Electrical fault

An electrical fault is a defect within an electric power system. This defect results in an abnormality of the electric current. When current flows in an unintended way, it is called a fault current. These events are critical because they can damage equipment or cause safety issues. The main goal of power-system protection is to design ways to detect and interrupt these faults. Engineers use protective devices like circuit breakers and fuses to limit the loss of service. By stopping the fault quickly, they prevent widespread damage to the grid.

Faults can be classified by how long they persist. A transient fault is a temporary problem. It may disappear if power is disconnected and then restored. Many faults in overhead power lines are transient. Examples include a bird touching a wire, a lightning strike, or a tree branch making momentary contact. Some systems use an automatic re-close function to restore power after these brief events. A semi-persistent fault might clear itself over time, such as a tree branch burning away. In contrast, a persistent fault remains until it is physically repaired. These are common in underground cables due to mechanical damage.

Engineers also group faults by their symmetry. A symmetric fault, or balanced fault, affects all phases of a system equally. These are rare, accounting for only 2% to 5% of all system faults. However, they can cause very severe damage to equipment. Most faults are asymmetric, also known as unbalanced faults. These affect the phases differently. In transmission lines, single line-to-ground faults are the most common, making up about 65% to 70% of cases. Line-to-line faults occur in 5% to 10% of cases. Double line-to-ground faults account for roughly 15% to 20% of transmission line faults.

Faults can also be categorized by their location relative to a device. An internal fault develops inside a specific piece of equipment. For example, a transformer might experience overpressure inside its containment vessel due to local overheating. An external fault occurs outside the device. An overload on that same transformer would be considered an external fault. Another specific type is a ground fault, or earth fault. This is any failure that allows current to flow into the earth. Ground faults can cause dangerous voltages on the housings of electrical equipment.

Sometimes, high voltage creates an arcing fault. An electric arc is a spark that forms between conductors and the ground. These arcs can have high impedance, making them difficult to detect with simple tools. An arc of a few hundred amperes might not trip a breaker, yet it can still cause massive damage. In residential wiring, arc-fault circuit interrupters are used to detect these small arcs. This prevents damage or fires before they start. A "bolted fault" is a theoretical extreme where conductors are connected to ground by a metallic conductor. This creates zero impedance and the maximum possible short-circuit current.

Analyzing these faults requires complex mathematical methods. For symmetric faults, engineers use software to perform power flow studies. They often use the principle of superposition to calculate voltages and currents. This involves assuming generators are in phase and calculating a base case. To find the fault result, they treat the fault location as a negative voltage source. For asymmetric faults, the math is more difficult because the system is unbalanced. Engineers use a method called symmetrical components. This views the system as a mix of positive-sequence, negative-sequence, and zero-sequence components.

Understanding the magnitude of these faults is vital for safety. Engineers must calculate the prospective short-circuit current to select the right protective devices. A fuse or breaker must be strong enough to withstand the current without being destroyed. It must also be sensitive enough to trip when the fault occurs. Fault currents vary wildly depending on the system. A domestic 230 V supply in the UK might see a few thousand amperes. Large low-voltage networks can reach fault levels of 300,000 amperes. Meanwhile, a high-resistance-grounded system might restrict a ground fault to only 5 amperes.