A bolted fault is a type of short circuit in an electrical system where the conductors—such as phase-to-phase or phase-to-ground—are connected by a conductor with zero impedance. In other words, it is a direct and solid connection between two or more points in a circuit where a fault occurs, without any arc resistance.

Unlike arcing faults, which involve electrical discharge through air or insulation and have variable impedance, bolted faults create a path of extremely low resistance. As a result, they produce the maximum possible fault current that the system can generate.

Why Is It Called a “Bolted” Fault?

The term “bolted” comes from the hypothetical idea that two conductors are physically bolted together—creating a perfect electrical connection. This term is used for analysis and system design purposes to simulate the worst-case fault condition.

Key Characteristics of a Bolted Fault

  • Zero or near-zero impedance
  • Very high fault current
  • No arc or electrical discharge (unlike arc faults)
  • Rapid system response required to isolate the fault
  • Highest mechanical and thermal stress on equipment

Impacts on Electrical Systems

Bolted faults can cause significant damage if not isolated quickly. Potential consequences include:

  • Circuit breaker trips and relay operations
  • Severe heating of conductors and components
  • Mechanical stress on busbars, transformers, and switchgear
  • Complete equipment failure if protections malfunction

Because of the extremely high current involved, bolted faults serve as the benchmark for sizing circuit breakers, fuses, and protection relays during system design.

Importance of Protection Systems

Modern power systems rely on protective relays and circuit breakers designed to detect and isolate bolted faults within milliseconds. Coordination of these systems ensures:

  • Personnel safety
  • Equipment protection
  • Minimal service interruption
  • Compliance with electrical codes and standards

A bolted fault represents the most severe form of short circuit in an electrical system. Its analysis is critical in designing protection schemes and ensuring operational safety in industrial and utility environments. Understanding its behavior helps engineers implement robust and responsive fault mitigation strategies.