Transcript of "Measurement of electrical quantities"
Electrical QuantitiesUNIT 3 MEASUREMENT OF ELECTRICAL
3.2 Moving Coil Galvanometer
3.3 Navigational Lights
3.3.1 Intensity of Lights
3.3.2 Horizontal Sectors
3.3.3 Vertical Sectors
3.4 Colour Specification of Lights
3.4.2 Visibility of Lights
3.8 Electrical Safety
3.10 Key Words
3.11 Answers to SAQs
In Unit 2, you have studied simple concepts of electricity and magnetic effect. You were
introduced to electrical quantities like voltage, current, elements like resistance,
inductance and capacitance, and their units like voltage, amp, ohm, henry and farad. In
this unit, we will learn some basic instruments used for measuring electric voltage and
current through use of voltmeter and ammeter. You will also be introduced to safety
norms, and safety control using different types of accessories such as switch, fuse, relay
and navigational lights.
After studying this unit, you should be able to
• understand the basic galvanometer,
• understand how a galvanometer is used as voltmeter or ammeter,
• understand the importance of navigational light,
• explain different types of light and colour specification,
• appreciate the importance of shapes and visibility features of light,
• describe the different types of switches in electrical control,
• understand the operations of relay and fuse, and
• appreciate the relevance and importance of safety standards in electrical
3.2 MOVING COIL GALVANOMETER
Galvanometer is an instrument used to determine the presence, direction, and strength of
an electric current in a conductor. All galvanometers are based upon the discovery by
Hans C. Oersted that a magnetic needle is deflected by the presence of an electric current
in a nearby conductor. When an electric current is passing through the conductor, the
magnetic needle tends to turn at right angles to the conductor so that its direction is
parallel to the lines of induction around the conductor and its north pole points in the
direction in which these lines of induction flow. In general, the extent to which the needle
turns is dependent upon the strength of the current. In the earlier galvanometers, a freely
turning magnetic needle was hung in a coil of wire; in later versions the magnet was
fixed and the coil made movable. Modern galvanometers are of this movable-coil type
and are called d'Arsonval galvanometers (after Arsène d'Arsonval, a French physicist). If
a pointer is attached to the moving coil so that it passes over a suitably calibrated scale,
the galvanometer can be used to measure quantitatively the current passing through it.
Such calibrated galvanometers are used in many electrical measuring devices. The DC
ammeter, an instrument for measuring direct current, often consists of a calibrated
galvanometer through which the current to be measured is made to pass. Since heavy
currents would damage the galvanometer, a bypass, or shunt, is provided so that only a
certain known percentage of the current passes through the galvanometer. By measuring
the known percentage of the current, one arrives at the total current. The DC voltmeter,
which can measure direct voltage, consists of a calibrated galvanometer connected in
series with a high resistance. To measure the voltage between two points, one connects
the voltmeter between them. The current through the galvanometer (and hence the
pointer reading) is then proportional to the voltage (Ohm’s law).
Ammeter is an instrument used to measure the magnitude of an electric current of several
amperes or more. An ammeter is usually combined with a voltmeter and an ohmmeter in
a multipurpose instrument. Most ammeters are based on the d′Arsonval galvanometer
and are of the analog type, i.e. they give current values that can vary over a continuous
range as indicated by a scale and pointer or digital readout.
Voltmeter, instrument used to measure differences of electric potential, commonly called
voltage, in volts or units that are multiples or fractions of volts. A voltmeter is usually
combined with an ammeter and an ohmmeter in a multipurpose instrument. Most
voltmeters are based on the d′Arsonval galvanometer and are of the analog type, i.e. they
give voltage readings that can vary over a continuous range as indicated by a scale and
pointer. However, digital voltmeters, which provide voltage readings that are composed
of a group of digits, are becoming increasingly common. Since an oscilloscope is capable
of giving a calibrated visual indication of voltage, it can be called a voltmeter.
The constructional feature of a typical galvanometer is shown in Figure 3.1. Between the
curved cylindrical pole pieces of a horseshoe magnet is placed a coaxial cylindrical soft
iron core to strengthen the field and render it absolutely radial in the air gap between the
core and the pole faces. A rectangular coil of several turns is also located between the
pole faces so that its opposite arms lie in the air gap between the core and the pole faces.
The axis of the radial magnetic field lies along the straight line joining the midpoints of
its other pair of opposite arms. When this coil is connected to the closed circuit in which
the current is to be detected by the galvanometer, it experiences a constant deflecting
torque given by
τ = B I A N
where B = Flux density,
I = Current,
A = Area of cross-section, and
N = Number of turns.
In the simplest form of the instrument, this coil rests on an agate knife edge and is
attached to one end of a spring whose other end is fixed to a rigid support. As the
deflecting torque turns the coil, the spring gets twisted and sets up a restoring torque
which is proportional to the angle of twist. The restoring torque, therefore, grows until it
is equal to the forward torque. About this position the coil oscillates under its own
angular inertia until the dissipation of energy finally brings it to rest. At the rest position,
B I A N = θ = k θ
where k is the torsional constant of the suspension (i.e. the restoring torque per unit
Since the flux density B, the area A of the coil and its number of turns N are also
constants, the deflection θ is proportional to the current, i.e. the instrument has a linear
scale. This is due to the radial field.
The deflection of the coil for each unit current flowing in it is called the current
sensitivity of the instrument and is given by
Pointer and Scale
CircuitSoft Iron Core
Agate Knife Edge
Wire Completing Circuit
Figure 3.1 : Constructional Features of a Galvanometer
The strong field magnet serves the dual purpose of rendering the earth’s magnetic field
negligibly small, and increasing the current sensitivity of the galvanometer. The current
sensitivity of the instrument is also enhanced by a coil of large number of turns of large
area and a spring of low torsional constant (k).
The cylindrical pole pieces create a radial field in which the deflecting couple is of
constant torque (BIAN) which gives rise to a linear scale (I ∝ θ) for the instrument as
shown in Figure 3.2.
Figure 3.2 : Linear Scale between Current and Deflection
The soft iron core coaxially placed with the coil and the field magnet strengthens the
magnetic field and further ensures that it is indeed radial in the airspace between the core
and the pole faces. Its weight is supported by a brass pin fixed to the body of the
instrument as shown in Figure 3.3.
Coaxial Soft Iron Core
Brass Pin Cylindrical Pole Pieces
Figure 3.3 : Flux Distribution
The rigid light frame on which the coil is wound prevents the coil from being distorted
by balanced forces acting on the other pair of arms. If the frame is made of metal then as
the coil swings in the field, the frame cutting through flux induces an alternating current
in the frame itself. These currents, called eddy currents, dissipate the mechanical energy
of oscillation and bring the coil rapidly to rest.
The spring sets up the restoring torque which opposes the deflecting torque. It carries the
pointer which moves over a scale to indicate the deflection of the coil. It further serves to
complete the electrical circuit. However, in the spring type of instrument the torsional
constant is rather high reducing current sensitivity at the expense of making the
Alternative to the spring type is the suspension type of moving coil galvanometer in
which the restoring torque is set up by the suspension wire made of phosphor bronze.
This wire completes the circuit, supports the weight of the coil and carries a small mirror
just above the coil. A beam of light reflects off this mirror and is made incident on a long
metre scale as shown in Figure 3.4. The mirror lamp and scale together constitute an
optical lever arrangement which greatly enhance the sensitivity of the instrument due to
the low torsional constant (k) of the wire. However, this type of instrument is easily
damaged by excess current.
Figure 3.4 : Optical Lever Arrangement
A single range “multi-meter” might be designed as illustrated in Figure 3.5.
Figure 3.5 : Circuit of a Multi-meter
An ammeter is an instrument for measuring the electric current in amperes in a branch of
an electric circuit. It must be placed in series with the measured branch, and must have
very low resistance to avoid significant alteration of the current it is to measure. This is
show in Figure 3.6. By contrast, voltmeter must be connected in parallel. The analogy
with an in-line flowmeter in a water circuit can help visualize why an ammeter must have
a low resistance, and why connecting an ammeter in parallel can damage the meter. This
is shown in Figure 3.7. Modern solid-state meters have digital readouts, but the
principles of operation can be better appreciated by examining the older moving coil
meters based on galvanometer sensors.
Ammeter in series
with resistor measures
current through the
Figure 3.6 : Ammeter Connection to Measure
Flowmeter A meter for volume
flowrate must be in
series to measure the
flow, but must not
appreciably affect the
Figure 3.7 : Flowmeter Analogy of an Ammeter
Ammeters, as well as voltmeters and ohmmeters, are designed with the use of a
sensitive current detector such as a galvanometer.
Design current I amperes Voltage across
and thus across
the shunt resistor
RP G IG
must be diverted
to the shunt to
Design task: Given the galvanometer
resistance and the current value which
will produce a full-scale reading on the
galvanometer, find the value of shunt
resistance which will permit the flow
of the design current and produce a
I RG G
I - Idesign G
Figure 3.8 : Ammeter Design
An ammeter is placed in series with a circuit element to measure the electric
current flow through it as shown in Figure 3.8. The meter must be designed to
offer very little resistance to the current so that it does not appreciably change the
circuit it is measuring. To accomplish this, a small resistor is placed in parallel
with the galvanometer to shunt most of the current around the galvanometer. Its
value is chosen so that when the design current flows through the meter it will
deflect to its full-scale reading. A galvanometer full-scale current is very small : of
the order of milliamperes.
Electricity If a galvanometer with GR = Ω, GI = mA is used to design an
ammeter for a full scale current I = amperes, the required shunt resistor
is given by
A 1 mA meter movement with an internal resistance of 100 Ω is to be converted
into a (0 – 100) mA ammeter. Calculate the value of shunt resistance required.
What particulars should be specified on the shunt.
(1 mA) (100 )
(100 mA 1 mA)
1.01PR = Ω
Voltage drop across the shunt = (1.0) (100)
= 100 mV
Equivalent resistance of shunt in parallel with meter
= = Ω
∴ Shunt should be specified as 1.0 Ω, 100 mA
or, 1.0 Ω, 100 mV
A voltmeter measures the change in voltage between two points in an electric circuit and
therefore must be connected in parallel with the portion of the circuit on which the
measurement is made as shown in Figure 3.9. In analogy with a water circuit, a voltmeter
is like a meter designed to measure pressure difference. It is necessary for the voltmeter
to have a very high resistance so that it does not have an appreciable affect on the current
or voltage associated with the measured circuit. Modern solid-state meters have digital
readouts, but the principles of operation can be better appreciated by examining the older
moving coil meters based on galvanometer sensors.
A voltmeter is
connected in parallel
to measure the
across a circuit
Figure 3.9 : Voltmeter Connection to Measure Voltage Across Circuit Element
A pressure gauge
is connected in
parallel to measure
the pressure drop
Figure 3.10 : Pressure Gauge Analogy of Voltmeter
Voltmeters, as well as ammeters and ohmmeters, are designed with the use of a
sensitive current detector such as a galvanometer.
Design Voltage V
IG I is the
resistor in series
Design task: Given the
resistance and the
current which causes
full scale reading on
the galvanometer, find
the value of the series
which will give full scale
reading with the design
voltage of the voltmeter
Figure 3.11 : Voltmeter Design
A voltmeter is placed in parallel with a circuit element to measure the voltage drop
across it and must be designed to draw very little current from the circuit so that it
does not appreciably change the circuit it is measuring. To accomplish this, a large
resistor is placed in series with the galvanometer as shown in Figure 3.11. Its value
is chosen so that the design voltage placed across the meter will cause the meter to
deflect to its full-scale reading. A galvanometer full-scale current is very small: of
the order of milliamperes.
If a galvanometer with Ω,GR = GI = mA is used to design
voltmeter for a full scale voltage V = volts, the required series resistor is
S G =
= − Ω
A moving coil meter has a resistance of 2 Ω and gives full scale defection with
20 mA. Show how it can be used to measure voltage upto 250 V.
Meter resistance, RG = 2 Ω
Meter current, IG = 20 mA = 0.02 A
Design voltage Vdesign = 250 V
∴ External resistance required to be connected in series with the instrument to
measure voltage upto 250 V
= − = − Ω
12,498SR = Ω
(a) Draw a circuit of a multimeter using galvanometer.
(b) A moving coil meter has a resistance of 5 Ω and gives a full scale deflection
with 10 mA. Show how it can be used to measure current up to 10A.
(c) A moving coil instrument has a resistance of 10 Ω and gives a full scale
deflection when carrying 50 mA current. Show, how it can be adopted to
measure voltage upto 750 V and current upto 100A.
3.3 NAVIGATIONAL LIGHTS
As these are vital circuits in a ship from the point of view of laws of navigation, they
require special attention. They should be connected to a distribution board, which does
not supply any other service, so that they cannot be put out of action by the inadvertent
opening of a wrong switch. Also they should have a change over switch so that they can
be transferred to another source of supply if the normal supply fails, for example by the
blowing of a fuse. Warning devices are also required to indicate the failure of any
individual navigation light. The indicating panel may also comprise the distribution
board previously mentioned, i.e. it may incorporate the fuse protection. The warning
device may be visual or audible or both. The requirements for navigation lights are
prescribed by the International convention, and lamps of special construction and
appropriate wattages and performance are necessary.
A typical arrangement is shown in Figure 3.12 from which it will be seen that if an open
circuit occurs, the relay will be de-energised and the contacts will close and operate the
buzzer. If the indicating lamp fails the circuit is maintained through the parallel
Figure 3.12 : Typical Arrangement of Light Relay Switch
(a) “Masthead light” means a white light placed over the fore and aft centerline
of the vessel showing an unbroken light over an arc of the horizon of
225 degrees and so fixed as to show the light from right ahead to
22.5 degrees abaft the beam on either side of the vessel.
(b) “Sidelights” means a green light on the starboard side and a red light on the
port side each showing an unbroken light over an arc of the horizon of
and so fixed as to show the light from right ahead to 22.5o
beam on its respective side. In a vessel of less than 20 meters in length the
sidelights may be combined in one lantern carried on the fore and aft
centerline of the vessel.
(c) “Sternlight” means a white light placed as nearly as practicable at the stern
showing an unbroken light over an arc of the horizon of 135o
and so fixed as
to show the light 67.5o
from right aft on each side of the vessel.
(d) “Towing light”, means a yellow light having the same characteristics as the
“sternlight” defined in (c) of this rule.
(e) “All-round light” means a light showing an unbroken light over an arc of the
horizon of 360o
(f) “Flashing light”, means a light flashing at regular intervals at a frequency of
120 flashes or more per minute.
3.3.1 Intensity of Lights
The minimum luminous intensity of lights shall be calculated by using the formula :
I = 3.43 × 106
× T × D2
× K −D
where I is luminous intensity in candelas under service conditions,
T is threshold factor 2 × 10− 7
D is range of visibility (luminous range) of the light in nautical miles, and
K is atmospheric transmissivity.
For prescribed lights the value of K shall be 0.8, corresponding to a meteorological
visibility of approximately 13 nautical miles.
3.3.2 Horizontal Sectors
(a) In the forward direction, sidelights as fitted on the vessel shall show the
minimum required intensities. The intensities shall decrease to reach
practical cut off between 1o
outside the prescribed sectors.
(b) For sternlights and masthead lights at 22.5o
abaft the beam for sidelights, the
minimum required intensities shall be maintained over the arc of the horizon
up to 5o
within the limits of the sectors prescribed in rule 21. From 5o
the prescribed sectors the intensity may decrease by 50 per cent up to the
prescribed limits; it shall decrease steadily to reach the practical
cut-off at not more than 5o
outside the prescribed sectors.
(c) All round lights shall be so located as not to be obscured by masts, topmasts
or structures within angular sectors of more than 6o
, except anchor lights,
prescribed in rule 30 which need not be placed at an impracticable height
above the hull.
3.3.3 Vertical Sectors
The vertical sectors of electric lights, as fitted with the exception of lights on sailing
vessels underway shall ensure that :
(a) At least the required minimum intensity is maintained at all angles from
above to 5o
below the horizontal.
(b) At least 60 per cent of the required minimum intensity is maintained from
above to 7.5o
below the horizontal.
(c) In the case of sailing vessels underway the vertical sectors of electric lights
as fitted shall ensure that :
(i) At least the required minimum intensity is maintained at all angles
above to 5o
below the horizontal;
(ii) At least 50 per cent of the required minimum intensity is maintained
above to 25o
below the horizontal.
(d) In the case of lights other than electric, these specifications shall be met as
closely as possible.
The maneuvering light shall be placed in the same fore and aft vertical plane as the
masthead light or lights and, where practicable, at a minimum height of
2 meters vertically above the forward masthead light, provided that it shall be
carried not less than 2 meters vertically above the after masthead light. On a vessel
where only one masthead light is carried, the manoeuvering light, if fitted, shall be
carried where it can best be seen, not less than 2 meters vertically apart from the
3.4 COLOUR SPECIFICATION OF LIGHTS
The chromaticity of all navigation lights conforms to the standards, specified for each
colour by the International Commission on Illumination (CIE). Normally used colours
are white, red, green and yellow.
Shapes shall be black and of the following sizes :
(a) A ball shall have a diameter of not less than 0.6 meter;
(b) A cone shall have a base diameter of not less than 0.6 meter and a height
equal to its diameter;
(c) A cylinder shall have a diameter of at least 0.6 meter and a height of twice
(d) A diamond shape shall consist of two cones as defined in (b) above having a
(e) The vertical distance between the shapes shall be at least 1.5 meter;
(f) In a vessel of less than 20 meters in length, shapes of lesser dimensions but
commensurate with the size of the vessel may be used and the distance apart
may be correspondingly reduced.
3.4.2 Visibility of Lights
(a) In vessels of 50 meter or more in length :
(i) A masthead light, 6 miles;
(ii) A sidelight, 3 miles;
(iii) A sternlight, 3 miles;
(iv) A towing light, 3 miles;
(v) A white, green, red or yellow all-round light, 3 miles.
(b) In vessels of 12 meters or more in length but less than 50 meters in length :
(i) A masthead light, 5 miles; except that where the length of the vessel is
less than 20 meters, 3 miles;
(ii) A sidelight, 2 miles;
(iii) A sternlight, 2 miles;
(iv) A towing light, 2 miles;
(v) a white, red, green, or yellow all-round light, 2 miles.
(c) In vessels less than 20 meters in length :
(i) A masthead light, 2 miles;
(ii) A sidelight, 1 mile;
(iii) A sternlight, 2 miles;
(iv) A towing light, 2 miles;
(v) A white, red, green or yellow all-round light, 2 miles.
(d) In inconspicuous, partly submerged vessels or objects being towed :
(i) A white all-round light, 3 miles.
(a) Discuss the importance of navigational lights.
(b) What is the visibility specification of light in navigation?
Electrical Quantities3.5 SWITCHES
Switches are devices used to allow electric current to flow when closed, and when
opened, they prevent current flow. Common switch types include pushbutton, rocker,
toggle, rotary coded DIP, keylock, slide, snap action, and reed.
Pushbutton switch is a mechanical switch defined by the method used to activate
the switch. The activation method is typically in the form of a plunger that in
pushed down to open or close the switch. These switches are shown in
Figure 3.13, and are classified as :
(a) PUSH ‘ON’, RELEASE ‘OFF’
(b) PUSH ‘OFF’, RELEASE ‘ON’
(c) PUSH ‘ON’, PUSH ‘OFF’,‘PUSH TO CHANGE OVER’:
Figure 3.13 : Type of Switches
Rocker actuators are familiar in many on-off switches; they rock or pivot about the
centerline, and include both maintained and momentary contact types.
Rocker switches, momentary contact, are defined by their motion and momentary
contact function. Rocker switches move like rocking chairs when pushed on either
side to open or close the circuit. In momentary contact, the switch is opened or
closed only during actuation. Rocker switches are often differentiated on by the
difference in their actuators.
The momentary contact switch function can be momentary ON (normally open),
momentary off (normally closed), three position momentary (center OFF), three
position momentary (center ON), and three position momentary (center
NEUTRAL). Momentary ON is a term used to describe contacts, which interrupt a
circuit when in their normal position. When actuated, the circuit is established, but
opened again when actuator is released. Momentary OFF is a term used to
describe contacts, which establish a circuit when in their normal position. When
actuated, the circuit is interrupted, but established again when actuator is released.
An example of three position momentary center OFF is Momentary
ON-OFF-Momentary ON. An example of three position momentary center ON is
an ignition switch (OFF-ON-Momentary Start). An example of three position
momentary center NEUTRAL is Momentary ON-NEUTRAL-Momentary OFF.
Important electrical switch specifications to consider when searching for Rocker
switches, momentary contact, include mechanical life, maximum current rating,
maximum AC voltage rating, maximum DC voltage rating, and maximum power
rating. Mechanical life is the maximum life expectancy of the switch. Often,
electrical life expectancy is less than mechanical life.
The toggle moves or swings to make or break the circuit; includes maintained
contact and momentary contact types.
This is often used on equipment as a power supply ‘ON-OFF’ switch, either in the
Single Pole Single Throw (SPST) form or Single Pole Double Throw (SPDT) form
or Double Pole Double Throw (DPDT) type as shown in Figure 3.14.
(a) SPST Switch (b) SPDT Switch
(c) DPDT Switch
Figure 3.14 : Toggle Switches
The rating for a particular switch depends on whether it is to be used in AC or DC
circuits. Life of switch depends on its rating. If rated values exceed, then life of
switch is shortened, when a circuit is switched off. Sparking occurs at the switch
contacts and vaporizes the metal. In general, switches last longer with DC than
AC. Physical appearance of different type of switches is shown in Figure 3.15.
Figure 3.15 : Various Kinds of Switches
Rotary Coded DIP Switches
Rotary coded DIP switches are activated by means of a rotating shaft that can have
several stop positions. For each position of the shaft (the input) the switch
generates an output binary code. A DIP (Dual In-line Package) switch is an
electronic package (circuit board) consisting of a series of tiny switchs. Typically
used in the configuration of computers and computer peripherals (for example,
circuit boards, modems etc.). Rotary switches move in a circle, and can stop in
several positions along its range.
Rotary Wafer Switches
One or more discs (wafers) of paxolin (an insulator) are mounted on a twelve
position spindle as shown in Figure 3.16. The wafers have metal contact strips on
one or both sides and rotate between a similar number of fixed wafers with springy
contact strips. The contacts on wafers can be arranged to give
1 pole – 12 way, 2 pole – 6 way, 3 pole – 4 way, or 6 pole – 2 way switching etc.
Figure 3.16 : Rotary Wafer Switches
Keylock switches have key as the means of activation. The key is turned in a
circle, and can stop in several positions in its range.
Key Board Switch
Switches used on computer key board, they are of SPST push type to make
momentary contact which can be mounted on a printed circuit board.
A slider moves linearly (slides) from position to position.
Change Over or Single Pole Double Throw (SPDT)
The poles are the number of separate circuits the switch makes or breaks at
the same time. The ‘Throws’ are the number of positions to which each pole
can be switched.
Figure 3.17 : SPDT Switch
Double Pole Double Throw Switch (DPDT)
Operates two circuits simultaneously. The circuits are those connected to X
and Y and each one can go to either of two positions – to P or Q for X and to
R and S for Y as shown in Figure 3.18.
Figure 3.18 : DPDT Switch
Snap Action Switch
Snap action switch is a mechanical switch that produces a very rapid transfer of
contacts from one position to another. They are useful in situations that require a
fast opening or closing of a circuit, such as a mouse button or appliance setting.
Reed switches are magnetically activated switches. They are typically
manufactured with two ferromagnetic reeds (contact blades), which are sealed in a
glass capsule. In the presence of a magnet, the blades (contacts) close.
Used for fast switching operations of a single circuit, e.g. line in a telephone
exchange. The reeds are thin strips of easily magnetisable and demagnetisable
material. They are sealed in a glass tube containing an inert gas such as nitrogen to
reduce corrosion of the contacts. A typical reed switch schematic is shown in
Figure 3.19 : Reed Switches
The switch is operated either by bringing a magnet near or by passing a current
through a coil surrounding it. In both cases the reeds become magnetized, attract
each other and on touching, they complete the circuit connected to the terminals.
They separate when the magnet is removed or the current stops flowing in the coil.
When the change over reed switch operates, the reed is attracted from the non-
magnetic contact to the magnetic area.
Pole and throw configurations for switches can be single pole single throw (SPST),
single pole double throw (SPDT), double pole single throw (DPST), or double
pole double throw (DPDT). SPST is a switch that makes or breaks the connection
of a single conductor in a single branch circuit. This switch typically has two
terminals. It is commonly referred to as a “Single-Pole” Switch. SPDT is a switch
that makes or breaks the connection of a single conductor with either of two other
single conductors. This switch typically has 3 terminals, and is commonly used in
pairs and called a “Three-Way” switch. DPST is a switch that makes or breaks the
connection of two circuit conductors in a single branch circuit. This switch
typically has four terminals. DPDT is a switch that makes or breaks the connection
of two conductors to two separate circuits. This switch typically has six terminals
and is available in both momentary and maintained contact versions. The switch
function can be maintained contact, momentary contact, or alternating contact. In a
maintained contact switch, an actuator stays in thrown position. This includes on-
off, but it also includes Three-Position – (Center-off) and Three Position – (No
Center-off) switches where the switch remains in its actuated position. In a
momentary contact switch, the switch must be held in position; it reverts to normal
position when actuating force is removed. Alternating action such as push on,
push off characterizes an alternating contact. A normally open (NO) switch has
contacts that are open or disconnected in their unactuated (normal) position. A
normally closed (NC) switch has contacts that are closed or connected in their
unactuated (normal) position.
Important electrical switch specifications to consider when searching for switches
include maximum current rating, maximum AC voltage rating, maximum DC
voltage rating, and maximum power rating. Other important parameters to consider
when searching for switches include the terminal type, construction materials,
common features, and environmental conditions.
What are the various types of switches used in electrical circuits? Briefly describe
A relay is a switch operated by an electromagnet. It is useful when a small current in one
circuit is required to control another circuit containing a device such as a lamp or electric
motor which requires a large current or several different switch contacts are to be
The structure of a relay and its symbol are shown in Figure 3.20. When the controlling
current flows through the coil, the soft iron core is magnetized and attracts the L shaped
soft iron armature. This rocks on its pivot and opens, closes or changes over, the
electrical contacts in the circuit being controlled. In the figure shown, it closes the
Figure 3.20 : Relay Configuration
The current needed to operate a relay is called the ‘pull in’ current and the ‘drop out’
current is the current in the coil when the relay just stops working.
Protective relays and monitoring relays detect or monitor for abnormal power system
conditions. Protective relays detect defective lines, defective apparatus, or other power
system conditions of an abnormal or dangerous nature and initiate appropriate control
circuit actions. Monitoring relays are used to verify conditions in the power system or in
the protective system. Monitoring relay functions include fault detection, voltage
checking, and direction-sensing that confirms power system conditions but does not
directly sense the fault or problem.
Protective relays and monitoring relays can be sensitive to voltages, power or phase,
current, or frequency. Important specifications to consider for voltage sensitive relays
include under voltage, over voltage, and differential voltage. An under voltage relay trips
when the voltage drops below a set point. An over voltage relay trips when a voltage
rises above a set point. A differential voltage relay responds to the difference between
incoming and outgoing voltage associated with the protected apparatus. A power or
phase sensitive relay can monitor phase sequence, phase reversal, ground or earth fault,
power factor, phase failure or loss, and phase unbalance. A phase sequence relay
monitors for correct phase sequence. A phase reversal relay monitors for a change of
one-half cycle or 180o
in phase. A ground earth fault relay monitors for any undesired
current path from a point of differing potential to ground. Power factor is the cosine of
the phase angle between the voltage and current in alternating-current power
transmission and distribution. A phase failure relay monitors for voltage with the
incorrect phase sequence or one or more phases open. A phase unbalance relay operates
when the magnitude of one current exceeds the magnitude of another current by a
predetermined degree. Voltage Balance operates similarly. Important specifications to
consider for current sensitive relays include undercurrent, over current, and differential
current. An undercurrent relay trips when the current drops below a set point. An over
current relay trips when a current rises above a set point. A differential current relay
responds to the difference between incoming and outgoing current associated with the
protected apparatus. Important specifications to consider for frequency sensitive relays
include under frequency, over frequency, and differential frequency. An under
frequency relay responds to a decrease in the frequency of an alternating electrical input
quantity. An over frequency relay responds to an increase in the frequency of an
alternating electrical input quantity. A differential frequency relay responds to the
difference between incoming and outgoing frequency associated with the protected
apparatus. Other protective functions include time delay and synchronous check.
What is a relay? What are the functions of a relay? State the applications where
relays are used.
A fuse is the most common type of protection against a short circuit fault in low voltage
distribution circuits, motor circuits and for portable appliances. When circuit parameters
are normal, it maintains continuity and ensures proper supply voltage to the equipment.
In case of abnormality when current in the circuit increases abnormally (equivalent of
almost short circuit current), the fuse melts off and breaks the circuit, thereby cutting off
the supply to the equipment, thus protecting the equipment and the circuit.
It is relatively simple, inexpensive and reliable. As rewirable fuses, tend to be less
reliable (fitting the wron size of the fuse wire) they are not recommended for marine
practice. High rupturing capacity (HRC) cartridge type fuse links as shown in
Figure 3.21 are normally used.
End Cap Metal
Figure 3.21 : Fuse
A disadvantage a fuse is its insensitivity to small over current. An HRC fuse will blow at
currents as low as 25% overload, but only after about 4 hrs.
The advantage of a fuse is its very high speed of operation (few milli seconds) at high
short circuit fault current – faster than a circuit breaker.
Fuses are fitted in circuits to give protection against short circuits. The fuse links provide
back up protection for the supply cables and equipment against short circuit fault.
Motor fuses are typically rated at 2-3 times the motor full load current in order to
withstand the large starting current surge (upto 6 times full load) of the motor.
In the event of a fuse blowing, the cause of the fault must be located and repaired before
the fuse link is replaced. The replacement fuse link must be of the correct current rating,
grade and type (identical to the blown fuse link).
In three phase circuits the fuses are to be replaced in all three phases even though only
one fuse is found blown.
End Cap Metal
Fuses provide protection descrimination in low voltage distribution systems. The current
ratings and time settings of the fuses are normally co-ordinated such that devices nearest
to the load having the lowest current rating and shortest operating time, those nearest the
main switch board or power board having the highest current rating and longest time. If
a short circuit fault occurs in a lamp holder, the fault current will be large enough to blow
out the lowest current rated fuse near the lamp holder circuit to clear the fault and leave
all other healthy circuits still connected.
What is a fuse? Give its application. State the types of fuse known to you.
Describe the advantages and disadvantages of fuse.
3.8 ELECTRICAL SAFETY
Inspect all electrical equipment your employees use to make sure the equipment is safe :
• Make sure all electrical equipment is used for its approved or listed purpose;
• Make sure electrical equipment used or located in wet or damp locations is
designed for such use;
• Make sure electrical equipment that is not marked by the manufacturer
cannot be used;
• Identify disconnecting means;
• Maintain electrical fittings, boxes, cabinets, and outlets in good condition;
• Maintain all flexible cords and cables in good condition and use safely;
• Guard electrical equipment to prevent your employees from electrical
• Make sure electrical equipment is effectively grounded; and
• Make sure electrical equipment has over current protection.
Inspect electrical equipment to make sure there are no recognized hazards likely to cause
death or serious physical harm. Determine the safety of the equipment by using the
following list :
• Has been approved or listed by a recognized testing laboratory;
• Is approved, or listed as approved, for the purpose it is being used;
• Has strong and durable guards providing adequate protection including parts
designed to enclose and protect other equipment;
• Is insulated;
• Will not overheat under conditions of use;
• Will not produce arcs during normal use;
• Is classified by :
Make sure all electrical equipment is used for its approved or listed purpose
• Electrical outlets are places on an electric circuit where power is supplied to
equipment through receptacles, sockets and outlets for attachment plugs; and
• Receptacles are outlets that accept a plug to supply electric power to
equipment through a cord or cable.
Different types of common electrical outlet configurations are shown in Figure 3.22.
Some Common Electrical Outlet (Receptacle) Configurations
20 Ampere 30 Ampere 50 Ampere
Note: A 20-ampere “T-solt” outlet or cord connector may accept a 15-ampere attachment plug of the same voltage rating.Note : A 20-ampere “T-solt” outlet or cord connector may accept a 15-ampere attachment plug of the same voltage rating.
Figure 3.22 : Electrical Outlet Configurations
• Make sure electrical outlets are rated equal or greater to the electrical load
• Make sure the proper mating configuration exists when connecting the
attachment plug to a receptacle; and
• Make sure when electrical outlets, cord connectors, and receptacles are
joined, they accept the attachment plug with the same voltage or current
Make sure electrical equipment used or located in wet or damp locations is designed
for such use
• Make sure fixtures and receptacles located in wet or damp locations are
approved for such use. They must be constructed or installed so that water
cannot enter or accumulate in wire ways, lamp holders, or other electrical
• Make sure cabinets, fittings, boxes, and other enclosures in wet or damp
locations are installed to prevent moisture or water from entering and
• In wet locations these enclosures must be weather proof; and
• Switches, circuit breakers, and switchboards located in wet locations must
be in weatherproof enclosures.
A typical switch board and covered outlet commonly observed are shown in
Figure 3.23 : A Typical Switch Board and Covered Outlet
Electrical Equipment used in Wet Locations must be Weatherproof
Make sure electrical equipment that is not marked is not used :
• Make sure markings are durable and appropriate to the environment;
• Voltage, current and wattage, or other ratings as necessary.
• Appropriate markings include :
The manufacturer’s name,
The organization responsible for the product.
A typical nameplate showing details of electrical tool with manufacturer’s details,
warning, rating etc. is shown in Figure 3.24.
3/8” Drill Double Insulated - Variable Speed
110/120 VOLTS AC ONLY 60HZ 4.0 AMPS
WEAR EYE PROTECTION. FOR SAFE OPERATION,
SEE OWNERS MANUAL. WHEN SERVICING, USE
IDENTICAL REPLACEMENT PARTS
MODEL 315 271490
MADE IN U.S.A.
SEARS, ROEBUCK AND CO.
Voltage Double Insulated Amperage
ManufacturerTesting Organisation Approval
Figure 3.24 : Electrical Tools and Equipment Marked to Show
Manufacturer Approvals and Power Requirements
Identify Disconnecting Means
• Make sure the disconnect means (such as on/off switches and circuit
breakers) is marked to show when it is open and closed and what equipment
it controls, unless located and arranged so the purpose is obvious; and
• Make sure each service, feeder and branch circuit is marked, at its
disconnecting means or over current device, to show when the circuit is
open and closed and what circuit it controls, unless located and arranged so
the purpose is obvious.
Electricity SAQ 6
State the safety measures in electrical equipment handling.
In this unit, we have introduced you with the basic concepts of electrical measurement,
safety, control and navigational lights.
The construction and principle of measuring voltage and current using galvanometer has
been introduced. How the value of resistance in series or shunt with the galvanometer can
be designed to extend the range of basic movement has also been dealt.
Also, you have been explained the role and importance of navigational lights and the
terms used. Intensity of lights by horizontal sector and vertical sector has been explained.
After a brief introduction of switching operation in electrical circuits, this unit gives the
details of different types of switches commonly used. Similarly, the importance and role
of relays and fuses in protection of electrical equipment against fault has been explained
Electrical safety is very important for man and machine. Hence, a detailed step-by-step
procedure for electrical safety has also been given.
Certain key words used in navigation have also been described.
3.10 KEY WORDS
Candella : The unit of luminous intensity. Symbol Cd.
Colour Temperature : Colour temperature of a light source is that of a
full radiator which would emit radiation of
substantially the same spectral distribution in the
visible region as the radiation from light source
and which would have the same colour.
Direct Lighting : A system of lighting in which the greater part of
the luminous flux after leaving the sources passes
directly towards the area Illuminated.
Indirect Lighting : A system in which the greater part of the luminous
flux reaches the area to be illuminated after
reflection from a ceiling or other object external to
Efficiency : Efficiency of a light source is the ratio of luminous
flux emitted to the power consumed by the source
and for electric lamps is expressed in lumens/watt.
Foot Lambert : A unit of luminance. lumen/sq.ft.
Full Radiator : A light source emitting radiation, the spectral
distribution of which is dependent on the
temperature only and not on the material and
nature of the source.
Luminance : Quotient of the luminous intensity of a surface in a
given direction of an infinitesimal element of the
surface containing the point under consideration
on a plane perpendicular to the given direction.
Lux : Unit of illumination. One Lumen/m². Symbol lx.
Luminosity : The attitude of visual perception in accordance
with which an area appears to emit more or less
Luminous Flux : Quantity characteristic of radiant flux which
expresses its capacity to produce visual sensation.
Luminous Efficiency : Quotient of luminous flux by the corresponding
radiant flux. Symbol k.
Radiant Flux : Power emitted or received in the form of radiation.
3.11 ANSWERS TO SAQs
(a) Refer figure.
(b) Total current to be measured, I = 10 A
Instrument current, IG = 10 mA
Instrument resistance, RG = 5 Ω
Resistance of ammeter shunt,
P PR R
(c) RG = 10 Ω
IG = 50 mA = 0.05 A
(i) Vdesign = 750 V
design 750 V
= − = − Ω
RS = 14990 Ω
(ii) Idesign = 100 A
0.05 A 10
100 A 0.05 A
Rp = 0.0005 Ω
(a) Refer section 3.3.
(b) Refer section 3.4.2.
Refer section 3.5.
Refer section 3.6.
Refer section 3.7.
Refer section 3.8.
Chakrabarti A. (2001), Circuit Theory, Dhanpat Rai and Co., New Delhi.
Del Toro V., Principles of Electrical Engineering, Prentice-Hall of India, New Delhi.
Huges E. Pearson, Electrical Technology, Longmans.
Richard J. Fowler (2003), Electricity, International Edition, Tata McGraw Hill.
Sahadev, Fundamentals of Electrical and Electronics Engineering, Dhanpat Rai
Van Valkerburg, Network Analysis, Prentice-Hall of India, New Delhi.
Electrical QuantitiesELECTRICITY AND ELECTRONICS
Addressed to you as a student of nautical science, this course on Electricity and
Electronics provides you with a coverage of the essential topics that would help you
acquire a proper understanding of the basic concepts of the electrical and electronics
systems that as a user you are likely to deal with in your profession.
The course ‘Electricity and Electronics’ has been designed to provide entry-level
competencies in a wide range of occupation in electrical and electronics field.
Block 1 of this course deals with the analysis of electrical circuits where you will be
explained the techniques to calculate the value of electric current, voltage, power, energy
in any electrical circuit using three basic laws – Ohm’s Law, Kirchhoff’s Voltage Law
and Kirchhoff’s Current law.
Electrical power is most important to industry. Second part of this block mainly covers
topics of electrical power engineering where you will learn the concepts related to
magnetic effect of electric current, concepts of inductance and capacitance. Further you
will be introduced with transformers and different types of electrical motor and
In last part of this block, you will learn about measurement of electrical quantities, safety
and control equipment and methods.
More than any other discipline, it is electronics which has had the greatest impact on
modern society. Its role in systems for communication and computation, control and
instrumentation, education and entertainment is increasing day-by-day.
In Block 2, you will get acquainted with fundamentals of transistors, use of transistors in
amplifier circuits, oscillators, modulation and demodulation concepts, working of radio
transmitter and receiver, antenna, sensors and transducers. The last part of this block
deals with radar, semiconductors, filter circuits and digital electronics.
Since concepts build from unit to unit and within a unit from section to section, students
are provided with a number of stepping points for self assessment questions (SAQs).
Answers to the SAQs are provided as the last item of each unit.
Electricity is so closely linked to our day-to-day life that living without electricity would
indeed be a stifling thought to many of us. Therefore, an exposure to principle of
electrical science is important to any applied science education.
This block consists of three units. In first unit, you will be introduced with basic
definitions of electric current, emf, power and energy. Circuit elements like resistance,
effect of temperature on resistance, heating effect of electric current will also be part of
You will get understanding of three basic law’s governing the current flow and voltage
drops like Ohm’s law and its limitations, Kirchhoff’s Voltage and Current Laws with
In second unit of this block, you will be introduced to the magnetic effect of electric
current, concept of self and mutual inductance, capacitance, fundamentals of alternating
current, resonance, power, power factor and quality factor.
In overall electric power scenario, transformers and rotating electrical machines play a
major role. In this unit, you will also be introduced with transformer, DC generator and
Finally, in third unit, you will acquaint yourself about the most common instruments used
to measure various electrical quantities. Here you will learn some basic instruments used
for measuring electrical voltage and current through voltmeter and ammeter. You will
also be introduced to safety norms and safety control using different types of accessories
such as switch, fuse relay and navigational lights.