Definitions
As discussed in the previous article, the definitions of terms associated with personal protective grounding create many misunderstandings. Let’s take another look at 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 ends of the cable or both sides of a work site.

Clearance: The certification by the system operator, or the person in charge, that a specified cable 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 pulling elbows, by opening switches, jumpers or taps, or by other means. De-energized cables and equipment can be electrically charged or energized through various means, such as trapped charge, electromagnetic field induction from other energized cables, backfeed or lighting. De-energizing cables 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.

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 cable due to the proximity of an adjacent or nearby energized cable.

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 use of a temporary ground grid (ground mat) positioned where the worker will be standing and connected to the cable’s concentric neutral, shield or system neutral, or the equipment’s ground bus.

Personal Protective Grounding: The combination of tripping grounds and personal grounds installed in a method that bonds the de-energized cables and equipment with all other conductive objects within the work site, 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 Ground Grid: A specialized mat with a conductive metallic grid that provides a connection point to the cable or equipment’s ground as specified in ASTM F2715. The temporary ground grid is used to establish an EPZ.

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 the cable’s phase conductor and concentric neutral or shield together. Tripping grounds are not used by themselves for worker protection.

Underground Voltage Hazards
Potentially hazardous voltage may appear in de-energized underground distribution cables and equipment due to:
• Accidental energization of cables or underground equipment by connecting to an energized source
• A backfeed through a transformer
• A ground fault rise impressed on the system neutral by a cable or equipment failure
• Human error
• Equipment failure

When an underground system fault occurs while one cable of the system is de-energized and grounded to the system neutral, a transfer of potential through the cable’s conductor and the cable’s concentric neutral to a work site, on the cable, is possible. This transfer of potential through the cable and concentric neutral results in a potentially hazardous voltage difference along the length of the cable between the cable and ground. If protective grounding is inadequate or improperly applied, workers contacting the cable, concentric neutral or a device connected to the cable could be subjected to hazardous potential voltages. To eliminate any hazard to workers in contact with de-energized cables or equipment, workers should follow one of the following procedures:
• Insulate themselves from any possible potential difference between the cable and earth
• Isolate themselves from any possible potential difference between the cable and earth
• Provide an EPZ at the work site

Insulation and Isolation
Workers can insulate themselves from any possible transfer of potential difference between grounded cables and earth by using – either by themselves or in combination – insulated rubber gloves, insulated footwear, insulated tools, insulated platforms or insulated mats. The insulation method is often difficult to use in many underground installations and has limited applications.

Workers can isolate themselves from any possible transfer of potential difference through a cable by first installing tripping grounds on both ends of the cable using an approved method. Next, remove both tripping grounds and park the elbows on insulated parking bushings or isolate the terminal ends. Finally, disconnect the cable’s concentric neutral or shield from the system neutral. Any connection of the cable’s concentric neutral or shield to other cables must be eliminated, such as bare concentric neutrals lying on, or in contact with, other concentric neutrals.

Establishing an EPZ
Workers can provide an EPZ at the work site by first installing tripping grounds at both ends of the cable using an approved method, then removing both tripping grounds and parking the elbows on insulated parking bushings or isolating the terminal ends. The worker can then install a temporary ground grid at the work site that covers the area where workers will be standing during contact with the cable. The temporary ground grid must be properly connected to the concentric neutral or shield of the cable being worked, thus creating an EPZ for workers in contact with the cable’s conductor and concentric neutral or shield. The temporary ground grid, when properly installed, works the same way as a cluster bar on overhead structures when used in conjunction with overhead equipotential grounding.

It should be noted that when the isolation or EPZ grounding method is used in underground distribution systems, a clearance must be issued and both ends of the cable must be tagged. Issuing a clearance on electrical systems energized at 50 volts or more is not only required by OSHA 1910.269(m) if the isolation or EPZ grounding method is used, but will also ensure the ends of the cables are not re-energized until all work is complete and workers are in the clear.

Fact vs. Fiction
Many believe that installing tripping grounds on both ends of a cable – commonly called bracket grounding in overhead systems – protects the worker from any accidental energization or a fault on the surrounding underground residential distribution (URD) system. The grounding procedure typically involves pulling the elbow on both ends of the cable and installing tripping grounds at each elbow. The belief is if the worker is in contact with the cable’s grounded conductor, somewhere between the two ground points, any voltage and current impressed on the surrounding URD system by an accidental energization or a fault will be shunted or bled off to ground at the tripping grounds. It is also believed the worker will not see any hazardous voltage or current at the work location.

However, what actually occurs during an accidental energization or a fault is a voltage rise on the URD’s system neutral, ground and the tripping grounds installed on both ends of the grounded cable. This voltage rise will also be seen along the entire length of the grounded cable, including the work site. How does this happen? Since the cable’s conductor and concentric neutral are bonded – using the tripping grounds at each elbow – to the URD’s system neutral, any voltage rise on the URD’s system neutral will be transferred down the grounded cable to the work location, energizing the work location and the worker.

How do we know that the voltage and current can continue through the tripping grounds to travel down the cable to the work site and potentially to the worker’s body? Documentation of real accidents and a series of tests conducted by San Diego Gas & Electric in 2001 clearly showed hazardous voltage and current do travel past the tripping grounds and energize the work site. If the worker is in contact with the cable’s conductor, concentric neutral or shield, and at that same moment an accidental energization or a fault occurs, lethal current can flow through the worker’s body. For additional information on this subject, refer to the IEEE/ESMOL Task Force 15.07.09.01 paper titled “Worker Protection While Working De-energized Underground Distribution Systems.”

OSHA Standards
OSHA 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.”

The above paragraph does not cover just overhead conductors and devices – it covers overhead, underground, substations and network systems. Creating an EPZ in the underground electrical system is a viable option for worker protection with the development of the temporary ground grid.
OSHA 1910.269(n)(8) states, “‘Additional precautions.’ When work is performed on a cable at a location remote from the cable terminal, the cable may not be grounded at the cable terminal if there is a possibility of hazardous transfer of potential should a fault occur.”
The above paragraph details OSHA’s concern about hazardous transfer of potential in underground electrical systems and supports the use of the isolation method.

What is common in work on underground cables? A cable will be de-energized and grounded at both ends, and somewhere along the cable, the worker will cut it open to repair a fault or install a new device. When the worker cuts the cable, what does he also do? He opens the system neutral. The cable’s concentric neutral is the system neutral. How often should a worker open an underground common (system) neutral? Never. OSHA 1910.269(t)(8) states, “‘Sheath continuity.’ When work is performed on buried cable or on cable in manholes, metallic sheath continuity shall be maintained or the cable sheath shall be treated as energized.”

Before any concentric neutral or shield of a cable connected to an energized URD system is opened, a bonding jumper must be installed across the point that will be opened.

Following is guidance on how to perform personal protective grounding of cable with 200-amp elbow systems using the isolation method and the EPZ method.

Isolation Method
The isolation method – detailed below – can be used on a single-phase bare concentric neutral cable; a three-phase bare concentric neutral cable system if all three conductors can be de-energized and their concentric neutrals isolated; and single-phase and three-phase jacketed cables. In any cable configuration, the concentric neutral(s) must be isolated from the system neutral. If this is not possible, the insulation method or EPZ method must be used.
1. Identify the cable.
2. Obtain a visible opening on both ends of the cable by parking the cable’s elbows on feed-thru bushings.
3. Obtain a clearance and install clearance tags on both elbows.
4. Use an approved voltage detector to assure the cable is de-energized by testing both ends of the cable.
5. Using a grounding elbow, ground both ends of the cable. Wait one minute to bleed off any capacitive charge left on the cable and then remove both grounding elbows, leaving the cable’s conductor ungrounded and isolated.
6. Disconnect the cable’s concentric neutral from the system neutral using rated rubber gloves. Ensure the concentric neutral is completely isolated on both ends. Caution: When opening a cable’s concentric neutral, do not place yourself in series with the concentric neutral.
7. If the cable is to be cut, install a concentric bonding jumper across the point of the cable to be opened before cutting the concentric neutral.
8. Cut the cable with hot cutters.
9. Test the cable ends with a voltage detector before contacting the cable.
10. Proceed with the planned work on the cable system.
11. When the work has been completed, reconnect both ends of the cable’s concentric neutral using rated rubber gloves.
12. Release the clearance on the cable and remove the clearance tags from the elbows.

EPZ Method
The EPZ method can be used on all types of cable systems.
1. Identify the cable.
2. Obtain a visible opening on both ends of the cable by parking the cable’s elbows on feed-thru bushings.
3. Obtain a clearance and install clearance tags on both cables.
4. Use an approved voltage detector to assure the cable is de-energized by testing both ends of the cable.
5. Using a grounding elbow, ground both ends of the cable.
6. If the system consists of a three-phase cable system, either identify the de-energized cable at the work site or de-energize and ground all three phases.
7. Wait one minute to bleed off any capacitive charge left on the cable and then remove the grounding elbows, leaving the cable’s conductor ungrounded but isolated. If the cable’s concentric neutral is intact and in good condition, the cable may be grounded.
8. At the work location, lay a ground grid where workers will be positioned when working on the cable. Connect the ground grid to the cable’s concentric neutral using a bonding jumper. If the cable is jacketed, cut off the jacket and install the bonding jumper to the cable’s concentric neutral.
9. If the cable is to be cut, install a concentric bonding jumper across the point of the cable to be opened before cutting the concentric neutral.
10. Cut the cable with hot cutters.
11. Test the cable ends with a voltage detector before contacting the cable.
12. Proceed with the planned work on the cable or elbow.
13. When the work is complete, release the clearance on the cable and remove the clearance tags from the elbows.

When an energized AC cable carries current, a magnetic field (flux) is developed around the energized AC cable. When a second cable (paralleling the first energized AC cable, and in relative close proximity) is de-energized and grounded at both ends, a conductive loop is created. The varying magnetic flux created by the AC current in the energized cable creates an induced voltage on the de-energized and multigrounded cable. In turn, this voltage will create current flow in the conductive loop. This process of inducing current and voltage into the de-energized and multigrounded cable is also called magnetic field induction or inductive coupling. This can occur on very long cable runs and in underground transmission cables carrying large currents.

Next Steps
The above discussion is a brief outline of what an underground distribution temporary grounding procedure should include. This discussion 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 underground grounding procedure. You may also consider retaining a subject matter expert on personal protective grounding to assist you in your review.

Also, as discussed in the previous grounding article, 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.

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.”