Short Answer:

A bolted fault is a type of short circuit where two or more conductors are solidly connected together with no resistance or arc between them. This means the fault path offers very low impedance, allowing the maximum possible fault current to flow instantly through the system.

Bolted faults are often used as a worst-case scenario in fault studies, especially for selecting the proper ratings for circuit breakers and protection devices. Even though bolted faults rarely occur in practice, they help engineers design equipment to withstand the highest stress levels.

Detailed Explanation:

Bolted fault

In electrical power systems, a bolted fault refers to a condition where there is a solid and direct connection between conductors (like phase-to-phase or phase-to-ground), creating a short circuit with zero or near-zero impedance. This name comes from the idea of the conductors being literally “bolted” together, allowing maximum current to flow without interruption or resistance.

Bolted faults are not natural faults like those caused by lightning or insulation failure. Instead, they are assumed faults used in engineering analysis to determine the maximum fault current that a system may experience.

Characteristics of bolted fault

1. Zero impedance path:
   • Since the fault path is assumed to have no resistance or reactance, the only impedance that limits the current is the system’s internal source impedance.
2. Highest fault current:
   • Among all types of faults, bolted faults cause the maximum current to flow, which may be several times the normal operating current.
3. No arcing or heat dissipation:
   • Unlike arc faults that have high resistance and heat generation, bolted faults are clean connections and don’t produce arcs.
4. Used in fault studies:
   • Engineers use bolted fault conditions to select breaker ratings, set relay parameters, and ensure equipment can withstand peak current levels.

Where bolted fault assumptions are applied

• Circuit breaker sizing:
  • Devices must be able to safely interrupt the highest fault current, which is calculated based on a bolted fault condition.
• Busbar and cable design:
  • The mechanical and thermal stresses on cables and busbars are analyzed using the extreme values of fault current from bolted fault analysis.
• Relay coordination:
  • Protective relays are adjusted so that they act quickly when bolted faults occur, ensuring minimal damage.
• System protection planning:
  • Engineers evaluate how much energy is released during such a fault and design enclosures and barriers to contain it.

Difference from other faults

• Arcing faults involve high resistance and lower current but create high heat and are more common.
• Bolted faults involve low resistance, very high current, and no arc.
• In practice, arcing faults occur more often, but bolted faults are used in worst-case design scenarios.

Practical examples

• A tool accidentally left between busbars causing a solid short.
• A maintenance error where conductors are mistakenly connected.
• Intentional testing where conductors are clamped or bolted for current injection testing.

Though such situations are rare, systems must still be designed to handle them safely.

Conclusion:

A bolted fault is a type of short circuit with zero impedance between conductors, resulting in the highest possible fault current. Although it rarely happens naturally, it is critical for designing protective devices and equipment that must endure the most severe conditions. Using bolted fault analysis ensures system safety, reliability, and effective fault-clearing capability.