Low voltage circuit breakers

explore all types of LV circuit breakers and how to size and select a suitable circuit breaker.

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
thermal thermal
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:
??????
�=
??????�??????.������(??????�)
3∗�
??????.??????(??????�)∗??????.??????

Multiply ??????
� by 1.25 to get ??????
2
Get the nearest nominal CB current more than ??????
2
Example:
For load 80 kw, p.f = 0.8
??????
�=
80
3∗0.38∗.8
=151.9 A → ??????
2= 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