Why is Developing an Equipotential Zone Critical for Effective TPG? Part 1

  • Post comments:0 Comments
Why is Developing an Equipotential Zone Critical for Effective TPG? Part 1

Introduction to EPZ

The subject of temporary protective grounding (TPG) often comes up during my high voltage and electric utility classes by students who are interested in exactly how standard TPG differs from ‘EPZ’ grounding.  EPZ is an acronym for ‘equipotential zone’, and, truthfully, there isn’t a difference between the two because they’re inseparable.

Because the topic of temporary protective grounding and, especially, establishing an equipotential zone is vast and expansive, we’ll break the articles up into a multi-part series. This will allow us to build our understanding based on the previously presented materials without overwhelming the reader with one huge paper.

Later in this Part 1, we’ll cover the federal regulations, standards and codes governing it.  Subsequent parts will proceed into a little history for temporary protective grounding; some of the technical aspects surrounding TPG, especially EPZ; then finally merge an event in my switchyard to show the parallels which should help you grasp the fundamentals of EPZ grounding and its critical parameters for keeping electrical workers safe.

TPG – the Final Step of Establishing an Electrically Safe Work Condition

For those who may not be acquainted with temporary protective grounding, it’s the vitally important last step when establishing an electrically safe work condition (ESWC) according to NFPA 70E®, article 120.5(8). It is also mandated by OSHA 1910.269(n)

TPG consists of specially designed cables, equipment and hardware that’s intentionally installed to the lines, bus segments or circuit parts which are energized during normal operation and used mainly with high voltage systems of greater than 600 volts.  Due to a variety of factors which we’ll cover later, TPGs are essential to worker safety if the “deenergized” circuit or equipment inadvertently becomes reenergized while workers are in contact with it.

EPZ as Related to Live Line Bare Hand Work

Many of my students are very familiar with temporary grounding, but when the term equipotential zone is mentioned, then the confused looks on faces abound. Therefore, to help readers gain a better grasp of EPZ grounding, I’ll relate key principles and elements needed during Live Line Bare Hand Work (LLBHW) – critical in keeping linemen from being electrocuted when touching energized lines at hundreds of thousands of volts –  with some of the same crucial attributes necessary when establishing an EPZ with temporary protective grounding.

Short Definition of Live Line Bare Hand Work

We’ll cover LLBHW in greater detail later, but here’s a quick explanation of what it is.  Sometimes called “bare hand work,” this activity is a very specialized work practice that’s limited only to overhead high voltage transmission lines, where highly trained linemen intentionally make physical contact with one phase of power lines and equipment energized at hundreds of thousands of volts. Some of you may have seen videos of linemen in special conductive suits touching or even climbing on top of energized power lines; that is LLBHW. But keep in mind, I stressed “one phase,” which will become more apparent as we progress through this article.

Temporary Protective Grounding in More Detail

The parallel analogy I’ll be sharing to support the efficacy of EPZ grounding is based on a close-call event that occurred inside the 525kV switchyard at my power plant in 2018 during the preparation for LLBHW that unexpectedly and catastrophically failed. Fortunately, it resulted in no injuries. 

So, was it a miracle or “dumb luck” that all workers involved with this near-death incident walked away without injury, or are there hidden factors, based on proven electrical theory, that assured their survival?  And can we take applicable learnings from this event and use them to help us effectively establish an EPZ during temporary protective grounding?  The unequivocal answer is yes. 

Some of you may be scratching your heads right about now trying to figure out how LLBHW on energized high voltage lines is somehow related to working on the same lines and equipment while deenergized and grounded.  This is a fair question, which I’ll be able to fully answer as we progress through this article.

For those who work outside of the electric utility sector, the use of temporary protective grounding may be somewhat unfamiliar territory.  On the other hand, linemen and electricians working for electric utilities are quite knowledgeable with its importance and practice.  In fact, a common saying in the trade is, “If It’s Not Grounded – It’s Not Dead.”  While true, it is only partially accurate because a more applicable statement would be, “If It’s Not Grounded Correctly – It’s Not Safe.” 

Training received by these utility workers for temporary protective grounding is sometimes inaccurate and based on outdated principles which leads to field practices contrary to OSHA regulations and, more importantly, compromises their safety.  The vast majority are very accustomed with using TPGs daily, but a good number are not well versed in the OSHA requirement, “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,” better known as equipotential zone (EPZ) grounding.

Fundamentally, applying TPG is very similar in concept with grounding and bonding installations as required by the National Electric Code® (NEC) used to permanently connect all non-current-carrying metallic parts together, such as the steel housing of panelboards, switchgear and motors.

The inside wiremen reading this article are very familiar with the many facets of Article 250 – Grounding and Bonding.  They understand the practice of grounding and bonding of external metal parts ensures safety if any non-current metal parts inadvertently become energized, e.g., when the wire’s insulation inside a motor termination box has been compromised due to vibration.  Once the insulation is rubbed through and the copper conductor contacts the housing, a significant ground fault follows.  If installed correctly, fault current is provided with many low impedance paths, one being the equipment grounding conductor (EGC), back to its source, quickly actuating the over current protection device (OCPD), such as breakers, fuses or protective relaying.  But if the motor housing wasn’t correctly grounded and bonded, the frame would become energized at dangerous voltages, waiting to injure an unsuspecting worker who touches it.

But the similarities between general grounding and bonding and TPG tend to end there because temporary protective grounding, or “safety grounds” as its name implies, is installed on a short-term basis and is intentionally attached to parts or conductors, e.g., lines, bus sections, etc., which are normally energized at full system voltage as the “normal current-carrying parts.”  TPG’s sole purpose is to allow work on these de-energized high voltage lines and equipment to be done safely. But, unlike standard grounding and bonding, if the line or equipment should become unexpectedly re-energized, electric current is offered only one to three paths back to its source to trip the OCPD and clear the fault, consisting of only the TPG cables and connectors.

Side Note

An intriguing side fact related to the term “equipotential zone” in context with temporary protective grounding is, interestingly, found in the NFPA 70® National Electrical Code.®   The NEC contains a few provisions where the term “Equipotential Plane” is applied, which is identical in concept to an EPZ. But it is not found in Article 250 as one would guess.  It’s defined in Article 100 and is employed by Articles 547.10 for livestock structures, 680.26 for swimming pools and 682.33 for electrical equipment installed near bodies of water to minimize step and touch potential hazards. While the NEC’s equipotential requirements are associated with permanent installations during initial construction rather than for temporary reasons, the similarities are fascinating nonetheless.

Regulations, Standards and Codes


From a regulatory position, temporary protective grounding for the protection of employees is mandated by OSHA Title 29 Code of Federal Regulations (CFR), primarily for the transmission, distribution and generation of electrical energy.  Subpart R, 1910.269(n) applies to general industry during maintenance and operations for all three sectors while Subpart V, 1926.962 applies to new construction of transmission and distribution systems but excludes generation in its application.

However, for companies that mainly fall under the scope of “electric utilization equipment” (1910 Subpart S for general industry), 1910.333(c)(3) has very little information regarding TPGs other than a brief statement when work is performed “near overhead lines.”  Interestingly, the note proceeding the overhead lines points back to 1910.269 for the temporary grounding practices of the lines.  The parallel construction standard 1926 Subpart K has no information related to TPGs.

NFPA 70E® and NESC®

Fortunately, NFPA 70E®, Standard for Electrical Safety in the Workplace applies to both operational facilities and those under construction, and can be used by employers with detailed instructions when installing temporary protective grounding in facilities and industries outside of electric utilities. 

But, unlike the OSHA requirements, 70E’s temporary grounding is somewhat conditional if “the possibility of induced voltages or stored energy exists or the conductors being deenergized could contact other exposed energized parts”.  Apart from this small conditional statement, the capacity criteria and impedance characteristics of temporary protective grounding devices under NFPA 70E® is identical to those in OSHA 1910.269(n).

Furthermore, the National Electric Safety Code® (NESC) C2-20117, Section 44, Rule 444 “De-energizing equipment or line to protect employee” also contains valuable information pertaining to temporary protective grounding.

For most employers, NFPA 70E® is not ‘The Law.’ However, it is recognized as the premier consensus standard for electrical safety nationally and internationally and can be used by OSHA under the “General Duty Clause” following an accident.

But the general duty clause is a reactive approach to safety after a worker is killed or suffers a serious life altering injury.  Obviously, being proactive in safety is a much better choice because it aims at preventing injuries before they happen by voluntarily adopting standards that go beyond the bare minimum mandated by OSHA.


For those in the mining industry, the Mine Safety and Health Administration (MSHA) did not directly implement NFPA 70E® under their Title 30CFR regulations, but the “Not Covered” industries listed under article 90 of 70E formerly included “Installations underground in mines and self-propelled mobile surface mining machinery….” But this was completely removed from the 2015 version at the request of MSHA.  I can’t speak for MSHA’s reasoning for this move, but it seems like they understand the value of NFPA 70E® for mining operations.

IEEE Standards Association

Furthermore, IEEE 1048 IEEE Guide for Protective Grounding of Power Lines and IEEE 1246 IEEE Guide for Temporary Protective Grounding Systems Used in Substations can also provide some additional guidance when using temporary protective grounding.

As for the question of what voltages are TPG-required, this is somewhat subjective because the corresponding OSHA regulations and standards do not list a specific voltage level when they have to be used.  Most electric utility companies and other industries set their mandatory TPG threshold for electric equipment and lines normally energized at greater than 600 volts.

Summing It Up

The one singular element that is common among all these regulations, standards and codes is found in this critically important statement: “Temporary protective grounding equipment shall be placed at such locations and arranged in such a manner as to prevent each employee from being exposed to shock hazard from hazardous differences in electrical potential.”  The key words consist of “shock hazard from hazardous differences in electrical potential.”

As we wrap up Part 1, we see temporary protective grounding is an essential part in keeping workers safe from electric shock hazards that’s mandated through many federal regulations and standards to address it.  Obviously, if OSHA health and safety regulations govern it, then there’s little doubt of its importance. 

In Part 2, we’ll delve into the history of temporary protective grounding and the various ways it has evolved over the years and expound further on why minimizing hazardous differences in electrical potential is so critical.

George Cole

George Cole joined the e-Hazard team in 2021 as an electrical safety instructor and consultant specializing in the electric utility industry. He has worked for the largest electric utility company in Arizona for over three decades, holding various technical roles in several departments (building electrical maintenance, T & D, radio telecommunications, electric power generation, etc.). George is currently assigned to the Palo Verde Nuclear Generating Station as their electrical safety consultant and is the “Subject Matter Expert” (SME) in all matters related to electrical safety. George holds credentials as a Certified Electrical Safety Compliance Professional (CESCP) and a Certified Electrical Safety Worker (CESW) from the NFPA and serves as a member of NFPA’s Certification Advisory Group (CAG) for the CESCP and CESW. He is also a member of the Electrical Safety Industry Working Group (IWG) within the nuclear power industry, where he is considered an electrical safety expert among his peers.

Leave a Reply