What You Need to Know About Arc Flash

Overview

The paper frames the hazard through incident energy, the NFPA 70E / IEEE 1584 framework and labelling. NFPA 70E 130.5(G) requires the incident energy analysis to be updated at least every 5 years. The arc-flash boundary is where incident energy equals 1.2 cal/cm2, and labels are required on equipment over 50 V that may be accessed energised. Single-phase-to-ground faults are 7.8 (junctions) to 73 (cable) times more likely than three-phase faults (IEEE 493-2007), but IEEE 1584-2002 gives no method for them.

Mitigation is grouped into reducing available fault current (open tie during maintenance, smaller or higher-impedance transformers, high-resistance grounding, current-limiting devices), shortening clearing time (zone-selective interlocking, bus differential, maintenance-mode settings, arc-quenching switchgear), remote operation, predictive maintenance and redirecting blast energy. Opening the tie between dual feeds during maintenance roughly halves available fault current.

Eaton recommends separate line-side and load-side calculations for the main breaker, because line-side-only values can exceed 40 cal/cm2 and wrongly bar energised work. Main breakers can take up to 4 cycles (67 ms) to clear, while arc-quenching switchgear contains an arc in under 4 ms.

NEC prohibits high-resistance grounding on systems serving line-to-neutral loads such as most servers, which limits HRG to motor-only portions.

Key points

1. NFPA 70E 130.5(G): the incident energy analysis must be updated at least every 5 years.
2. Single-phase-to-ground faults are 7.8 (junctions) to 73 (cable) times more likely than three-phase faults (IEEE 493-2007); IEEE 1584-2002 gives no method.
3. The arc-flash boundary is where incident energy equals 1.2 cal/cm2; labels are required on equipment over 50 V accessed energised.
4. Eaton recommends separate line-side and load-side calculations for the main breaker; line-side-only values can exceed 40 cal/cm2 and wrongly bar energised work.
5. Opening the tie between dual feeds during maintenance roughly halves available fault current.
6. NEC prohibits HRG on systems serving line-to-neutral loads such as most servers; HRG is limited to motor-only portions.
7. Main breakers can take up to 4 cycles (67 ms) to clear; arc-quenching switchgear contains an arc in under 4 ms.

Using it in design

• Add arc-flash attributes per switchboard (available fault current, clearing time, maintenance-mode/ZSI availability, working distance) and compute incident energy per IEEE 1584 for line and load side.
• Warn when incident energy exceeds 40 cal/cm2 or when breakers above the threshold ratings lack energy-reducing maintenance switching.
• Prevent HRG selection on boards feeding line-to-neutral IT loads.
• Flag closed-tie (double-ended) maintenance scenarios and generate label data.

Who should read it

Electrical engineers designing boards and maintenance procedures, and operational safety managers.

Related standards

• NFPA 70E
• IEEE 1584
• IEEE 493
• NEC 240.87
• OSHA 29 CFR 1910.269

Publication details

Authors

Adam Reeves; David G. Loucks

Publisher

Eaton

Year

2018

Document number

WP083033EN

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation