How ETAP DC arc flash analysis addresses safety risks during generator brush collector ring maintenance

ETAP DC Arc Flash methods and their capabilities can be utilized to expand and evaluate incident energy at the generator brush/collector rings assembly. Collaboration with the university to support and perform research in this area is vital to helping companies conduct significant research and development.
By Mr. Zia Salami, Senior Management Specialist at CDM Smith

Maintenance on generator brush and collector ring assemblies is often performed while the generator remains energized. Workers must physically approach energized DC components to inspect or replace graphite brushes, exposing them to severe shock and DC arc flash hazards—a risk not adequately covered by current industry standards. This study demonstrates how ETAP’s DC Arc Flash tools, combined with Transient Stability analysis, can estimate incident energy at the collector ring location and support safer maintenance practices in applications where no established DC arc flash standards exist.

CDM Smith is a global, privately owned engineering and construction firm. It provides water, environment, transportation, energy, and facilities engineering services to public and private clients worldwide. The company is headquartered in Boston, Massachusetts, and employs over 5,000 people. As a full-service engineering and construction firm, CDM Smith stays ahead of tomorrow’s changing needs and implements lasting impacts today that propel their client services, quality results, and enduring value across the project life cycle.

Location: Charlotte, North Carolina, United States

Year: 2022

Ensure safety maintenance of generators according to standards

Challenges

1. Lack of DC arc flash standards

IEEE 1584 defines AC arc flash calculations only. There is currently no recognized DC method for quantifying incident energy in collector ring applications or determining proper PPE levels.

2. Two independent DC energy sources must be evaluated

During a brush or collector ring fault:
  • The generator field winding discharges through the fault.
  • The exciter/regulator injects additional current until its protective device trips.

Both must be modeled and combined.

3. Wide variation in excitation system designs

Solid-state exciters, brushless exciters, and shaft-driven AC exciters have very different discharge characteristics, making generic assumptions unsafe.

4. Worker proximity to energized components

Brush replacement requires personnel to be within inches of energized DC circuits, increasing the consequences of incorrect incident energy evaluation.

Which solutions did they choose?

Selected applications

ETAP transient stability

Used to determine the field discharge current and time constant under a simulated fault such as loss of excitation. This provides a realistic DC source contribution based on the actual machine model.

ETAP DC arc flash module

Applies Stokes, Paukert, and Maximum Power methods to estimate incident energy from DC fault currents and protective device clearing times.

IE subtraction method

When two DC sources feed the same fault location, this method considers the tripping device’s clearing time and subtracts non-contributing energy, yielding a more realistic estimate than the overly conservative Max Power method.

Why do they use ETAP?

Main customer benefits

Accurate representation of field discharge energy. 

  • ETAP derives field currents directly from Transient Stability, capturing generator time constants and realistic decay behavior.

Comprehensive incident energy calculation. 

  • Combining field discharge IE (~6 cal/cm²) and exciter/regulator IE (~0.43 cal/cm²) provides the total DC hazard at the brush assembly.

Better selection of PPE and safety procedures. 

  • In the absence of standards, ETAP provides a structured method to quantify potential hazard levels and guide safe work practices.

Appropriate handling of multi-source DC faults. 

  • The IE Subtraction approach prevents excessive conservatism and reflects the actual clearing action of protective devices.

Support for advanced research and industry development. 

  • ETAP’s collaboration with UNC Charlotte helps develop better methodologies for DC arc flash analysis, benefiting the wider industry.

 

Conclusion

DC arc flash hazards at generator brush and collector ring assemblies are poorly addressed by existing standards, yet represent a significant maintenance risk. ETAP provides a practical methodology to evaluate incident energy from both field winding discharge and exciter/regulator current, enabling safer work planning until formal DC arc flash standards are established.

What do they think about ETAP?

Customer perspectives

Generator brush collector rings do not fall under IEEE 1584. We must identify the factors impacting incident energy when brushes or rings fault during energized maintenance, and ETAP helps quantify both field discharge and regulator contributions.
Combining ETAP Transient Stability and DC Arc Flash modules allows us to estimate incident energy while accounting for real system characteristics and protective device timing.
By Mr. Zia Salami, Senior Management Specialist at CDM Smith


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Generator Brush Collector Ring’s Arc Flash Hazard, A Safety Concern!

For some types of generator exciter systems, collector rings and brushes are used to provide energy from the exciter to the rotating field. The brushes wear down and constantly need to be replaced with the unit on-line and while brush/ring assembly is still energized, a risky and unsafe maintenance operation and may cause a serious harm. A potential shock and arc-flash hazard. This presentation addresses the important and dangerous maintenance of worn-down collector rings and brushes, electrical safety, lack of standards and guidelines, and a first possible calculation method.


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