Protective Devices: Characteristics and Principles of Operation and Application
I.
Definition
A device used to protect equipment, machinery, components and devices, in electrical
and electronic circuit, against short circuit, over-current and earth fault.
Importance of a Protective Device:
A.
B.
C.
D.
II.
Short Circuit
Abnormal variations in the supply voltage
Overloading of equipment
To protect operator against accidental contact with the faulty equipment, falling
which the operator may get a severe shock.
Types of Protective Devices
There are various types of protective devices, which are as follows:
A. Fuse
1. Definition
A fuse is an electrical safety device built around a conductive strip that is
designed to melt and separate in the event of excessive current.
2. Characteristics
a. Current Rating
A nominal current rating which a fuse can continuously carry without
overheating or melting. It is based on the temperature rise of the fuse element
and its environment. It is recommended to operate the fuse at not more than
the 75% of the nominal current rating.
b. Voltage Rating
The voltage rating of the fuse must be greater or equal to that of the
circuit voltage. It is because the fuse can safely interrupt an abnormal current. It
relates to the ability of the fuse to function and extinguish arcs when it opens.
c. Breaking Capacity
The breaking capacity is the maximum current that the fuse can safely break at
rated voltage. It is also known as the interrupting rating or short circuit rating.
d. Ambient Temperature
It is the temperature of the surrounding components such as fuse
element, fuse holders etc., where the fuse is installed. The time-current
characteristics of fuses are affected by ambient temperature. Higher the
ambient temperature, the hotter the fuse will operate and it shorter its life.
e. Melting Integral I2t Rating
It is the amount of energy required to melt the fuse element to
interrupt the current. It is the function of current squared and time. It is
expressed as ampere squared seconds (A2 Sec).
f. Fusing Current
It is the minimum amount of current at which the fusing element melts.
g. Temperature Derating
In order to prolong device life, a device is operated less than its rated
maximum power rating is called derating. It is recommended that when a fuse is
operated in an ambient temperature for 25°C, the fuse current rating should be
derated.
h. Normal Operating Current
The current rating of a fuse is typically derated 25% for operation at
25ºC to avoid nuisance blowing. For example, a fuse with a current rating of 10A
is not usually recommended for operation at more than 7.5A in a 25ºC
temperature.
i. Time-Current Characteristics
The time required to blow out the fuse depends upon the magnitude of
excessive current. So, the greater the current, the melting time taken by the
fuse reduces (the fuse will melt faster the more overcurrent is). Hence the blow
time of fuse is inversely proportional to the current flowing through the fuse
element.
3. Principles of Operation
The working principle of the fuse depends on the heating effect of the
current. The fuses are connected in series with the circuit and voltage source. In
the event of short circuit or overload conditions, the amount of current in the
circuit increases, therefore the amount of heat increases and thus the fuse
element melts. This is because the fuse element has a low melting point. Now
the fuse is blown out and opens the circuit by preventing the device.
4. Application
a. Power Transformers
b. Electrical Appliances, like ACs (Air Conditioners), TV, Washing Machines, Music
Systems
c. Electrical Cabling in Home
d. Mobile Phones
e. Motor starters
f. Laptops
g. Power Chargers
h. Cameras, Scanners, Printers, and Photocopiers
i. Automobiles, electronic devices and Gaming’s
B. Breakers
1. Definition
Circuit breakers are essentially resettable fuses that are automatically operated
electrical switches that cut off the circuit whenever the current jumps above a safe
level, avoiding overheating, melting, and potential fires.
2. Characteristics
a. Rated Voltage Ue
This is the voltage at which the circuit-breaker has been designed to
operate, in normal (undisturbed) conditions.
b. Rated Current In
This is the maximum value of current that a circuit-breaker, fitted with a
specified overcurrent tripping relay, can carry indefinitely at an ambient
temperature stated by the manufacturer, without exceeding the specified
temperature limits of the current carrying parts.
c. Tripping-current-level adjustment ranges for overload protection (Ir[1] or Irth[1])
and for short-circuit protection (Im)
Apart from small circuit-breakers which are very easily replaced,
industrial circuit-breakers are equipped with removable, i.e. exchangeable,
overcurrent-trip relays. Moreover, in order to adapt a circuit-breaker to the
requirements of the circuit it controls, and to avoid the need to install oversized cables, the trip relays are generally adjustable. The trip-current setting Ir
or Irth (both designations are in common use) is the current above which the
circuit-breaker will trip. It also represents the maximum current that the circuitbreaker can carry without tripping. That value must be greater than the
maximum load current IB, but less than the maximum current permitted in the
circuit Iz
Short-circuit tripping relays (instantaneous or slightly time-delayed) are
intended to trip the circuit-breaker rapidly on the occurrence of high values of
fault current. Their tripping threshold Im is:
 Either fixed by standards for domestic type CBs, e.g., IEC 60898, or,
 Indicated by the manufacturer for industrial type CBs according to
related standards, notably IEC 60947-2.
d. Short-circuit current breaking rating (Icu for industrial CBs; Icn for domestic-type
CBs) (Icu (rated ultimate s.c. breaking capacity) and Ics (rated service s.c.
breaking capacity)
The short-circuit current-breaking rating of a CB is the highest
(prospective) value of current that the CB is capable of breaking without being
damaged. The value of current quoted in the standards is the rms value of the
AC component of the fault current, i.e., the DC transient component (which is
always present in the worst possible case of short-circuit) is assumed to be zero
for calculating the standardized value. This rated value (Icu) for industrial CBs
and (Icn) for domestic-type CBs is normally given in kA rms.
3. Principles of Operation
Circuit breaker essentially consists of fixed and moving contacts. These
contacts are touching each other and carrying the current under normal
conditions when the circuit is closed. When the circuit breaker is closed, the
current carrying contacts, called the electrodes, engaged each other under the
pressure of a spring.
During the normal operating condition, the arms of the circuit breaker
can be opened or closed for a switching and maintenance of the system. To
open the circuit breaker, only a pressure is required to be applied to a trigger.
Whenever a fault occurs on any part of the system, the trip coil of the
breaker gets energized and the moving contacts are getting apart from each
other by some mechanism, thus opening the circuit.
4. Application
a. They are used for switching of loads in Industries, Buildings, Commercial
complexes, Hotels etc.
b. Air C/B is used for the protection of plants, electrical machines, transformers,
capacitors and generators.
c. Air Blast C/B is used in Indian Railways for electrification.
C. Lightning Arresters
1. Definition
a. A protective device used to protect the circuit from lightning strokes having high
transient voltage surges, surge currents due to lightning, spark and isolation arcs
etc.
b. A device used to protect the electric circuit and connected devices from the
lightning strikes having high voltage transient surges. Lightning arresters are
installed outside to ground the harmful effects of lightning spikes.
2. Characteristics
3. Principles of Operation
Once the voltage surge travels throughout the conductor then it
reaches the location of the arrestor where it is installed. So, it will break down
the insulation of the lightning arrestor for a moment, so voltage surge can be
discharged toward the ground. Once the voltage of the system falls under the
fixed value, then the insulation will be restored among the ground & conductor.
Further, the current flow toward the ground will be stopped.
4. Application
a. Tall buildings to prevent lightning from causing physical damage to
infrastructure.
b. Power lines to protect the cables from lightning.
c. Power outlets to protect electronic devices from surges of electricity.
III.
References
A. https://automationforum.co/electrical-protective-device-types-of-protective-device/
B. https://www.codrey.com/electrical/electrical-fuse/
C. https://www.allaboutcircuits.com/textbook/direct-current/chpt-12/fuses/
D. https://www.elprocus.com/what-is-fuse-different-types-of-fuses-and-applications/
E. https://electronics.howstuffworks.com/circuit-breaker.htm
F. https://www.dfliq.net/blog/different-types-of-circuit-breakers/
G. https://www.electrical-installation.org/enwiki/Fundamental_characteristics_of_a_circuitbreaker
H. https://circuitglobe.com/circuit-breaker.html
I.
J.
K.
L.
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https://www.eaton.com/us/en-us/products/electrical-circuit-protection/circuitbreakers/circuit-breakers-fundamentals.html
https://slideplayer.com/slide/12736142/
https://www.electrical4u.com/electrical-circuit-breaker-operation-and-types-of-circuitbreaker/
https://electricalfundablog.com/circuit-breaker-works-typesapplications/#4_Applications_of_Circuit_Breaker
https://circuitglobe.com/lightning-arrester.html
https://www.electricaltechnology.org/2020/03/surge-arrester-suppressor-lightningarrester-rod.html