FENTONS LTD. Introduction ELECTRICAL DIVISION Since 1921 the division undertakes contracts in the entire field of electrical power engineering. SUPPLY / INSTALL / MAINTENANCE •Electrical installation of industrial, commercial & domestic buildings. •Lighting Control Systems. •Design, implementation and maintenance of Mini-Hydro projects. • HT and LT electrical projects. •Design and build solutions on electrical installation. •Industrial generators. •Consultancy on all aspects of electrical instal lations and applications. •Electronic lighting control systems. •Lightning / surge protection system. Repairs to electrical appliances. •Capacitor banks. •Rewinding & repairs of industrial motors. •Electric fences. The Report on Specialized Industrial Training1
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The most common form of conduit used today is screwed steel with a welded seam or
solid drawn (used in hazardous areas where there is a high risk of fire and explosion). A
light gauge conduit is also available with its use restricted to providing protection for
flush PVC cable installations. Two finishes for conduit are: black enamel (dry situations)and galvanized (for out doors and situations where dampness is present).
The main advantages of steel conduit include its ability to give conductors good
protection against mechanical damage; it allows easy rewiring; fire risks are minimized;
and the conduit can be used as a circuit protective conductor (CPC), through it is
common practice to run a separate CPC in the conduit.
PVC Conduit:
Where appropriate, PVC conduit is a popular, and inexpensive alternative to steel
conduit. It is available in both light and heavy grades and does not need to be threaded
unless so specified by the job. The conduit is available as rigid, semi-rigid, flexible round
(for surface and embedded work) and in an oval shape (for switch drops). Grades of PVC
conduit include super high impact, standard impact, and high temperature (up to 850C)
Because the expansion rate for PVC conduit is around five times that of its steelequivalent, expansion couplers are needed in long runs (at every 8m). Where the conduit
is to be used in damp situations, a special temperatures etc.
Flexible metallic or PVC conduit is often used to make a suitable connection between
rigid conduit systems and, for example, a motor which may be required to be moved for
belt tensioning, a separate circuit protective conductor is needed, run either inside the
conduit or externally.
2. Trunking
Trunking is a fabricated casing for conductors and cables, generally rectangular in shape
with a removable lid which allows the conductors to be laid in rather than be drawn in as
is the case with conduit it is used where a large number of conductors are to be carried, or follow the same route. Both steel and PVC Trunking are available, with a wide range of
such accessories as bends, T’s ,flanged adaptors, risers and reducers.
The variety of trunking includes plain section, compart mented , skirting, bench, floor
trunking, and busbar trunking. Trunking is not necessarily a complete wiring system in it
self and is thus associated with conduit and MI cables to allow connection to wiring
accessories and their mounting boxes.
Finishes on steel trunking include gray enamel, galvanized and silver enamel on zinc-coated mild steel.
Compartmented trunking: Is allows wiring at different voltages to be segregated but
carried within the same unit run. This prevents services at one voltage accidentally
becoming live to a higher voltage in the event of a fault.
Skirting trunking: Is used in offices where the services (socket outlets, switches, etc.)
can be sited on the perimeters of rooms.
Bench trunking: Is commonly found in school and laboratories where access to a large
number of socket outlets is required. As the name implies, the trunking units are mounted
on benches.
Floor trunking: Is an alternative to skirting trunking. There are three types: under floor
(where the trunking is set in a concrete floor with flush with the floor surface), and flush
duct trunking (where the lid is mounted flush with the screen) and a finish (such as
parquet wood or tiles) are placed directly on to it.
This joint involves the use of molten metal introduced to the two surfaces to be jointed so
that they are linked by a thin film of the metal which has penetrated in to the surfaces.
The metal used for joining copper surfaces is solder, which is an alloy of tin and led. It
melts at a comparatively low temperature. The grade of solder most suitable for electrical
joints istinman’s solder (60% tin, 40% led; melting point is about 200 C). The
disadvantage of soldering is that it makes the joint a non-separable contact. Soldered
joints in bus bars must be reinforced by bolts or clamps.
2. Welding
This process is some times used for large-section conductors such as bus bars. Welding is
the joining of two metal surfaces by melting adjacent portions so that there is a definite
fusion between them to an appreciable depth. The heat is supplied by a gas torch or an
electric arc. Again the welding joint is a non-separable contact.
3. Clamping
A clamped joint is easy to make, no particular preparation being required, through the
extra mass of metal round. The joints of termination make a larger bulk. How ever the
joint or termination is cooler in operation. This method provides a separable contact.Surfaces must be clean and in definite mechanical coated. Precautions must be taken to
ensure that the bolt and nuts of the clamp are locked tight.
4. Bolting
This is method involves drilling holes in the material and has the obvious disadvantage of
reducing effectiveness of the material. Contact pressure also tends to be less uniformly
distributed in a bolted joint that in one held together by clamps. Spring washers are
needed to allow for expansion and contraction as the material temperature varies with thecurrent carried.
5. Riveting
If well made, riveted joints make a good connection. There is the disadvantage, however
that they cannot easily be undone or tightened in-service.
Electricity that is supplied to homes under specific conditions of current and voltage.
Voltage, measured in volts (V), causes electric current, measured in amperes (A), to flow
in a conducting material such as copper wire. For practical and commercial reasons a“harmonized” 230-V system is used in homes in member countries of the European
Union. In the United States, domestic voltage is 120 V. Various sizes of cable are used,
depending on the circuit requirements. Typically these are: 6 A for lighting; 16 A for
water heaters; 32 A for sockets (the “ring main”); 40 A for ovens; 63 A for showers.
Electric current flows when a circuit is continuous and unbroken—for example, when a
switch is closed to the “on” position, enabling an electrical appliance to be operated.
Because conductors carry electric current, they must be insulated in order to prevent
potentially fatal contact with them. The usual insulating material is PVC(polyvinylchloride), a plastic that is flexible and mechanically strong, and can be made
thick enough to prevent a short-circuit between the conductor and any adjacent metal
parts.
For identification purposes, insulation is colour-coded. In the permanent “hidden” house
wiring, “live” cables are coloured red and are used for the supply to the appliance.
Neutral cables are coloured black and are used to complete the circuit from the appliance
back to the supply. A third cable, identified by green and yellow stripes, is used to
connect the exposed metallic parts of the appliance to earth, so that if a fault develops in
the appliance, any small fault current will flow to earth, and the exposed parts will remain
at earth potential. If there is a more serious fault, the current flowing to earth will operate
a protective device. The three cables are grouped together and further insulated by a grey
PVC outer covering. Flexible cables running from an appliance to its plug are also
colour-coded. The live cable is brown, the neutral one is blue, and the earth is again green
and yellow.
Electric current can cause fires in property, and electric shock to human beings and
animals. Fires are caused by overloading circuits attempting to take more current from
the circuit than it is designed to support. Electric shock is experienced when current passes through a living body. The result is, at best, an unpleasant experience; or burns
(which can be both external and internal), or death.
While a current of a few amperes is sufficient to cause a fire, voltages in excess of 50 V
and current in excess of 50 mA (1 mA, or milliampere, is one-thousandth of an ampere)
can prove fatal to human beings. (A 25-V shock for domestic pets can be fatal.)
Consequently, the electrical installations in homes require some form of protection to
safeguard property and lives. This is the function of the consumer unit, which is used to
divide the incoming electrical current between circuits, each carrying an appropriatecurrent, and to provide protection in each individual circuit against the hazards of shock
and fire. Protection devices are designed to sense the development of a dangerous
situation and operate to cut off the electrical supply to that circuit before the danger
reaches an unacceptable level.
Such protective devices include miniature circuit-breakers (MCBs), which prevent
circuits from being overloaded; and residual-current devices (RCDs), which protectagainst earth faults, which can cause an electric shock. An earth fault is a condition in
which current flows to earth through a conducting pathway, which could be a human
body.
Consumer units with these protective devices have superseded outdated fuse boards,
although many older properties still have fuse boards
Some appliances do not include an earth wire. This is because the appliances are double-
insulated to prevent accidental contact with the live parts inside. It is a legal requirement
in the United Kingdom for appliances to be supplied with a 13 A plug connected. These plugs have rectangular pins and are fitted with a fuse in order to protect the appliance
from damage. It is essential that the correct rating of fuse is used.
The power used by an appliance (the rate at which it consumes energy) is measured in
watts (W). The amount of power used by a particular appliance must be shown on it.
Appliances rated up to 720 W must be protected by a 3-A fuse; between 720 W and
1,200 W by a 5-A fuse; and from 1,200 W to 3,000 W by a 13-A fuse.
Fuse
Safety device used to protect an electrical circuit from the effect of excessive current. Its
essential component is usually a strip of metal that will melt at a given temperature. A
fuse is so designed that the strip of metal can easily be placed in the electric circuit. If the
current in the circuit exceeds a predetermined value, the fusible metal will melt and thus
break, or open, the circuit. Devices used to detonate explosives are also called fuses.
A cylindrical fuse consists of a ribbon of fusible metal enclosed in a ceramic or fiber
cylinder. Metal end caps fastened over the cylinder make contact with the metal ribbon.
This type of fuse is placed in an electric circuit so that the current must flow through the
metal strip to complete the circuit. If excess current surges through the circuit, the metallink will heat to its melting point and break. This action will open the circuit, stop the
current flow, and thus protect the circuit.
Recent fuse developments include types that will permit a momentary overload without
breaking the circuit. These are necessary for circuits that are used to power air
conditioners, because initial surges of power can be expected with such appliances.
Another recently developed type of fuse contains several links that can be selected by the
flip of a switch. If the fuse is blown, another link can be switched in without replacing the
fuse.In high-voltage circuits, subject to frequent interruptions, and increasingly in residential
wiring, protection is provided by circuit-breakers instead of fuses.
Switch designed to control an electrical power system by switching power on or off,
under conditions of either normal or excessive load, in order to protect the electrical
system in which it is connected. It is easier to break an alternating current (AC) than a
direct current as an AC current passes through zero twice in each cycle. The circuit- breaker may be controlled manually or automatically.
Operating conditions are unusually demanding, as a circuit-breaker may on the one hand
be called upon to open under conditions of a short circuit on the load, requiring it to break
a current that is many times the normal load current, and on the other may be required to
close on to a short-circuited system in order to confirm that a fault exists. The circuit-
breaker must therefore be reliable under static conditions, yet must operate virtually
instantaneously when called upon to do so after a long quiescent period.
When switch contacts open an arc is formed in the medium between the contacts. Thismedium is often air, but may be oil, a high-pressure gas, or a vacuum. To prevent damage
because of arcing, an air breaker may have a main set of flat contacts held together under
pressure when the switch is closed; a secondary set where the arc forms, because they are
arranged to open after the main contacts; and an arc chute, consisting of insulated parallel
metal plates, to spread and extinguish the arc.
A mechanical system of levers serves as a latch to keep the switch closed under
mechanical pressure, and a sensitive trigger, operated mechanically or electrically, is used
to release it. Electrical operation may be controlled by an electromagnet, using either the
load current itself, or some other electrical signal derived from the network, to trip the
trigger. In this way the switch can act as an overload trip.
Alternatively, the circuit-breaker can be arranged to use some other electrical quantity,
such as an imbalance in current between the load connections, which might represent
leakage to earth. In this case the circuit-breaker becomes a residual current device (RCD),
which can provide protection against serious electric shock. Miniature circuit-breakers
and RCDs have largely replaced fuses and conventional isolating switches in domestic
consumer units because they are much more accurate and reliable.
Switch
Part of an electronic or electric circuit that controls the flow of electric current. In its
simplest form, a switch consists of two metal contacts that are held together so that
current flows through them from, for example, a battery to a bulb (for example, in a
torch). In this case, as current flows through the bulb and back to the battery, the bulb is
illuminated. When the metal contacts are not held together there is a gap in the circuit and
so current cannot flow.
The mechanical contacts may be held together in different ways, depending on the
purpose of the switch and the way in which it has been designed. For example, when a
For other protective devices the breaking capacity must be adequate for the prospective
short- circuit current at the point.
Protection against Electric Shock
Direct Contact:
Electrical insulation and enclosures and barriers give protection against direct contact.
Non-sheathed insulated conductors must be protected by conduit or trunking or be with in
a suitable enclosure. A 30A RCD may be provided to give supplementary protection
against direct contact, but must not to be relied upon for primary protection.
Indirect contact:
Protection against indirect contact is given by limiting to safe values the magnitude and
duration of voltages that may appear under earth fault conditions between simultaneously
accessible exposed- conductive parts or earth. This may be effected by the:
(a) Co-ordination of protective devices and circuit impedances, or
(b) Use of RCD’s to limit the disconnection time, or
(c) Use of class 2 equipment or equivalent Insulation.
SELV and PELV
Separated Extra Low Voltage (SELV) system
(a) Are supplied from isolated safety sources such as a safety isolating transformer
to BS 3535
(b) Have no live part connected to earth or the protective conductor of another
system.
(c) Are enclosed in an insulating sheath additional to their basic insulation(d) Have no exposed-conductive parts or protective conductors of other systems
or extraneous-conductive parts.
Protective Extra- Low Voltage (PELV) system
PELV systems must meet all the requirements for SELV except that the circuits are not
electrically separated from earth.
For SELV and PELV systems protection against direct contact need not be provided if
Molded case circuit breakers have two tripping mechanism
1. Normal tripping
2. shunt tripping
2. Miniature Circuit Breaker (MCB)
MCB’s have two tripping mechanism
(a) The Bi-metal over load trip
(b) The electromagnetic short-circuit trip
The Bi-metal over load trip
The over load tripping depends on the operation of the thermally operated bi-metal strip,
which consists of two different metals rolled on each other. Due to the different
coefficient of thermal expansion, the two metals expand differently when heated (for
instance by an electric current flowing through), which results in a deflection. The
deflection depends directly on the duration. After a predetermined deflection (or
temperature), the bi-metal will activate the tripping mechanism. Normally the bi-metal is
selected to carry the line current and can be directly heated, for lower current ratings itmight be necessary to use indirect heating via a heater tape which is wound around the bi-
metal.
Some older designs of miniature circuit breakers still in use, extend the function of the bi-
metal tripping system to trip on short- circuit conditions as well as to support the bi-metal
for faster bending on high short- circuit current, an iron core is attached to the bi-metal.
Such systems normally cause the bi-metal to be over heated and result in an over
stretching. After the fault has been cleared, the bi-metal does not return to its next fault
situation the miniature circuit breaker will trip much earlier than it was design for,however the distortion is irreversible such circuit breakers may comply with standards
like NEMA (American standards) or JIS (Japanese standards), but would not pass the
more stringent requirements of standards like IEC (International standards), SS
For server over load or short-circuit conditions, miniature circuit breakers should provide
an instantaneous tripping facility.
The electromagnetic tripping system consists essentially of a solenoid coil through which
the load current flows. The coil has a fixed iron core plus a movable armature. If the
current exceeds a predermined value the coil produces sufficient electromagnetic force to
attract the movable armature against the for of the re-set spring. The switching
mechanism is activated by the tripping liver to open the contacts
This classical method is used in the so called zero point extinguishing MCBs (ZPE).
ZPE MCBs operate with an arc voltage which is much lower than the supply voltage.
This allows the short-circuit current to flow practically uninfluenced or impeded for the
first half wave of the a.c. cycle only just near the zero or cross over point of the a.c. sine
wave, the arc can be extinguished, in some cases it may even re-ignite. Electromagnetic
short-circuit trip shown in
3. Residual Current Circuit Breaker (RCCB)
RCCB’s have a two pole and 4 poles. Phase and Neutral going through the 2 pole RCCB.
3 phase and neutral go through the 4 pole RCCB.
Residual current devices provide the functions of isolation switching and earth leakage protection of electrical circuits (no over load and short-circuit protection).
They have a residual current operated electromechanical release which operates without
any auxiliary source of supply to open a circuit automatically in the case of an earth
leakage fault between phase & earth greater than or equal to a threshold of 30,100 or
300mA.
Mechanism of the RCCB
A residual current device (RCD) is a measurement device connected to a torrid sensor
surrounding the active conductors of a circuit, it’s function is to detect a difference
incurrent, i.e. a residual current caused by in insulation fault between an active conductor
and the frames or earth, and to automatically interrupt the supply with in a delay that is
compatible with people safety.
4. Earth Fault Relay (EFR)
Earth fault relay suitable for protection of all electrical circuit. This relay is extremely
accurate easy to set, compact and easy to install with rear terminal connection.
If the voltage is with in the control window, both out put relays are energized, the “Min”
and “Max” LED’s are off.
II. Maximum voltage trip
When the voltage exceeds the “Max voltage” limit of the control window and the over
voltage remains for more that time “Delay max” the “Max” out put relay de-energizesand the “Max” out put relay energizes and the “Max” LED switch off.
III. Minimum Voltage trip
When the voltage exceeds the “Min voltage” limit of the control window and the under
voltage remains for more than time “Delay min”, the “Min” out put relay de-energizes
and the “Min” LED switches on. When the voltage returns to a value more than “Min
voltage” +3% (hysterics). The reset is automatic, the “Min” out put relay energizes and
the “Min” LED switches off.
3.4 EARTHING
Every exposed-conductive-part (a part which may become live under earth fault
conditions) shall be connected by a protective conductor to the main earthing terminal
3.4.1Types of earthing system
1. TT system
2. IT system
3. TN-C system
4. TN-S system
5. TN-CS system
The correct selection of protective devices and their current/voltage ratings depend on,
among other factors, the earthing arrangement of the electrical installation system. The
distribution systems ate classified according to IEC 60364-3 by the method of systemearthing. The basic definition of the system is denoted by using two letters. An addition
of one or two letters may be necessary to indicate neutral and protective conductor
arrangements as well.
The first letter indicates the relationship of the power source and earthing.
T- Direct connection to earth
N- All live parts isolated from earth or one point earthed through impedance
The second letter indicates the relationship of the exposed conductive parts of theinstallation and the earthing.
Monthly safety meeting - safety meeting for all workers
shall be held monthly in order to educate workers for improvement of safety at
site.
Daily meeting - at daily meeting for tomorrow’s
activities, what to take care for safety shall be discussed.
6.0 Specification
Specification generally consisted of two sections, one being the requirements for
Stranded of workmanship and the other being a specific requirement for the electrical
wiring of the installation. Particular requirements were detailed in individual clauses.
The Electrical Consultant had the responsibilities of ensuring that the Specification
correctly identified and details the work that the electrical contractor was to undertaken;
there be clauses requiring him to accept responsibility for the satisfactory design of the
installation, as well as clauses requiring him to point out any alleged deficiencies or error
in the design at the tender stage of the project. In addition to this, was the general
responsibility upon the electrical installation were safe and designed to a satisfactory
standard.
It some times happens that the client changed the requirement about some aspect of the
work being undertaken or that an incorrect detail was discovered on a drawing or in the
specification. Such variance (as they were called) were generally advised by the
consulting engineer to the architect who then issued an architect’s instruction to the main
contractor who in turn informs the electrical contractor to carry out the extra work in
accordance with terms and condition.
The permanent site staff consists of project Manager, Engineers, Assistant EngineersQuantity Surveyors, Administrative Officer, & Storekeepers. Duties and responsibilities
of these officers at the work site distribute as follows.
6.1 DUTIES & RESPONSIBILITIES OF PROJECT MANAGER
He should responsibilities for around all section of the site administration,
technical and other parts.
He should prepare programs & progress charts for the site organization.
He should co- ordinates between head office and site and also between the client& contractor
C-Bus is a microprocessor-based control and management system for buildings and
homes. It is used to control lighting and other electrical services such as pumps,
audiovisual devices, motors, etc. Whether simple ON/OFF control of a lighting circuit, or
variable (analogue) type control, such as electronic dimmable fluorescent ballasts, C-Bus
can be used to easily control virtually any type of electrical load..
To ensure fast and reliable operation, each C-Bus device has its own in-built
microprocessor and “intelligence”, allowing units to be individually programmed.
C-Bus uses a patented method for updating the status of units. This method does not
require a central computer or central controller to handle databases or lookup tables tooperate. The status of each C-Bus unit is initiated at specific time intervals, without the
need of a central controller. Each device is allocated a specific time frame to broadcast its
status, synchronized by a self- generated system clock pulse. This allows large amounts
of data to be transmitted in a very small time frame, effectively and reliably on the
network, leading to low processing overheads and low bandwidth requirements.
There are many reasons to use C-Bus:
It is a highly robust and reliable control system, with a low cost per node. A wide range of tools is available, allowing third party companies to interface
with both PC based and embedded systems.
A single C-Bus cable connection can control many devices.
C-Bus offers the ultimate flexibility in switching and control. Functions can be
changed, added, removed, moved, reprogrammed, at any position on the network,
at any time — without any cumbersome hard-wiring.
C-Bus is simple to install and commission.
C-Bus can control any type of load, digital and analogue.
Electrical wiring practices have not changed much since the introduction of insulated
multicore cabling. However, wiring requirements in commercial buildings have changed
rapidly since that innovation. The additions of fire and smoke detection, security and
energy management systems have placed high demands on electrical installations.
The need for central monitoring and control of these extra systems may result in massive
networks of wires emanating from the control area.
Conventional wiring practice requires current to flow through both a switch and its load.
This requires heavy conductors to run from the switchboard to the load and, from the
load to the controlling switches. These aspects add to wiring complexity, increasing
installation time, documentation control and overall system cost. Maintenance and
system flexibility can be problematic.
The C-Bus network overcomes these problems. It uses a twisted pair of wires such as
Unshielded Twisted Pair (UTP) Category 5 (Cat-5) Local Area Network (LAN) cable, to
communicate between a building’s light switches and load controlling devices. This same
cable pair also provides the DC supply voltage to the C-Bus devices.
This greatly reduces the number of heavy wires in an installation, while enabling easy
central monitoring and system control.
C-Bus can be expanded to control and monitor a building’s electrical appliances from a
personal computer. Security, air conditioning and other systems can be programmed to
turn on or off at specific times or events. Lighting and temperature can be variedaccording to ambient conditions. Inputs, switches and loads can be reconfigured without
reconnecting a single wire.
C-Bus Communications
When a button is pressed on an input unit, a measurement is made of its press duration.
This measurement influences the message that the unit issues in response to the button
press (depending on its programming). This is illustrated in Figure 1.
Figure 6 shows how the same two-way control is wired using C-Bus (pink wiring). Thecontrol circuitry is simpler than the conventional method. If a four or eight button switch
is used instead of the two button, the wiring remains the same. Just two conductors are
required to link the C-Bus control.
DESIGN PHILOSOPHY
There are several methods of designing and installing C-Bus. An overview of theinstallation approach is shown below.
Installation
Once the design phase has been completed, installation may
begin. Several
simple steps are typically followed:
• Implementation of Programming Requirements of the Design
on a Personal Computer (Build C-Bus Database).
• Unit Initialisation and Programming (One at a Time).
• Cabling and Electrical Installation of the Hardware.
• Finalization and Further Programming of Units on the Network as required.
The details of the design are first input into a personal
computer using the C-Bus Installation Software. Hardware
should then be initialised (on a Unit by Unit basis). This
involves the assignment of a Unit Address to each Unit, one ata time. In this way Units can be uniquely identified once
installed on the Network. It is recommended that each Unit be