tiny interesting device called an mcb
protects you and your house from
electrical mishaps this breaker saves
you from two situations
first short circuit and second overload
conditions
in a short circuit scenario the mcb
trips in less than three milliseconds
and isolates the internal connections
let's see how this smart device detects
current chaos so quickly and precisely
a simple device called a fuse which has
a low melting point wire
was used for home protection in earlier
days
in both the fault conditions mentioned
previously the current spikes up
resulting in overheating and fusing the
wire or
breaking the circuit however every time
the fuse would blow
you had to manually replace it until you
could do that
good luck with the blackout this
commonly occurring situation is why
circuit breakers were invented
the circuit breaker is an automatic on
off mechanism
that keeps one side of the wire moving
and the other fixed
during an electric fault if the circuit
opens wide we are done with the mcb
design
after the fault ends simply manually
turn it on
before getting more into the mcb design
a small note about the current direction
an mcb gets activated in less than three
milliseconds
which is way lower than the half cycle
time period of an alternating current
so it is okay to show the current flow
in a single direction
throughout the analysis
now the question is how does this system
recognize the occurrence of an electric
fault
the answer is that it has sensing
elements which activates a triggering
mechanism
let's first see how we can design a
super quick triggering mechanism which
opens wide
a clever mechanism to achieve this
objective is shown here
this mechanism has a lever to which a
rectangular ring is connected
as shown the rectangular ring is
connected by two springs and a central
lever
in this position both the springs are in
a neutral state
when the lever moves up the bottom
spring compresses and the top spring
expands
the forces of both these springs are
eventually transferred to the lever
via the rectangular ring and this force
will be in the upper direction
as you can see the rectangular ring is
initially at a negative offset to the
center of the lever
the torque produced by the force acting
on the rectangular ring
will be in the clockwise direction if
you release the lever in this position
this clockwise torque will bring the
lever back to its initial position
however if you push the lever farther up
you notice anything peculiar about the
offset distance
it suddenly becomes positive which means
that after a critical limit
the torque will act on it in a
counterclockwise direction
when the mcb is on this will be the
position of the lever
the structure of the mcb will block the
lever from rotating further
counterclockwise
now if an external trigger turns the
lever slightly
the torque on the lever suddenly becomes
clockwise and the circuit opens
wide quickly without the need of an
external force
here as the operator pushes the lever
down you can see in
ultra slow motion how the mechanism
operates in practice
after a small angle the lever generates
its own torque and then there is no need
to apply external force
now the only question is how can we
generate this small
input trigger or input motion when a
fault occurs
the best answer is with an electromagnet
this coil produces magnetic fields
proportional in nature to the current
passing through them
as the current increases the magnetic
field becomes stronger
throughout the video note that in the
case of a short circuit the current
rises up to a thousand times in
magnitude within milliseconds
this result generates a very strong
magnetic field
an iron cylinder and pin is placed
inside this electromagnet on a spring
slightly offset from its center
the strong magnetic field pulls the
cylinder downward
thus pushing the pin this tiny movement
of the pin
is the input trigger of the mechanism
and it leads to opening the circuit as
we discussed earlier
you might be wondering in normal current
flow why isn't this cylinder attracted
down
well the force in normal current flow is
not high enough to overcome spring
tension
whereas short circuit current can go up
to 10 to 100 times the normal current
the force is quite high so the circuit
trips
the danger of fault is not over yet
folks
as soon as the contacts are opened the
current doesn't just stop flowing
the fault current is of large value and
will lead to air discharge
or current will flow through the air it
is an amazing to watch and yet hazardous
arc
to extinguish the ark a component called
an arc runner or
arc chute is used the arc chute is an
arrangement of parallel plates arranged
at small intervals
as the contacts separate the heavy fault
current flowing as an arc
triggers a huge temperature rise that
can cause damage
therefore this arc should be killed
resistance increases with length and
decreases with area
here we need increased resistance of the
current which passes through air
which is why the distance..
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