by , on June 20, 2018
Danger High Voltage sign on chain-link fence at substation

First Things First: What is High Voltage?

When we examine industrial on-site electrical distribution systems, we immediately notice that a large majority of these sites distribute power throughout the facility with voltage levels exceeding 600 volts. This puts the site directly in line with OSHA 1910.269, which defines any voltages above 600 Volts as High Voltage. Before anyone says anything, I fully realize that most electrical professionals do NOT refer to every voltage above 600 as “High”, as most follow one of several formats. I was taught early on that the following breakdowns were the norm as per ANSI C84.1-1989:

  • 600 V and below – “Low Voltage,”
  • 600 V to 69 kV – “Medium Voltage,”
  • 69 kV-230 kV – “High Voltage”
  • 230 kV-1,100 kV – “Extra High Voltage”
  • 1,100 kV – “Ultra High Voltage”

Other breakdowns exist. According to the particular group you adhere to, such as IEEE or the National Electrical Code, the definition of high voltage changes. For the purposes of this article, we will turn to OSHA’s definition in 1910.269 (601V and higher), and look at the concept of Equipotential Grounding.

Equipotential Zone in 1910.269

This is what1910.269(n)(3)says:

Equipotential zone. Temporary protective grounds shall be placed at such locations and arranged in such a manner that the employer can demonstrate will prevent each employee from being exposed to hazardous differences in electric potential.”

A note refers the reader to Appendix C of this section.  The appendix refers to the requirement of establishing an Equipotential Grounding scenario. This will ensure that a properly-grounded job site will minimize any current flow through employees in the area utilizing the formula I=116/√t, where t is the duration of the current flow in seconds.

In a situation where the time factor is considered “unlimited”, anything more than 6 milliamps is considered unacceptable. In situations where upstream trip devices are likely to actuate, a current flow of 1 milliamp through an assumed value of 500 ohms is the maximum acceptable level.

OSHA assumes in 1910.269 that an electrical worker will represent a 500 ohm resistance. The 1 mA and 6 mA values above represent the current flow through an electrical worker who is unprotected from “involuntary muscle reaction due to shock”. These levels are all based upon proper design and installation of equipotential grounding prior to beginning work.

Feel free to read Appendix C for more details.

An Industrial 15kV Distribution System

Now let’s look at a typical industrial 15 kV distribution. Note here that we are discussing industrial-owned equipment and not utility-owned equipment (although safety principles are the same in both situations). A typical medium-sized industrial system may have the following:

  • (4) 1500 KVA transformers being fed from a local utility.
  • Underground cable vaults, direct-burial, or overhead lines to distribute the 15kV across the locations (No Tesla-based wireless transmission of power – yet!)

Eventually, the distribution of 15 kV power will feed other transformers, bringing the voltage levels down to a reasonable use level (4160 V for large motors, 480V for industrial plant distribution, etc.) Beginning at the company’s Point of Service, where utility ownership ceases and the company takes ownership, responsibility for electrical safety in accordance with 1910.269 also transfers to the company.

The same starting point of responsibility applies for OSHA standards 1910.331-335 as well as other applicable standards and “best practice” guidelines like NFPA 70E, the NESC, and applicable portions of OSHA 1910.269.

Switching of 15 kV and 4160V is routinely done on industrial sites. Lockout/tagout/verification of these systems must include grounding of all three phases in almost all cases per OSHA 1910.269. It is highly recommended at all times if feasible. One clear requirement in this same OSHA standard is the creation of an equipotential work zone.

Equipotential Rings

As an example, let’s look at a typical wooden power pole supporting 15 kV lines on your industrial property. This pole is 30 years old, and unfortunately, has seen very little maintenance over the years. As the insulators fail over time, more current begins to flow down the pole structure and into the surrounding soil. This failure from pole insulators will cause a voltage drop from the wooden pole to the surrounding soil, resulting in voltage shells (also known as voltage gradients). These gradients are affected by many conditions, including soil resistivity (which varies with soil condition), nearby buried metal objects, iron deposits, and a host of other factors.

Theoretically, if your site has uniform soil properties, about half of the source voltage drops within the first three feet of the defective wooden pole. This pattern continues every three feet beyond that first point, with the voltage dropping by one-half the previous value.

Often what we call 15 kV actually refers to system insulation ratings, when actual voltage may be 12.47kV, 13.2 kV, or 13.8 kV. In a 12.47kV, voltage to ground on a faulty pole may reach 7.2 kV. Following the 1/2 reduction pattern, a worker standing 3 feet from the pole with feet together would be exposed to 3.6 kV. If this worker stepped closer toward the pole or further away from the pole, he or she would experience a potentially lethal shock.

  • Equipotential Ring 1: 7.2 kV – 3.6 kV = 6 kV
  • Equipotential Ring 2: 3.6 kV – 1.8 kV=8 kV

The distance of every three feet from the pole would see this repetition, so at 9 feet from the source, employees would be subject to 900V. Keep in mind that any metal objects or structures in the area would also be subject to similar voltage gradients.

Note that this Rule of 3 is only a thumb rule, and that non-uniform soil can distort that pattern greatly. According to Temporary Grounding for Lineworker Protection by Alexander Publications, voltage gradient patterns can flatten out, stretching into oblong shapes that can present lethal voltages up to 50 feet from the source voltage.

Voltage Gradient

Let’s apply the same theory to a 12.47 kV metal-clad outdoor-rated switchgear. The gear was installed in the early 1970s, and the site has gone through multiple acquisitions. Historical data on switchgear maintenance, repairs, failures, etc. simply does not exist. Your company now owns the property. If this outdoor switch suffers a failure, a voltage gradient could form around the metal clad enclosure and surrounding area. The failure could be an internal insulator failure, a ground grid failure from lack of maintenance over time, or other potential failure mechanisms.

The gradient that forms around the switchgear would be similar to the equipotential rings around the wooden pole in the previous example. The voltage level would decrease the further away from the switchgear a person is standing. Again, though, be aware that if gradient levels flatten out, lethal voltages will be found at further distances.

Proper installation, maintenance, and testing are required to prevent these types of failures from creating voltage gradients. These procedures are very important, considering that these types of failures do not always yield enough current flow back to the source to trip a protective relay or blow an upstream fuse.

Proper Equipotential Zone Creation

The grounding requirements inside of section (n) go further than just saying, “If it’s grounded, it’s dead.” Proper EPZ creation requires a little more planning and knowledge than just hanging a ground cluster on that outdoor switch and jumping in to “git er done.”

The employer must ensure that electrical procedures and approved job plans are appropriate, clearly written, and followed. This is of tantamount importance on equipment greater than 600 volts. These practices ensure a safe electrical working space by creating a place that does not allow for voltage gradients or for voltage across equipment, adjacent structures, and the like.

The Problem

A prime example of this was found at a work site during an audit. Without going into too many details, picture a 12.47 kV outdoor metal-clad switch mounted on a concrete pad just the perfect size for the switch to sit on. Installed many years ago, the switchgear was obviously leaning to the left by at least 10 degrees. Over the years, the soil had shifted, and likewise, supply and feed cables had been stressed. No apparent ground cable emerged from the concrete pad to bond the metal clad structure to the concrete and its (hopefully) existent tie to the site’s ground grid (that also hopefully existed). No ground grid testing had been performed at the site for twenty-five years.

Let’s assume that your company assigns two electricians to go and throw this outdoor switch and lock/tag/verify/ground the incoming power for an upcoming weekend outage.  The questions for you as the employer are these: How do the electricians safely approach the switch, open the enclosure, operate the switchgear handle, and apply grounds? And even once grounds are applied, has the company fulfilled its requirements to create an equalized potential zone?

Remember, the concrete pad is only wide enough to fit the gear, placing the electricians standing on the grassy area in front of the enclosure. A potential issue already exists due to lack of equalized potential from the ground to the switchgear housing. This could be true even in normal operating conditions, and especially in conditions of deranged equipment as described above. Your electricians are well-trained and smart enough to take along a proximity detector and check for voltage on the equipment door handle and frame prior to touching the enclosure.

Finding none, they “safely” open the outdoor metal-clad door to access the switching mechanism handle. They keep their Class 2 voltage-rated gloves with protectors on, along with the other required shock and arc-flash-rated PPE. The electricians stand to the side, clear the area, and operate the handle to fully open to switch. They check the window for blade operation, which clearly indicates proper operation, and then proceed to open the inner door, exposing the energized conductors and terminations.

After verification with the proximity detector that the load side of the switch is in fact “dead,” they hang grounds by applying the grounding cluster to the bottom ground bar, and one leg to each phase. Upon completion, they barricade the area, hang a warning sign on the switch that states, “WARNING – SAFETY GROUNDS APPLIED,” and close the door to prevent moisture (possible rainfall, for example) from entering the enclosure. Viola – job well done! Right?

Some questions remain: has an equipotential zone been created? Can you as the employer ensure that this arrangement, once grounds are hung, creates a safe electrical work space? What if, during the outage, a decision is made to clean the bottom insulators of the 12.47 kV switch? What if non-electrical personnel are near this switch, performing yard maintenance on nearby trees and shrubs? The list could go on and on. The bottom line is this: the employer must ensure that an Equipotential Zone is created for every situation.

In this example, too many factors show a potential for a serious electrical hazard, thus increasing the risk of an unsafe working condition.  If anything, a false sense of security is given. With no obvious ground wire coming up from the bottom of the enclosure, what exactly did grounding accomplish? Does the concrete pad have a ground mesh? Does the surrounding area have a grid? Safety risks skyrocket if these unanswered questions are ignored.

Possible Solutions

What helps with proper Equipotential Zone creation? Let’s consider a few options:

  • Install a grounding mat– a portable or a permanent one. A grounding mat or metal grate is placed in front of outdoor equipment rated higher than 600 volts. It is bonded to the high-voltage equipment enclosure and grounding bar to make sure that both the mat and the equipment remain at the same potential, even in changing conditions.*

IEEE defines this ground mat concept as follows:

“a solid metallic plate or a system of closely spaced bare conductors that are connected to and often placed in shallow depths above a ground grid or elsewhere at the earth’s surface, in order to obtain an extra protective measure minimizing the danger of the exposure to high step or touch voltages in a critical operating area or places that are frequently used by people. Grounded metal gratings, placed on or above the soil surface, or wire mesh placed directly under surface material, are common forms of a ground mat.”

One note of caution when using grounding mats: the grounding mat simply shifts the area of step potential away from the equipment to the earth around the mat. Approaching and stepping on to a grounding mat presents step-potential issues if the equipment is experiencing a failure. This is why we always teach the “shuffle method” when approaching grounding mats. The shuffle method involves shuffling the feet without lifting them so that there is never more than 1/2 shoe length in front of the other shoe to ensure lethal voltages are not impressed across the worker’s feet. Some utilities instruct employees to start shuffling at no closer than ten feet away. Others mandate thirty feet from the grounding mat if not in a substation proper.

  • Dig an area around the switchgear ground grid and fill with rock. Just like in substation design, the area can be dug out, a proper local grounding grid installed and tested, and then the area is filled with rock to a depth specified in IEEE  Standard 80-2013 Guide for Safety in AC Substation Grounding. You find substations with rock as a top layer because of the high resistivity of rock or even gravel. IEEE 80-2013 gives a table that shows the resistivities of a variety of surface materials, including crushed granite with fines, washed #4 granite, asphalt and concrete. In most situations, this layer should be at least 4″ thick to provide appropriate resistivity to prevent voltage building up any greater than 50 V across a human in the area. With this setup, and with the ground grid tied to the switchgear ground bar, the OSHA-required safety grounding process mentioned above certainly enhances employee safety.
  • Re-pour the concrete pad so that it is large enough to allow personnel to stand on the same surface as the outdoor switchgear. Install an appropriate ground grid in the concrete pour and bond the grid to the switchgear and any nearby metal structure(s). This solution will effectively place the switch operator on the same electrical voltage plane as the equipment being operated.

Conclusion

Having traveled extensively throughout the United States, and performed audits in several countries abroad, I have seen some alarming electrical situations involving lack of proper grounding, bonding, and Equipotential Zone around equipment greater than 600 volts. It is imperative that we as electrical professionals understand and apply these concepts to all electrical installations. Doing anything less does the very thing I never want to do – put people at risk of electrical shock or electrocution. Ensuring a properly planned and executed creation of an equalized potential zone takes care of this issue, thus reducing employee risks to shock/electrocution hazards. As I always say at the end of my classes: be safe out there, and go home to your family to enjoy another day!

 

*If you need more information on grounding and bonding, you can read our blog post Why is Grounding and Bonding So Important? 


Ken Sellars
About author:
Ken Sellars is an instructor of electrical safety, NEC, Grounding/Bonding and Arc Flash Safety courses nationwide. Read more about Ken.

3 Comments on "Equipotential Grounding: Let’s Review"

Robert LeRoy - 25 June 2018 Reply

Well written Ken. Great job!

    Ken Sellars
    Ken Sellars - 26 June 2018 Reply

    Thanks so much, Bob, for the nice comment. Much appreciated. Safe travels, my friend!

JULIAN AGUILAR - 10 July 2018 Reply

Great information Ken.

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