with their operational
characteristics. The combination of knowledge and direct practical experience with the equipment is a powerful aid to fault
finding.
Diagnostic performance
In addition to the necessary skills of the diagnostician,
systematic use of “job aids” will improve fault finding method.
Examples are:
- Fault charts.
A list of typical symptoms and faults for a particular
equipment plus suggested remedies.
These lists should be updated according to experience
to show the most probable faults;
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- FACERAP.
The seven letters of the mnemonic “FACERAP” are the
key steps to logical fault finding:
F
(fault)
the name and classification of a fault
A
(appearance)
the description of the fault
or its related symptom
C
(cause)
the operational reason for the fault
E
(effect)
the consequential effect of the fault
R
(responsibility)
the correct person to take
remedial action
A
(action)
the standard procedure adopted
to rectify the fault
P
(prevention)
the procedure to avoid repetition
of the fault
Search strategy
Once the diagnostician can visualize the circuit or machine as a series of functions and / or use a job aid, a search
strategy can be applied to locate the fault in the minimum time.
A “six step approach” is summarized as:
1. Collect evidence (stop and think).
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2. Analyze evidence.
3. Check assumptions.
4. Locate fault (inspect and test).
5. Determine and remove cause.
6. Rectify fault.
7. Check system.
Conclusion:
Fault finding is not easy!
However, a logical approach supported by knowledge
and experience will certainly help.
Answer the questions:
1. Must an engineer know the power system and its
equipment?
2. How can the maintenance be classified?
3. What is breakdown maintenance?
4. What is planned maintenance?
5. What is condition monitoring?
6. What mental abilities does a good diagnostician use?
7. What are the key steps to logical fault finding?
8. Is fault finding easy?
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2. GENERATORS
AND MAIN CIRCUIT BREAKERS
Words and expressions (2.0, 2.1)
review
обзор, рассмотрение
to be backed
поддерживаться,
подкрепляться
lead-acid
свинцово-кислотный
alkaline-cells
щелочные ячейки
power demand
потребность в мощности
2.0. Introduction
The electrical power demand aboard ship will vary according to the ship type (tanker, bulk carrier, ro-ro, container,
ferry, cruise liner, offshore support etc.) and its day-to-day operational needs (at sea or in port). To meet the power demand,
two or more main generators are used which are backed up by
an emergency generator and an emergency battery service.
The construction, operation, protection and maintenance of generators is described together with a review of main
circuit breakers and the main switchboard.
30
2.1. AC generator operation
Main generator power ratings range from, typically,
100 kW to 2 MW at 440 V, 60 Hz a.c. or 380 V, 50 Hz a.c.
driven by diesel, steam turbine, gas turbine or propulsion shaftdriven prime movers. As the demand for increased electrical
power installations arise (e.g. for specialist offshore vessels and
cruise liners) it is necessary to generate at high voltage (HV)
with voltages typically at 6.6 kV, 60 Hz but 3.3 kV and 11 kV
are also used.
An emergency generator, typically 20 kW to 200 kW at
440 V or 220 V, will be diesel driven and fitted with an automatic start facility.
Battery supplies from lead-acid or alkaline cells, usually
rated at 24 V d.c., provide sufficient power for the emergency
alarm and communication systems together with some lighting
and power essential for safety during a main power failure.
As the vast majority of ships use alternating current
(a.c.) generators (sometimes called alternators), the principles
and operational features will cover this type only, and ignore
the direct current (d.c.) type.
The basic principle of an a.c. generator is very simple.
Pairs of electromagnetic poles are driven (by the prime mover)
past fixed coils of wire on the stator.
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