Ground Potential Rise
Ground Potential Rise (GPR) is the transient increase in local earth voltage at a faulted installation relative to remote earth, caused by fault current flowing through the grounding electrode impedance. It is calculated as GPR = Z_grid × I_fault, where Z_grid is the ground grid impedance to remote earth and I_fault is the fault current. GPR creates hazardous step and touch voltages, requiring engineering controls per IEEE 80 and OSHA 1910.269.
On the shop floor, GPR hazards arise from high-voltage faults at substations or plant distribution systems, raising the entire local ground grid potential. Workers touching grounded structures or standing on surfaces with potential gradients face step and touch voltage risks. In mining and heavy industrial sites, CSA M421 limits GPR at mobile equipment to 100 V, requiring low-impedance bonding and insulating joints on metallic services. OSHA mandates equipotential zones and protective grounding to minimize potential differences. Practical controls include bonding all metallic structures, using insulating mats, and ensuring fast fault clearing. Communication cables entering GPR zones need isolation and surge protection.
How is GPR quantitatively evaluated for a substation or industrial plant, and how does that tie into OSHA 1910.269?
GPR evaluation follows IEEE Std 80 and 367: determine single line-to-ground fault current, calculate ground grid impedance to remote earth, compute GPR_max = Z_grid × I_fault, model surface potential distribution, and compare step/touch voltages with allowable limits. OSHA 1910.269 Appendix C requires grounding to minimize potential differences in work areas, making GPR analysis essential for identifying and mitigating step/touch hazards in HV workplaces.
What does CSA M421's 'GPR and transfer of potential at moveable or mobile equipment shall not exceed 100 V' practically mean?
It means during a worst-case ground fault, the potential difference between mobile equipment frames and earth or other conductive objects a worker can touch must not exceed 100 V. This is achieved by bonding equipment to the local grid with short, low-resistance leads, using insulating joints on metallic services leaving the zone, and implementing touch voltage protection systems. Failure to comply is a non-conformance with CSA M421 and a potential WorkSafeBC violation.
How are step and touch voltage limits derived and how do they interact with GPR in risk assessments?
Limits are derived from body resistance models, acceptable current/time exposure curves, soil resistivity, and fault clearing time per IEEE 80. GPR sets the upper bound of station potential, while the surface potential distribution determines step/touch voltages at worker locations. A well-designed grid can keep touch voltages safe even with high GPR. Risk assessments should confirm GPR/step-touch studies exist and map controls like fencing, bonding, and PPE to WorkSafeBC electrical safety expectations.