Medium Voltage Electrical Safety Tips | MV HV

Published 07 May 2020

MV and LV Electrical Substation Hub: Courtesy OSHA

• Guest Article By Sam Pherwani. P.Eng, CESCP Engineer, Instructor, Entrepreneur

Medium Voltage Electrical Safety

Five years+ of doing this, I’m still a student.

However been grateful to my customers for the exposure to windfarms, oil and gas, substations, pulp/paper meeting many fine electricians and learning from spirited discussions on how industry in US and Canada interprets Electrical Safe Work practices. Of course, NFPA 70E/NEC/CSA Z462 form the basis of Electrical Safety on this side of the pond.

These few pointers come out of experience in the Medium Voltage (per ANSI C84.1) Electrical Systems practically (2.4kV-69kV). Hope we see more of these best practices considered and frequently I might add, as the new world rolls into a better future.

1. Electrical Safety Footwear – Conventional Electrical Hazard rated Safety Boots most electrical workers have are rated to ASTM F2413-11 and CSA Z195-14 offer very little electrical shock protection, 600VAC or less if in wet conditions. Most well written MV electrical safety Risk Assessment forms (JHA/JSAs) call for secondary protection such as dielectric insulating mats or dielectric insulating safety overshoes but human error, inertia sometimes causes these steps overlooked. ASTM F1117 Class 2 AC EN 50321 dielectric boots now offer 17kV/20kVAC protection, comparable to Class E hard hats, making it a better fit for MV workplace.
2. Arc Rated Suits & Arc Flash Protective Clothing  – Until NFPA 70E-2015 the soft “do not exceed” threshold for unsafe energized work was 40 cal/cm2 of incident energy at working distance (3-4 feet, length of hot-stick away from equipment). Since then, PPE manufacturers making arc flash suits rated to as high as 140 cal/cm2 have influenced the standards to soften stance on that verboten verbiage. Their rationale – no OSHA recorded fatalities for arc blast at these higher cal ratings. My take – first of all, probably none of the mavens have experienced a 100 cal blast first hand from within the arc flash suit or PPE. Secondly, G-forces of even a 1.5 cal/cm2 blast could rattle human vitals like jello, permanently marring a victim with weakened resistance to other adverse forces soft or direct, possibly even a shorter lifespan if not an instant injury. Personally, I’d find a way to minimize high incident energy exposure regardless of the arc flash protective clothing.
3. Current Probe – So you want to work on MV Medium Volage electrical equipment (make modifications) … you de-energize the system, test for absence of voltage, and apply grounds, You troubleshoot with a voltage detector, not clearly distinguishing between nominal, stray, backfed or induced voltages. Would be nice to have a current meter clamped device hotstick mountable to gauge trace currents. Can’t find one? Until we do, here is one good at least for measuring current draw on troubleshooting. An amp-probe such as this one works well in tandem with a tick tracer. https://sensorlink.com/products/ampstik-plus. Consider adding one to your toolbox.
4. Rescue Hooks – OSHA 1910.269, 70E-110 are not clear on when rescue hooks or insulating sticks are to be used for shock mitigation. The urban legend of using a 2×4 lumber on an electrocution won’t die, the victim might, especially in an MV environment. In addition to switching bare overhead substation equipment, racking of breakers, 4160V MCCs justify use of rescue shepherd hooks. The second Qualified Electrical person should secure the worker with a rescue hook and correct body position, weight pulling outward passively. Many a site has a rescue hook sitting in a warehouse collecting dust, just for the fact nobody included it in their electrical safety program conditions.
5. Infrared Thermograhy – 70E/Z462 very clearly say equipment may not be in normal operation if equipment is not properly maintained. So you buy a maintenance contract (per NFPA 70B, CSA Z462, NERC) for peace of mind. You still could have a potential situation where a transformer or breaker is in imminent danger of failure owing to end of life, service fatigue, process upsets or transients. Infrared thermometers, prohibitively expensive 20 years ago are now very feasible tools, reasonable in price and are way preferable to contact troubleshooting methods. A big plus is predictive maintenance if baseline images/videos have been established. Care is required to use rated instruments in outdoor MV environments, outside restricted approach shock boundary.
6. Performing Ground Resistance Studies – You inherit a substation, so you’re only worried about above ground equipment maintenance right? Whats under the ground can hurt you … Copper theft, corrosion, increase of plant or station capacity from initial Ufer/ring ground design, improper grounding and bonding could result in fault currents light up the ground in ripples (step & touch potential hazards). Yes, those yellow sleeves on guy wires have a real purpose. NEC 250 requires 25 ohms or under as ground resistance for industrial sites but utilities, power generation consultants usually count five ohms or less as rule of thumb. Its a good idea every few years to perform a ground resistance test – three/four point method, preferably during a shutdown in vicinity of mains transformer.

Further Reading

• Electrical Safety – Arc Flash Accidents & Electrocution In LV-HV Installations
  
• Arc Flash Calculation – Selecting Clothing & PPE To Protect Lives Against Arc Hazard
• IEC 61482-2:2018 – Get Up To Speed With The New Arc Flash Standard
• Arc Flash PPE | 7 Top Considerations
• Arc Flash The Basics

Insulating Matting | Voltage Detectors | Substation Kits | Phase Comparators

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Thorne & Derrick are national distributors of LV, MV & HV Cable Installation, Jointing, Substation & Electrical Equipment – servicing businesses involved in cabling, jointing, substation, earthing, overhead line and electrical construction at LV, 11kV, 33kV, 66kV and EHV. Supplying a complete range of power cable accessories to support the installation and maintenance of low/medium and high voltage power systems:

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