Electrical System Protection: Faults, Concepts, and Devices
Understanding Electrical Concepts & Protection
Voltage (Tension)
Voltage (V) is the electrical potential difference between two points in a circuit. It exists when there is an imbalance between two points, with one having a higher electrical potential than the other. Voltage is measured in volts. It is present in both DC (Direct Current) and AC (Alternating Current) systems, including single-phase and three-phase configurations. The relationship between voltage, current, and resistance is described by Ohm’s Law: V = IR (Voltage = Current × Resistance).
Current (Intensity)
Current (I), also known as intensity, is the circulation of electric charge through a conductive material. It is measured in amperes (A). To measure current, an ammeter is used, connected in series with the circuit. The conventional direction of current flow is from positive to negative.
Common Electrical Installation Defects
Over time, and due to operational and climatic conditions, electrical installations can develop defects. It is extremely important to identify these factors and implement appropriate protective measures to ensure safety and reliability.
Overcurrent
An overcurrent occurs when the current flowing through an installation exceeds its nominal (rated) intensity under normal operating conditions. If not properly calculated or protected, an overcurrent can lead to an overload. An overload occurs when the current circulates for a long enough duration to cause excessive heat, potentially damaging the insulation of conductors.
Short Circuit
A short circuit occurs when an unintended, low-resistance path allows a very large current to flow. The installation is not designed to withstand this high intensity. If the current is not interrupted immediately, there is a significant risk of destroying the installation and even causing a fire.
Discrimination (Selective Coordination)
Discrimination (also known as selective coordination) is the ability of a protective device to isolate a fault without affecting upstream devices. This is achieved by coordinating automatic circuit breakers such that the downstream device trips for a fault within its zone, while the upstream device remains operational. This ensures that only the faulty section is disconnected, minimizing power outages and achieving significant economic savings.
Schematic Representation
Electrical installations are represented using diagrams, primarily of two types:
Multifilar (Stranded) Diagrams
These diagrams represent the symbols of the apparatus with all its poles and lines, showing all phases individually.
Unifilar (Single-Line) Diagrams
These diagrams use a simplified symbol with a single pole and represent the wiring through a single line, often with small oblique lines indicating the number of phases.
Insulation Defects
Proper insulation ensures there is no unwanted contact between different elements connected to varying electrical potentials. If insulation fails, two main issues can arise:
Short Circuit
Two active (energized) conductors can come into direct contact.
Ground Fault
An active conductor can come into contact with an unenergized conductive part, such as a metallic enclosure. The resulting currents are called ground leakage currents.
Voltage Surges
Voltage surges occur when the voltage exceeds the nominal grid voltage. They can be categorized as:
Transient Surges
These are high-value surges occurring over a very short period. They are often caused by atmospheric phenomena (e.g., lightning strikes) or switching operations of highly inductive loads.
Temporary or Permanent Surges
These are lower-value surges (e.g., more than 10% above nominal voltage) but lasting for a longer duration. An example is the loss of a neutral connection in a transformer, which can lead to overvoltage on certain phases.
Fuse Operation
Fuses are essential protective elements in electrical installations, always connected in series. Their primary objective is to be the first point to heat up due to an excessive current. If the temperature is sufficient, the fuse element melts, interrupting the circuit and protecting the downstream equipment. Externally, it’s often not possible to see if a fuse has blown, though some types include a ‘striker pin’ or ‘hammer device’ which, through a colored element, indicates the fuse’s status. Each time a fuse blows, it becomes useless and must be replaced with a new one.
Types of Fuses
Cylindrical Fuses
These are traditional cartridge fuses, commonly used in various applications.
Blade or NH Fuses
Designed for low voltage and high breaking capacity applications. They have current ratings typically up to 1250A and voltage ratings up to 690V. Their breaking capacity can reach up to 120kA. When equipped with an indicator, it often appears as a small tab parallel to the fuse body.
D-Type (DIAZ) Fuses
Of German origin, these fuses are also known as ‘bottle type’ fuses. They have current ratings typically between 2A and 100A and a nominal breaking capacity up to 50kA.