Step Voltage
Step voltage, or step potential, is the difference in electric potential between two points on the ground approximately one meter apart, experienced by a person standing on energized ground with feet separated and not touching any grounded object. It arises when fault current flows into the earth, creating a ground potential rise and voltage gradient, potentially driving current through the body and causing electric shock.
On a manufacturing shop floor, step voltage hazards occur when fault current energizes the floor, yard, or surface, such as from downed overhead lines, damaged equipment, or high-voltage equipment yards. Engineering controls include low-resistance grounding grids, equipotential bonding, and surface treatments like crushed rock to limit gradients. Administrative controls involve minimum approach distances (e.g., 10 m from downed lines per BC Hydro) and emergency procedures requiring workers to shuffle or hop with feet together to minimize voltage difference. Training ensures workers recognize energized ground and apply correct movement techniques.
- Treating energized ground as only a contact hazard, ignoring step voltage risks from standing or walking near a fault.
- Improper movement in energized zones, such as running or striding, which increases step voltage; correct technique is shuffling or hopping with feet together.
- Inadequate grounding system design or testing for step voltage control, such as omitting IEEE 80 step and touch voltage analysis in industrial plants.
How is step voltage quantitatively defined and calculated (IEEE 80 context)?
Step voltage is defined as V_step = V(x) - V(x + 1 m), where V(x) is ground surface potential at distance x from the fault electrode. IEEE 80 computes ground potential rise (GPR) during a fault and surface potential distribution from the grounding grid, then compares calculated step voltages to allowable limits based on human tolerance, body weight, fault duration, and surface resistivity.
How do OSHA and WorkSafeBC treat step voltage in regulations and guidance?
OSHA 1910.269 Appendix C addresses step and touch potentials, requiring employers to ensure employees are at distances where step voltages are insufficient to cause injury. WorkSafeBC OHS Regulation Part 19 focuses on electrical safety and minimum approach distances, with practical guidance from BC Hydro and CASA defining step potential hazards and safe movement techniques.
What are typical safe thresholds for step voltage, and how do they relate to human physiology?
Engineering guides often treat step voltages above 40-50 V as potentially hazardous, depending on fault duration and conditions. IEEE 80 uses detailed formulas factoring body weight, shock duration, and surface resistivity to determine maximum allowable step voltage. High gradients near HV faults can produce thousands of volts over a step, driving dangerous currents that can cause ventricular fibrillation.