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TRAINING PROGRESS REPORT FOR:

MUNDENDA P

POST GRADUATE TRAINEE

(ELECTRICAL ENGINEERING)

Z.E.T.D.C- EASTERN REGION

NTC
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The induction programme at the National Training Center was conducted for 11
weeks from 05/04/11 to 20/06/11 and covered the following aspects:

 First Aid

 Protection

 Cable Jointing

 Domestic Wiring

 Motor Control and Wiring

 Familiarisation with 33 KV
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Electric Motor Control Circuits

The course was conducted by Mr Mutsago for 2weeks and covered the following:

Contactors

The contactor consists of a coiled conductor, or solenoid and magnetic circuit. The magnetic circuit is
in two parts with one fixed and the moving one held apart by a spring. The moving contacts are
mounted on the spring-loaded part. This is attracted by the fixed part when the coil is energised. This
motion closes the gap between the moving and fixed contacts and compresses the spring. At de-
energising, the magnetic field is lost and the spring pushes the armature from the fixed part of the
magnetic circuit, opening the contacts.

The contacts on the contactor are the three main ones for Power Circuit three phase supply and one
auxiliary contact used in control circuit. The Auxiliary contact can be used to substitute for the start
button if required.

Thermal Overload Relay

This device opens the Control Circuit supplying the contactor in the event of a current higher than
that on the device setting flows into the power circuit. This de-energises the contactor, tripping the
power circuit. Such a high current can be due to the motor being overloaded or results from a fault
in the Power Circuit.

Current in the Power Circuit’s three phases passes through three sets of heating elements and
bimetallic strips. The devices do not interrupt the power circuit, but simply lets the current through
it. In the event of a high current in any of the phases, the elements get hot and the heat in turn
affects the bimetallic strip. As the bimetallic strip bends it pushes a component linked to a rotating
arm that triggers the opening of the control circuit contacts. This opens the control circuit supplying
the main contactor and trips the power circuit.

Start and Stop Push Buttons

The start push button is used initially to supply the contactor with a current. It has normally open
contacts that close when the button is pressed. The start push button contacts are held apart by the
force of the spring and will open as the button is released. The action energises the contactor for as
long as the button is pressed. Continuity of current supply is achieved through auxiliary contacts
(13;14) of the contactor when wired in parallel to the start button contacts.

During the course we also covered the following control circuits:

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Control Circuit with Start Button Only

Fuse

Overload N/C Contact

Start Button

Contactor

In the circuit above, the contactor gets energised only when the start button contacts make. As soon
as the start button is released, the contacts open as the spring forces them apart. In the power
circuit the main contacts of the contactor will remain closed for as long as the start button in the
control circuit is pressed. Within this time the motor or whatever equipment supplied by the power
circuit functions. The motor or supplied equipment stops as soon as the button is released. Such a
circuit is used when the motor is required to function momentarily, like with motor inching (or
jogging circuits)

Control circuits with hold-in Contact

In the case that the motor must remain running when the start button is released, the normally open
(N/O) auxiliary contacts of the contactor are used to substitute for the start button contacts in the
circuit. The circuit is wired with the normally open contacts in parallel to those of the start button.
This gives the current alternative paths to follow when the start button is released. The contactor
remains energised and the circuit requires a stop push button to interrupt the control circuit
current,de-energising the contactor and opening the power circuit.

Both the power and control circuits require their own protection in case they develop faults. A fuse
or fused isolator at the circuits starting points provides the necessary protection.

Winding Terminals
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The terminations of the motor windings are made in the terminal box. Each of the three sets of
windings has a starting and end terminal with an appropriate marking. The motor Power circuits are
made using six points. In a star circuit the supply line feeds the points marked as 1 and those marked
as 2 are made common by use of links.

Forward Reverse Power Circuit

The forward and reverse operation of an electric motor is achieved by swapping any two of the three
phase supplies. This reverses the rotational sequence of the stator magnetic field.

Winding set A is supplied from the Red Phase, winding B from the yellow phase and C from the blue
phase. The rotary motion of the stator magnetic field is as a result of the changing instantaneous
current values within the windings. If the three phases are supplied such that the changing
instantaneous values reach a certain value, the magnetic field in the stator would follow the same
sequence. The resultant motion of the rotor would also be in the direction as determined by the
field.

Forward Reverse Principle

Swapping the supplies to the motor terminals will only change the stator windings being supplied by
the three phases at those instances. Winding set A is supplied from the Red Phase, while winding B is
supplied from the Blue phase and C from the yellow phase. If the sequence of instantaneous current
change is maintained, the rotating magnetic field reverses to anticlockwise

Star to Delta Changeover Circuit

Most motor power circuits are preferred to be in delta in which the current and subsequently the
torque generated by the motor is greater. The only challenge that might occur is that of an
excessively high starting current. To counter this, the Star delta starter circuit is used. In this circuit
the motor starts to run in star and draws less current than it would in delta. After the starting
current diminishes, the supply circuit is changed to delta.

Star Delta Forward Reverse Circuit

This is achieved by combining the concept of the forward and reverse rotation and the star to delta
changeover circuits. In this circuit the two main contactors in the motor supply circuit remain and
the other two at the winding terminals are also incorporated.

When the circuit is run in either the forward or reverse direction, it runs in the same manner as a
star delta circuit. The only difference between the two circuits is that in the star delta forward
reverse circuit the changeover has two main contactors one for the forward and another for the
reverse operation that cannot be energised at the same time.
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Direct On Line (DOL)

The control circuit for a direct on line power star or delta circuit is the basic control circuit to switch
on and off a single contactor that opens and closes the power circuit contacts. The circuit starts with
the live conductor supplying the circuit through a fuse, fuse isolator or mcb for its protection.

After the isolator the circuit passes through the overload’s N/O contacts. Next are the circuit’s stop
followed by the start push buttons. The start button is in parallel with the contactor’s N/O auxiliary
contacts, referred to as the hold-in contact,

There are no interlocks in the basic stand alone DOL circuit.

33 KV SWITCHING FAMILIARISATION

The course was conducted for 1 week by Mr Verengai and covers the following:
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Purpose of electrical safety rules

-Protection of the Authority’s staff and other persons against loss of life or limb

-Damage to person or property

-Ensure a safe and secure environment for the efficient control, operation and maintenance of the
Authority plant and equipment.

Categories of authorized Persons

Competent Person: A person over 18 years of age who has sufficient technical knowledge and
experience to safely carry out specific tasks/functions.

Junior Authorised Person: A competent person appointed in writing by the Authority to carry out
specific work on the Authority’s Power System in accordance with his/her Certificate of
Authorisation, He or She shall not issue or cancel a Permit-to-work or live line Permit-to-work or
sanction-for-test or limitation-of-Access documents.

Senior Authorised Person: A competent person appointed in writing by the authority to carry out
work and all forms of switching on the authority’s power system in accordance with his/her
certificate of authorisation. He or She issue or cancel Permit-to-work, Live line Permit –to-work,
sanction-for-test and limitation-of-access documents.

Contractor’s Representative: A competent person, other than a person employed by the authority,
who is required to work on the Authority’s plant and equipment.

Controller: A Senior authorised person appointed in writing by the Authority to control power in one
or more of the specific fields of Generation, Transmission, Subtransmission or Distribution and
whose duties are to maintain maximum safety at all times to personnel and plant and equipment on
the Authority’s Power System.

Safety Documents

Limitation of Access: A form of declaration signed and given by a senior authorized person to a
competent person in charge of work in any generating station, switching station or substation
(where a Permit-to-Work, Live-Line Permit-to Work or Sanction-for-Test is not applicable) defining
the limits of the area within which the work is to be performed.

Live Line Permit-to-Work: A form of declaration signed and given by a Senior Authorised Person to a
live line linesman in charge of the live line work to be carried out on the Authority’s plant and
equipment for the purpose of making known to that person exactly what points of the supply have
had placed on them a Live line caution notice, what remote control devices had a live line caution
notice placed on them, what auto-reclosing devices, including remote control have been switched
off or disabled and what equipment he is permitted to work on.
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Permit-to-Work: A form of declaration signed and given by a Senior Authorised Person to a

competent Person in charge of work to be carried out on the Authority’s high voltage plant and
equipment for the purpose of making known to that person exactly what equipment is dead,
isolated from all points of supply and feedback, earthed and on which it is safe to work.

Sanction-for-Test: A form of declaration signed and given by a Senior Authorised Person to a Senior
Authorised Person in charge of testing of the Authority’s high voltage plant and equipment for the
purpose of making known to such person exactly what plant and equipment is to be tested and the
condition under which testing is to be carried out.

Visitor’s Live Enclosure Permit: A formed signed by visitors acknowledging their understanding of the
danger of entering a live enclosure and to indemnify the Authority against injury whilst they are in or
about the live enclosure.

Earthing

Additional Earthing: Any earth connection which is applied after the issue of a Permit-to-work or
Sanction-for-test.

Circuit Main Earth: Any earth connection which is applied on the instruction of a Controller before
the issue of a Permit-to-work or Sanction-for-test.

Conditions for working on High voltage Plant and Equipment

No person shall carry out work on any part of high voltage plant and equipment unless this part of
the plant or equipment is

 Dead
 Isolated and all necessary steps have been taken to lock off from live plant or equipment.
 Efficiently connected to earth at all points of disconnection of supply to the plant or
equipment or between such points and point(s) of work.
 Screened where necessary to prevent danger and Caution and Danger Notices fixed,
 Released for work by the issue of a Permit-to0work or Sanction-for-test to a person who is
fully conversant with the nature and the extent of work to be done.

Work on High Structures and Overhead Lines

All persons while at work on poles and high structures shall make proper use of their safety belts and
other safety equipment, wear safety clothing and shall be in visual range of a second person.

Before any pole is climbed it shall be sounded or tested. No pole badly impaired by decay or damage
shall be climbed until it has been supported by approved means, and then only climbed by one
person.

Work in Substations and Switching Stations Containing Exposed Live Conductors

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When work is to be carried out in a substation in which there are exposed high voltage conductors,
then, unless the whole plant or equipment is dead, the section which is to be made dead for work
shall be defined as far as possible by the use of barriers, notice boards or roping arranged so that the
minimum clearance from the nearest exposed live conductor to ground level or to any platform or
access way which may be required be as specified in the safety handbook.

Work on High Voltage Cables and Overhead Lines

When any high voltage cable is to be cut or joint opened, a Senior Authorised Person shall satisfy
himself/herself that the cable has been made dead, indentified and spiked to prove dead with
approved devices before the issue of a Permit-to-work.

Work on High Voltage Plant or Equipment and Ancillary Plant or Equipment Containing Sulphur
Hexafluoride (SF6)

For any plant or equipment containing sulphur hexafluoride gas relevant operating instructions must
be available at the point of work stating the method of dealing with the gas and associated
compounds. The plant or equipment shall bear a label stating that it contains sulphur hexafluoride.

Work in Oil Tanks

No person shall enter a vessel which has been empted of oil or any other inflammable substance
until a senior authorised person is satisfied that all dangerous vapours have been expelled.

CABLE JOINTING

Electric power can be transmitted or distributed either by overhead system or by underground

cables.

Advantages of underground cables over over-heads

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 Less liable to damage through storms or lightning

 Low maintenance costs
 Less chance of faults
 Smaller voltage drop
 Better general appearance

Disadvantages

 Greater installation costs

 Introduce insulation problems at high voltages compared with the equivalent overhead
system

The chief use of underground cables for many years has been for distribution of electric power in
congested urban areas at comparatively low or moderate voltages.

An underground cable essentially consists of one or more conductors covered with insulation
.Although several types of cables are available, the type of cable to be used will depend upon the
working voltage and service requirements. In general, a cable must fulfil the following necessary
requirements:

The conductors used in cables should be tinned stranded copper or aluminium of high conductivity.

Stranding is done so that the conductor may be flexible and carry more current.

 The conductor size should be such that the cable carries the desired load current without
overheating and causes voltage drop with permissible limits

 The cable must have proper thickness of insulation in order to give high degree of safety and
reliability at the voltage for which it is designed.

 The cable must be provided with suitable mechanical protection.

 The materials used in the manufacture of cables should be such that there is complete
chemical and physical stability throughout

Construction of Cables

Core of conductors: A cable may have one or more than one core depending upon the type of
service for which it is intended.

Insulation: Each core or conductor provides a suitable thickness of insulation, the thickness of
layer depending upon the voltage to be withstood by the cable. The commonly used materials
for insulation are impregnated paper, varnished cambric or rubber mineral compound.

Metallic Sheath: In order to protect the cable from moisture, gases or other damaging liquids in
the soil and atmosphere, a metallic sheath of lead or aluminium is provided over the insulation.
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Bedding: Over the metallic sheath is applied a layer of bedding which consist of a fibrous
material like jute or hessian. The purpose of bedding is to protect the metallic sheath against
corrosion and from mechanical injury due to the surrounding.

Armouring: Over the bedding armouring is provided which consists of one or two layers of
galvanised steel wire or steel tape its purpose is to protect the cable from mechanical injury
while laying it and during the course of handling.

Serving: In order to protect the armour from atmospheric conditions a layer of fibrous materials
similar to the bedding is provided over the armour.

Insulating materials of cables

Properties

 High insulation resistance to avoid leakage current

 High dielectric strength to avoid electrical breakdown of cable

 High mechanical strength to withstand the mechanical handling of cables

 Non-hygroscopic

 Non-flammable

 Low cost

 Non-flammable

 Unaffected by acids and alkalis

Classification of Cables

Cables for underground service may be classified in two ways according to:

 Type of insulating material used in their manufacture

 The voltage for which they are manufactured

Low-Tension cables-up to 1000V

High-Tension cables-up to 11 000V

Super-Tension cables-from 22KV to 33KV

Extra high tension cables-from 33KV to 66KV

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Extra super voltage-beyond 132KV

Types of low voltage cables

 PVC ( Polyvinyl chloride) :operating temperature 65 to 70 degrees C.

 PCP ( Polychloroprene sheathed)

 PVC-SWA

 TRS

 MIMS

Cables for 3 Phase Service

The following types of cables are generally used for 3 phase service

Belted cables-up to 11 KV

Screened cables-from 22KV to 66KV

Pressure cables-beyond 66KV

Belted Cables: These cables are used for voltages up to 11KV but in extraordinary cases their use
may be extended to 22KV.The cores are insulated from each other by layers of impregnated paper.
Another layer of impregnated paper, called paper belt is wound round the grouped insulated cores.
The gap between the insulated cores is filled with fibrous insulating material so as to give the circular
section of the cable. The cores are generally stranded and may be of non-circular shape to make use
of available space.

The belt is covered with lead sheath to protect the cable against ingress of moisture and mechanical
injury. The lead sheath is covered with one or more layers of armouring with an outer serving. The
belted construction is suitable only for low and medium voltages as the electrostatic stresses
developed are more or less radial. These stresses act along the layers of paper insulation. As the
insulation resistance of paper is quite small along the layers, therefore tangential stresses set up
leakage current along the layers of paper insulation. The leakage current causes local heating
resulting in the risk of breakdown of insulation at any moment.

Screened Cables: They conduct leakage currents through metallic screens

Pressure Cables: For voltages beyond 66KV, solid type cables are unreliable because there is danger
of breakdown of insulation due to the presence of voids. When the operating voltages are greater
than 66KV, pressure cables are used.

PAPER INSULATED CABLE (PILC)

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The construction details of a typical insulated cable for use at 11KV consist of the following:

 Conductor: Copper or aluminium conductors which are stranded and shaped

 The conductors are stranded with a built in prespiral

 The conductors are insulated by application of supervolt papers to the required thickness

 The insulated cores are laid up and held together by a belt consisting of insulated papers.
Screened cables have an aluminium or copper tape soon after the insulation

 A lead sheath, either pure lead or lead alloy

 A bedding of jute or PVC

 An outer serving of hessian or PVC

CROSS LINKED POLYETHYLENE (XLPE)

 The construction of a typical cable comprises the following;

 Conductor: The conductor still consists of stranded copper or aluminium, but it is always
circular in the case of XLPE

 Insulation: Over the conductor is applied a semiconductor extruded layer followed by the
dielectric of cross linked polyethylene.

 Screening: The construction continues with the application of a numbered semiconductive

black non-woven tape, followed by a layer of copper tape over each individual core.

 Lay up: Three such cores are laid up and suitable fillers are included to keep the cable
circular

 Bedding: A layer of PVC for bedding

 Steel wire Armour

 A layer of PVC material for outer sheath

 BASIC FITTING

The course was conducted by Mr Shonhihwa for 2 weeks and covered the following:

Chipping

When a piece of metal needs to be reduced, cold, to given form and dimensions the hand processes
employed are chipping and filling.
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Chipping is performed with cold chisels driven by succession blows of a hand hammer and is for the
purpose of roughing off the great bulk of surplus material prior to filling to secure more minutely
exact form and dimensions.

Chipping Chisels: Are forged out of hexagonal or octagonal steel. They are about 8 inch long.

Flat and Cross-cut Chissels: The two most commonly used forms are the flat and cross-cut chisels.
The flat chisel is used for chipping flat and narrow surfaces. The cross cut is used for such work as
cutting grooves and key ways.

Round nosed chisel: Is used for cutting curved-bottom grooves

Diamond-point Chisel: Is used for cutting V-shaped grooves and for squaring round holes.

Filing

Files are distinguished according to:

 Length

 Cut

 Sectional form

Cut of file: The coarser cut files are used to remove a maximum quantity of material and the finer
cuts to produce a smoother and true surface and for draw filing and polishing.

Sectional forms of files: Files are made in a variety of sectional forms adapted for use in every
possible form of work.

The following are the most usual forms.

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Use of Files on different metals: Some amount of discrimination has to be exercised in the selection
of files for use on different metals.

Drilling

The operation of boring holes in a metal is called drilling and may be performed in the lathe, by
drilling machines or by hand-drilling tools.

Drills

In diagram below a and b shows the flat and twist drill. The twist drill is more expensive than the flat
but have the following advantages;

It bores a perfectly regular and parallel hole

When boring small holes the flat requires to be frequently removed from the hole in order that the
borings are cleared out, but with the twist drill the boring automatically travel along the flutings of
the drill.
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Points to Observe in Drilling

 During the process of drilling the work should be accurately centered

 The feed to the drill should be even

 When boring wrought iron or steel the cutting-edge of the drill should be kept constantly
lubricated, otherwise it will become heated and softened.

Threading, marking out and parallelism covered out during the course.

Fundamentals of Welding

Welding is a materials joining process in which two or more parts are joined together by application
of heat and pressure.The assemblage of parts that are joined by welding are called weldment.

Many welding process are accomplished by heat alone with no pressure applied,others by a
combination of heat and pressure and still others by pressure alone.

In some welding processes a filler material is added to facilitate coalescence.

Welding is most commonly associated with metal parts, but the process is also used for joining
plastics.

Advantages

Welding provides a permanent joint

Welding is usually the most economical way to join components in terms of materials usage and
fabrication.

Welding is not restricted to the factory environment

Disadvantages

Most welding operations are perfomed manually and are expensive in terms of labour cost.

Most welding processes are inherently dangerous

Since welding establishes a permanent bond between the components it does not allow for
convenient disassembly

The welded joint can suffer from certain quality defects that are difficult to detect

Fusion Welding

Uses heat to melt the base metals. In many fusion operations a filler metal is added to the molten
pool to facilitate the process and provide bulk and strength to the welded joint.
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Arc Welding: Arc welding refers to a group of welding processes in which heating of the metals
accomplished by an electric arc.

Oxy-Acetylene welding

This joining process used a mixture of oxygen and acetylene to produce a hot flame for melting the
base metal and filler metal is used. It can also be used for metal cutting.

First Aid

The course was conducted for 1 week by first aid officials from St Annes and covered the following:
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What is First Aid?

First Aid is the emergency care given to an injured or suddenly ill person using
readily available materials, while awaiting the arrival of
professional medical assistance.
What are the objectives of First Aid?
As a First Aider, you have three main objectives:
Preserving life
Preventing injuries or sudden illness form becoming worse and
Promoting recovery
What are the responsibilities of a First Aider?
As a First Aider, you are undertaking to provide any assistance that can be given safely,
and to remain on the scene until the casualty is handed over to professional medical
assistance or the proper authorities.
It is important to remember that an injured person has the right to accept or refuse your
help:
If your offer assistance and the casualty does not object, or gives you permission to
initiate First Aid, you have their consent.
It is assumed that an unconscious adult would consent to your help if possible,
therefore it is assumed that you have their implied consent to administer First Aid.
Should a person refuse your aid, stay with them and keep a close eye on their
condition until medical assistance arrives. If they become unconscious or their condition
worsens to the stage where their life may be in danger – apply First Aid
Care of the unconscious casualty
Check carefully for neck and back injuries.
Place an unconscious, breathing person on their side in the recovery position to
prevent them from swallowing their tongue, or to prevent obstruction of the airway in
case of vomiting.
Monitor breathing be prepared to assist if breathing becomes difficult or stops.
Assess the casualty for other injuries that might have caused them to lose
consciousness and apply First Aid if necessary.
Make a note of any changes in the level of consciousness, including the times at
which these changes occurred.
Do not give anything to eat or drink.
Do not leave an unconscious casualty unattended except in extreme emergencies. If
you must leave to attend to other casualties, summon a bystander to keep an eye on the
unconscious person

The ABC of breathing emergencies

Sufficient oxygen supply
Airway
If the airway is not open, a person cannot breathe.
Breathing
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There must be adequate oxygen available. If the person is trapped in a confined area,
ensure enough clean air is getting to them.
Circulation
Oxygen gets transported to the vital organs via blood; therefore normal blood flow must
be maintained to carry oxygen to the tissues.
Based on the above, breathing emergencies are therefore caused by the following
situations:
Obstruction of the airway
Lack of oxygen in the air
Interruption of the heart- lung action
NB: If the brain is deprived of oxygen for more than four minutes, brain damage may
occur!
Artificial respiration
1. Gently tap casualty on the shoulder and shout: “Are you OK?”
2. If there is not response, look, listen and feel for signs of breathing.
3. If none of these signs are present, call out for help to attract attention of anyone who
may be able to assist.
4. Position the casualty. Place casualty on their back, supporting the head and neck. Use
extreme care as neck or back may be injured.
5. Open the airway. Gently tilt the head back by placing one hand on the forehead and
the other under the chin.
6. Reassess breathing. Look, listen and feel again for signs of breathing for up to 10
seconds – opening the airway may have restored normal breathing.
7. If no breathing is detected, start mouth-to-mouth resuscitation by gently pinching
nose closed and giving two slow, full breaths.
8. Check the pulse. Allow 5 to 10 seconds to detect a possible weak pulse.
9. Send for help. Do not leave a non-breathing casualty alone while calling for medical
assistance!
10. Resume mouth-to-mouth resuscitation. Ventilate the lungs every 5 seconds for an
adult, and every 4 seconds for a child.
11. Recheck the pulse after the first minute of artificial respiration.
NB: if possible use a disposable oval mouthpiece for protection

Severe bleeding

Treatment
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 Put on disposable gloves.

 Apply direct
pressure to the wound with a pad (e.g. a clean
cloth) or fingers until a sterile dressing is available.

 Raise and support

the injured limb. Take particular care if you suspect
a bone has been broken.

 Lay the casualty

down to treat for shock.

 Bandage the pad or dressing

firmly to control bleeding, but not so tightly that it
stops the circulation to fingers or toes. If bleeding
seeps through first bandage, cover with a second
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bandage. If bleeding continues to seep through

bandage, remove it and reapply.

 Treat for shock.

 Dial 999 for an

ambulance.
Remember: protect yourself from infection by wearing disposable gloves and
covering any wounds on your hands.
If blood comes through the dressing do not remove it – bandage another over the
original.
If blood seeps through both dressings, remove them both and replace with a fresh
dressing, applying pressure over the site of bleeding

SHOCK
It is a life threatening condition that occurs when the vital organs, such as the brain and
heart are deprived of oxygen due to a problem affecting the circulating system.

 Treat any possible causes of shock

 Help them to lie down
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 Raise and support their legs

 Loosen tight clothing

 Keep them warm.

Fractures

Introduction

Complete or incomplete break or a crack in a bone due to an excessive amount of force.

Recognition and treatment

 Swelling
 Unnatural range of movement

 Immobility

 Grating noise or feeling

 Deformity

 Loss of strength

 Shock

 Twisting

 Shortening or bending of a limb

PROTECTION

The course was conducted for 1 week and encompassed familiarisation of 330/132 Warren
Substation and NTC Dummy Substation. The course also covered the following:

Introduction to Protection
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In a power system consisting of generators, transformers and transmission and distribution circuits, it
is inevitable that sooner or later some failure will occur somewhere in the system.

When a failure occurs on nay part of the system, it must be quickly detected and disconnected from
the system.

There are two principal reasons for it. Firstly, if the fault is not cleared quickly, it may cause
unnecessary interruption of service to customers. Secondly, rapid disconnection of a faulty section or
apparatus limits the amount of damage to it and prevents the effect of fault from spreading into the
system. The detection of a fault and disconnection of a fault section can be achieved by using fuses
or relays in conjunction with circuit breakers. A fuse performs both detection and interruption
functions but its use is limited for the protection of low voltage circuits’ only. For high voltage circuits
relays and circuit breakers are employed to serve the desired functions of automatic protection gear.

The relay detects the fault and supply information to the circuit breaker which performs the functions
of circuit interruption.

Protective Relays

It is a device that detects the faults and initiates the operation of the circuit breakers to isolate the
defective element from the rest of the system. The relays detect the abnormal conditions in the
electrical circuits by constantly measuring the electrical quantities which are different under normal
and fault conditions. The electrical quantities which may change under fault conditions are voltage,
current, frequency and phase angle. Changes in one or more of these quantities the faults signal
their presence, type and location to the protective relays. Having detected the fault, the relay
operates the trip circuit of the breaker. This results in the opening of the breaker and disconnection
of the faulty circuit.

The relay circuit connections can be divided into 3 parts:

 The first part is the primary winding of a current transformer which is connected in series
with the line to be protected.
 Second part consists of secondary winding of a CT and the relay operating coil.

 Third part is the tripping circuit which may be either a.c or d.c. It consists of a source of
supply, the trip coil of the circuit breaker and relay stationary contacts.

Fundamental Requirements of Protective Relays

A protective relay must have the following qualities.

Selectivity: It is the ability of the protective system to select correctly that part of the system in trouble
and disconnect the faulty part without disturbing the rest of the system.

Speed: The relay system should disconnect the faulty section as fast as possible for the following
reasons:

 Electrical apparatus may be damaged if they are made to carry the faulty currents for a long
time.
 A failure on the system leads to a great reduction in the system voltage.

 The high speed relay system decreases the possibility of development of one type of fault
into more faults.
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Sensitivity: It is the ability of the relay system to operate with low value of actuating quantity.

Reliability: It is the ability of the relay system to operate under the pre-determined conditions

Simplicity: The relay system should be simple so that it can be easily maintained.

Economy

Important Terms

Pick-up Current: It is the maximum current in the relay coil at which the relay starts to operate.

Current Setting; It is often desirable to adjust the pick-up current to any required value

Time Setting Multiplier (TSM);A relay is generally provided with control to adjust the time of operation.
This adjustment is known as time-setting multiplier.

Plug-Setting Multiplier (PSM): It is the ratio of the fault current in the relay coil to the pick-up current.

Functional Relay Types

Induction type overcurrent relays (non-directional)

This type of relay works on the induction principle and initiates corrective measures when current in
the circuit exceeds the predetermined values. The actuating source is a current in the circuit supplied
to the relay by the current transformer.

Induction type directional power relay

This type of relay operates when power in the circuit flows in a specific direction. Unlike, a non-
directional overcurrent relay, a directional power relay is designed that it obtains its operating torque
by the interaction of magnetic fields derived from both voltage and current source of the protected
circuit.

Distance or Impedance Relay

The operation of this relays is governed by the ratio of the applied voltage to current in the protected
circuit. In an impedance relay, the torque produced by a current element is opposed by the torque
produced by the voltage element. The relay will operate when the ratio V/I is less than a
predetermined value.

Differential-Relays

Is one that operates when the phase difference of two or more similar electrical quantities exceeds a
pre-determined value.

Types of Protection

Primary Protection

It serves as the first line of defence of a power system.

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Back-Up Protection

It is the second line of defence in case of failure of the primary protection. It is designed to operate
with sufficient time delay so that the primary relay will be given enough time to function if it is able to.

Protection of Alternators

Failures which can occur in an alternator

Failure of Prime mover

When input to the prime mover falls, the alternator runs as a synchronous motor and drains some
current from the supply system. The motoring condition is called inverted running.

Failure of Field

Overcurrent; It occurs mainly due to partial breakdown of winding insulation or due to overload of the
system.

Overspeed: The chief cause of overspeed is the sudden loss of all or major part of the load on the
alternator.

Overvoltage: The field excitation of modern alternators is so designed that overvoltage conditions at
normal running speed cannot occur. However, overvoltage in an alternator occurs when speed of the
prime-mover increases due to sudden loss of alternator load.

Unbalanced Loading: Means that there are different phase currents in the alternator. Unbalanced
loading rises from faults to earth or between phases on the circuit external to the alternator. The
unbalanced currents, if allowed to persist, may either severely burn the mechanical fixing of the motor
or damage the field winding.

Stator Winding Faults: These faults occur mainly due to the insulation failure of the stator winding.

Differential Protection of Alternators

The most common system used for the protection of stator winding faults employs circulating-current
principle. In this scheme of protection, currents at the two ends of the protected section are compared.
Under normal operating conditions these currents are equal but may become unequal on the
occurrence of a fault in the protected section. The difference of the currents under fault conditions is
arranged to pass through operating coil of the relay. The relay then closes its contacts to isolate the
system’s protected section. The form of protection is also known as Metz-Price circulating current
scheme.

Balanced Earth Fault

In small-size alternators, the neutral ends of the three-phase windings are often connected internally
to a single terminal. Therefore, it is not possible to use Mertz-Price principle. Under these
circumstances, it is considered sufficient to provide protection against earth faults by the balanced
earth fault method.
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Transformer Protection

Common transformer faults

Open Circuiting

An open circuit in one phase of a 3 phase transformer may cause undesirable heating. On the
occurrence of such a fault, the transformer can be disconnected manually from the system.

Overheating

Overheating of the transformer is usually caused by sustained loads or short-circuiting and very
occasionally by the failure of the cooling system.

Winding Short-circuiting

Winding short-circuiting in the transformer can cause insulation deterioration due to overheating or
mechanical injury.

Transformer Protection System

Bucholz Relay: It provides protection against all kinds of incipient faults. Faults such as insulation
failure of winding, core heating, fall of oil level due to leaking.

Earth-fault Relay: Provides protection against earthfault.

Overcurrent relays: Provides protection against phase-to-phase faults and overloading.

Differential System

Busbar and Line Protection

Busbars and lines are important elements of electric power system and acquire immediate attention of
protection such as differential protection to guard against possible faults occurring on them.

DOMESTIC WIRING

The course was conducted by Mr Mutsago for a week and covered the following:

Domestic wiring materials

 Cables

 Mcbs

 Spraigue tubes
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 Bulk heads

 Saddles

 Conduits

 Round boxes

 Distribution boards

 Earthing

 Nipples

 Couplings

Threading of steel conduits was also done and the threading process is as follow:

 Cut the conduit with a saw

 Press the die head against the conduit

 Stop the cutting as soon as the die has taken hold and apply oil to the die and the area to be
threaded

 Thread one thread short of the end of the chaser

 Remove the die head and clean chips from the thread.

During the course we also performed domestic wiring practical’s involving both pvc and steel conduits.
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ZIMBABWE ELECTRICITY TRANSMISSION &
DISTRIBUTION COMPANY