Circuit Breakers
Circuit Breakers
High Voltage Medium Voltage Low Voltage Extra Low Voltage
> 33𝑘𝑉 1000𝑉 − 33𝑘𝑉 50𝑉 − 1000𝑉 0𝑉 − 50V
High/Medium Voltage Circuit Breakers
OCB – Oil Circuit Breakers
SF6 – Sulphur Hexafluoride
VCB – Vacuum Circuit Breakers
Low/Extra Low Voltage Circuit Breakers
ACB – Air Circuit Breakers (> 1000𝐴)
MCCB – Moulded Case Circuit Breakers (40A-1000A)
MCB – Miniature Circuit Breakers (5A-63A)
Why do we need a circuit breaker?
Circuit breakers are used to protect the downstream equipment and cables from excessive
current than the rated.
What are the reasons to create excessive currents (over-currents) in a circuit?
Basically excessive currents are categorized into three
1. Over Load Current
2. Short Circuit Current
3. Earth Fault Current
Over Load Current
An overload current in a conductor when too many devices are operated in a single circuit
or when electrical equipment is made to work beyond its rated capabilities.
Short Circuit Current
An over current resulting from a fault negligible impedance between line conductors having
different potential.
, Circuit Breaker Construction
Circuit breakers are constructed from the following major components.
Frame
Contacts
Arc Chute Assembly
Tripping Mechanism
Operating Mechanism
Circuit Breaker Ratings
Circuit breaker ratings can be categorized into two as
1. Voltage Ratings
2. Current Ratings
Voltage ratings of a circuit breaker
𝒖𝒆 - Rated Operational Voltage
This is the normal line-to-line voltage of the system in normal operation. If a circuit breaker
is exposed to the supply voltage for a prolonged period, the circuit breaker should withstand
for voltage limit indicator.
𝐮𝐢 -Rated Insulation Voltage
This is the voltage assigned by the manufacturer to which dielectric test and creep age
distance are referred. Unless otherwise stated the rated insulation voltage is the value of
the maximum rated operational voltage of the circuit breaker.
𝒖𝒊𝒎𝒑 – Rated Impulse Voltage
It is the value of the transient peak voltage.
Current ratings of the circuit breaker
𝑰𝒏 – Rated Operating Current
The current that a circuit breaker can carry continuously without temperature rise in the
circuit breaker.
Circuit Breakers
High Voltage Medium Voltage Low Voltage Extra Low Voltage
> 33𝑘𝑉 1000𝑉 − 33𝑘𝑉 50𝑉 − 1000𝑉 0𝑉 − 50V
High/Medium Voltage Circuit Breakers
OCB – Oil Circuit Breakers
SF6 – Sulphur Hexafluoride
VCB – Vacuum Circuit Breakers
Low/Extra Low Voltage Circuit Breakers
ACB – Air Circuit Breakers (> 1000𝐴)
MCCB – Moulded Case Circuit Breakers (40A-1000A)
MCB – Miniature Circuit Breakers (5A-63A)
Why do we need a circuit breaker?
Circuit breakers are used to protect the downstream equipment and cables from excessive
current than the rated.
What are the reasons to create excessive currents (over-currents) in a circuit?
Basically excessive currents are categorized into three
1. Over Load Current
2. Short Circuit Current
3. Earth Fault Current
Over Load Current
An overload current in a conductor when too many devices are operated in a single circuit
or when electrical equipment is made to work beyond its rated capabilities.
Short Circuit Current
An over current resulting from a fault negligible impedance between line conductors having
different potential.
, Circuit Breaker Construction
Circuit breakers are constructed from the following major components.
Frame
Contacts
Arc Chute Assembly
Tripping Mechanism
Operating Mechanism
Circuit Breaker Ratings
Circuit breaker ratings can be categorized into two as
1. Voltage Ratings
2. Current Ratings
Voltage ratings of a circuit breaker
𝒖𝒆 - Rated Operational Voltage
This is the normal line-to-line voltage of the system in normal operation. If a circuit breaker
is exposed to the supply voltage for a prolonged period, the circuit breaker should withstand
for voltage limit indicator.
𝐮𝐢 -Rated Insulation Voltage
This is the voltage assigned by the manufacturer to which dielectric test and creep age
distance are referred. Unless otherwise stated the rated insulation voltage is the value of
the maximum rated operational voltage of the circuit breaker.
𝒖𝒊𝒎𝒑 – Rated Impulse Voltage
It is the value of the transient peak voltage.
Current ratings of the circuit breaker
𝑰𝒏 – Rated Operating Current
The current that a circuit breaker can carry continuously without temperature rise in the
circuit breaker.
