Touch Voltage
Touch voltage is the potential difference between simultaneously accessible conductive parts that a person can contact during an earth fault, typically between an energized metallic enclosure and ground or another earthed part. In OSHA/WorkSafeBC context, it is often called touch potential: the voltage between an energized grounded object and the feet of a person in contact with that object. Standards like IEC 61140 set permissible limits, e.g., 50 V AC in dry conditions and 25 V AC in wet locations.
On a manufacturing shop floor, touch voltage is critical for assessing electric shock risk from equipment frames, enclosures, and bonded structures during faults. In TN systems, touch voltage is calculated as U_t = I_f × R_PE, where I_f is fault current and R_PE is protective conductor resistance. Designers use this to size conductors and ensure fault clearing times keep touch voltage within safety curves (e.g., below 50 V). Ground grid studies per IEEE 80 calculate touch and step voltages to verify safe earthing. Day-to-day controls include equipment grounding verification, bonding of all metal parts, inspection of wet locations (where limits drop to 25 V), and use of GFCI/RCD protection. Lockout/tagout procedures recognize that energized grounded parts may present hazardous touch potential, requiring insulating gloves and mats. For new installations, step and touch voltage analyses around MCCs, transformers, and outdoor equipment ensure touch voltages remain below conventional limits for expected fault duration.
How do I quantify touch voltage for a machine on a TN system in a shop?
For a low-voltage TN system, assume a line-to-frame fault. Touch voltage U_t at the frame relative to the worker's feet is approximated by U_t = I_f × R_PE, where I_f is earth fault current and R_PE is protective earth conductor resistance. More detailed models include source impedance and soil/grid impedance. Compare calculated U_t to permissible limits (e.g., 50 V for dry conditions, lower for wet) and ensure automatic disconnection time is short enough to keep body current below fibrillation thresholds.
How is touch voltage different from step voltage in OSHA/IEEE terms?
Step potential is voltage between the feet of a person standing near an energized grounded object, related to surface potential gradient. Touch potential is voltage between the energized grounded object and the feet of a person in contact with that object. IEEE and OSHA emphasize that touch potential can be nearly the full voltage across the grounded object if grounding is remote. On the shop floor, step voltage matters near ground faults in earth grids, while touch voltage is critical at equipment frames, enclosures, and structures workers physically touch.
What limits or criteria should I use when evaluating touch voltage for compliance?
While WorkSafeBC Part 19 does not specify explicit touch voltage limits, engineering practice references IEC 61140: 50 V AC in dry conditions, 25 V AC in wet locations, and lower values for high-risk environments. NFPA 70E and IEEE 80 treat 50 V and above as hazardous, requiring controls. For SafeDesk-aligned programs, treat any condition where a worker could experience ≥50 V between hand and feet or between two metal parts as a significant shock hazard, with more conservative thresholds (25 V) in wet or conductive floor areas.
What engineering controls directly reduce touch voltage risk on the shop floor?
Key controls include robust equipment grounding and equipotential bonding to minimize R_PE and equalize potentials; rapid automatic disconnection using properly rated breakers and ground-fault protection; ground grid design and verification per IEEE 80 to keep touch voltage below limits; use of insulating barriers and PPE like mats and gloves to break the hand-to-foot path; and environmental control to manage water, conductive dust, and corrosion that reduce body impedance.
How should SafeDesk-style procedures integrate touch voltage into WorkSafeBC compliance?
Integrate touch potential scenarios in hazard assessments, evaluating where workers touch metal enclosures or structures while standing on concrete or metal. Document grounding/bonding inspections as critical controls for touch voltage. Align internal design criteria with IEC/NFPA/IEEE limits (50 V/25 V thresholds). Train workers on touch potential, explaining that any conductive object contacting an energized line or faulted circuit can create dangerous touch voltage, as highlighted by BC Construction Safety Alliance examples.