Content:
1. Design, function and types of
circuit breakers
1.1. Summary
1.2. Types of switches
1.2.1. Manual motor starter and protector or
circuit breaker with motor protective characteristics 1.2
1.2.2. Circuit breaker 1.2
1.2.3. Load break switch 1.6
1.2.4. Disconnector 1.6
1.2.5. Main switch 1.7
1.2.6. Emergency OFF-switch 1.8
1.2.7. Summary: Circuit breaker as load break switch 1.8
1.3. Design of a circuit breaker 1.9
1.3.1. The current path of the circuit breaker
1.3.2. Thermal overload release 1.10
1.3.3. Electromagnetic overcurrent release
1.3.4. Main contact system 1.12
1.3.5. Auxiliary contacts 1.15
1.3.6. Operating mechanism 1.15
1.4. Functions of a circuit breaker 1.15
1.4.1. Interrupting short-circuit current 1.16
1.4.2. Reliable protection of motors 1.17
1.4.3. Protection of leads and its optimum utilisation 1.18
1.4.4. Protection of installations 1.19
1.4.5. Integration in the control circuit 1.19
1.4.6. Switching under normal service conditions 1.20
1.4.7. Disconnecting function 1.20
1.4.8. Locking out with a padlock 1.20
2. Circuit breaker technology 2.1
2.1. Summary 2.1
2.2. Short-circuit current in supply systems 2.2
2.2.1. Types of short-circuit 2.2
2.2.2. The peak value of the short-circuit current 2.3
2.2.3. Calculation of the short-circuit current
close to the transformer
2.2.4. Calculation of the short-circuit current in
radial supply systems 2.7
2.2.5. Dynamic stress on the connecting leads
in the case of a short-circuit 2.16
2.3. Short-circuit protection 2.18
2.3.1. The principle of current limitation 2.18
2.3.2. Breaking capacity 2.26
2.3.3. Electrical life (durability) of
circuit breakers 2.26
2.4. Short-circuit co-ordination 2.28
2.4.1. Definitions in accordance with
the IEC 947-4-1 2.28
2.4.2. Conclusions drawn from the
definitions for the user 2.28
2.4.3. Physical significance of the short-circuit
co-ordination 2.30
2.4.4. Requirements of a circuit breaker for a simple
co-ordination of type "2" 2.33
3. Fields of application of circuit breakers 3.1
3.1. General procedure for the selection of
correctly rated circuit breakers 3.1
3.2. Circuit breakers for motor protection 3.2
3.2.1. Protection of motors with direct-on-line starting 3.3
3.2.2. Protection of motors with star-delta starting 3.4
3.2.3. Protection during heavy-duty starting 3.8
3.2.4. Circuit breaker with a motor protective
device connected downstream 3.8
3.2.5. Protection of motors in explosive environments 3.12
3.2.6. Protection of motors with phase controlled
starting (soft starter) 3.13
3.2.7. Protection of frequency controlled motors
(frequency converter) 3.14
3.3. Circuit breakers for the protection of
connecting leads and for group protection 3.16
3.3.1. Protection of the connecting leads 3.16
3.3.2. Group protection
3.4. Circuit breakers for capacitors 3.17
3.5. Circuit breakers for transformers 3.18
3.5.1. Protection of transformer: primary side 3.18
3.5.2. Protection of transformer: secondary side 3.18
3.6. Circuit breakers for generators 3.18
3.7. Circuit breakers for special supply frequencies 3.19
3.7.1. Breaking capacity at frequencies below 50/60Hz 3.19
3.7.2. Breaking capacity at frequencies above 50/60Hz 3.20
3.8. Interruption of direct current 3.20
3.9. Breaking capacity at higher supply voltages 3.21
3.10. Selectivity (discrimination) 3.21
3.10.1. Selectivity between circuit breakers 3.21
3.10.2. Selectivity between circuit breaker and fuse 3.24
3.10.3. Selectivity between fuses 3.25
4. Arguments in favor of the circuit breaker 4.1
4.1. Summary 4.1
4.2. Comparison of the functions:
circuit breaker / fuse 4.2
4.2.1. Time-current characteristics 4.2
4.2.2. Comparison of Joule-integrals 4.3
4.2.3. Comparison of the ultimative tripping current 4.4
4.2.4. Table of comparison 4.4
4.3. Arguments in favour of the circuit breaker 4.6
4.3.1. Prevention of accidents with the help of
circuit breakers 4.6
4.3.2. Ready to be switched on again without delay 4.8
4.3.3. All pole interruption 4.9
4.3.4. No ageing 4.9
4.3.5. Reduction of the conductor cross-section 4.10
4.3.6. Simplified planning of installations 4.14
4.3.7. Reduction of costs of installations and
optional costs
A circuit breaker, as we shall understand in the following text consists of a
thermal overload release, an electromagnetic short-circuit release, a tripping
(operating) mechanism, the main contact system and the auxiliary contacts.
These are the most important functional blocks.
By integrating all these functional blocks in a single unit, it is possible to replace
many individual components in an installation with one single device, viz. the
circuit breaker. The combination of fuse, contactor and thermal overload relay
will be replaced by the starter combination of circuit breaker and contactor.
One single device, the circuit breaker, fulfils the following functions :
• Short-circuit protection
• Motor protection
• Protection of connecting leads
• Protection of installations
• Signalisation of the switching state
Tripping indication
• Switching under normal service conditions
• Remote switching
• Disconnecting
• Locking out with padlock (mandatory for main switch)
Hence, it can be used not only as a circuit breaker, but also as circuit breaker for
motor protection, as load-break switch or as disconnector.
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