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Файл:English for marine electro-technical officers. Supplementary book. Учебное пособие
.pdf
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Electrical power on board ship is commonly generated
at 440 V, 60 Hz (sometimes 380 V, 50 Hz). Ships with a very
large electrical power demand will require generators that operate at a high voltage (3,3 kV, 6,6 kV or 11 kV) to limit the
size of normal load current and the prospective fault current.
1.1. Electrical safety
Safety rules
Large power equipment and processes utilize high forces. Electrical, mechanical, thermal and chemical changes produce the desired operation. Very high values of voltage, current, power, temperature, force, pressure etc. create the possibility of danger in an engineering system.
To minimize the safety risk to personnel and equipment
a system must be designed and manufactured to the latest high
standards and be correctly installed. During its working life the
equipment must be continuously monitored and correctly maintained by professionally qualified personnel who understand its
operation and safety requirements.
Before attempting any electrical work, there are some
basic safety precautions you must bear in mind. The possible
dangers arising from the misuse of electrical equipment are
well known. Electric shock and fire can cause loss of life and
damage to equipment.
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Regulations exist to control the construction, installation, operation and maintenance of electrical equipment so that
danger is eliminated as far as possible. Minimum acceptable
standards of safety are issued by various bodies including national governments, international governmental conventions
(e.g. SOLAS), national and international standards associations
(e.g. BS and IEC), learned societies (e.g. IEE), classification
societies (e.g. Lloyds), etc. Where danger arises it is usually
due to accident, neglect or some other contravention of the
regulations.
Ships' staff must operate equipment in a safe manner
and maintain it in a safe condition at all times. Failure to do so
will cause danger with serious consequences arising. Keep in
mind an essential list of DO's and DO NOT's when working
with electrical equipment:
DO 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.
DO operate equipment according to the manufacturer's
recommendations.
DO maintain equipment according to the manufacturer's
recommendations or the shipowner's maintenance procedures.
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DO ensure that all guards, covers and doors are securely
fitted and that all bolts and fixings are in place and tight.
DO inform the Officer of the Watch before shutting
down equipment for maintenance.
DO switch off and lock-off supplies, remove fuses, and
display warning notices before removing covers of equipment
for maintenance.
DO confirm that circuits are DEAD (by using an ap-
proved voltage tester) before touching conductors and terminals.
Electric shock
Nearly everyone has experienced an electric shock at
some time. At best it is an unpleasant experience, at worst it is
fatal.
Anyone who has access to live electrical equipment
must be fully aware of first-aid and safety procedures related to
electric shock as described in relevant safety acts. Copies of
these safety procedures should be displayed on board ship.
Electric shock is due to the flow of current through your body.
This is often from hand to hand or from hand to foot. A shock
current as low as 15 mA a.c. or d.c. may be fatal. Obviously the
size of shock current is related to the applied voltage and your
body resistance. Unfortunately, your body resistance goes
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down as the applied voltage goes up. This means that the shock
current is further increased at high voltages. The size of your
body resistance also depends on other factors such as your state
of health, the degree of contact with live wires and the perspiration or dampness on your skin. Typical dry full-contact body
resistance is about 5 000 < 2 at 25 V falling to about 2 000 Q at
250 V.
Voltages of about 60 V and below are regarded as reasonably safe for portable hand tools. This is why special stepdown isolating transformers are used with portable tools and
handlamps. These transformers supply the tool or lamp at
110 V a.c. but because the secondary winding is center-tapped
to earth, the maximum shock voltage to earth is 55 V a.c.
Electric shock is often accompanied by falling, which
may cause additional physical injury and require first-aid action. If the shock victim is unconscious, resuscitation must take
priority over first aid methods. Check the resuscitation techniques described on the electric shock posters displayed on
your ship.
Insulation resistance
AU electrical equipment has insulation. The purpose of
the insulation is to keep electric currents in the conductors and
to prevent contact with live wires. The electrical resistance of
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insulation must be very high (MQ) to prevent current leaking
away from conductors. Insulation resistance is measured between:
- Conductors and Earth;
- Conductors.
The insulation resistance includes the resistance of the
insulation material and also the resistance of any surface deposits of dirt, oil, moisture, etc. Surface deposits can reduce the
insulation resistance.
The flow of leakage currents through such surface deposits is called tracking which is also affected by the creepage
and clearance distances between terminals. Equipment must be
maintained in a clean condition to prevent tracking and to
maintain a high value of insulation resistance (usually at least
1 MQ).
Insulation materials are non-metallic and have very few
of the generally good physical properties associated with metals. Insulation is adversely affected by many factors such as
humidity, temperature, electrical and mechanical stress, vibration, chemicals, oil, dirt and, of course, old age.
Traditional insulation materials include cotton, silk, paper, etc. They may be either dry or treated with suitable varnishes or resins to exclude moisture and other harmful substances. Other materials include mica, glass fibre, etc., and
more modem materials such as PVC and other plastics and
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compounds. An extensively used medium not normally considered as an insulation material is the air surrounding the electrical components.
The majority of insulation materials in common use
cannot withstand temperatures much in excess of 100 °C.
All electrical equipment heats up when carrying load
current with the consequent rise in temperature. This temperature rise is above that of the ambient cooling air temperature.
All marine electrical equipment is constructed and rated
to work satisfactorily in a maximum ambient air temperature of
45 °C (Lloyds). Under these conditions the expected tempera-
ture rise will not exceed the permitted temperature limit set for
the insulation material. It is therefore the insulation material
that dictates the maximum permitted operating temperature of
the electrical equipment.
Answer the questions:
1. What do the emergency generator and emergency
switchboard maintain?
2. What may the generator be driven by?
3. A cargo ship has two main generators rated from
350 to 1 000 kW, hasn’t she?
4. What is electrical power on board ship generated at?
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5. What creates the possibility of danger in an engineer-
ing system?
6. Who must maintain the equipment during the ser-
vice life?
7. What must one do when working with electrical
equipment?
8. What mustn’t one do when working with electrical
equipment?
9. Whom are minimum acceptable standards of safety
issued by?
10. What would the equivalent shock current levels be
at 25 V and 250 V?
11. What is the insulation of all electrical equip-
ment for?
12. What is insulation resistance measured between?
13. Surface deposits can reduce the insulation re-
sistance, can’t they?
14. What is insulation affected by?
15. Is all electrical equipment constructed and rated to
work satisfactorily in a maximum ambient air temperature of
45 degrees C?
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Words and expressions (1.2, 1.3, 1.4, 1.5, 1.6)
clampmeter
токоизмерительные клещи
charger
зарядное устройство
incorporate
включать, объединять
continuity
непрерывность
strap
ремень
range
диапазон
pointer and scale
указатель и масштаб
live-line tester
тестер для работы
под напряжением
light up
загораться
fuse
плавкий предохранитель
finger guard
защита пальцев
to be fitted
быть установленным
home-made
самодельный
TESTING
1.2. Circuit testing
This section looks at the various electrical circuit testing
operations you may need to carry out, and at the instruments
you will need.
The main tests are for:
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Insulation Resistance
(IR)
Using a (megger) tester
(at 500 V d.c. for
a 440 V circuit)
Do not use a multimeter
for this task
Continuity Resistance
(Low 0)
Typically using
a multimeter
Component Resistance
(£1 or k£2)
Voltage (a.c. or d.c.)
Current
Using a clampmeter
(or multimeter for small cur-
rents)
1.3. Insulation testing
A measurement of the insulation resistance (IR) gives
one of the best guides to the state of health of electrical equipment. The resistance should be measured between insulated
conductors and earth, and between conductors.
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An insulation tester is a high reading resistance meter
using a high-test voltage — usually 500 V d.c. The test voltage
is produced either by an internal hand-driven generator or by
a battery and electronic voltage charger. A test voltage of
500 V d.c. is suitable for testing ships' equipment rated at 440
V a.c. Test voltages of 1 000 V and 5 000 V are used for high
voltage (HV) systems on board ship.
1.4. Continuity testing
An insulation tester normally also incorporates a low
voltage continuity test facility. This is a low resistance instrument for measuring the continuity (or otherwise) of conductors.
It can be used to measure the low resistance of cables, motor
windings, transformer windings, earthing straps, etc. The procedure for use is similar to that for the insulation tester.
Prove the correct operation of the instrument.
Isolate and lock off the equipment to be tested.
Prove the equipment to be dead.
Switch the instrument to “O” or “continuity”.
Connect the probes to the circuit.
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