Why is Developing an Equipotential Zone Critical for Effective TPG? Part 5

  • Post comments:0 Comments
Why is Developing an Equipotential Zone Critical for Effective TPG? Part 5

Safety Measures to Remember

In this conclusion of our five-part journey to help us better understand temporary protective grounding (TPG) and its nonnegotiable practice to develop an equipotential zone (EPZ), we’ll now transition to a few other important factors related to grounding for the protection of workers. 

These additional safety measures are also required pursuant to OSHA 29CFR 1910.269(n)(4) through (n)(8) and NFPA 70E® article 120.5(8). They should be discussed in this article at a high level.

Sizing and Constructing TPGs

Correctly sizing temporary grounding equipment is imperative; otherwise, there’s a high probability it will fail when needed the most.  While 2 AWG copper is the minimum size cable permitted by OSHA 1910.269(n)(4)(ii), it must be balanced with its preceding caveat to be “capable of conducting the maximum fault current that could flow at the point of grounding for the time necessary to clear the fault,” according to 1910.269(n)(4)(i) and 70E® article 120.5(8)(b).  This second mandate will necessitate an engineering analysis to determine how much fault current is available from the various sources so accurate sizing can be accomplished.

In my experience, I’ve heard very intelligent individuals make some very unintelligent statements about temporary protective grounding because they falsely presume it’s pretty basic and elementary.  Comments such as, “Every journeyman electrician should know how to install grounds,” or “I can use car battery jumper cables for grounding,” or “I can make safety grounds from parts we have in the shop,” and even “Mousing wire can be used to ground and trip the breakers” unfortunately abound within certain sectors of the electric utility industry. 

For those who may not know, the term “mousing wire” is a slang in electric utilities to mean a very small diameter solid copper wire (18 to 22 AWG) that’s used to “short, bond and ground” all metal parts of spare electric equipment together, such as bushings and frame on large transformers, during storage in areas with electromagnetic flux fields, like those inside of high voltage substations and switchyards.

But the most frequent safety oversight is the belief that common grounding and bonding components, such as single or double-hole crimp type power lugs, bronze bonding clamps, split bolts, spring loaded clamps, welding clamps, welding cables and other similar items are adequate for constructing temporary protective grounding assemblies.  This is because, while these electrical parts are rated and listed for a particular grounding and bonding need, they are not designed for nor can they provide the protection needed if the deenergized lines or equipment being worked on becomes unexpectedly reenergized from its normal source. 

Some overlook the fact temporary grounding is applied to lines and equipment that are normally energized at full system voltage during normal operations, unlike grounding and bonding of non-current-carrying parts, such as metal enclosures and housing.  This is by far the most common, albeit dangerous, practice when technically savvy workers are tasked with making their own temporary protective grounds.

What these individuals have forgotten is what happens during ground faults and short circuits, especially with high voltage distribution systems, where tremendous energies are imposed upon the entire temporary grounding assembly.  Obviously, electrical stresses will consist of arcing and heat generated by the massive amount of current flowing through the connection points.

But thermal energy alone isn’t the only factor that must be considered.  Substantial mechanical forces will also be generated and then released upon the entire grounding assembly as well.   This mechanical energy can bend, twist off, break and even cause the cable to fail catastrophically, i.e., “explode.” 

The combination of these dangerous energies is called the electromechanical force, known as the  ratio or  factor (“reactance over resistance”).  Therefore, all components (not just the cable), such as the clamps, ferrules, etc.  and even the termination methods used to fabricate and construct the grounding equipment must be specifically designed and selected to meet this threat.

For this reason, employers must use ASTM F855, Standard Specification for Temporary Protective Grounds to Be Used on De-energized Electric Power Lines and Equipment, when selecting temporary grounding needs.  ASTM F855 uses different “grades,” numbering from grade 1 through grade 7, listed in Table 1 to designate the temporary grounding components’ ability to deal with the electromechanical forces that can be imposed on the assembly.  For locations with unusually high X/R characteristics (X/R  = 30), the preference is for parts with grade “H” designations (grade 1H through grade 7H), according to Table 2.

This also means the various methods used to connect and even terminate the individual components together must be in strict accordance with the manufacturer’s instructions, such as the number of crimps on the ferrule’s barrel, the correct type of crimping dies used, adherence with torque specification of bolts and screws, strip length of cable’s jacket, the use of heat shrink tubing, etc.

TPG Must Have Low Impedance

The next important attribute for temporary protective grounding is found in OSHA 1910.269(n)(4)(iii) and 70E® article 120.5(8)(c), “Protective grounds shall have an impedance low enough so that they do not delay the operation of protective devices in case of accidental energizing of the lines or equipment.”  (emphasis added)

Impedance is represented by the symbol “Z,” which is the sum of resistance (R) and reactance (X).  For temporary protective grounding, inductive reactance (XL) will be the main type of reactance for concern.  Impedance is found by the simple equation Z = XL + R

Most electrical workers have a good understanding of resistance and its role to restrict current flow.  High resistance is primarily found in the connection points of the grounding assembly and where the connection is made between the grounding clamp and the line or electric equipment being grounded.  The presence of resistance can be from a variety of factors, such as dirty connections; corrosion; dirt; bird droppings; paint; loose bolts, screws and connections; inadequate crimps and a host of other contributors. 

Reducing resistance is fairly simple to correct, with activities like thoroughly cleaning all attachment points with wire brushes and tightening mechanical connections to the proper torque values.

However, inductive reactance (XL) can be somewhat challenging to understand.  Because this article is not an electrical engineering class and I’m only an electrician by trade, I’ll address it at only a fairly high level.

XL is a property that’s only found in alternating sine-wave (ac) circuits, whereas resistance (R) affects both ac and dc circuits alike.  Inductive reactance restricts the flow of current but does so based on the frequency (‘f,’ measured in hertz or cycles) and its interaction with the inductance (‘L,’ measured in henrys) inherent to ac circuits.  Inductive reactance is the product of the equation XL = 2πfL  and is also measured in ohms because it, too, opposes alternating current and is developed through inductors, chokes and coils.  When a conductor or wire is wound in coils, it becomes an inductor, and when ac voltage is applied and current starts flowing through the wire coils, the magnetic flux properties in the ac circuit opposes the current flow with a voltage drop across the inductor very similar to a resistor.

Do you recall the mnemonic “ELI the ICE man” during your apprenticeship or college engineering classes?  It was used to help us easily remember the relationship between voltage and current with reactance.  ELI is the acronym for an inductive circuit where voltage (E) leads current (I) by 90° in an inductor (L).  Whereas ICE is used when current (I) in a capacitive (C) circuit leads voltage (E) by 90°.  All this is shared to show that any restriction to current flow, be it resistance, reactance or impedance, negatively affects how quickly an overcurrent protection device (OCPD) will operate in response to a ground fault and/or short circuit.  If the protective device can’t or won’t trip the OCPD to clear the fault within the prescribed time limit, then the temporary grounding devices may not be able to safely carry the fault current for the longer duration, increasing the probability of failure and injury to the worker.

And if the inductive reactance is substantial, it can restrict current flow significantly to the point the cable itself will catastrophically fail.

Electrical Testing Ground Cables and Ground Sets

Once again, OSHA doesn’t provide any direct specification or periodicities when grounding cables and assemblies must be tested or retested.  However, we can extrapolate some information from 1910.269(n)(4)(i) “Protective grounding equipment shall be capable of conducting the maximum fault current that could flow at the point of grounding for the time necessary to clear the fault” and 1910.269(n)(4)(iii) “Protective grounds shall have an impedance low enough so that they do not delay the operation of proactive devices in case of accidental energizing of the lines or equipment.”

From these two regulations, it’s obvious some type of reoccurring electrical testing is necessary to ensure long term sustainability with these OSHA imperatives.  This is where ASTM F2249-20, Standard Specification for In-Service Test Methods for Temporary Grounding Jumper Assemblies Used on De-Energized Electric Power Lines and Equipment, can be very helpful.    

Several manufacturers make portable test equipment specifically for temporary grounding cables and assemblies which are relatively simple to use requiring only a moderate level of training.  The equipment injects a premeasured amount of current through the cable at low voltages, then displays the corresponding voltage drop across the entire cable, which determines the overall resistance.  The technician then compares the voltage drop against a “pass/fail” chart, factoring in the length of the cable and the size of the wire to make the final determination of its condition. 

There are several advantages to this type of test equipment: relative ease of use, minimal training needed, no risk of electric shock, equipment is portable, and it can quickly test the entire length of the grounding cable with a simple pass or fail verdict.

The disadvantage is if the cable assembly should fail, the test set can’t pinpoint the location of where the high resistance resides at, such as bad crimps, corrosion in the mating surfaces of threaded connections or terminations, loose parts, etc.  Locating these compromised areas may require the use of additional equipment, such as a digital low resistance ohmmeter (DLRO) or alternate means.  Once located, the compromised areas will have to be disassembled, cleaned and/or re-crimped, and then tested again to ensure they will pass.

Most companies schedule routine testing of their temporary grounding assemblies on an annual or biennial periodicity.

Testing for the Absence of Voltage – Live-Dead-Live Prior to Attaching Grounds

OSHA 1910.269(n)(5) requires “…unless a previously installed ground is present, employees test lines and equipment and verify the absence of nominal voltage before employees install any ground on those lines or that equipment.”  NFPA 70E® applies this through the prescribed eight step sequence when establishing and verifying an Electrically Safe Work Condition.  Article 120.5(7) requires the use of rated test devices to test each phase conductor for the absence of hazardous voltage. Then the next step 120.5(8) has the worker installing temporary protective grounding.  If the eight steps of article 120.5 is “performed in the order presented,” then the lines or equipment will be tested prior to installing temporary protective grounds.

The permissive action of “unless a previously installed ground is present” in OSHA is omitted from 70E®, but the premise behind this caveat is if a TPG has already been installed, then the line or equipment is obviously free of nominal system voltage, so an additional absence of voltage test is not required. 

However, industry best practices will require that the previously installed TPG must be physically visible to the worker and within his line of sight at the work location; otherwise, another absence of voltage test must be performed.  This is especially important when working within switchgear and bus ducts and other enclosed equipment.  There have been many incidents where a worker wrongly believed a TPG was installed but couldn’t visually see them.  They incorrectly assumed they were on the correct component – then connected their ground to an energized part.  You can imagine the serious consequences that followed.

Another point I wish to make about testing prior to hanging grounds is to follow 70E®, Article 120.5(7) and use an “adequately rated test instrument….”  In the “good old days” as recent as the 1990’s, a practice called “fuzzing the line” was commonly employed to determine if it was energized or not.  This involved using a hotstick with a ferrous piece of metal attached to it, like a wrench, which was moved in proximity to a bare conductor.  If it was energized at nominal system voltages, then small sparks and/or an audible buzzing sound would be emitted.  However, with the availability of many types of high voltage testers readily on the market, there is absolutely no reason to “fuzz the line” in today’s world of electrical safety.  Such unsafe and outdated practices are incapable of determining with 100% certainty the line or equipment is deenergized.

Additionally, the practice of verifying the test instrument is working  – before and after the absence-of-voltage check – must be followed.  This is commonly called “Live-Dead-Live Testing.” While OSHA 1910.333(b)(2)(iv)(B), requires “the test equipment shall be checked for proper operation immediately after this test” and only for circuits over 600 volts, NFPA 70E®, on the other hand, mandates the operation of the test instrument be verified on a known energized source “before” and “after” testing on any circuits at 50 volts or greater.

Most low voltage electricians are very familiar with the requirement to test “phase to phase” and “phase to ground” during “Live-Dead-Live” checks. However, with higher voltage equipment energized at 1kV or more, this may be difficult to do, if not impossible.  For high voltage circuits, Exemption No. 2 to 7, under article 120.5, precludes the checks across potentials: “On electrical systems over 1000 volts, noncontact capacitive test instruments shall be permitted to be used to test each phase conductor.”

Safe and Correct Sequence when Attaching and Removing TPGs

OSHA regulations contain very detailed instructions for the correct sequence when attaching and when removing TPGs to and from electric lines and equipment for a very good reason.

During the installation of TPGs according to 1910.269(n)(6)(i), the ground-end shall be attached first.  Then the other end, which is attached to the normally energized lines or parts, shall be accomplished by using a live-line tool, i.e., “hotstick.”

The removal process according to 1910.269(n)(6)(ii) requires a reversal of the sequence.  The grounding device shall first be removed from the normally energized line or part using a live line tool; then the ground-end shall be removed.

If TPGs are installed on lines or equipment energized at 600 volts or less, then OSHA permits the use of “insulating equipment other than a live-line tool,” meaning the appropriate class of rubber insulating gloves with leather protectors. But the order for attaching and removing remains unchanged.

The reason for this very detailed sequence is to protect the worker from any dangerous currents that can be induced on the deenergized line or equipment from adjacent energized lines through electromagnetic coupling.  If the employee installs or removes the TPG in the incorrect order, he will place himself in series with the TPG where deadly electric current will flow through his body, mainly from a hand-to-hand contact.  Many electrical workers have been seriously injured and even killed because they made this small but fatal mistake.

For some unknown reason, the important order when attaching and removing TPGs is not contained in the current version of NFPA 70E®; however, I’ve submitted a public comment #19 to have this very important practice added during the 2024 revision.  Let’s hope the 70E® technical committee members will see the wisdom and value from this update.

Required PPE when Attaching and Removing TPGs

OSHA doesn’t spell out any specific PPE requirements for electric shock and arc flash protection, but we can deduce a couple of known requirements from within the regulations.

Live-line tools are not PPE but are required when installing and removing TPGs from the normally energized lines or equipment, with the exception of those energized at 600 volts or less.  Since OSHA clearly states for employees to be able to work on lines and equipment as deenergized, it must be deenergized (isolated from all sources), then placed under the protection of a clearance or LOTO to ensure it stays in that condition and must be properly grounded.  In other words, if “It’s Not Grounded, Then it’s not Dead.” 

Therefore, the lines or equipment must be treated as energized, meaning the minimum approach distance (MAD) is in effect until the TPGs are installed.   OSHA 1910.269(I)(3(iii) and 1910.333(c)(3)(ii) essentially state, “The employer shall ensure that no employee approaches or takes any conductive object closer to exposed energized parts than the employer’s established minimum approach distance,” unless the employee is insulated from the energized part or the energized part is insulated from the employee. 

The live-line tool must be of adequate length to keep the worker’s hands and other body parts (which are conductive) from crossing into the MAD unless rubber insulating gloves and sleeves are worn while using the hotstick.  This OSHA requirement is nearly identical to those found in NFPA 70E® article 130.4(G) as it applies to the restricted approach boundary (RAB)

Rubber insulating gloves and sleeves are designed to protect against electric shock, as are live-line tools. But the additional length of live-line tools can theoretically provide some level of arc flash protection by increasing the distance of the worker from the source of an arc flash if one should occur.

However, other than electric shock protection, OSHA is fairly silent on arc flash protection when installing or removing TPGs.  This is where 70E® provides the additional guidance for arc flash PPE with TPGs.  When we review Table 130.5(C) “Estimate of the Likelihood of Occurrence of an Arc Flash Incident for ac and dc Systems,” we find the task of “Application of temporary protective grounding equipment after voltage test.”  The Equipment Condition is identified as “ANY” and the “Likelihood of Occurrence” (of an arc flash) is annotated as a “YES.”  This means arc flash PPE must be worn when applying TPGs after the absence of voltage test has been performed. 

But what about arc flash PPE requirements when removing TPGs?  Buried in Table 130.5(C) is another task listed under “Arc-resistant equipment with DOORS CLOSED and SECURED” consisting of “(3) Insertion or removal (racking) of ground and test device.”  These devices, sometimes called ground trucks, grounding breakers or ground buggies, are used primarily with metal clad switchgear.  In this case, the Likelihood of Occurrence is “YES” if the Equipment Condition is “ABNORMAL.” 

While there is no description of “abnormal” within NFPA 70E®, article 110.4(D) provides us with its opposite, a “Normal Operating Condition.”

A normal operating condition exists when all six of the following are in place:

  • The equipment is properly installed.
  • The equipment is properly maintained.
  • The equipment is used according to its listing and labeling and manufacturer’s instructions.
  • The equipment doors are closed and secured.
  • All equipment covers are in place and secured.
  • There is no evidence of impending failure.

If any one or more is missing, then the equipment is in an “Abnormal” condition by default, which means there’s a likelihood of an arc flash, necessitating the use of arc flash PPE. 


I hope I’ve been successful in imparting to you some very important information and learnings about the effectiveness of EPZ grounding.

But I also wish to share these closing reminders:

  • Technically accurate training of employees by qualified instructors in proper EPZ grounding is essential,
  • Plan for success but prepare for failure,
  • Always expect the unexpected,
  • Avoid assumptions, overconfidence, and complacency, and
  • Never stop learning…

…because if temporary protective grounds are correctly “placed at such locations and arranged in such a manner,” then “each employee will not be exposed to any hazardous difference of electric potential.”


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