Effects of Shock
Hazards of human exposure to electricity depends more on the current than the voltage. The current resulting from a given voltage depends on the resistance of the body - typically tens of thousands of ohms for unbroken, dry skin. Can be as low as one thousand ohms if the skin is punctured or wet.
Current values (milliamperes)
Effects AC 25 Hz to 400 Hz DC
Perception 0-1 0-4
Surprise 1-4 4-15
Reflex action 4-21 15-80
Muscular inhibition 21-40 80-160
Respiratory block 40-100 160-300
Usually fatal Over 100 Over 300
Mil-Std-454 TABLE 1-I. Probable effects of shock.
Current values (milliamperes)
Effects AC 25 Hz to 400 Hz DC
Perception 0-1 0-4
Surprise 1-4 4-15
Reflex action 4-21 15-80
Muscular inhibition 21-40 80-160
Respiratory block 40-100 160-300
Usually fatal Over 100 Over 300
Mil-Std-454 TABLE 1-I. Probable effects of shock.
Other Hazards
Reaction hazard
Caused by involuntary reaction to a shock. Not a direct problem if the current is low, but can be a safety problem if reaction causes a hazard (striking, falling, dropping, losing control).Mitigations: Lockout/Tagout, proper grounding design.OSHA Lockout/Tagout. Also requirements for a Job Hazard Analysis (OSHA). Requirements do protect against contact with voltages over ~50 volts.Stray voltages caused by large ground currents can be a source of exposure. (Large ground currents can be caused by fault conditions or utility problems.)
Shock hazard
Caused by driving enough current through the body to cause ventricular fibrillation or tissue damage resulting in injury or death. Typically takes 10 or more milliamps driven through the body to start causing problems.Typical mitigations: Use as low a voltage as possible. Prevent exposure with distance, insulation, grounded enclosure, interlocks or active detection systems (GFCIs). NFPA’s NEC and IEEE’s NESC provide most of the requirements. OSHA picks up requirements from both and from NFPA 70E. NEC requires grounded enclosures (conduit and boxes) to provide protection in normal and faulted conditions.
Requirements include: PPE (esp. high voltage), prevent contact, provide warning signs, training. Also Lockout/Tagout requirements (OSHA and KNPR 8715.4 KSC Lockout/Tagout Program Procedural Requirements). NEC has requirements for grounding, enclosing, protecting etc. and for GFCIs in specified locations. Separate requirements for specific locations (like lighting near pools, disconnects for air conditioning units) NESC takes over above 600 v.
Arc-flash
Explosion or arc (plasma at several thousand degrees) and spark (molten metal particles) caused by short circuit or the opening of a high voltage circuit. Can cause burns or start fires.Mitigations: De-energize, Barriers, PPE.Requirements: NFPA and OSHA have requirements for training and PPE. Five levels of PPE based on possible energy release.
Main Requirements
NFPA
National Electrical Code (NFPA 70) – Covers buildings divided by “occupancy”.
Standard for Electrical Safety in the Workplace (NFPA 70E) – Written for OSHA.
IEEE National Electrical Safety Code – Covers work done by linemen and electricians.
OSHA Subpart S – Workplace safety; picks up parts of NFPA 70, 70E and NESC.NASA NPR 8715.3 section 3.6 requires NFPA 70, Mil-Std-454 or Center-Specific requirements.
UL, ANSI and NEMA have lower-level standards for components, warning signs and enclosures.
Medical devices have their own standards with actual current limits.
Caused by involuntary reaction to a shock. Not a direct problem if the current is low, but can be a safety problem if reaction causes a hazard (striking, falling, dropping, losing control).Mitigations: Lockout/Tagout, proper grounding design.OSHA Lockout/Tagout. Also requirements for a Job Hazard Analysis (OSHA). Requirements do protect against contact with voltages over ~50 volts.Stray voltages caused by large ground currents can be a source of exposure. (Large ground currents can be caused by fault conditions or utility problems.)
Shock hazard
Caused by driving enough current through the body to cause ventricular fibrillation or tissue damage resulting in injury or death. Typically takes 10 or more milliamps driven through the body to start causing problems.Typical mitigations: Use as low a voltage as possible. Prevent exposure with distance, insulation, grounded enclosure, interlocks or active detection systems (GFCIs). NFPA’s NEC and IEEE’s NESC provide most of the requirements. OSHA picks up requirements from both and from NFPA 70E. NEC requires grounded enclosures (conduit and boxes) to provide protection in normal and faulted conditions.
Requirements include: PPE (esp. high voltage), prevent contact, provide warning signs, training. Also Lockout/Tagout requirements (OSHA and KNPR 8715.4 KSC Lockout/Tagout Program Procedural Requirements). NEC has requirements for grounding, enclosing, protecting etc. and for GFCIs in specified locations. Separate requirements for specific locations (like lighting near pools, disconnects for air conditioning units) NESC takes over above 600 v.
Arc-flash
Explosion or arc (plasma at several thousand degrees) and spark (molten metal particles) caused by short circuit or the opening of a high voltage circuit. Can cause burns or start fires.Mitigations: De-energize, Barriers, PPE.Requirements: NFPA and OSHA have requirements for training and PPE. Five levels of PPE based on possible energy release.
Main Requirements
NFPA
National Electrical Code (NFPA 70) – Covers buildings divided by “occupancy”.
Standard for Electrical Safety in the Workplace (NFPA 70E) – Written for OSHA.
IEEE National Electrical Safety Code – Covers work done by linemen and electricians.
OSHA Subpart S – Workplace safety; picks up parts of NFPA 70, 70E and NESC.NASA NPR 8715.3 section 3.6 requires NFPA 70, Mil-Std-454 or Center-Specific requirements.
UL, ANSI and NEMA have lower-level standards for components, warning signs and enclosures.
Medical devices have their own standards with actual current limits.
