Internal Arc Fault: Switchgear Protection Design Under UL 891

Internal arc faults in low-voltage switchgear are rare but can cause severe damage. When an arc occurs due to insulation failure, foreign objects, or operational errors, temperatures can reach 20,000°C within milliseconds, and pressure can rise to tens of thousands of Newtons per square meter. Without proper protection, hot plasma and molten metal may escape the enclosure, endangering nearby equipment and personnel.

UL 891 provides key requirements for internal arc fault protection. This article explores how structural design—covering fault mechanisms, protection principles, design requirements, and modern technologies—can deliver effective protection.

Physical Characteristics of Internal Arc Faults

Definition

An internal arc fault refers to an unintended arc discharge inside switchgear caused by insulation breakdown between live conductors or between a conductor and ground. Unlike switching arcs generated during normal operation, internal arc faults are characterized by their uncontrollable nature and high energy density.

Energy Release

Taking a typical industrial power distribution system as an example, the energy released by a 20kA arc fault lasting 100ms can be estimated using the following formula:

Energy (Joules) = Current × Arc Voltage × Duration

Based on an arc voltage of 500V:
20,000A × 500V × 0.1s = 1,000,000 Joules

This is equivalent to the energy of approximately 250 grams of TNT explosives being released instantaneously within a confined space.

Four Physical Effects

UL 891 Protection Design Principles

UL 891 adopts a design principle combining isolation and pressure relief for internal arc fault protection.

Compartmentalization

UL 891 requires switchgear to be divided into independent compartments using grounded metal barriers. Typical partitioning includes:

Bus Compartment: Space housing main and branch buses

Breaker Compartment: Space for circuit breaker functional units

Cable Compartment: Space for incoming and outgoing cable connections

Low-Voltage Compartment: Space for control, metering, and protection components

Metal barriers at least 2.0mm thick physically separate compartments, confining an arc fault to its origin and preventing it from spreading to adjacent areas.

Pressure Relief System

Each compartment is equipped with a pressure relief device at the top or rear. When an arc fault occurs inside a compartment, the high temperature causes rapid air expansion and a sharp pressure increase. The pressure relief device automatically activates upon reaching a set threshold (typically 10–20 kPa), directing high-temperature, high-pressure gases to a safe area behind or above the enclosure for discharge.

Types of pressure relief devices include:

The opening direction of the pressure relief device must not face the operating front, a basic safety requirement of UL 891.

Structural Strength Requirements

The barriers and enclosure structure must withstand the mechanical stress generated by an arc fault. Key design considerations include:

Barrier material thickness not less than 2.5mm

Door locking mechanisms capable of withstanding internal pressure impacts

Bus support spacing meeting short-circuit electrodynamic force withstand requirements

Arc Flash Protection System: Active Protection Technology

In addition to passive structural protection, modern switchgear can be equipped with an arc flash protection system to achieve early fault detection and rapid fault clearing.

Operating Principle

The arc flash protection system consists of arc sensors, a protection device, and a high-speed circuit breaker:

Arc Sensors: Continuously monitor light intensity inside the enclosure and detect characteristic spectra of arcs.

Protection Device: Receives sensor signals while monitoring current changes, confirms arc fault identification, and issues a trip command.

High-Speed Circuit Breaker: Rapidly interrupts the fault current upon receiving the command.

Response Time Comparison

By clearing the fault within 15 ms, the arc energy is insufficient to form destructive pressure and temperature, significantly reducing equipment damage and ensuring personnel safety.

Relationship with Passive Protection

The arc flash protection system does not replace the structural protection requirements of UL 891. The relationship between the two is as follows:

Passive Protection (compartmentalization + pressure relief): Provides physical barriers to ensure that even if a fault occurs, hazards are contained within a defined range

Active Protection (arc flash protection): Clears the fault as early as possible to reduce energy release and damage

The optimal design combines both approaches to form a dual-layer protection system.

Summary of Key Design Requirements

Based on UL 891 requirements, switchgear designed with internal arc fault protection capabilities should meet the following key points:

Conclusion

Internal arc faults are a critical risk in low-voltage switchgear. UL 891 provides foundational passive protection through compartmentalization and pressure relief requirements. Adding an arc flash protection system further enhances safety by enabling early fault detection and rapid clearing.

For critical power distribution, selecting switchgear with internal arc fault protection is essential to ensure personnel safety and reliable operation.