Preventing arc flash in mission critical facilities
Consulting-Specifying Engineer · Kenneth Kutsmeda
Kenneth Kutsmeda explains why data centres, with large UPSs, big transformers and closed-transition generator paralleling, tend to have high available fault current and arc-flash exposure. He identifies arcing time as the dominant variable and reviews mitigation options.
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Overview
The article appeared in Consulting-Specifying Engineer in 2016. Kutsmeda explains that data centres, with large UPSs, big transformers and closed-transition generator paralleling, tend to have high available fault current and arc-flash exposure.
The author stresses that arcing time is the variable that dominates arc energy. Per IEEE 1584-2002, incident energy depends on arcing current, working distance and arcing time; energy is linear with duration, so clearing time is the key lever. Disabled or delayed instantaneous trips can push clearing to about 2 seconds in the time-delay region.
Mitigation options include revisiting coordination settings, electronic trip units, zone-selective interlocking (ZSI), differential protection, optical sensing, arc-resistant gear, maintenance-mode switches and remote operation. ZSI is recommended for arc-flash mitigation only on UL 1558 switchgear with full section barriers. NEC 240.87 (2014) also requires arc energy reduction on overcurrent devices rated 1,200 A or higher.
In a 14 MW facility case study, a white-space RPP was reduced from 17 cal/cm2 (Level 3 PPE, 91 in. boundary) to 0.17 cal/cm2 (Level 0) by replacing a fixed thermal-magnetic main with a lower-instantaneous breaker, without losing coordination (selectivity). The design recommendations include a preliminary arc-flash study during design and a bypass source so that portions can be de-energized for maintenance.
Key points
- Incident energy depends on arcing current, working distance and arcing time; per IEEE 1584-2002 energy is linear with duration.
- Disabled or delayed instantaneous trips can push clearing to about 2 s in the time-delay region.
- NEC 240.87 (2014) requires arc energy reduction on overcurrent devices rated 1,200 A or higher.
- ZSI is recommended for arc-flash mitigation only on UL 1558 switchgear with full section barriers.
- Case study: white-space RPP reduced from 17 cal/cm2 (Level 3 PPE, 91 in. boundary) to 0.17 cal/cm2 (Level 0).
- The reduction came from a lower-instantaneous breaker, with coordination retained.
- Do a preliminary arc-flash study during design and provide a bypass source for maintenance.
Using it in design
- Run a preliminary arc-flash estimate at design stage and flag white-space panels above about 1.2 cal/cm2 or above PPE category 2.
- Require arc-energy reduction (ERMS/ZSI/differential) on breakers of 1,200 A or more.
- Prefer electrical galleries over white-space placement of high-energy gear.
- Provide a bypass source so portions can be de-energized for maintenance.
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Who should read it
Engineers designing data center power distribution and protection, and safety and operations managers.
Related standards
- NFPA 70E-2015
- IEEE 1584
- NFPA 70 Art. 240.87
- UL 1558
Publication details
- Authors
- Kenneth Kutsmeda
- Publisher
- Consulting-Specifying Engineer
- Year
- 2016
- Document language
- English
- Source type
- Expert / community
- Access
- Free to read
- Licence
- Copyright retained; citation only — the publisher’s terms apply to the original document
Citation
Kenneth Kutsmeda (2016). Preventing arc flash in mission critical facilities. Consulting-Specifying Engineer. https://www.csemag.com/preventing-arc-flash-in-mission-critical-facilities/
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