Week 9
Practical Work
4. Protective and Control Device
We will choose the circuit breaker as protective device and the
contactor as control device.
4.1 Circuit breaker
Circuit breaker is used to protect any electrical installation from the
danger of over currents.
4.1.1 Disassembling and re-assembling Circuit breaker

Introduction
Circuit breaker is a piece of
equipment which is designed to
protect an electrical apparatus
from damage caused by overload
or short circuit. Unlike a fuse
which operates once and then has
to be replaced, a circuit breaker
can be reset (either manually or
automatically) to resume normal
operation.
Circuit
breakers
implemented
with
are
a
often
solenoid
(electromagnet) whose strength
increases as the current increases
1. lever
5. Bimetallic strip
2. lever
6. adjusting screw
mechanism
7. Solenoid
3. Contacts
8. Arc divider
4. Terminals
Figure(4-1) : Construction of the
circuit breaker
and eventually trips the circuit
breaker. Alternatively a bimetallic strip may be used which heats and bends
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Week 9
Practical Work
with increased current. Some circuit breakers incorporate both techniques.
This allows the properties of the circuit breaker to be tailored to suit the
application, with the electromagnet generally responding to short, large surges
in current (short circuit) and the bimetallic strip responding to smaller but
longer-term (overload) over current conditions, the construction of circuit
breaker is shown in figure (5-1)
 Apparatus
1. One circuit breaker.
2. Tools box
 Procedure :
o Disassembling the circuit breaker
o Identify the all parts of the circuit breaker as shown in figure (51).
o re-assembling the circuit breaker
4.1.2 Use the Circuit breaker in the protected circuit
 Introduction
To discriminate between the current the circuit breaker can handle and
the current it has to interrupt.
 Apparatus
3. 15 rating circuit breaker.
4. 0 to 100 Ω variable resistors (high power rating).
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Week 9
Practical Work
5. 240 V ac power supply.
6. Connecting wires.
7. Ammeter.
 Procedure :
1. Connect the circuit as shown in fig. (4-2).
supply
I>
Circuit
breaker
Figure (4-2): circuit diagram of the protected circuit
2. Put the resistance (R) at its maximum value, and then turn on the
power supply.
3. Perform the adequate variation of the circuit breaker rating
before any alteration of the load resistance, according to the
above table. Close the circuit breaker.
4. Start from full resistance to zero resistance, decreasing it in 10 steps.
54
Week 9
Practical Work
5. Fill, in table in your worksheet, the circuit breaker tripping time, until
you reach zero resistive load.
6. Turn off the system, by opening the circuit breaker.
7. Put the load resistance 'to zero, then turn "ON" the circuit breaker and
note the tripping time.
Worksheet 09
Solve the following questions:
1. Calculate the various values of
Current
(Amps)
current (I) if we are to start from
Resistance
(Ohms)
full resistance to zero resistance,
decreasing it in 10 steps. Tabulate
the values obtained in table (5-1).
Table (4-1)
55
Tripping
Time
(seconds)
Week 9
Practical Work
2.Write your own conclusion regarding the mechanisms that tripped the
circuit
breaker in step 5 and in step 7.
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