CIRCUIT BREAKERS
A circuit breaker is a circuit protection device( like a fuse )
stop current in the circuit if there is a direct short, excessive current
or
excessive heat .
Circuit breakers can also be used as circuit control devices .
Circuit breakers are available in a great variety of sizes and types .
DC circuit breaking-effect of decreasing current & increasing arc length
AC circuit breaking-current-zero period
How to select a circuit breaker?
This depends on
application type where you are using this C.B.
Whether we require some delay in tripping after the overcurrent
or
frequent operation is needed.
-If a delay is needed (use thermal relay which has some delay and low
trip current at high temp.
-Frequent operation (use magnetic type relay.
-also operating environment
- nominal current rating and max peak current bearing capacity
- delay time of the circuit breaker.
Ratings of circuit breakers
Rated voltage
Rated insulation
Rated frequency
Rated normal current
Rated short circuit breaking current
Rated short circuit making current
Rated opening sequence for auto-reclose CBs
Rated transient recovery voltage for terminal faults
CB interrupting time-its components in relation to fault clearing time
Single-pole auto reclosing & its effects on system performance
Low-Voltage Power Circuit Breakers
A low-voltage power circuit breaker has two elements
• A set of contacts with a mechanical linkage to open or close
the electric circuit rapidly
• An abnormal-condition-sensing element, called a trip device
• Low-voltage power circuit breakers may have continuous-load
ratings of 15-6000 amperes
MCB (Miniature Circuit Breaker)—rated current not more than 100 A .
Moulded Case and Miniature Circuit - Breakers
They are designed for hand operation only but have built-in protective tripping
arrangements IEC 60947-1
Moulded Case and Miniature Circuit - Breakers
It covers all ratings from 3A to 1000A
LV switchboards or switchgear cabinets
Low voltage molded case circuit breakers with high breaking capacity
100kA
Magnetic circuit breaker
Use a solenoid (electromagnet) whose pulling force increases with the current.
The circuit breaker contacts are held closed by a latch .
Circuit breaker components .
Circuit breakers have five main components .
Frame
Operating mechanism
Arc extinguishers
Contacts
Terminal connectors, and the trip elements
Arc extinguisher action
Arc extinguishers are generally used in circuit breakers that control a large amount of power,
such as those found in power distribution panels. Small power circuit breakers
(such as those found in lighting panels) may not have arc extinguishers .
Air magnetic breaker
Arc Is extinguished
by:
- Cooling
- Increaslgpathreslstance and cooling it
"magnetic blow"
It has large insulated arc chutes
(insulated fins and "blow out" Coil)
Usually installed in
"Metal chad" Lineups.
Applied on systems
in the 15 kV and below
Thermal inspection of a molded-case circuit breaker terminal
Modern Vacuum Distribution Switchgear
Vacuum Interruption
Lighter and smaller than air magnetic breakers.
Often installed in metal - clad line-ups.
Vacuum extremely
high dielectric strength than air.
Not used a system above 30kV.
Arc Extinction is simple and quick
Oil Breaker
Small Oil Volume Breaker
Circuit Breaker Maintenance
The rules:
- Keep it Tight
- Keep it Clean
- Keep it Dry
- Keep it Dry
- Keep it Dry
- Keep it friction free
- Detect loosing connections in order to prevent unnecessary failures
- Dirt
- Grime
SF6
non toxic & non flammable
5 times heavier than air/ one of the geaviest known gases
Higher capacity ratings possible and chemical properties
SF6 dielectric strength is much greater than air or nitrogen @ atmospheric pressure
Strength of air is increased by the addition of SF6
Advantages of SFG gas
Space saving design
Completely enclosed design
Improved worker safety
Elimination of the hazard of fire
Light weight and hence low cost
Low maintenance cost
Chemical properties:
S + 6 F SF6 + 524 Kcal
The dissociation products before interruption of the arc
Thionyl fluoride SOF2
Sulfur fluoride SO2F2
Sulfur tetra fluoride SF4
Sulfur deca fluoride S2F10
Thionyl tetra fluoride SOF4
The dissociation products after interruption of an arc
Sulfur fluoride SO2F2 Carbon tetra fluoride CF4 Silicon tetra fluoride SIF4 Sulfurous anhydride SO2
SF6 and the environment
Apparatus and omponents
Fixed Contact
Moving Contact
Puffer Designs
Rated Short Circuit Breaking Current
The rated short-circuit Breaking- Current of a circuit breaker is the highest value of short circuit current which a circuit breaker is capable of breaking under specified conditions of transient recovery voltage and power frequency voltage
When the circuit breaker close on an existing fault, the current increase to the maximum value.
The circuit breaker should be able to withstand the high mechanical forces during a closure.
The rated short circuit making current should be at least 2.5 times the r.m.s value of a.c component of rated breaking current
SF6 Testing
Dew point
Is the temperature at which the vapour in a space will start to condense
The dew point temperature varies as a function of the gas pressure .
Moisture content in parts per million ( PPM ) is independent of pressure and can be measured directly .
Humidity
Is used to describe water vapour or moisture present in atmosphere
Volume- Percentage Measuring Device
The principle of measurement is based on the evaluation of the different velocities of sound of gas
A microprocessor converts the measured values automatically into the SF6 – volume percentage
The following table is necessary of measurement of the SF6 concentration in an SF6 / CF4 gas mixture
SF6 Gas Handling Procedures
Evacuation of Service Truck
by using vacuum pump < 1 m bar
Synchronizing and Synchronizing Equipment
Application of Synchronizing Equipment
Three phase contactor principle
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