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HEBREWS 11: 1

- Excess current protection is one of the requirements of statutory and I.E.E Regulations. The
IEE Regulation states that; - every consumer’s installation shall be adequately controlled by
switchgears that are readily accessible to the consumers which shall incorporate means of
excess current protection.
- The means of excess current protection required by the regulations shall comprise of a fuse
placed in each live conductor of the supply or a circuit breaker having an excess current
release fitted in each live conductor of the supply.
- The means of excess current referred above may be omitted if: -
i. The rating of all cables connected between the supply undertaking fuse or circuit breaker
and the consumer’s sub-circuit fuses or CBs is not loss than the rating of the supply
undertaking’s fuse or CBs,
ii. The excess current protective devices protecting all circuits controlled by the switchgear
are located within the same enclosure as the switchgear or alternatively are located
immediately adjacent to it.
- All circuit’s requires protection against the effects of excess current which includes increased
temperatures in conductors (leading to deterioration of insulation), hence increased fire
hazards, excess voltage drop and deterioration of conductor joints and termination.
TERMS USED IN ELECTRICAL PROTECTION

a) A fuse; - a device for opening a circuit by means of a fuse element designed to melt when an
excessive current flow through it. It consists of a fuse base and a fuse link.
b) A circuit breaker; - a mechanical device for making and breaking a circuit both under
normal and under-abnormal circumstances such as those of a short-circuit. The circuit is
being broken automatically.
c) Current rating; - this is a current, less than the minimum fusing current stated by the
manufacturers as current that the fuse can carry continuously without deterioration of the
fusing element or the maximum current that a fuse will carry indefinitely without undue
deterioration of the fuse-element.
d) Fusing/fuse element; -

e) Fusing current; - this is the minimum current that will cause the fuse to operate in a
specified time under the prescribed conditions, or the minimum current that will blow the
fuse.

f) Fusing factor; - this is the ratio, greater than the unity, of the rated minimum fusing current
to the current rating.

Fusing factor = minimum fusing current / current rating

g) Breaking capacity; - this is the greatest prospective current that may be associated with a
fuse under prescribed conditions of voltage and power factor or time constant.

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h) Discrimination; - the ability of fuses and circuit breakers to interrupt the supply of a faulty
circuit without interfering with the source of supply of the healthy circuits. This requires that
a larger fuse near the source supply will remain unaffected by fault currents which would
cause a smaller fuse, for that form the same, to operate.

i) Close excess current protection; - excess current protection which will operate four hours at
1.5 times the designed load current of the circuit which it protects.

j) Coarse excess current protection; - excess current protection which will not operate within
four hours at 1.5 times the designed load current of the circuit which it protects.

VARIOUS FORMS OF ELECTRICAL PROTECTION

- This are the precautions taken to prevent damage to the various parts of an electrical circuit
such as the; - wiring systems, electrical accessories, heating appliances and other electrical
apparatus.
- Protection entails prevention of damage, generally of physical nature.
- The various forms of protection comprise; -
 Protection against mechanical damage,
 Protection against heat or fire,
 Protection against corrosion,
 Protection against excess current.
PROTECTION AGAINST MECHANICAL DAMAGE
- Mechanical damage is basically the term used in describing the physical damage and harm
sustained on some portion or various parts of electric circuits generally by impacts of hitting
the cables with hammer or by abrasion or by collistance.
- The IEE Regulation B25-30 states that cables must be prevented against mechanical damage
during their normal condition of service. The conduits, ducts, trunking and casing must be
additionally protected.
- Protection against mechanical damage is an important factor in the choice of a wiring system
or fitting for a particular situation.
REGULATIONS FOR THE PROTECTION OF CONDUCTORS AND CABLES
AGAINST MECHANICAL DAMAGE
i. Cables installed under floors should either be in position where they are not liable to damage
by contact with the floor.
ii. All non-sheathed cables must be enclosed in a conduit, duct or trunking.
iii. Non-metal sheathed cables exposed to direct sunlight should be covered with a black type of
P.C.P. / P.V.C.
iv. Cables which form part of a lift installation and are running in the lift should be armoured or
copper sheathed.
v. In order to prevent damage to the cables, during installation, holes in structural metal work
through which cables pass must be brushed

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HEBREWS 11: 1

PROTECTION AGAINST HEAT OR FIRE

- Electric fires are generally caused by; -
 A fault, defect or omission in the wiring system.
 Faults or defects in the appliances.
 Operation or abuse of electrical circuits through overloading.
- Protection against heat or fire entails that the installation must be fire proof or flame proof.
- The IEE Regulation B A0 requires that when installing conductors in any building, it is
necessary to avoid gaps or holes in the walls or floors which may assist the spread of fire.
- The IEE Regulation B 3A-38 indicates that the installation must be prevented against the risk
of overheating.
- Holes and other openings must be made good with an incombustible material.
- Where vertical cables duct is installed, non-ignitable materials must be fixed at intervals
inside the ducts.
- There are two types of flame prove apparatus; -
i. Mining gear – which is used with armoured cables.
ii. Industrial gear – which is used with solid drawn steel conduits.
- Mining gear is known as group one gear and comes into contact with only fire hazards
(methyonel)
- Industrial gear is installed in situation where a wide range of explosive gases and liquids are
present e.g.,
a) explosive gases and vapors
b) Inflammable liquids
c) Explosive ducts
- When considering the protection of cables against damage by heat or fire, three possible
sources of heat are considered; -
a) The ambient temperature of the surrounding,
b) Rise in temperature of the cables,
c) Transfer of heat from terminal apparatus
PROTECTION AGAINST CORROSION
- Corrosion is the electrochemical process or reaction by which metal reverts. In the presence
of moisture to a more stable form usually of the type in which it is found in nature.
- Corrosion is normally caused by the flow of direct electrical current which may be self-
generated or imposed from external source e.g., earth leakage current.
PRECAUTION AGAINST OCCURRENCE OF CORROSION
i. Prevention of contact between two dissimilar metals e.g., copper and aluminium.
ii. The prohibition of soldering fluxes which remains acidic or corrosive at the completion
of soldering operation.
iii. The protection of cables, wiring systems and equipment’s against the corrosive action of
water, oil and dampness.

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iv. The protection of metal sheath of cables and metal conduits where they come int contact
with lime, cement and plaster hardwood.
Classes of fuses

FUSING FACTOR

CLASS EXCEEDING NOT EXCEEDING

P 1.0 1.25

Q1 1.25 1.50

Q2 1.50 1.75

R 1.75 2.50

PROTECTION BY USE OF FUSES

- Fuse offers a means of excess current protection in its basic form; the fuse consists of a short
length conductor of a suitable material. In most cases, it is in the form of a wire which has a
very small cross-sectional area compared to the circuit conductor.
- A current flow which is greater than the current rating of the wire (fusing element); the fusing
element(wire) will get very hot and eventually melt.
TYPES OF FUSES
i. Cartilage fuses; -
- It consists of a ceramic or porcelain tube containing the fusing element.
- The fuse is filled with a non-fusible sand which serves the purpose of quenching the
resultant heat produced(generated) when the element melts, the fusing factor is between 1.25
to 1.75 depending on the type of the fusing/ fuse elements.

Fig 8. 3 a typical cartilage fuse

Advantages of cartilage fuses

a) The fuse is designed in such a way that it is enclosed hence no destruction of the fuse
element.
b) The fuse rating is accurately given.
Disadvantages of cartilage fuses

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a) The fusing element is more expensive to replace.

b) They are designed in such a way that they cannot be interchanged expect with their own
group.
c) They are unsuitable for use for high rated voltage-current circuits.
ii. Rewireable fuses; -
- A rewireable fuse is a simple and relatively cheaper type of over-current protective device.
- It consists of a porcelain bridge and a fuse base.
- The fusing/fuse element is connected between the terminals of the bridge.
- An asbestos tube is fitted in the fuse to minimize the effects of arcing formed when the fuse
element melts.
- The fusing factor of a rewireable fuse is roughly 2 with a protective asbestos pad which
eventually reduces the factor to 1.9. This implies that a fuse-element rated at 10A will melt
when 10 x 2 = 20A flows in the circuit. It also implies that a 1.00mm2 conductor (which has a
current rating of 11A) may, in an overload condition, be made to carry as much as 50 percent
overload without the fuse coming into action; the cable is thus run on overload which may
lead eventually to a fault.

Fig 8. 4 A typical rewireable fuse

Advantages of a rewireable fuses

a) It is not expensive.
b) It is easy to replace the fusing element.
c) It is simple in construction.

Disadvantages of a rewireable fuses

a. The ease with which an inexperienced person can replace a ‘blown’ fuse element with a
wire of incorrect gauge or type.
b. It takes time before blowing or melting hence it can cause physical damage to circuit
conductors and equipment’s being protected.
c. It doesn’t offer discrimination between a surge and a continuous fault current; - this
means it is possible, in certain installation conditions, for a 15A fuse-element to melt
before a 10A fuse-element. Also, a rewireable fuse is not capable of discriminating
between a momentary high current e.g. motor starting current and a continuous fault
current.
d. It’s very difficult to know the rating, this renders this fuse unsuitable for circuits which
requires discrimination protection.
e. The fuse element is always at a fairly high temperatures when in use, this leads to it’s
oxidization hence the fuse element deteriorates very fast.

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f. They are classified by offering course and close excess current protection i.e., the device
will take longer than 4 hours to come into operation when a current of 1.5 times the
designed low current of the associated circuit flows through the fuse element.
iii. HIGH BREAKING CAPACITY FUSE (H.B.C)
- It has ceramic with some metal cups.
- The fuse element is silver of special shape stripes. The stripes are surrounded by chemically
purified silver.
- When an overload occurs, breaking the fuse element, there is formation of an ore.
- These fuses are commonly used to protect current in large industry, mains cables, motor
circuit for back-up protection for machinery.

Advantages of high breaking capacity fuse (H.B.C)

a) There are discriminative in nature (will not blow when there is a surge).
b) Has a short time operation, hence can be used for short circuits.
c) Is able to
Disadvantages of high breaking capacity fuse (H.B.C)
a) It very expensive
b) Requires an experienced person to replace.
iv. Protection by circuit breakers
- It is a mechanical device for making and breaking of circuit both under normal and abnormal
conditions.
- The circuit breaker is selected for a particular duty taking the following into consideration; -
 The normal current it will have to carry,
 The amount of current which the supply system will feed into the circuit fault.
Functions of a circuit breakers
 It permits the installation of appliances it will protect to be used up to its full rated
capacity.
 To protect devices to be used in dangerous conditions.
 They are to provide a closer and more accurate degree of excess current protection.

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- The circuit breaker has a mechanism which when it is in a closed position, holds the contact
together. The contacts are separated when the release mechanism of a circuit breaker is
operated by hand or automatically by magnetic or thermal means.

Magnetic tripping
- It employs a solenoid which is a coil with an iron slug. The normal circuit current which
flows through the coil is not sufficiently strong to produce a significant magnetic flux.
- As the circuit current increases the magnetic field strength increases to cause the iron slug to
move within the solenoid and collapses the attached tripping linkage to open the contact.
Thermal tripping
- It uses a heat sensitive bimetallic element. When the element is heated to a predetermined
temperature, the resultant deflection is arranged to trip the circuit breakers.
- The time taken to heat the element to this temperature provides the necessary time-delay
characteristics.
- The bimetal element may be arranged to carry the circuit current, and so be directly heated.
When a heavier overload occurs, the magnetic trip coil operates quickly to disconnect the
faulty circuit.
v. D

TOPIC 9

BATTERY CHARGING

Objectives

By the end of the topic the learner should be able to: -

i) Explain the constant voltage charging methods
- Constant current charging
- Constant voltage charging
- Float battery charging
- Trickle battery charging
ii) Describe the maintenance of various batteries
- Lead-acid cells
- Alkaline cells
- Zinc air

Battery charging methods

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A rechargeable battery, storage battery or secondary cell is a type of electrical battery which can
be charged, discharged through a load and recharged many times as opposed to a disposable or
primary battery, which is supplied fully charged and discarded after use.
Some of the charging methods includes: -
i. Constant current charging method
ii. Constant voltage charging method
iii. Float charging method
iv. Trickle charging method
v. Booster charging method
vi. Taper charging method
vii. Iui charging method
viii. Burp charging method
ix. Pulsed charging method
1. Constant current charging method
The charging current is kept constant by varying the supply voltage to voltage to overcome the
increased back e.m.f of cells. If a charging booster (which is just a shunt dynamo directly driven
by a motor) is used, the current supplied by it can be kept constant by adjusting its excitations. If
charged on a d.c. supply, the current is controlled by varying the rheostat connected in the circuit.
The value of charging current should be chosen appropriately to avoid excessive gassing during
final stages of charging. This also maintains the cell temperature not avoid 45 degrees Celsius
The method takes comparatively longer time.

Figure 9. 1 constant-current charging system

2. Constant-Voltage charging method

Here voltage is maintained constant but it results in very large charging current in the beginning
when the back e.m.f of the cell is low and a small current when their back e.m.f increases on
being charged.
When a secondary cell is being charged, then the e.m.f of the cell acts in opposition to the
applied voltage. Taking V as the supply voltage and I as the charging voltage against the back
e.m.f Eb, then input is VI but power send in overcoming the opposition is EbI. The power EbI is
converted into chemical energy which is stored in the cell.
The charging current is then given as: -

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