■ Glossary
Glossary.
Definitions from NFPA 70E Article 100 (2027 edition), plus the data center and regulatory terms that surround them. Where a term changed in 2027, it is marked.
§ Electrical safety
| Term | Meaning |
|---|---|
| Arc flash boundary | The approach limit from an arc source at which incident energy equals 1.2 cal/cm² (5 J/cm²) — the onset of a second-degree burn on bare skin |
| Arc rating | The value attributed to materials describing their performance on exposure to an electric arc discharge, in cal/cm², derived from ATPV or EBT, whichever is lower |
| Arc flash suit | A complete arc-rated clothing and equipment system covering the entire body except hands and feet |
| Balaclava | Arc-rated head-protective fabric protecting the neck and head except a small portion of the facial area |
| Contact thermal hazard NEW 2027 | The risk of burn injury from touching an energized or overheated surface. Distinct from arc flash and from shock. Triggers thermal hand protection at 130.7(C)(7)(e) |
| De-energized | Free from any electrical connection to a source of potential difference and from electrical charge; not having a potential different from that of the earth |
| Electrically safe work condition (ESWC) | A state in which the conductor or circuit part has been disconnected from energized parts, locked/tagged, tested to verify the absence of voltage, and grounded if determined necessary |
| Incident energy | The amount of thermal energy impressed on a surface, at a certain distance from the source, generated during an electrical arc event. Expressed in cal/cm² |
| Limited approach boundary | An approach limit at a distance from an exposed energized conductor or circuit part within which a shock hazard exists |
| Restricted approach boundary | An approach limit at a distance from an exposed energized conductor or circuit part within which there is an increased likelihood of electric shock due to arc-over combined with inadvertent movement |
| Qualified person | One who has demonstrated skills and knowledge related to the construction and operation of the electrical equipment and installations, and has received safety training to identify and avoid the hazards involved. 2027 restores explicit task- and equipment-specific language |
| Unqualified person | A person who is not a qualified person |
| Safeguarding NEW 2027 | Means of protecting persons from exposure to energized conductors |
| Shorting stick NEW 2027 | A device used to dissipate stored charge before contact work |
| Working distance | The distance between a person's face and chest area and a prospective arc source. Incident energy values are stated at a working distance |
| Competent person REMOVED 2027 | Deleted from Article 100 because it conflicted with OSHA's construction-context use. Only qualified and unqualified person remain |
§ Data center
| Term | Meaning |
|---|---|
| 2N | Fully duplicated infrastructure — two complete, independent paths. Every load is fed from both |
| N+1 | One more unit than the minimum required to carry the load |
| Concurrent maintainability | The ability to remove any single capacity component or distribution path from service for maintenance without impacting the critical load. A reliability property — not a safety justification |
| Fault tolerance | The ability to sustain an unplanned failure of any single component without impacting the critical load |
| White space | The data hall — the room containing IT equipment |
| Grey space | The support infrastructure — electrical rooms, UPS rooms, mechanical plant |
| PDU / RPP | Power distribution unit / remote power panel — the distribution between the UPS output and the racks |
| Busway | Overhead distribution with removable tap boxes, common in modern halls |
| Static bypass | A UPS path that transfers load to the bypass source automatically on module failure. Energized independent of the module |
| Maintenance bypass | A manual path around the UPS for servicing. Does not de-energize the module input or the DC bus |
| BESS | Battery energy storage system. Governed by NEC Art. 706, NFPA 855 and 70E Art. 360 |
| Commissioning L1–L5 | Factory witness, installation verification, pre-functional static, functional performance, integrated systems testing |
| DCOA | Designated critical operations area — the trigger for NEC Article 708 |
§ Regulatory
| Term | Meaning |
|---|---|
| AHJ | Authority having jurisdiction — the organisation or individual responsible for enforcing code requirements or approving equipment and installations |
| General Duty Clause | OSH Act Section 5(a)(1). Requires employers to furnish employment free from recognized hazards. How OSHA uses it → |
| Consensus standard | A standard developed by a recognized body through a consensus process. Not law unless adopted, but establishes industry recognition |
| Multi-employer worksite | OSHA policy under which creating, exposing, correcting and controlling employers may each be cited |
| Host employer | The employer who owns or operates the facility where contract employees work — 110.5 |
| Contract employer | An employer whose employees perform work at a host employer's facility |
| Recordable / reportable | Recordable under 1904; reportable means fatality within 8 hours, or hospitalization, amputation or loss of an eye within 24 hours |
Terms in context, not on a page
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