Electric potential hazard protection

Step potential is the voltage difference between the feet of a person standing near an energized grounded object, and touch potential is the voltage between that object and the feet of a person touching it; both can cause hazardous current to flow through a worker and lead to shock or electrocution.

• The appendix explains that voltage dissipates from a grounding electrode and creates a ground potential gradient, so voltages vary with distance from the grounded object, creating step and touch hazards (1910.269AppC).
• The illustrations and discussion in the appendix show that touching a grounded object or standing near it can expose a worker to nearly the full fault voltage depending on grounding and distance (1910.269AppC).

An equipotential zone protects employees by making the entire work surface essentially the same voltage, so there is no dangerous voltage difference between a worker and the objects they contact.

• The appendix says an equipotential zone can be established using a metal mat or grounding grid bonded to the grounded object to equalize voltages within the protected area (1910.269AppCIII.C).
• Equipotential zones only protect workers inside the zone and will not protect anyone who is partially or wholly outside it, so the zone must fully encompass the worker while they perform tasks (1910.269AppCIII.C).

The employer must demonstrate that temporary protective grounds are placed and arranged so they will prevent exposure of each employee to hazardous differences in electric potential during work on deenergized lines.

• Paragraph (n)(3) requires temporary protective grounds to be placed at locations and arranged in a manner the employer can demonstrate will prevent hazardous potential differences (1910.269(n)(3)).
• Appendix C Section III.D provides guidelines to help make that demonstration, and Section III.D.2 lists grounding methods employers can use instead of a full engineering analysis; compliance with the appendix criteria will be considered as meeting 1910.269(n)(3) (1910.269AppCIII.D, 1910.269AppCIII.D.2).

You can use the IEEE equation I = 116/√t to estimate the ventricular fibrillation threshold for a worker when the shock duration t (in seconds) is limited; this gives a current level below which 95.5% of adults (≥50 kg) are unlikely to fibrillate.

• Appendix C cites IEEE Std 1048-2003 and provides the equation I = 116/√t for shock durations between 0.0083 and 3.0 seconds; I is body current in milliamperes and t is time in seconds (1910.269AppCIII.D.1).
• The employer must also account for the expected duration of fault current and choose insulating or grounding measures so currents remain below this limit, and must protect against hazards from involuntary reactions (e.g., falls) as required by the appendix (1910.269AppCIII.D.1).

Appendix C recommends using 1000 ohms as a typical total-body resistance for setting limits, but it warns employers that body impedance can be much lower (for example, 610 ohms hand-to-hand or about 500 ohms internal resistance) and to consider lower values when necessary.

• The appendix states IEEE Std 1048-2003 usually takes total body resistance as 1000 Ω for determining body current limits, but it also notes measured minimum resistances (610 Ω hand-to-hand, 500 Ω internal) and explains when a lower value is more appropriate (1910.269AppCIII.D.1).
• Employers should use a conservative resistance value when worker skin may be wet, broken, or when internal resistance may dominate, because that increases the current for a given voltage (1910.269AppCIII.D.1).

Insulating equipment must be rated for the highest voltage that can be impressed on the grounded object under fault conditions—not simply the system's nominal voltage.

• Appendix C directs employers to select insulating equipment rated for the highest voltage that could appear on the grounded objects after considering fault conditions and any residual voltages remaining after grounding/bonding (1910.269AppCIII.C.2).
• The required rating should come from an analysis (or conservative assumptions described in the appendix) of the maximum voltage a grounded object might acquire during a fault so the insulating gear provides adequate protection (1910.269AppCIII.C.2).

Restricting employees from areas where hazardous step or touch potentials could arise is acceptable to protect workers who are not directly involved in the operation, provided the employer ensures those employees are kept at a safe distance.

• Appendix C says restricting access protects employees not involved in the operation and requires the employer to ensure employees on the ground near transmission structures are at distances where step voltages would be insufficient to cause injury (1910.269AppCIII.C.3).
• The employer should base the restricted distance on an engineering analysis or conservative assumptions about potential gradients so excluded personnel are not exposed to hazardous voltages (1910.269AppCIII.C.3).

You establish an equipotential zone by placing a metallic mat or grounding grid connected to the grounded object so the mat/grid equalizes the voltage across the work surface and provides attachment points for grounds.

• Appendix C defines a ground mat (grounding grid) as a metallic mat or grating that establishes an equipotential surface and provides connection points for attaching grounds (1910.269AppCIII.C).
• The mat or grid should be bonded to the grounded object and designed so the worker and tools are within the equalized potential area; remember the appendix warns that objects bonded outside the work area can increase touch potential to that object (1910.269AppCIII.C).

If you do not perform an engineering analysis, Appendix C Section III.D.2 provides specific grounding practices that, when followed, OSHA will consider as satisfying the demonstration required by 1910.269(n)(3).

• Appendix C explicitly states that employers who comply with the criteria in Section III.D.2 will be considered as meeting 1910.269(n)(3) (1910.269AppCIII.D.2).
• The appendix lists grounding configurations and placement methods intended to minimize potential differences; follow those methods and the related guidance in III.D.1 and III.D.3 to show compliance without a full engineering study (1910.269AppCIII.D.2).

Even well-grounded objects can develop hazardous voltages during a ground fault because current flowing through the grounding path creates voltage drops across impedances, which can raise the potential of the grounded object above true earth ground.

• Appendix C explains that even faults to well-grounded transmission towers or substation structures (which have low impedance to ground) can result in hazardous voltages; the hazard depends on fault current magnitude and exposure time (1910.269AppC).
• The appendix emphasizes that the voltage impressed on a grounded object depends on the line voltage, the impedance of the faulted conductor, and the impedance to absolute ground represented by the object (1910.269AppC).

The employer should analyze the power system under fault conditions to determine the voltages that will appear on conductive objects in the work area and how long those voltages will be present, then select protective measures and equipment accordingly.

• Appendix C advises employers to use an engineering analysis of the power system under fault conditions to determine voltages on conductive objects and the duration of those voltages so appropriate measures (equipotential zones, grounding, or insulating equipment) can be selected (1910.269AppCIII.B).
• The analysis helps employers choose insulating equipment ratings and grounding/bonding arrangements that will keep worker exposure below hazardous levels (1910.269AppCIII.B).

Bonding electrically connects conductive parts to maintain the same potential (minimizing voltage differences between them), while grounding connects parts to earth so fault current can return to ground; bonding normally does not carry substantial fault current but grounding cables do.

• Appendix C defines "bond" as an interconnection to maintain a common electric potential and defines a "grounding cable" as one that connects a deenergized part to ground and carries fault current, whereas a "bonding cable" connects conductive parts to each other and generally does not carry substantial fault current (1910.269AppCIII.A).
• The appendix also notes that a cable that bonds two parts but carries substantial fault current should be treated as a grounding cable, so proper sizing and placement matter for safety (1910.269AppCIII.A).

Employees must not handle grounded conductors or equipment likely to become energized to hazardous voltages unless they are within an equipotential zone or are protected by insulating equipment rated for the maximum impressed voltage.

• Appendix C states explicitly that employees must not handle grounded conductors or equipment likely to become energized unless they are within an equipotential zone or protected by insulating equipment (1910.269AppC).
• This requirement supports the protective grounding rules in 1910.269(n) for transmission and distribution grounding operations (1910.269(n)).

Appendix C recommends following additional safety practices such as ensuring grounds are visible and clearly marked, connecting grounds in a safe sequence, using appropriately rated equipment, and protecting workers from mechanical hazards and environmental conditions that could affect grounding performance.

• Section III.D.3 discusses other safety considerations intended to help employers meet broader requirements of 1910.269(n), including procedural and equipment precautions to reduce risks when a deenergized and grounded line becomes energized (1910.269AppCIII.D.3).
• Employers should integrate the appendix guidance with the temporary protective grounding rules in 1910.269(n) and the grounding-placement guidance in III.D.1 and III.D.2 to build complete procedures (1910.269(n)).

A hazard exists if induced voltage could cause a current of 1 milliampere through a 500-ohm resistor (the threshold of perception) when the employer has not adequately protected workers from involuntary reactions due to electric shock.

• Appendix C says that if precautions do not adequately protect employees from involuntary reactions and the induced voltage is sufficient to pass 1 mA through a 500 Ω resistor, then a hazard exists; the 500 Ω resistor represents the employee and 1 mA is the perception threshold (1910.269AppCIII.D.1).
• This criterion is separate from the ventricular fibrillation threshold and applies when involuntary reaction hazards have not been otherwise addressed (1910.269AppCIII.D.1).

The 6 milliamp threshold applies when the employer has protected workers from involuntary reactions but the potential fault current duration is unlimited (i.e., it will not be cleared by protective devices); a hazard exists if the resultant current would exceed 6 mA, the recognized let-go threshold.

• Appendix C explains that if workers are protected from injury due to involuntary reactions but the duration of shock could be unlimited (fault current not cleared), then currents above 6 mA constitute a hazard because 6 mA is the let-go threshold (1910.269AppCIII.D.1).
• Employers must therefore consider whether protective devices will clear faults and select grounding/insulation so steady-state or long-duration currents remain below that threshold (1910.269AppCIII.D.1).

Step potential decreases with increasing distance from the grounding electrode because the voltage dissipates rapidly as you move away from the electrode, so two feet spaced apart at different distances can have a dangerous voltage difference.

• Appendix C explains that voltage decreases rapidly with increasing distance from the grounding electrode, producing a ground potential gradient; step potential is the voltage between two points (feet) at different distances from the electrode (1910.269AppCII.B).
• The appendix uses figures and examples to show how a person standing near an energized grounded object can be at risk simply by standing on the ground because of these gradients (1910.269AppCII.B).

A crane grounded to system neutral that contacts an energized line can have nearly the full fault voltage impressed on it, exposing anyone touching the crane or its uninsulated load line to that touch potential.

• Appendix C gives this specific example: a crane grounded to the system neutral that contacts an energized line would expose a person in contact with the crane or its uninsulated load line to a touch potential nearly equal to the full fault voltage (1910.269AppC).
• That example illustrates why employers must use equipotential zones, insulating equipment rated for impressed voltages, or grounding/bonding methods to protect workers (1910.269AppCIII.C).

Yes—OSHA states employers that comply with the criteria in Appendix C Section III.D.2 will be considered as meeting the demonstration required by 1910.269(n)(3).

• Appendix C explicitly says the Agency will consider employers that comply with the criteria in Section III.D.2 as meeting 1910.269(n)(3), so following those grounding methods is an acceptable way to demonstrate protection against hazardous potential differences (1910.269AppCIII.D.2).
• Employers should document how their chosen grounding methods meet the appendix criteria and ensure equipment, placement, and procedures match the guidance in III.D.1–D.3 (1910.269AppCIII.D).

A ground mat is a metallic mat or grating that establishes an equipotential surface and gives attachment points for grounds so workers standing on it are at nearly the same potential as the grounded object.

• Appendix C defines a ground mat (grounding grid) as a temporarily or permanently installed metallic mat or grating that creates an equipotential surface and provides connection points for attaching grounds (1910.269AppCIII.A).
• When properly bonded to the grounded conductor or structure, the mat equalizes voltages under fault conditions within the zone so step and touch potentials for workers inside the mat are minimized (1910.269AppCIII.C).

Under 1910.269AppC, you should ground a deenergized circuit to the grounded system neutral if one is present because it provides the lowest impedance and maximizes fault current.

• If a grounded system neutral is not available, ground to a substation grounding grid when working in a substation because it also provides extremely low impedance.
• Use remote system grounds (pole or tower grounds) only if lower-impedance options are not available.
• If none of the above exist, a temporary driven ground at the worksite may be used.

This approach helps the protective devices clear faults faster by maximizing fault current and follows the guidance in 1910.269AppC.

Under 1910.269AppC, the employer must short circuit all three phases to ensure faster fault clearing and to reduce the current (and resulting voltage) carried by the grounding cable at the worksite.

• Shorting all phases increases available fault current so protective devices operate sooner.
• Reducing current through the grounding cable lowers voltage drop across the cable and reduces hazardous potential differences in the work area.
• The short circuit does not have to be located at the worksite, but any conductor not grounded at the worksite must be treated as energized during a fault.

See the explanation in 1910.269AppC for practical effects on clearing time and grounding cable amperage.

Under 1910.269AppC, the employer must bond all conductive objects in the work area so their potentials are as close as possible.

• Use bonding cables to connect conductive objects unless the objects are already bonded by secure metal-to-metal contact.
• Ensure metal-to-metal contacts are tight and free of corrosion or contamination that could raise resistance.
• Verify all clamps and bonding connections are clean, tight, and low-resistance to prevent dangerous voltage differences.

These steps create an equipotential zone and reduce hazardous touch and step potentials as described in 1910.269AppC.

Under 1910.269AppC, an employer must either provide a conductive platform bonded to the grounding cable for the worker to stand on or use cluster bars to bond the wood pole to the grounding cable.

• If using cluster bars, mount them below and close to the worker's feet and ensure they make conductive contact with the more conductive inner portion of the pole (see next answer about depth).
• If using a conductive platform, bond the platform directly to the grounding cable so the worker stands in an equipotential area.

These options are described in 1910.269AppC to minimize dangerous potential differences on wood poles.

Under 1910.269AppC, a cluster bar must make conductive contact with the inner portion of the wood pole at least as deep as a worker's climbing gaffs will penetrate.

• Acceptable methods include mounting the cluster bar on a bare pole ground wire fastened with nails or staples that penetrate to the required depth.
• Alternatively, temporarily nail a conductive strap to the pole that reaches the required depth and connect the strap to the cluster bar.

This ensures the bonding reaches the more conductive inner wood and is described in 1910.269AppC.

Under 1910.269AppC, the employer must provide an equipotential zone at the worksite by connecting the deenergized cable to ground at the worksite and having the worker stand on a conductive mat bonded to that cable.

• If the cable is cut, install a bond across the opening or install a bond on each side so the separate ends share the same potential.
• Protect the worker from hazardous potential differences any time there is no bond between the mat and the cable (for example, before the bonds are installed).

These precautions are required to prevent dangerous step and touch potentials as explained in 1910.269AppC.

Under 1910.269AppC, the employer must maintain grounding equipment so clamps, cables, and attachment surfaces remain low-resistance and reliable.

• Inspect clamps and clamp surfaces for corrosion or oxidation and clean them as needed to keep resistance low.
• Check grounding cables for damage that could reduce current-carrying capacity and replace damaged cables.
• Ensure each clamp is tight and secure so it cannot separate during a fault.
• Establish a regular inspection and maintenance schedule and document repairs.

Good maintenance prevents increased resistance and hazardous potential differences as described in 1910.269AppC.

Under 1910.269AppC, grounding cables should be as short as practicable and routed so they will not injure workers if they move violently during a fault.

• Shorter cables reduce electromagnetic forces during a fault, lowering the chance of violent movement or cable/clamp failure.
• Position grounding cables carrying high fault current away from worker pathways and secure them to limit movement.
• Keep attachment points close to the work area to minimize length, subject to practical and safety considerations.

These measures reduce the risk of flying cables and damage during fault conditions as explained in 1910.269AppC.

Under 1910.269(n)(3) and 1910.269AppC, if an employer's grounding practices do not follow the two principles (maximize clearing speed and minimize potential differences), the employer must perform an engineering analysis to demonstrate that their protective grounds will prevent hazardous potential differences to each employee.

• The engineering analysis should document how the chosen grounding locations and arrangements will protect employees under foreseeable fault conditions.
• The analysis should include calculations or test data on grounding impedances, fault currents, and expected potential gradients.
• Keep the analysis as part of the employer's safety documentation to show compliance with 1910.269(n)(3).

Under 1910.269AppC, an equipotential zone is a work area where conductive objects are bonded so every point is approximately at the same electric potential, minimizing touch and step voltages.

• In practice, current flowing through grounding and bonding elements creates some potential differences.
• If those potential differences are large enough to be hazardous (for example, they could cause dangerous current through a worker), the employer may not treat the area as an equipotential zone.
• Employers must measure or analyze potential differences and take additional bonding, grounding, or isolation steps if the differences are unsafe.

See the discussion of equipotential zones in 1910.269AppC.

Under 1910.269AppC, remote grounds like pole and tower grounds have higher impedance than grounded system neutrals and should be used only when lower-impedance grounds are not available.

• Prefer grounding to a grounded system neutral or a substation grid because they provide extremely low impedance to the source ground.
• Use remote grounds only as a fallback; recognize that higher impedance increases voltage drop and may lengthen clearing times.
• If only remote grounds are available, consider an engineering analysis per 1910.269(n)(3) to demonstrate employee protection.

This hierarchy and guidance are described in 1910.269AppC.

Under 1910.269AppC, grounding to the substation grid is acceptable because the grid typically has an extremely low impedance to system ground and is often connected to the grounded system neutral.

• The low impedance of the substation grid maximizes fault current and aids faster clearing of faults.
• Grounding to the grid reduces voltage rises on grounding conductors and helps keep potential differences in the work area low.
• Where a grounded system neutral exists, the substation grid is usually equivalent or preferable to remote grounds.

See the explanation in 1910.269AppC.

Under 1910.269AppC, an employer must treat any conductor that is not grounded at the worksite as energized because it may be at fault voltage during a remote fault.

• Do not assume remote grounding elsewhere protects the conductor at the worksite—ungrounded conductors can become energized by faults on other parts of the system.
• Use insulating procedures, barriers, or additional grounding to ensure employee safety if the conductor cannot be grounded at the worksite.

This conservative approach is discussed in 1910.269AppC.

Under 1910.269AppC, employers should minimize grounding cable length and secure or route cables away from worker areas to reduce the chance of violent movement during a high fault current event.

• Keep grounding cables as short as practicable to lower electromagnetic forces during a fault.
• Securely fasten cables and clamps so they cannot separate or whip during a fault.
• Place cables where they will not injure workers if they move — e.g., route them behind barriers or away from work positions.

These precautions are recommended in 1910.269AppC to prevent cable damage and personnel injury.

Under 1910.269AppC, short circuiting all phases (on three-phase systems) and using the lowest-impedance ground available reduce the current through the grounding cable and therefore lower the voltage across that cable.

• Ensuring the grounding point is as close to a grounded system neutral or substation grid as possible decreases impedance.
• Shorting phases reduces the share of fault current that must flow through the grounding cable, lowering voltage rise across the cable.
• Properly sized and maintained cables and clamps help carry expected fault currents without excessive voltage drop.

These measures are described in 1910.269AppC.

Under 1910.269AppC, employers should design grounding and bonding so that currents that could flow through a worker stay well below the recognized "let-go" threshold of about 6 milliamperes.

• The let-go threshold is the current at which a worker may not be able to voluntarily release an energized object; keeping currents below this level reduces the risk of prolonged contact and injury.
• Use low-impedance grounding, full bonding of conductive objects, appropriate insulation or barriers, and ensure proper maintenance so fault currents do not cause hazardous body currents.

See the discussion of effects of current and safe potential control in 1910.269AppC.

Under 1910.269AppC, wood poles are considered conductive because they can absorb moisture and conduct electricity, so employers must either provide a bonded conductive platform or bond the pole with cluster bars placed below and close to the worker's feet.

• Recognize that moisture increases pole conductivity; do not assume a pole is insulating.
• Ensure cluster bars contact the inner, more conductive wood by penetrating to the depth of climbing gaff penetration or use a conductive strap/nail method as described in the appendix.
• Always bond the platform or cluster bar to the grounding cable to create an equipotential zone for the worker.

This guidance is in 1910.269AppC.

Under 1910.269AppC, if a grounding clamp is attached to a corroded or oxidized surface, the employer must clean or otherwise ensure a low-resistance connection because corrosion increases resistance and raises hazardous potential differences.

• Remove corrosion or oxidation at the clamp attachment point and verify a tight mechanical connection.
• If cleaning is impractical, select an alternative attachment point or use a bonding method that guarantees low resistance (e.g., drill and install a penetrating connector when appropriate and safe).
• Reinspect connections regularly and document maintenance to ensure continued effectiveness.

These maintenance and connection quality requirements are emphasized in 1910.269AppC.