The two main hazards
Shock
Current through the body. At 480 V, contact can cause fatal ventricular fibrillation or deep burns. Shock risk is managed with approach boundaries, insulated tools and rubber insulating gloves.
Arc flash and arc blast
A fault through air creates an arc that can reach several times the temperature of the sun's surface, along with pressure waves, molten metal and shrapnel. Three-phase systems with high available fault current can release very high incident energy, measured in cal/cm².
Hierarchy of risk control
- Elimination: de-energize and establish an electrically safe work condition.
- Substitution: e.g., lower-energy designs or current-limiting devices.
- Engineering controls: remote racking, arc-resistant gear, maintenance switches.
- Awareness: labels, signs and barricades.
- Administrative controls: procedures, permits, training.
- PPE: the last line of defense.
Establishing an electrically safe work condition
- Identify all sources of supply using up-to-date drawings, including backfeeds and generators.
- Interrupt load current, then open the disconnecting device.
- Visually verify the blades are open or the drawout breaker is withdrawn, where possible.
- Apply lockout/tagout devices (OSHA 1910.147 / 1910.333).
- Test for absence of voltage on each phase, phase-to-phase and phase-to-ground, with a meter verified on a known source before and after.
- Apply temporary protective grounds where induced voltage or stored energy could be present.
Approach boundaries
| Nominal voltage | Limited (exposed movable conductor) | Limited (fixed part) | Restricted |
|---|---|---|---|
| 50–150 V | 10 ft 0 in | 3 ft 6 in | Avoid contact |
| 151–750 V | 10 ft 0 in | 3 ft 6 in | 1 ft 0 in |
The arc flash boundary is calculated separately. It's the distance at which incident energy equals 1.2 cal/cm², and it often extends beyond the shock boundaries. Always check the current edition of NFPA 70E and the equipment label.
PPE basics
| Arc flash PPE category | Minimum arc rating | Typical items |
|---|---|---|
| 1 | 4 cal/cm² | AR shirt and pants or coverall, face shield, hard hat, safety glasses, hearing protection, leather gloves |
| 2 | 8 cal/cm² | As above, plus AR balaclava or hood |
| 3 | 25 cal/cm² | AR flash suit, hood, gloves |
| 4 | 40 cal/cm² | AR flash suit, hood, gloves |
Use either the incident energy analysis on the equipment label or the PPE category tables, not both. Rubber insulating gloves with leather protectors are chosen by voltage class and tested on a schedule.
Grounding and standards
Proper system grounding lets protective devices clear faults quickly. See Grounding Three-Phase Systems. For the codes that apply (NEC, NFPA 70E, OSHA, IEEE), read Standards & Regulations.