This document discusses circuit breakers, including their purpose, construction, types, sizing and selection factors. It provides information on: - Circuit breakers are used to protect electrical systems and people from faults and overloads. They have advantages over fuses in many applications. - The main types discussed are miniature circuit breakers (MCB), moulded case circuit breakers (MCCB), earth leakage circuit breakers (ELCB), motor protection circuit breakers (MPCB) and air circuit breakers (ACB). - Selection of a circuit breaker depends on the load magnitude and nature, as well as the system component being protected. Standards like rated current and short circuit capacity must

2.  Identification on distribution system general components.
(Fig.1)
 Why we use circuit breakers?
 Circuit breaker construction& principle of operation.
 Main circuit breaker parameters.
 Circuit breaker types.
 Circuit breaker sizing.
 Factors which determines selection the type of CB.

3. To protect the different system components
and persons from the destroying effect of the
faults and overloads.
There are a lot of applications at which the
circuit breaker is preferred more than fuse.

7. Rated Current or Ampere Trip (AT)
Characteristic Curve
Ampere Frame (AF)
Short Circuit Capacity
Adjustability Of C.B
Number Of Poles

8. 10 16 20 25 32 40 50 63 80 100
125 160 200 250 400 630 800 1000 1250 1600
2000 2500 3200 4000 5000 6300
What is the value of current which makes
the CB trips??

9. magnetic
thermalthermal
magnetic

10.  The Ampere frame is a number which indicates the
maximum size of a new CB which can be installed
instead of the existing circuit breaker.
 It shows the required space for each CB in the DB.
 For any CB, AF can take values more than or equals
the AT
 Example : the CBs with ratings from 40 to 100 A can
take ampere frame of 100 A.

11.  It is the max. fault current (KA) which the CB can
carry and trip without damaging.

12.  In some applications, an adjustable CB is needed
due to the changing operating conditions of some
applications.

13.  Most types of circuit breakers are available with
different number of poles (1,2,3or 4) to suit all types
of feeder systems.

14.  Its rating can range from 10 to 125 A
 Its typical use is for the final circuits(lighting &
power circuits). (fig.2)

15.  It can be used in final circuits which are located in wet areas
to protect from electric shocks. (fig.3)
 Also it is used in main circuits as a ground fault indicator in
systems which have low levels of ground fault currents.
 It have either 2 or 4 poles.

16.  It is a circuit breaker which is designed especially to protect
electric motors.
 It contains thermal protection with delayed time to allow
starting current, it also contains magnetic protection.
 It can trip the circuit in case of three phase unbalance and
phase failure.

17.  Its rating can range from 16 to 1600 A
 It is used as a main CB for panels. (fig.4)
 More details (pic.1)

18.  Its rating can range from 630 to 6300 A
 It is used at main incoming for MDBs. (fig.5)

19.  Calculating the size of a circuit breaker depends on the
magnitude of the electric load which the CB will protect.
 First, get the design current:
𝐼 𝑏 =
𝑀𝑎𝑥.𝑑𝑒𝑚𝑎𝑛𝑑(𝐾𝑊)
3∗𝑉 𝑙.𝑙(𝐾𝑉)∗𝑃.𝐹
 Multiply 𝐼 𝑏 by 1.25 to get 𝐼2
 Get the nearest nominal CB current more than 𝐼2
 Example:
 For load 80 kw, p.f = 0.8
 𝐼 𝑏 =
80
3∗0.38∗.8
= 151.9 A → 𝐼2= 1.25*151.9 = 189.9 A
 Select a CB with nominal current = 200 A with type MCCB

20.  1.The magnitude of load which determine the required
CB size.
 2.The nature of load and the position in the system.
Lighting& power circuits: MCB
Motor loads: MPCB
Circuits in wet areas: ELCB
Main CB for a distribution board: MCB or MCCB
Main CB for a main distribution board: ACB