Insulation: Workers can insulate themselves from any possible potential difference between lines and equipment, and ground, by using insulated rubber gloves, insulated tools or the live-line bare hand work method. Some companies de-energize their lines and equipment and use the insulation method instead of grounding.
Isolation: Workers can use the isolation method when working on lines and equipment by first de-energizing the lines and equipment, obtaining a clearance, installing temporary protective grounding equipment, and then finally removing the temporary protective grounding equipment and working the line or equipment as isolated. To use the isolation method, the lines and equipment must have:
• Been de-energized under the provisions of the company’s switching and clearance procedures
• No possibility of contact with another energized source
• No possible hazards of induced voltage
It should be noted that the isolation method may be an acceptable work method in some instances; however, this work method must be used with extreme caution and only after review by your company’s management and safety and engineering group to ensure the isolation method is a safe option.
Personal Protective Grounding: Workers can install a personal protective grounding system, sometimes called equipotential grounding (EPZ), at the work site to limit the voltage difference between any two accessible points to a safe value within the work site.
In this article we will review current industry-accepted methods of installing a personal protective grounding system on overhead distribution and transmission systems. I hope to follow this article with a discussion on underground distribution and transmission personal protective grounding, then hazards of induction and mechanical equipment grounding.
The definitions of terms associated with personal protective grounding create numerous misunderstandings. Let’s start by defining a number of terms that will be used in this article.
Bracket Grounding: A grounding method in which temporary protective grounding equipment is installed on both sides of a work site.
Clearance: The certification by the system operator, or the person in charge, that a specified line or piece of equipment is de-energized from all normal sources of electrical energy; a clearance tag has been placed at all clearance points; and the transfer of authority from the system operator, or person in charge, to the clearance-holder has been completed.
Cluster Bar: A terminal temporarily attached to a structure to support and provide a connection point to accommodate grounding cables. It may also be used to establish an equipotential zone.
De-energized: Disconnected from all intentional sources of electrical supply by opening switches, jumpers, taps or other items. De-energized lines and equipment can be electrically charged or energized through various means, such as induction from energized circuits, portable generators or lighting. De-energizing lines and equipment does not allow workers to enter minimum approach distances unless the workers are insulated, isolated, or the lines and equipment have been properly grounded.
Electric Field Induction (Capacitive Coupling): The process of generating a voltage or current in an isolated conductive object or electric circuit by means of time-varying electric fields.
Electromagnetic Field Induction (Electromagnetic Coupling): The process that employs both electric and magnetic fields to generate a circulating current between two grounded sites of a line due to the proximity of an adjacent or nearby energized line.
Energized: Electrically connected to a source of potential difference, or electrically charged so as to have a potential different from that of the earth.
Equipotential Zone (EPZ): The state of maintaining a near-identical electrical potential between two or more items, as compared to the nominal voltage present.
Exposure Voltage: The voltage impressed across a worker’s body, either hand to hand or hand to foot, when the worker comes in contact with objects at the work site that are not at the same potential.
Ground (Ground Source): Earth, or a conductive body of relatively large extent that serves in place of earth. Ground normally provides a reference to zero volts – no voltage – for electrical circuits. Under fault conditions, ground may rise in voltage to a level above zero volts near an intentional or accidental connection of an electrical circuit to ground.
Grounded (Grounding): A means of connecting an electrical circuit or electrical equipment to ground (see definition of “ground”) whether intentional or accidental.
Minimum Air Insulation Distance (MAID): The shortest distance in air between an energized line or equipment and a worker’s body at different potential. This distance does not take into account a floating electrode in the gap or any factor for inadvertent movement.
Minimum Approach Distance (MAD): MAID plus a factor for inadvertent movement.
Personal Grounds: The combination of a cluster bar and a grounding jumper from the cluster bar to the tripping grounds.
Personal Protective Grounding: The combination of tripping grounds and personal grounds installed in a method that bonds the de-energized lines and equipment with all other conductive objects within the work site – including the structure – limiting the exposure voltage to a safe value.
Qualified Employee (Worker): One knowledgeable in the construction and operation of the electric power generation, transmission and distribution equipment involved, along with the associated hazards. An employee must have the training required by OSHA 1910.269(a)(2)(ii) in order to be considered a qualified employee.
Temporary Protective Grounding Equipment: A system of ground clamps, ferrules, cluster bar(s) and cables designed and suitable for carrying fault current as specified in ASTM F855.
Tripping Grounds: Temporary protective grounding equipment installed in a manner that bonds the ground source and phase conductor(s) together. Tripping grounds are not used by themselves for worker protection.
Not long ago, the electrical industry believed that installing “shorts” – better known today as tripping grounds – between the work site and the energy source protected the worker from any accidental re-energization of the lines and equipment. If the lines or equipment could be accidentally re-energized from either side of the work site, bracket grounds were installed. The belief was that the voltage and current would travel down the line toward the work site, but before they could reach the work site and the worker, they would be shunted or bleed off to ground through the shorts. Thus, the worker would not see any hazardous voltage or current at their work location. Sounds like a reasonable assumption until basic electrical theory – probably more correctly referred to as electrical fact – is applied to this idea. Electrical theory provides us with two simple facts:
1. Current takes the least resistive path to ground.
2. Current takes all paths to ground.
It is true that tripping grounds are a very low-resistance path to ground and the current will want to take this low-resistive path to ground as stated in No. 1 above. But current also takes all paths to ground. If the worker has their hand on the conductor and is working off of a wood pole or steel structure, for example, there is a path to ground through the worker’s body and down the structure to ground. One might argue that the path is high-resistive and very little current will flow. True, the path is high-resistive, but it is a path that must be considered.
How much current entering the worker’s hand, traveling through their body and down that structure is hazardous? Electrical engineering expert Charles Dalziel and the “IEEE Guide for Safety in AC Substation Grounding” tell us as little as 82 volts and 164 mA (0.164 amps) can be lethal current and voltage for a human. Can a worker who is standing on a structure and contacting an overhead conductor, which accidentally becomes energized, see lethal voltage and current? Yes, if the structure is conductive at all, lethal voltages and currents can easily travel through the worker’s body. Wooden structures can have resistances anywhere from 3 million ohms to as few as 5,000 ohms. What can influence the resistance of the wood pole or structure? The resistance can be greatly reduced by many things including moisture, treatment and a pole ground.
How do we know that the voltage and current will continue past the tripping grounds to enter the work site and potentially the worker’s body? Test results going back to 1954 clearly show hazardous voltage and current do travel past the tripping grounds installed between the work site and the source of energy, and enter the work site. If the worker is in contact with the conductor at that same moment, lethal current can flow through the worker’s body and down the structure.
The first people to discover the failure of tripping grounds to provide worker protection were E.J. Harrington and T.M.C. Martin, who conducted research and published “Placement of Protective Grounds for the Safety of Linemen” in 1954. Harrington and Martin found tripping grounds and bracket grounds did not provide protection for workers as the industry once believed. Their research clearly showed that bonding the structure to the tripping grounds created an EPZ. Harrington and Martin dubbed their new method “single point grounding.” The idea of single point grounding was to attach all the temporary protective grounding equipment to a single point – the structure. Some companies embraced the idea of single point grounding and grounding equipment manufacturers promoted the concept, but the industry was slow to accept the theory and its application. Many more tests and research have been conducted since 1954 that support Harrington and Martin’s original findings that, in fact, tripping grounds installed between the work site and the source do not protect the worker.
A research project completed by J.T. Bonner, B. Erga, W.W. Gibbs and V.M. Gregorius in 1985 produced an IEEE paper titled “Tests Results of Personal Protective Grounding on Distribution Line Wood Pole Construction,” which reaffirmed EPZ grounding and its ability to work at distribution voltages. Recent tests have provided similar results and are currently being reviewed by the industry.
In 1994, OSHA published 29 CFR 1910.269, “Electric Power Generation, Transmission, and Distribution.” Section 1910.269(n)(3) states, “‘Equipotential zone.’ Temporary protective grounds shall be placed at such locations and arranged in such a manner as to prevent each employee from being exposed to hazardous differences in electrical potential.”
Developing a Grounding Procedure
Taking into account the 57 years of industry research on personal protective grounding and the requirements of OSHA 1910.269(n)(3), what is the best way to install temporary protective grounding equipment to protect the worker? I have assisted in developing grounding procedures for a number of utilities throughout the industry and I suggest the following items – as just a few of many – be considered when developing your grounding procedure:
• When lines and equipment that are or may be energized at more than 50 volts are removed from service for operation, maintenance or construction, they must be considered energized until a clearance has been issued; the lines and equipment have been tested; and temporary protective grounding equipment has been installed to develop a personal protective grounding system (EPZ).
• Conductors and devices must be tested and grounded only after proper clearances have been issued.
• A job briefing must be held with all workers prior to beginning any job in order to discuss potential hazards. When the work includes installing a personal protective grounding system, all involved in the work process must discuss the grounding process and understand the value and limitations of the work method.
• Depending on the work location, lines and equipment must be connected to ground using the following sources in descending priority:
i. Substation ground mat
ii. Multigrounded common neutral system
iii. Multigrounded static wire (overhead ground wire)
iv. Structure ground (pole ground, tower ground, footing ground)
v. Temporarily driven ground rod
• Temporary protective grounding equipment must be visually inspected each day before use. This includes visually checking grounding jumpers for broken or loose fittings and chafed or cut insulation. The grounding clamp jaws should be clean and the cable ferrules tightened each day. The grounding clamp jaws should be wire brushed with inhibitor before each use. If any damage is found, repair or replace the equipment.
• Do not ground through fuses, transrupters, power circuit breakers, switches, power transformers or other types of devices.
• An approved voltage detector, rated for the system voltage, must be used to verify the line or equipment is de-energized. “Fuzzing” the line is not an approved method of testing lines or equipment. The voltage detector should be tested before and after each use to ensure the device is working properly.
• The ground-end clamp of the grounding cable must always be connected to the ground first and removed last. The conductor end of the ground cable must be connected and disconnected with hot-line tools.
• Workers on the ground may be exposed to step and touch potentials when all types of protective grounding procedures are used. While work is in progress, ground personnel should stay a minimum of 10 feet from the structure being worked on and any driven ground rod. If ground workers must contact the structure, approved insulated rubber gloves, insulated overshoes or conductive mats should be used.
• Not all work will allow the procedures detailed below to be used. If the work requires applying alternative work methods, the person in charge shall receive approval from management, engineering and the safety department before making any revisions to these procedures.
The steps involved in installing a personal protective grounding system include:
i. Obtaining a clearance as specified in your company’s clearance and switching procedures
ii. Testing the line or device to be sure it is de-energized using an approved voltage detector
iii. Installing a cluster bar on the pole below the work area
iv. Clamping one end of a properly sized grounding jumper of correct length to the cluster bar and the other end to the approved ground as specified above
v. Installing a properly sized grounding jumper of correct length from the cluster bar, or ground, to the closest phase conductor using hot-line tools, then jumpering the other phases together working from the nearest to the farthest away
vi. Removing personal protective grounds after the work is completed in the exact reverse order
When developing a personal protective grounding system for transmission lines and equipment, it is imperative that the lines and equipment be grounded to the best available ground as specified above. A low-resistance ground source will greatly reduce any electric field induction impressed on the transmission line. A similar process as listed above for distribution lines and equipment should be used when installing a personal protective grounding system.
Ensure the personal protective grounds are installed as close to the work site as possible. When the worker contacts the conductor while positioned on the structure, a conductive loop is developed and the voltages generated can be as much as three times the voltage developed across the temporary protective grounding equipment.
Overhead transmission lines should be considered to have dangerous levels of electric field induction and magnetic field induction until they have been thoroughly assessed, tested and found safe, or proper work methods are used to eliminate the induction hazard.
The Influence of Induction
Let’s briefly discuss what is commonly called induction in the electric utility industry – technically defined as electric field induction and magnetic field induction – and the influence it has on nearby de-energized lines.
Electric fields and magnetic fields are generated when current flows in an energized AC system. The electric and magnetic fields generated by this energized AC system can induce a charge into nearby de-energized conductors through what is called capacitive coupling and inductive coupling. The intensity of the electric and magnetic fields is directly related to the energized AC system’s voltage level, current flow and the proximity of the de-energized lines to the energized AC system. It is often thought that workers are safe contacting a de-energized line, located in the vicinity of an energized AC system, if the de-energized conductor has been grounded. In fact, the process of grounding a de-energized line may increase the hazard to workers if grounding procedures are applied incorrectly.
Electric field induction can be present anytime two conductive objects are separated by a dielectric medium, such as air, creating a simple capacitor. When a de-energized line is separated by air from a nearby energized line, a process called capacitive coupling induces a capacitive voltage in the de-energized line.
Any line energized with an AC voltage creates an electric field between the energized conductor and all other objects at different potential. An electric field will be present due to the voltage on the energized line whether current is or is not flowing in the energized conductor. This electric field is measured in volts per meter.
When a de-energized conductive object – such as an overhead conductor, cable, shield wire, vehicle, tool, equipment or worker’s body – is positioned near the energized conductor, the electric field induces a voltage onto the de-energized object through a process referred to as electric field induction.
When an energized AC transmission line carries current, magnetic field (flux) is developed around the energized AC transmission line. When a second transmission line, paralleling the first energized AC transmission line and in relatively close proximity, is de-energized and grounded at two distant locations, a conductive loop is created. The varying magnetic flux created by the AC current in the transmission line creates an induced voltage on the de-energized and multigrounded transmission line. This voltage in turn will create current flow in the conductive loop. This process of inducing current and voltage into the de-energized and multigrounded transmission line is also called magnetic field induction or inductive coupling.
The above information about personal protective grounding is a brief outline of what a temporary grounding procedure should include. It does not cover the many exceptions and adjustments that might need to be made to fit your system. It is recommended that your company review all industry-accepted and published standards, guides and papers related to personal protective grounding when reviewing and revising your grounding procedure. You may also consider retaining a subject matter expert in personal protective grounding to assist you in your review.
Additionally, the size and rating of the temporary protective grounding equipment used on your system must be rated for the maximum available fault current and duration. Refer to ASTM F855 - 09, “Standard Specifications for Temporary Protective Grounds to Be Used on De-energized Electric Power Lines and Equipment,” for detailed information on application of temporary protective grounding equipment.
After I recently delivered a grounding training session to a group of electrical workers, their safety professional stood up and told the group that the statement, “If it is not grounded, it is not dead,” needs to be revised to, “If it is not equipotentially grounded, you may be dead.” When I look back on all the fatal accidents I have been involved with over the past 26 years in the EPZ process, that revised statement is very true.
About the Author: Brian Erga, president of ESCI Inc., has more than 36 years of electric utility expertise and holds a BSEE degree. An expert on safety practices and work methods related to the electric utility industry, he is a member of IEEE/ESMO, NSC, NFPA and ASTM F18, and a member of NESC Subcommittee 8, responsible for NESC Part 4 “Rules for the Operation of Electric Lines.”