27-03-2012, 04:43 PM
circuit breaker full report
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Introduction
Welcome to another course in the STEP series, Siemens
Technical Education Program, designed to prepare our
distributors to sell Siemens Energy & Automation products
more effectively. This course covers Basics of Circuit Breakers
and related products.
Upon completion of Basics of Circuit Breakers you will be able
to:
• Explain the need for circuit protection
• Identify various types of overcurrent protective devices
• Explain the basic electro-mechanical operation of a circuit
breaker
• Identify various types of Siemens circuit breakers
• Identify circuit protection ratings for various types of
Siemens circuit breakers
• Describe time-current characteristics on a time-current
curve
• Explain the benefits and function of circuit breaker
coordination
Need for Circuit Protection
Current and Temperature Current flow in a conductor always generates heat. The greater
the current flow, the hotter the conductor. Excess heat is
damaging to electrical components and conductor insulation.
For that reason, conductors have a rated continuous current
carrying capacity or ampacity. Overcurrent protection devices,
such as circuit breakers, are used to protect conductors from
excessive current flow. These protective devices are designed
to keep the flow of current in a circuit at a safe level to prevent
the circuit conductors from overheating.
Conductor Insulation
Motors, of course, are not the only devices that require circuit
protection for an overload condition. Every circuit requires some
form of protection against overcurrent. Heat is one of the major
causes of insulation failure of any electrical component. High
levels of heat can cause the insulation to breakdown and flake
off, exposing conductors.
Short Circuits
When two bare conductors touch, either phase to phase or
phase to ground, a short circuit occurs. When a short circuit
occurs, resistance drops to almost zero. Short circuit current
can be thousands of times higher than normal operating
current.
Circuit Breaker Operation
In the following illustration, an AC motor is connected through
a circuit breaker to a voltage source. When the circuit breaker is
closed, a complete path for current exists between the voltage
source and the motor allowing the motor to run. Opening the
circuit breaker breaks the path of current flow and the motor
stops. The circuit breaker will open automatically during a fault,
or can be manually opened. After the fault has been cleared, the
breaker can be closed allowing the motor to operate.
Ground Fault Sensor
An external ground fault sensor is available for the ED
frame circuit breakers. The relay functions to de-energize a
circuit within an established period of time when the current
to ground exceeds a predetermined value. This is done by
sensing the difference between two or more load conductors
passing through the sensor. A 5 mA ± 1 mA current difference
(for circuits requiring personnel protection) or a 30 mA ±
6mA current difference (for circuits requiring only equipment
protection) is the threshold for a ground fault.
Review 6