Electrical Safety
The rules that govern the work
Electrical safety questions come from two sources: OSHA (29 CFR 1910 Subpart S and 1926 Subpart K) and NFPA 70E for work practices. Know the three boundaries on the 70E approach chart. The limited approach boundary is the line only qualified persons may cross. The restricted approach boundary requires shock protection such as insulated gloves and tools. The arc flash boundary is a separate thermal limit, not a shock limit, and questions routinely blur the three.
Core practices
- De-energize and verify. Lockout/tagout follows 1910.147 and uses a meter rated for the available fault current (CAT III or IV). Test before you touch.
- GFCI protection trips at 5 mA plus or minus 1 mA (4 to 6 mA), far below the 100 mA that can cause ventricular fibrillation. A GFCI does not protect against line-to-line contact or series arcs.
- PPE is chosen from an incident-energy analysis or the 70E tables. Arc-rated clothing and face protection, not just safety glasses.
- Assured equipment grounding conductor program is the permitted alternative to GFCI protection on construction sites.
Rule of thumb
50 volts is the shock-hazard threshold and 5 mA is the GFCI trip level. If a live part must be exposed, the question is almost always about boundaries, PPE, or lockout, not about being careful.
Worked example
An unqualified helper must hand a tool to a qualified worker at a 480 V motor control center. Because the helper is unqualified, the limited approach boundary (3 ft 6 in for fixed 480 V parts) cannot be crossed. Correct answer: set the tool down outside the boundary rather than putting on gloves.