Workshop Practice Slides for Electrical and Industrial Engineers

1. Workshop Practice
Lab # 03
Recognize the basic Electrical safety devices

2. Aims and objectives
In this lab student will learn about
Safety devices and their working

3. Importance of safety devices
The safety features are inbuilt with electric power distribution.
• Insulators like PVC, paper or rubber are provided across the path through which the current
is not expected to flow.
• The circuit may also carry under short circuit much more than normal value of the current.
• The inbuilt safety features will isolate the faulty circuit from the rest of the supply.
• Protective devices are needed to break short circuit and overload currents
• In case of an overload or a short on that circuit, the breaker or fuse trips automatically shuts
off to that circuit.
• Fuses are commonly used protection devices to protect components like wires, transformers
etc. against overload.

4. Types of safety devices
• Fuse
• Circuit breakers
• Earthing
Basically, two types of protections are provided in the power supply of
domestic consumers
• Protection from over current
• Protection from leakage current due to failure of insulation or
inadvertent contact with live conductors by the user

5. Over current and short circuit
The electrical wiring is rated for certain maximum current.
When the wiring heats too much, it will cause the melting of cable insulation
causes fire. If there is something flammable near the cable and even melt the
copper conductors in the cable.
• When two conductors of an electric circuit touch each other without any
load this will cause short circuit.
• The current flow caused by a short circuit is usually high and rapid and is
quickly detected and halted by conventional circuit protective devices such
as fuses or circuit breakers.

6. Protection against over current
Every electrical circuit shall be protected against over current by suitable over
current devices.
These devices could be
1. Miniature circuit breaker (MCB)
2. Mildewcide circuit breaker
3. High rupturing capacity (HRC) fuses

7. Fuse
• A fuse is usually a thin piece of wire that will melt and break the circuit
if too large a current flow through it.The thicker the wire the more
current is needed to melt it and higher the ‘rating’ of the fuse.The
purpose of a fuse is to protect the wiring and the equipment.

8. Common types of fuses
• Re-wire able fuse
• High Rupturing fuse
• Cartridge fuse
• Enclosed Cartridge Fuse

9. Re-wire able fuse
• Wire of different diameters made of lead and
tin are used in a circuit.
• Fuse wire is carried in a removable fuse link.
• The fuse link is made of porcelain or other
suitable insulating material.
• The fuse carrier is push-fitted to the fuse base
to make the connection through
• An advantage of this type is that the blown fuse
wire can be replaced with negligible cost.
• It may sometimes lead to fire hazards, when the
fuse wire blows.

10. Cartridge fuse
• Cartridge fuse consist of a tube with metal end
caps at both ends.
• Since the tube is made of glass, the fuse
element can easily be inspected for breakage.
• The advantage of the cartridge fuses is quick
and easy replacement and the fuse rating is
marked on the end cap of the cartridge itself.

11. Enclosed Cartridge Fuse
• This is completely enclosed cartridge type of fuse.
• Generally, it is used in high power circuits
• HRC fuse consists of a porcelain tube with metal end caps and fixing tags
• The tube is filled with the silica sand and granulated quartz.
• The silica inside the tube prevents the formation of and arc.
• HRC fuse links are available in a range of 10A to 800A.

12. Enclosed Cartridge Fuse

13. Wood saw
• It is used to cut wooden boxes,
sticks and round blocks for the
required size.

14. Hack saw
• It is used to cut metal pipes and
metal frames.The frame of hack
saw is made up of iron and the
handle is made up of wood. A clip
is fixed on its other end to adjust
the length

15. Circuit Breakers
• The circuit breaker is an automatically operated electrical switch designed
to protect an electrical circuit from damage caused by overload or short
circuit.
• Its basic function is to detect the fault condition and interrupt current flow
unlike a fuse which operates once and then must be replaced, a circuit
breaker can be replaced (either manually or automatically) to resume
normal operation.
• Circuit breakers are made in various sizes, from small devices that protect
the individual household appliance up to large switch gear design to protect
high voltage circuit feeding an entire city.

16. Principle operation of circuit
breaker
• The basic circuit breaker consists of a simple switch connected to either bimetallic strip or
an electromagnet.
• The hot wire in the circuit connects to the ends of the switch.
• When the switch is flipped to the on position, electricity can flow from the bottom terminal
through electromagnet up to the moving contact across to the stationary contact and out
to the upper terminal.
• The electricity magnetizes the electromagnet increases the current boosts the
electromagnet’s magnetic force and decreasing current lowers the magnetism.
• When the current jumps to unsafe levels the electromagnet is strong enough to pull down a
metal level connected to the switch linkage.
• The entire linkage shifts tilting the moving contact away from the stationary contact to
break the circuit.
• The electricity shuts off.A bimetallic strip design works on the same principle
except of energizing an electromagnet, the high current bends a thin strip to move the
linkage.

17. Principle operation of circuit
breaker

18. How an RCD works?
• When properly connected, an electrical current flow through the live wire on
its way to the appliance and returns via the neutral wire.
• If for some reason, the current leaks (through dampness, frayed wire, faulty
connection etc.), it will take etc. the most direct path to earth often through
a person’s body.
• If that power is not cut off instantly, there is a strong possibility that a person
will die.The path taken by the current also contribute to the shock’s severity.
• The shock which flows from the hand to the arms and chest will effect both
breathing and heartbeat.

19. How an RCD works?
• The time that a person is exposed to the shock is the greatest factor in the
severity of the accident.
• An RCD cuts off the supply in 30 thousandths of a second-that’s less than the
time of a single heartbeat.
• A leakage as little as 10mA (thousandths of one Amp) is sufficient to trip the
RCD and cut off the power to the appliance – that’s a minute amount of
current when you consider that many appliances use as much as 10 Amps.

20. Step by step explanation
• Under normal operation when
there is no magnetic field in the
core, no electricity flows through
the search coil or relay.

21. Step by step explanation
• Suppose you cut through the live
wire(green) with your garden
shears and suppose it takes a
Tenth of a second for you to chop
right through.

22. Step by step explanation
• During that tenth of a second, just
you are starting to cut there is a
current imbalance between the
live and neutral cables.
• More current flows through the
neutral wire(orange) than
through the live wire(green), so
the neutral wire produces a
greater magnetic field (red arrow)
in the iron core the live wire does
(blue arrow).

23. Step by step explanation
• The two magnetic fields no longer
cancel out.
• The net magnetic field in the core
causes and electric current to flow
in the search coil(grey), which
activates the relay(blue).

24. Step by step explanation
• The relay snaps open breaking the
incoming circuit cables and
stopping all power from flowing in
as little as 30 milliseconds- far
faster than you can cut the cable.

25. Conclusion