(1) Miniature Circuit Breakers (MCBs) are devices that protect electrical circuits from overloads and short circuits. They contain a switch and trip unit that can open and close a circuit under rated current. (2) MCBs have standardized modular dimensions and fit into distribution boards. They use a bimetal strip to detect overloads and a magnetic trip to detect short circuits. (3) The tripping characteristics of MCBs can be selected using different curves (B, C, D, etc.) to provide appropriate protection for different circuit types like transformers or motors.

1. Miniature Circuit Breakers (MCB)
CAREER POINT UNIVERSITYCAREER POINT UNIVERSITY
MAJOR ASSIGNMENTMAJOR ASSIGNMENT
Submitted By:Submitted By:
Chhotray Tiyu(K12279)Chhotray Tiyu(K12279)
Guided By:Guided By:
Mr. Ravindra Meena(HOD)Mr. Ravindra Meena(HOD)

2. Customer Training - Freddy Suhartono - April 2000 - English 2
 Protect cables against
overloads and short
circuits
 Let the current flow
without overheating on
cables
 Open and close a circuit
under rated current
 Protect against insulation
faults (see earth leakage
protection module)
Definition
Main functions
Miniature circuit = switch + trip unit breaker (MCB) 
 Symbol

3. Customer Training - Freddy Suhartono - April 2000 - English 3
 A mechanical system to open and close the
circuit
 A bimetal strip to act against overloads
 A magnetic trip to act against short circuits
Definition
A Multi 9 circuit breaker has:
Miniature circuit = switch + trip unit breaker (MCB) 

4. Customer Training - Freddy Suhartono - April 2000 - English 4
 It has standardized modular dimensions
 ≤ 125A
 ≤ 50 kA
 ≤ 440 V CA
Definition
Main performances
It fits in your pocket... 

5. Customer Training - Freddy Suhartono - April 2000 - English 5
Definition
 Multi 9 standard
 Multi 9 switchgear comes in 9 mm pitch 
width
symmetrical
rail
external
partdepth
6010 maxi
3545
maxi
44 maxi
21.3
81
 The modular design meets 2
criteria:
 mounting on a 35 mm
symmetrical rail
 dimensions
- modules in widths of
17.5 mm + 0.5 - 0
- external part = 45 mm
- depth between 55 and 70 mm

6. Customer Training - Freddy Suhartono - April 2000 - English 6
The technology
Active parts
C60N NC100 / NC125
terminal
fixed contact
moving
contact
strip braid
magnetic
trip
load side
terminal
terminal
bimetral strip
strip braid
moving
contact
magnetic trip
load side
terminal
strip braid
bimetal strip


7. Customer Training - Freddy Suhartono - April 2000 - English 7
Overload protection
bimetal
tripping bar
bimetal
tripping bar
screw
calibration
strip
braid
NC45 NC100 / NC125
 A bimetal strip is used:
 to protect the cable against overloads
symbol:
 principle of the bimetal: in a Multi 9 circuit
breaker 

8. Customer Training - Freddy Suhartono - April 2000 - English 8
 Thermal tripping curve
 the "thermal" part of the
tripping curve must be in
accordance with the
standard
Overload protection
Legend:
In = rated current
Ind = no tripping current
Id = tripping current 
tripping
no
tripping
destruction
of bimetal
accuracy
t (s)
I/InIn Ind Id

9. Customer Training - Freddy Suhartono - April 2000 - English 9
Overload protection
Simultaneous multi-pole tripping 
 Temperature derating
 our circuit breakers do not have temperature
compensation
 if the calibration temperature is different from
the ambient temperature: derating is required

10. Customer Training - Freddy Suhartono - April 2000 - English 10
Short-circuit protection
Short-circuit current is given as RMS or peak
(Isc or Îsc)
RMS = Root Mean Square in symmetrical DC
(with PF = I)
Short-circuit protection must limit thermal stress in the
cable in order to prevent cable damage (which may
reach vaporization of the copper in some cases)
Comments slide 29

11. Customer Training - Freddy Suhartono - April 2000 - English 11
Short-circuit protection
"By-pass" technique for the
protection of sensitive parts
of the circuit breaker DC
breaking is difficult
"By-pass" technique for the protection of
sensitive parts of the circuit breaker (magnetic
trip)
The arc in Multi 9 circuit breakers
2 breaking techniques used by Multi 9 
arcing
horn arcing
horn
NC45 NC100 / NC125

12. Customer Training - Freddy Suhartono - April 2000 - English 12
Short-circuit protection
Magnetic tripping curve
Our curves are instantaneous tripping curves 
Legend:
Ind = no tripping current
Id = tripping current
destruction
of the solenoid
tripping
no
tripping
t (s)
I/InInd Id

13. Customer Training - Freddy Suhartono - April 2000 - English 13
 B curve
 3 and 5 In protection
low short-circuit current
generators
The magnetic tripping levels with high inrush
current:
like LV/LV transformers
 C Curve
 5 and 10 In control and protection of
circuits with high short-circuit current
in industrial and domestic distributions
 D and K curves tripping betwen
 tripping: 10 to 14 In; more particulary
adopted for installations with high
inrush currents like LV/LV transformers
 Z curve
 2.4 to 3.6 In protection against
electronic circuits
 MA curve
 12 In; protection only against
short-circuit and high inrush currents
such as motors, coil 
G
M

14. Customer Training - Freddy Suhartono - April 2000 - English 14
 Side panel marking:
Marking
 Front panel marking:
Circuit breaker marking is governed by strict
rules in accordance with the standards and
decisions made by the manufacturer 

15. Customer Training - Freddy Suhartono - April 2000 - English 15
THANK
YOU