Electrical Protection and Safety: A Comprehensive Guide
Electrical Protection and Safety
Electrical protection systems are designed to safeguard individuals, facilities, and equipment from hazards arising from electrical malfunctions.
Effects of Electrical Current on the Human Body
The severity of electrical shock depends on two primary factors:
- Intensity of electric current
- Duration of current flow
Types of Electrical Contact
Direct Contact
Direct contact occurs when a person touches live parts of an electrical installation or equipment.
Indirect Contact
Indirect contact happens when a person touches a conductive part that has become energized due to an insulation fault, exposing them to voltage with respect to ground.
Grounding
Grounding provides a direct electrical connection between a circuit’s conductive parts and the earth. This connection enables the safe passage of ground fault currents and the discharge of atmospheric electrical surges.
Grounding Systems
TN-C Scheme
In a TN-C system, the neutral and protective functions are combined within a single conductor throughout the entire electrical scheme.
TT Scheme
A TT scheme features a separate grounding point for the electrical supply neutral (generally connected directly to ground) and the masses of the installation, which are connected to a local grounding system.
Protection Against Overcurrents
Two main types of overcurrents require protection: overloads and short circuits.
Overloads
Overloads result from excessive current demand exceeding the circuit’s rated capacity. Their duration can range from seconds to hours or even days.
Short Circuits
Short circuits occur due to accidental contact between two points with different electrical potentials within an installation. These events generate extremely high currents, potentially thousands of times the circuit’s rated current, and typically last from milliseconds to a second.
Fuses
Fuses are protective devices designed to interrupt overcurrents caused by overloads or short circuits. They offer several advantages, including high breaking capacity, enabling them to detect and interrupt short-circuit currents up to 120kA or more.
Components of a Fuse
A fuse typically consists of a cartridge, fuse link (melting element), and fuse base. The cartridge houses the fuse link and filling material, while the base provides a secure mounting point within the electrical circuit.
Fuse Operation
When the current exceeds the fuse’s rated value, the fuse link melts and evaporates. This action creates a rapid temperature rise and interrupts the current flow, protecting the circuit.
Fuse Selection and Types
When selecting a fuse, consider the following factors:
- Rated voltage: The fuse’s voltage rating must match or exceed the application’s rated voltage.
- Rated current: The fuse’s current rating must match or exceed the application’s nominal current.
- Breaking capacity: The fuse’s breaking capacity must be equal to or greater than the anticipated short-circuit current in the circuit.
Common fuse types include:
- Type gG: General purpose fuses
- Type aM: Fuses designed for motor protection, typically used in conjunction with thermal overload relays
- Type aR: Semiconductor fuses, specifically designed to protect sensitive electronic components
Circuit Breakers
Circuit breakers are mechanical switching devices capable of establishing, carrying, and interrupting currents under both normal and abnormal conditions. They provide protection against overloads and short circuits.
In a typical residential electrical panel, three main types of circuit breakers are commonly used:
- Main circuit breaker (ICP): Limits the instantaneous power drawn from the utility company and is determined by the user’s contracted power rating.
- General protection circuit breaker (IGA): Provides overall protection for all circuits within the home’s electrical installation.
- Individual circuit breaker (PIA): Protects specific circuits or branches within the home’s electrical system.
Components of a Circuit Breaker
Key components of a circuit breaker include:
- Body: Encloses and houses all internal components.
- Connection terminals: Provide connection points for incoming and outgoing conductors.
- Operating lever: Allows manual connection and disconnection of the circuit.
- Contacts (fixed and mobile): Establish and interrupt the electrical path.
- Magnetic trip coil: Triggers the breaker’s tripping mechanism in response to a short circuit, creating a rapid magnetic force that separates the contacts.
- Bimetallic thermal trip: Responds to overload conditions by deforming under heat generated by excessive current flow, eventually tripping the breaker.
- Arc chute: Suppresses and extinguishes the electrical arc generated during contact separation.
Circuit Breaker Operation
Circuit breakers employ two main tripping mechanisms: magnetic and thermal.
Magnetic Mechanism
The magnetic mechanism utilizes a coil with multiple turns of insulated copper wire. During a short circuit, the high current creates a strong magnetic field that forces a steel cylinder within the coil to move, striking the movable contact and interrupting the circuit.
Thermal Mechanism
: It consists of a sheet also called bimetal. When an overload, the bimetal is heated by passing electric current and deforms to activate a trigger that releases the movable contact and causes the opening of the circuit. The slow speed or off is inversely proportional to the current flowing.