California Code of Regulations › Title 8 - Industrial Relations › Division 1 - Department of Industrial Relations › Chapter 4 - Division of Industrial Safety › Subchapter 5 - Electrical Safety Orders › Group 2 - High-Voltage Electrical Safety Orders › Article 36 - Work Procedures and Operating Procedures › Appendix E - Protection from Hazardous Differences in Electric Potential

Current through Register 2024 Notice Reg. No. 28, July 12, 2024
I. Introduction
Current passing through an impedance impresses voltage
						  across that impedance. Even conductors have some, albeit low, value of
						  impedance. Therefore, if a "grounded" [FN1] object, such as a crane or
						  deenergized and grounded power line, results in a ground fault on a power line,
						  voltage is impressed on that grounded object. The voltage impressed on the
						  grounded object depends largely on the voltage on the line, on the impedance of
						  the faulted conductor, and on the impedance to "true," or "absolute," ground
						  represented by the object. If the impedance of the object causing the fault is
						  relatively large, the voltage impressed on the object is essentially the
						  phase-to-ground system voltage. However, even faults to grounded power lines or
						  to well grounded transmission towers or substation structures (which have
						  relatively low values of impedance to ground) can result in hazardous voltages.
						  [FN2] In all cases, the degree of the hazard depends on the magnitude of the
						  current through the employee and the time of exposure. This appendix discusses
						  methods of protecting workers against the possibility that grounded objects,
						  such as cranes and other mechanical equipment, will contact energized power
						  lines and that deenergized and grounded power lines will become accidentally
						  energized.
II. Voltage-Gradient Distribution
A.
Voltage-gradient distribution
						  curve.
Absolute, or true, ground serves as a reference and always has
						  a voltage of 0 volts above ground potential. Because there is an impedance
						  between a grounding electrode and absolute ground, there will be a voltage
						  difference between the grounding electrode and absolute ground under
						  ground-fault conditions. Voltage dissipates from the grounding electrode (or
						  from the grounding point) and creates a ground potential gradient. The voltage
						  decreases rapidly with increasing distance from the grounding electrode. A
						  voltage drop associated with this dissipation of voltage is a ground potential.
						  Figure 1 is a typical voltage-gradient distribution curve (assuming a uniform
						  soil texture).
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B.
Step and touch potentials.
Figure 1
						  also shows that workers are at risk from step and touch potentials. Step
						  potential is the voltage between the feet of a person standing near an
						  energized grounded object (the electrode). In Figure 1, the step potential is
						  equal to the difference in voltage between two points at different distances
						  from the electrode (where the points represent the location of each foot in
						  relation to the electrode). A person could be at risk of injury during a fault
						  simply by standing near the object.
Touch potential is the voltage between the energized
						  grounded object (again, the electrode) and the feet of a person in contact with
						  the object. In Figure 1, the touch potential is equal to the difference in
						  voltage between the electrode (which is at a distance of 0 meters) and a point
						  some distance away from the electrode (where the point represents the location
						  of the feet of the person in contact with the object). The touch potential
						  could be nearly the full voltage across the grounded object if that object is
						  grounded at a point remote from the place where the person is in contact with
						  it. For example, a crane grounded to the system neutral and that contacts an
						  energized line would expose any person in contact with the crane or its
						  uninsulated load line to a touch potential nearly equal to the full fault
						  voltage.
Figure 2 illustrates step and touch potentials.
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III. Protecting Workers From Hazardous
						  Differences in Electrical Potential
A.
Definitions.
The following
						  definitions apply to section III of this appendix:
Bond. The electrical interconnection of conductive
						  parts designed to maintain a common electric potential.
Bonding cable (bonding jumper).
A
						  cable connected to two conductive parts to bond the parts together.
Cluster bar.
A terminal temporarily
						  attached to a structure that provides a means for the attachment and bonding of
						  grounding and bonding cables to the structure.
Ground.
A conducting connection
						  between an electric circuit or equipment and the earth, or to some conducting
						  body that serves in place of the earth.
Grounding cable (grounding jumper).
A
						  cable connected between a deenergized part and ground. Note that grounding
						  cables carry fault current and bonding cables generally do not. A cable that
						  bonds two conductive parts but carries substantial fault current (for example,
						  a jumper connected between one phase and a grounded phase) is a grounding
						  cable.
Ground mat (grounding grid).
A
						  temporarily or permanently installed metallic mat or grating that establishes
						  an equipotential surface and provides connection points for attaching
						  grounds.
B.
Analyzing the hazard.
The employer
						  can use an engineering analysis of the power system under fault conditions to
						  determine whether hazardous step and touch voltages will develop. The analysis
						  should determine the voltage on all conductive objects in the work area and the
						  amount of time the voltage will be present. Based on the this analysis, the
						  employer can select appropriate measures and protective equipment, including
						  the measures and protective equipment outlined in Section III of this appendix,
						  to protect each employee from hazardous differences in electric potential. For
						  example, from the analysis, the employer will know the voltage remaining on
						  conductive objects after employees install bonding and grounding equipment and
						  will be able to select insulating equipment with an appropriate rating, as
						  described in Section III.C.2 of this appendix.
C.
Protecting workers on the ground.
The employer may use several methods, including equipotential zones, insulating
						  equipment, and restricted work areas, to protect employees on the ground from
						  hazardous differences in electrical potential.
1. An equipotential zone will protect workers within it
						  from hazardous step and touch potentials. (See Figure 3.) Equipotential zones
						  will not, however, protect employees located either wholly or partially outside
						  the protected area. The employer can establish an equipotential zone for
						  workers on the ground, with respect to a grounded object, through the use of a
						  metal mat connected to the grounded object. The employer can use a grounding
						  grid to equalize the voltage within the grid or bond conductive objects in the
						  immediate work area to minimize the potential between the objects and between
						  each object and ground. (Bonding an object outside the work area can increase
						  the touch potential to that object, however.) Section III.D of this appendix
						  discusses equipotential zones for employees working on deenergized and grounded
						  power lines.
2. Insulating equipment, such as rubber gloves, can
						  protect employees handling grounded equipment and conductors from hazardous
						  touch potentials. The insulating equipment shall be rated for the highest
						  voltage that can be impressed on the grounded objects under fault conditions
						  (rather than for the full system voltage).
3. Restricting employees from areas where hazardous
						  step or touch potentials could arise can protect employees not directly
						  involved in performing the operation. The employer shall ensure that employees
						  on the ground in the vicinity of transmission structures are at a distance
						  where step voltages would be insufficient to cause injury. Employees shall not
						  handle grounded conductors or equipment likely to become energized to hazardous
						  voltages unless the employees are within an equipotential zone or protected by
						  insulating equipment.
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D.
Protecting employees working on deenergized
						  and grounded power
lines.
This Section III.D of
						  Appendix E establishes guidelines to help employers comply with requirements in
						  Section 2940.15 for using protective
						  grounding to protect employees working on deenergized power lines. Section
						  2940.15 applies to grounding of
						  transmission and distribution lines and equipment for the purpose of protecting
						  workers. Subsection (e) of Section
						  2940.15 requires temporary
						  protective grounds to be placed at such locations and arranged in such a manner
						  that the employer can demonstrate will prevent exposure of each employee to
						  hazardous differences in electric potential. [FN3] Sections III.D.1 and III.D.2
						  of this appendix provide guidelines that employers can use in making the
						  demonstration required by Section
						  2940.15(e).
						  Section III.D.1 of this appendix provides guidelines on how the employer can
						  determine whether particular grounding practices expose employees to hazardous
						  differences in electric potential. Section III.D.2 of this appendix describes
						  grounding methods that the employer can use in lieu of an engineering analysis
						  to make the demonstration required by Section
						  2940.15(e). The
						  Division of Occupational Safety and Health will consider employers that comply
						  with the criteria in this appendix as meeting Section
						  2940.15(e).
Finally, Section III.D.3 of this appendix discusses
						  other safety considerations that will help the employer comply with other
						  requirements in Section
						  2940.15. Following these guidelines
						  will protect workers from hazards that can occur when a deenergized and
						  grounded line becomes energized.
1.
Determining safe body current
						  limits.
This Section III.D.1 of Appendix E provides guidelines on how
						  an employer can determine whether any differences in electric potential to
						  which workers could be exposed are hazardous as part of the demonstration
						  required by Section
						  2940.14(e).
Institute of Electrical and Electronic Engineers (IEEE)
						  Standard 1048-2003,
IEEE Guide for Protective Grounding of Power
						  Lines
, provides the following equation for determining the threshold
						  of ventricular fibrillation when the duration of the electric shock is
						  limited:
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where
I
is the current through the
						  worker's body, and t is the duration of the current in seconds. This equation
						  represents the ventricular fibrillation threshold for 95.5 percent of the adult
						  population with a mass of 50 kilograms (110 pounds) or more. The equation is
						  valid for current durations between 0.0083 to 3.0 seconds.
To use this equation to set safe voltage limits in an
						  equipotential zone around the worker, the employer will need to assume a value
						  for the resistance of the worker's body. IEEE Std 1048-2003 states that "total
						  body resistance is usually taken as 1000 [OMEGA] for determining ... body
						  current limits." However, employers should be aware that the impedance of a
						  worker's body can be substantially less than that value. For instance, IEEE Std
						  1048-2003 reports a minimum hand-to-hand resistance of 610 ohms and an internal
						  body resistance of 500 ohms. The internal resistance of the body better
						  represents the minimum resistance of a worker's body when the skin resistance
						  drops near zero, which occurs, for example, when there are breaks in the
						  worker's skin, for instance, from cuts or from blisters formed as a result of
						  the current from an electric shock, or when the worker is wet at the points of
						  contact.
Employers may use the IEEE Std 1048-2003 equation to
						  determine safe body current limits only if the employer protects workers from
						  hazards associated with involuntary muscle reactions from electric shock (for
						  example, the hazard to a worker from falling as a result of an electric shock).
						  Moreover, the equation applies only when the duration of the electric shock is
						  limited. If the precautions the employer takes, including those required by
						  applicable standards, do not adequately protect employees from hazards
						  associated with involuntary reactions from electric shock, a hazard exists if
						  the induced voltage is sufficient to pass a current of 1 mill amperes through a
						  500-ohm resistor. (The 500-ohm resistor represents the resistance of an
						  employee. The 1-milliampere current is the threshold of perception.) Finally,
						  if the employer protects employees from injury due to involuntary reactions
						  from electric shock, but the duration of the electric shock is unlimited (that
						  is, when the fault current at the work location will be insufficient to trip
						  the devices protecting the circuit), a hazard exists if the resultant current
						  would be more than 6 milliamperes (the recognized let-go threshold for workers
						  [FN4]).
2.
Acceptable methods of grounding for
						  employers that do not perform
an engineering
						  determination.
The grounding methods presented in this section of this
						  appendix ensure that differences in electric potential are as low as possible
						  and, therefore, meet Section
						  2940.15(e) without
						  an engineering determination of the potential differences. These methods follow
						  two principles: (i) The grounding method shall ensure that the circuit opens in
						  the fastest available clearing time, and (ii) the grounding method shall ensure
						  that the potential differences between conductive objects in the employee's
						  work area are as low as possible.
Subsection (e) of Section
						  2940.15 does not require grounding
						  methods to meet the criteria embodied in these principles. Instead, the
						  subsection requires that protective grounds be "placed at such locations and
						  arranged in such a manner that the employer can demonstrate will prevent
						  exposure of each employee to hazardous differences in electric potential."
						  However, when the employer's grounding practices do not follow these two
						  principles, the employer will need to perform an engineering analysis to make
						  the demonstration required by Section
						  2940.15(e).
i.
Ensuring that the circuit opens in the
						  fastest available clearing time.
Generally, the higher the fault
						  current, the shorter the clearing times for the same type of fault. Therefore,
						  to ensure the fastest available clearing time, the grounding method shall
						  maximize the fault current with a low impedance connection to ground. The
						  employer accomplishes this objective by grounding the circuit conductors to the
						  best ground available at the worksite. Thus, the employer shall ground to a
						  grounded system neutral conductor, if one is present. A grounded system neutral
						  has a direct connection to the system ground at the source, resulting in an
						  extremely low impedance to ground. In a substation, the employer may instead
						  ground to the substation grid, which also has an extremely low impedance to the
						  system ground and, typically, is connected to a grounded system neutral when
						  one is present. Remote system grounds, such as pole and tower grounds, have a
						  higher impedance to the system ground than grounded system neutrals and
						  substation grounding grids; however, the employer may use a remote ground when
						  lower impedance grounds are not available. In the absence of a grounded system
						  neutral, substation grid, and remote ground, the employer may use a temporary
						  driven ground at the worksite.
In addition, if employees are working on a three-phase
						  system, the grounding method shall short circuit all three phases. Short
						  circuiting all phases will ensure faster clearing and lower the current through
						  the grounding cable connecting the deenergized line to ground, thereby lowering
						  the voltage across that cable. The short circuit need not be at the worksite;
						  however, the employer shall treat any conductor that is not grounded at the
						  worksite as energized because the ungrounded conductors will be energized at
						  fault voltage during a fault.
ii.
Ensuring that the potential differences
						  between conductive objects
in the employee's work area are as
						  low as possible.
To achieve as low a voltage as possible across any
						  two conductive objects in the work area, the employer shall bond all conductive
						  objects in the work area. This section of this appendix discusses how to create
						  a zone that minimizes differences in electric potential between conductive
						  objects in the work area. The employer shall use bonding cables to bond
						  conductive objects, except for metallic objects bonded through metal-to-metal
						  contact. The employer shall ensure that metal-to-metal contacts are tight and
						  free of contamination, such as oxidation, that can increase the impedance
						  across the connection. For example, a bolted connection between metal lattice
						  tower members is acceptable if the connection is tight and free of corrosion
						  and other contamination. Figure 4 shows how to create an equipotential zone for
						  metal lattice towers.
Wood poles are conductive objects. The poles can absorb
						  moisture and conduct electricity, particularly at distribution and transmission
						  voltages. Consequently, the employer shall either: (1) Provide a conductive
						  platform, bonded to a grounding cable, on which the worker stands or (2) use
						  cluster bars to bond wood poles to the grounding cable. The employer shall
						  ensure that employees install the cluster bar below, and close to, the worker's
						  feet. The inner portion of the wood pole is more conductive than the outer
						  shell, so it is important that the cluster bar be in conductive contact with a
						  metal spike or nail that penetrates the wood to a depth greater than or equal
						  to the depth the worker's climbing gaffs will penetrate the wood. For example,
						  the employer could mount the cluster bar on a bare pole ground wire fastened to
						  the pole with nails or staples that penetrate to the required depth.
						  Alternatively, the employer may temporarily nail a conductive strap to the pole
						  and connect the strap to the cluster bar. Figure 5 shows how to create an
						  equipotential zone for wood poles.
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NOTES:
1. Employers shall ground overhead ground wires that
						  are within reach of the employee.
2. The grounding cable must be as short as practicable;
						  therefore, the attachment points between the grounding cable and the tower may
						  be different from the shown in the figure.
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Figure reprinted with permission from Hubbel Power
						  Systems, Inc. (Hubbell).
OSHA revised the figure from Hubbell's original.
For underground systems, employers commonly install
						  grounds at the points of disconnection of the underground cables. These
						  grounding points are typically remote from the manhole or underground vault
						  where employees will be working on the cable. Workers in contact with a cable
						  grounded at a remote location can experience hazardous potential differences if
						  the cable becomes energized or if a fault occurs on a different, but nearby,
						  energized cable. The fault current causes potential gradients in the earth, and
						  a potential difference will exist between the earth where the worker is
						  standing and the earth where the cable is grounded. Consequently, to create an
						  equipotential zone for the worker, the employer shall provide a means of
						  connecting the deenergized cable to ground at the worksite by having the worker
						  stand on a conductive mat bonded to the deenergized cable. If the cable is cut,
						  the employer shall install a bond across the opening in the cable or install
						  one bond on each side of the opening to ensure that the separate cable ends are
						  at the same potential. The employer shall protect the worker from any hazardous
						  differences in potential any time there is no bond between the mat and the
						  cable (for example, before the worker installs the bonds).
3. Other safety-related considerations. To ensure that
						  the grounding system is safe and effective, the employer should also consider
						  the following factors: [FN5]
i.
Maintenance of grounding equipment.
It is essential that the employer properly maintain grounding equipment.
						  Corrosion in the connections between grounding cables and clamps and on the
						  clamp surface can increase the resistance of the cable, thereby increasing
						  potential differences. In addition, the surface to which a clamp attaches, such
						  as a conductor or tower member, shall be clean and free of corrosion and
						  oxidation to ensure a low-resistance connection. Cables shall be free of damage
						  that could reduce their current-carrying capacity so that they can carry the
						  full fault current without failure. Each clamp shall have a tight connection to
						  the cable to ensure a low resistance and to ensure that the clamp does not
						  separate from the cable during a fault.
ii.
Grounding cable length and
						  movement.
The electromagnetic forces on grounding cables during a
						  fault increase with increasing cable length. These forces can cause the cable
						  to move violently during a fault and can be high enough to damage the cable or
						  clamps and cause the cable to fail. In addition, flying cables can injure
						  workers. Consequently, cable lengths should be as short as possible, and
						  grounding cables that might carry high fault current should be in positions
						  where the cables will not injure workers during a fault.[FN1]
This appendix generally uses the term "grounded" only
						  with respect to grounding that the employer intentionally installs, for
						  example, the grounding an employer installs on a deenergized conductor.
						  However, in this case, the term "grounded" means connected to earth, regardless
						  of whether or not that connection is intentional.[FN2]
Thus, grounding systems for transmission towers and
						  substation structures should be designed to minimize the step and touch
						  potentials involved.[FN3]
The protective grounding required by Section
						  2940.15 limits to safe values the
						  potential differences between accessible objects in each employee's work
						  environment. Ideally, a protective grounding system would create a true
						  equipotential zone in which every point is at the same electric potential. In
						  practice, current passing through the grounding and bonding elements creates
						  potential differences. If these potential differences are hazardous, the
						  employer may not treat the zone as an equipotential zone.[FN4]
Electric current passing through the body has varying
						  effects depending on the amount of the current. At the let-go threshold, the
						  current overrides a person's control over his or her muscles. At that level, an
						  employee grasping an object will not be able to let go of the object. The
						  let-go threshold varies from person to person; however, the recognized value
						  for workers is 6 milliamperes.[FN5]
This appendix only discusses factors that relate to
						  ensuring an equipotential zone for employees. The employer shall consider other
						  factors in selecting a grounding system that is capable of conducting the
						  maximum fault current that could flow at the point of grounding for the time
						  necessary to clear the fault, as required by Section
						  2940.14(f)(2).
						  IEEE Std 1048-2003 contains guidelines for selecting and installing grounding
						  equipment that will meet Section
						  2940.14(f)(2).
1. New
						  Appendix E filed 2-27-2018; operative 4-1-2018 (Register 2018, No.
						  9).
Note: Authority cited: Section
							 142.3, Labor
							 Code. Reference: Section
							 142.3, Labor
							 Code.