MCB

Working Principle Miniature Circuit Breaker

There are two arrangement of operation of miniature circuit
breaker.
One due to thermal effect of over current and other due to
electromagnetic effect of over current. The thermal operation of
miniature circuit breaker is achieved with a bimetallic strip
whenevercontinuous over current flows through MCB, the
bimetallic strip is heated and deflects by bending. This deflection
of bimetallic strip releases mechanical latch. As this mechanical
latch is attached with operating mechanism, it causes to open
the miniature circuit breaker contacts. But during short
circuit condition, sudden rising of electric current, causes
electromechanical displacement of plunger associated with
trippingcoil or solenoid of MCB. The plunger strikes the trip lever
causing immediate release of latch mechanism consequently
open the circuit breaker contacts. This was a simple explanation
of miniature circuit breaker working principle.
 Thermal / Magnetic trip units
ABB Current Limiting Breakers use an
electromechanical (Thermal /Magnetic)
trip unit to open the breaker contacts during a
overcurrent event. The thermal trip unit
is temperature sensitiveand the magnetic trip
unit is current sensitive.
Both units act independently and mechanically
with the breakers trip mechanism to open the
breakers contacts.
Overload protection
 The thermal trip unit protects against a continuous overload.
The thermal unit is comprised of a bimetal element located behind
the circuit breaker trip bar and is part of the breakers current
carrying path.
When there is an overload, the increased current flow heats the
bimetal causing it to bend. As the bimetal bends it pulls the trip bar
which opens the breakers contacts. The time required for the
bimetal to bend and trip the breaker varies inversely with the
current. Because of this, thetripping time becomes quicker as
current increases in magnitude.Overload protection is applicable to
any installation, conductor, or component which can be subjected
to low-magnitude but longtime over-currents.
Low-magnitude, long-time over-currents can be dangerous because
they reduce the life of theelectrical installation, conductor, and
components and if left unchecked could result in fire.
 Components of a magnetic trip unit

Magnetic trip units (short circuit protection)

The Magnetic trip unit protects against a short circuit.
The magnetic trip unit is comprised of an
electromagnet and an armature.
When there is a short circuit, a high magnitude of
current passes through the coils creating amagnetic
field that attracts the movable armature towards the
fixed armature. The hammer trip is pushed against the
movable contact and the contacts are opened.
The opening of the breakers contacts during a short
circuit is complete in .5 milli-seconds.
 Arc runners / Arc chutes

During an overload or short circuit event, the contacts

of the breaker separate and an electrical arc is formed
between the contacts through air. The arc is moved
into the arch chute by running the arc down
the interior of the breaker along the arc runner. When
the arc reaches the arc chute it is broken into small
segmented arcs. The segmented arcs split the overall
energy level into segments less than 25V.
Each 25V segment does not have a high enough energy
level to maintain an arc and all energy is naturally
dissipated.
REGIONS OF PROTECTION
 Thermal Trip Unit (region one)
OPERATE in: 2 seconds and no slower than 1 minute.

Magnetic Trip Unit (region two)

instantaneous trip unit interrupts a short circuit in 2.3 to
2.5 milliseconds.

Breaker Contacts (region three)

This region of the curve is the time required for the
contacts of the breaker to begin to separate. The contacts
will open in less than .5 milliseconds and is graphically
represented by the bottom vertical portion of the curve.
 (ELCB) and Residual Current Device
(RCD)
An Earth Leakage Circuit Breaker (ELCB) is a
device used to directly detect currents leaking to
earth from an installation and cut the power
and mainly used in TT earthing systems.
There are two types of ELCBs:
1. Voltage Earth Leakage Circuit Breaker (voltage-
ELCB)
2. Current Earth Leakage Current Earth Leakage
Circuit Breaker (Current-ELCB).
 CHARACTERISTICS
Phase (line) and Neutral both wires connected through RCD.
It trips the circuit when there is earth fault current.
The amount of current flows through the phase (line) should return
through neutral .
It detects by RCD. any mismatch between two currents flowing
through phase and neutral detect by -RCD and trip the circuit within
30Miliseconed.
If a house has an earth system connected to an earth rod and not
the main incomingcable, then it must have all circuits protected by
an RCD (because u mite not be able to get enough fault current to
trip a MCB)
RCDs are an extremely effective form of shock protection
 CHARACTERISTICS
Phase (line), Neutral and Earth wire connected through
ELCB.
ELCB is working based on Earth leakage current.
Operating Time of ELCB:
The safest limit of Current which Human Body can
withstand is 30ma sec.
Suppose Human Body Resistance is 500 and Voltage to
ground is 230 Volt.
The Body current will be 500/230=460mA.
Hence ELCB must be operated in 30maSec/460mA =
0.65msec
 Limitation of RCCB
Standard electromechanical RCCBs are designed to
operate on normal supplywaveforms and cannot be
guaranteed to operate where none standard
waveforms are generated by loads. The most common
is the half wave rectified waveform sometimes called
pulsating dc generated by speed control devices, semi
conductors, computers and even dimmers.
Specially modified RCCBs are available which will
operate on normal ac and pulsating dc.
RCDs dont offer protection against current overloads:
 Nuisance tripping of RCCB: Sudden changes in
electrical load can cause a small, brief current
flow to earth, especially in old appliances.
RCDs are very sensitive and operate very
quickly; they may well trip when the motor of
an old freezer switches off. Some equipment is
notoriously `leaky, that is, generate a small,
constant current flow to earth. Some types of
computer equipment, and large television
sets, are widely reported to cause problems.
 RCD will not protect against live-neutral
shocks, because the current in the live and
neutral is balanced. So if you touch live and
neutral conductors at the same time (e.g.,
both terminals of a light fitting), you may still
get a nasty shock.
 Difference between ELCB and RCCB

ELCB is the old name and often refers to voltage operated

devices that are no longer available and it is advised you
replace them if you find one.
RCCB or RCD is the new name that specifies current
operated (hence the new name to distinguish from voltage
operated).
The new RCCB is best because it will detect any earth fault.
The voltage type only detects earth faults that flow back
through the main earth wire so this is why they stopped
being used.
The easy way to tell an old voltage operated trip is to look
for the main earth wire connected through it.
RCCB will only have the line and neutral connections.
 ELCB is working based on Earth leakage current. But RCCB is
not having sensing or connectivity of Earth, because
fundamentally Phase current is equal to the neutral current
in single phase. Thats why RCCB can trip when the both
currents are deferent and it withstand up to both the
currents are same. Both the neutral and phase currents are
different that means current is flowing through the Earth.
Finally both are working for same, but the thing is
connectivity is difference.
RCD does not necessarily require an earth connection itself
(it monitors only the live and neutral).In addition it detects
current flows to earth even in equipment without an earth
of its own.
 This means that an RCD will continue to give
shock protection in equipment that has a faulty
earth. It is these properties that have made the
RCD more popular than its rivals. For example,
earth-leakage circuit breakers (ELCBs) were
widely used about ten years ago. These devices
measured the voltage on the earth conductor; if
this voltage was not zero this indicated a current
leakage to earth. The problem is that ELCBs need
a sound earth connection, as does the equipment
it protects. As a result, the use of ELCBs is no
longer recommended.
 MCB Selection

The first characteristic is the overload which is

intended to prevent the accidental overloading of
the cable in a no fault situation. The speed of the MCB
tripping will vary with the degree of the overload. This
is usually achieved by the use of a thermal device in
the MCB.
The second characteristic is the magnetic fault
protection, which is intended to operate when the fault
reaches a predetermined level and to trip the MCB
within one tenth of a second. The level of this magnetic
trip gives the MCB its type characteristic as follows:
 MCB Selection

Type Tripping Current Operating Time

Type B 3 To 5 time full load 0.04 To 13 Sec
current
Type C 5 To 10 times full 0.04 To 5 Sec
load current
Type D 10 To 20 times full 0.04 To 3 Sec
load current
 MCB Selection
The third characteristic is the short circuit
protection, which is intended to protect against
heavy faults maybe in thousands of amps caused
by short circuit faults.
The capability of the MCB to operate under these
conditions gives its short circuit rating in Kilo
amps (KA). In general for consumer units a 6KA
fault level is adequate whereas for industrial
boards 10KA fault capabilities or above may be
required.