Working on Exposed Energized Parts

Under 1926.960(c)(1)(i) the employer must establish and use minimum approach distances that are at least as large as those computed by Table V–2 so employees can work safely without risk of sparkover. Employers should follow the methodology in Appendix B when calculating those distances and may rely on the electrical component values (air insulation distances) stated there.See 1926.960(c)(1)(i) and Appendix B to Subpart V of Part 1926 for the required procedures.

Under 1926.960(c)(1)(ii) the employer must determine the maximum anticipated per‑unit transient overvoltage, phase‑to‑ground, either by engineering analysis or by assuming the values listed in Table V‑8 for the voltage range involved. If you do not use the Table V‑8 maximums for voltages over 72.5 kV, you must have a qualified person perform the engineering analysis to determine the transient overvoltage.See 1926.960(c)(1)(ii) and the guidance in Appendix B to Subpart V of Part 1926 on when a knowledgeable person must be used.

Under 1926.960(c) an employer may rely on air as the insulating medium only when employees are not using electrical protective equipment and the minimum approach distance provides sufficient air clearance to withstand the maximum transient overvoltage that can reach the worksite. The minimum distance must be calculated so that sparkover is prevented using the methods in 1926.960(c)(1)(i) and Appendix B.See 1926.960(c)

Under Appendix B, you calculate the electrical component of the minimum approach distance for 5.1–72.5 kV by: (1) converting phase‑to‑phase to phase‑to‑ground, (2) converting rms to peak, (3) multiplying by the maximum per‑unit transient overvoltage (3.0 for this range), (4) dividing by 0.85 to get the critical sparkover voltage, and (5) interpolating the appropriate gap spacing from the IEEE table (Table 1) to get the air distance. Follow the step‑by‑step procedure and interpolation example in Appendix B, and reference 1926.960(c)(1)(ii) for the transient overvoltage factor guidance.See Appendix B to Subpart V of Part 1926.

Under Appendix B, use an electrical component (air insulation distance) of 20 millimeters (1 inch) for voltages from 301 to 5,000 volts. OSHA explains that test data at 5,000 volts show a 20 mm minimum is protective and that this distance is conservative for lower voltages in that range.See Appendix B to Subpart V of Part 1926 and 1926.960(c)(1)(i).

Under Appendix B, for 72.6–800 kV you calculate the electrical component using Equation 1: D = 0.3048 (C + a) V_L‑G T, where D is meters of air gap, C is a voltage‑dependent correction (0.01 or 0.011), a is the surge factor computed from Table 3 equations, V_L‑G is the line‑to‑ground rms kilovolts, and T is the maximum transient overvoltage factor in per unit. Use the specified values and the Table 3 equations for a and follow Appendix B for when to use C = 0.01 versus 0.011.See Appendix B and 1926.960(c)(1)(ii).

Under Appendix B the employer must use C = 0.011 if the approach gap may include objects other than only air (for example, an insulated tool spanning the gap or a large conductive object in the gap); otherwise use C = 0.01 when the gap consists only of air. The Appendix explains this choice reflects differences in sparkover behavior when insulation or objects are present in the gap.See Appendix B to Subpart V of Part 1926 and 1926.960(c)(1)(ii).

Under Appendix B you decide the exposure type by the employee's potential relative to other conductors: exposure is phase‑to‑ground when the worker is at ground potential (or at the energized part potential during bare‑hand work with respect to ground), and phase‑to‑phase when the worker or tool is at the potential of a different energized part. It matters because the calculation of the electrical component (and the surge factor a) differs for phase‑to‑ground versus phase‑to‑phase exposures and can yield different minimum approach distances.See Appendix B and 1926.960(c)(1)(ii).

Under 1926.964(c) the employer must use the technical criteria and methods in Appendix B when establishing minimum approach distances for live‑line work in accordance with 1926.960(c)(1)(i) and the related tables (such as Table V‑2 and Table V‑7). In short, when you are setting MADs for work on energized parts you must apply Appendix B's procedures unless you rely on the specific tabulated distances allowed by the standard.See 1926.964(c) and Appendix B.

Under Appendix B an employer can use normal system protection (such as lightning arrestors) or temporary measures (e.g., portable protective gaps, preventing automatic reclosing) to limit maximum transient overvoltages, and if those measures reduce the maximum overvoltage reaching the worksite, they can be accounted for in the MAD calculation. Any reduction must be justified by engineering analysis showing the maximum transient overvoltage used in the MAD calculation reflects those protective measures.See 1926.960(c)(1)(ii) and Appendix B guidance in which using system or temporary protection is discussed.See Appendix B to Subpart V of Part 1926.

Under Appendix B a "person knowledgeable" and competent in transmission and distribution system design must be used to determine the maximum transient overvoltage unless the employer uses the maximum transient overvoltages specified in Table V‑8 for voltages over 72.5 kV. In other words, if you do not accept Table V‑8 values you must have a qualified engineer perform the analysis.See Appendix B and 1926.960(c)(1)(ii).

Under Appendix B, Table 1 (the IEEE sparkover gap data) provides the relationship between peak kV and required rod‑to‑rod gap in centimeters; you use the computed peak phase‑to‑ground transient voltage (including the 3.0 per‑unit factor), convert it to the equivalent critical sparkover voltage (divide by 0.85), and then interpolate Table 1 to determine the electrical component (air gap) of the MAD. The Appendix directs using IEEE‑based test data for this voltage range.See Appendix B and 1926.960(c)(1)(ii).

Under Appendix B you must apply an altitude correction factor to the electrical component of the MAD because air density (and thus dielectric strength) decreases with altitude; Appendix B describes how to adjust the computed air gap value upward for higher altitude sites before you add any insulating component. Always follow the altitude correction procedure in Appendix B when your worksite is at elevation above which the correction becomes significant.See Appendix B to Subpart V of Part 1926 and 1926.960(c)(1)(i).

Under Appendix B the term 'guarded' means covered or protected by suitable barriers or covers to minimize inadvertent contact; 'insulated' means separated by a dielectric (including adequate air space) providing high resistance to current; and 'isolated' means not readily accessible unless special means are used. These definitions come from 1926.968 and affect whether parts are treated as exposed energized parts for MAD calculations.See 1926.968 and Appendix B.

Under Appendix B and the definitions in 1926.968, insulated wires that are not otherwise protected are not considered 'guarded'—insulation alone does not meet the guarded definition unless additional protective measures are present. Employers must therefore base MADs and work practices on whether parts are guarded, insulated, or exposed consistent with those definitions.See 1926.968 and Appendix B.

Under 1926.960 the hazard for 50–300 volt installations is similar to other workplaces: employees must avoid contact with exposed parts and use protective equipment suitable for those voltages (for example, rubber insulating gloves). The Appendix notes that while hazards exist, the complexity of protection is less than for high‑voltage systems, but PPE must provide the necessary insulation for the voltages involved.See 1926.960 and Appendix B.

Under Appendix B the 'statistical withstand voltage' is a transient overvoltage level with a 0.14‑percent probability of sparkover (three standard deviations), and it is used to represent a conservative withstand voltage for design of minimum approach distances so that sparkover is very unlikely under maximum transient conditions. Appendix B explains statistical definitions and how they inform selection of withstand versus critical sparkover voltages used in the MAD procedures.See Appendix B to Subpart V of Part 1926.

Under Appendix B the employer may use the tabulated minimum approach distances provided in the standard (such as Table V‑2 and Table V‑7) when those tables apply, but if the employer must establish MADs in accordance with 1926.960(c)(1)(i) that are not covered by the tables, the Appendix B methodology must be used to compute the electrical component and overall MAD. In short, use the tables when applicable; otherwise perform the Appendix B calculations.See 1926.960(c)(1)(i) and Appendix B.

Under Appendix B employers must recognize that an insulated tool spanning a phase‑to‑phase gap can change the gap behavior; the Appendix directs using the phase‑to‑ground equations (which yield larger 'a' values and larger MADs) unless the employer can demonstrate no insulated tool spans the gap and no large conductive object is present. If that demonstration cannot be made, use the more conservative phase‑to‑ground criteria.See Appendix B to Subpart V of Part 1926 and 1926.960(c)(1)(ii).

Under Appendix B multiplying the phase‑to‑ground rms voltage by √2 converts it to peak voltage, and multiplying by the transient overvoltage factor (T) accounts for the maximum per‑unit transient overvoltage expected at the site; together these yield the peak transient phase‑to‑ground voltage used to select the appropriate sparkover gap from IEEE data. This step is required in the Appendix B calculation procedure for 5.1–72.5 kV systems.See Appendix B and 1926.960(c)(1)(ii).

Under Appendix B the engineering analysis must be performed by persons knowledgeable in the Appendix's techniques and competent in electric transmission and distribution system design—practitioners who can determine maximum transient overvoltages, protective device performance, and relevant system behavior. Appendix B requires use of such qualified individuals unless the employer simply uses the Table V‑8 transient values for voltages over 72.5 kV.See Appendix B and 1926.960(c)(1)(ii).

Under 1926.960(c)(1)(ii) the employer must determine the maximum anticipated per-unit transient overvoltage, phase-to-ground, either by doing an engineering analysis or by using the conservative values in Table V‑8. See the requirement in 1926.960(c)(1)(ii).

• If you use an engineering analysis and that analysis assumes system modifications or operating limits (for example, disabling reclosing or installing protective gaps), you must ensure those conditions are actually in place during the work.
• If you cannot justify a lower value by analysis, use the Table V‑8 assumptions to calculate the minimum approach distances.

Use the IEEE-derived conversion: T_L-L = 1.35 × T_L-G + 0.45, and then use that value as T in Equation 1 for phase-to-phase exposures. This formula is described in the appendix guidance and referenced by 1926.960(c)(1)(ii).

• Example: if T_L-G = 2.0 per unit, then T_L-L = 1.35×2.0 + 0.45 = 3.15 per unit.
• After computing T (either T_L-G or T_L-L as appropriate), use Equation 1 and the applicable tables to find the minimum approach distance.

The ergonomic component is an extra distance added to the electrical component to account for inadvertent worker movement and to create a comfortable, safe work zone, and employers must add it to get the total minimum approach distance. See the explanation of the ergonomic component in 1926.960 and the appendix guidance at 1926.960(c)(1)(ii).

• The ergonomic component is based on a response time–distance analysis: the distance a worker might travel while perceiving the hazard and stopping movement.
• It protects body parts not covered by insulating equipment (for example, the face) and compensates for slips, tool adjustments, and other unanticipated motions.

The ergonomic component distances you must add to the electrical component are: 0.31 m (1.0 ft) for system voltages 0.301–0.750 kV, 0.61 m (2.0 ft) for 0.751–72.5 kV, and 0.31 m (1.0 ft) for 72.6–800 kV. These values are given in the appendix guidance (Table 4) and explained in 1926.960.

• Note: the appendix explicitly states the employer must add this ergonomic distance to the electrical component to obtain the full minimum approach distance.

Compute the total MAD by adding the electrical component (from the applicable table or Equation 1 using the transient overvoltage T) plus the ergonomic component specified for the voltage range. OSHA explains this combined requirement in 1926.960 and the appendix guidance.

• Steps: determine system voltage and transient T (see 1926.960(c)(1)(ii)), compute the electrical approach distance from the tables/Equation 1, then add the ergonomic component from Table 4.

At 751 V to 72.5 kV the calculated electrical component of the approach distance is often smaller than the ergonomic component, so the ergonomic component (0.61 m / 2 ft) dominates to protect uninsulated body parts like the face. The appendix guidance explains this rationale in 1926.960.

• For example, at 72.5 kV the electrical component can be only about 0.3 m (1 ft), which would not protect the worker's face; thus the larger ergonomic component is required.

For 72.6–800 kV the ergonomic component is 0.31 m (1 ft) because workers typically use live-line tools (hot sticks) and work methods that tightly control worker movement, keeping employees farther from the energized parts; this is explained in the appendix guidance and 1926.960.

• Hot sticks maintain a constant distance between the worker and the energized part, reducing the need for a larger ergonomic buffer.
• The chosen working position and methods must still allow the worker to perform required movements while maintaining the MAD.

If your engineering analysis assumes controls such as disabling reclosing or functioning insertion resistors, you must ensure those controls are actually in effect during the live-line work and institute procedures reflecting those conditions. OSHA explains this requirement in 1926.960(c)(1)(ii).

• That may require new live-work procedures, operator coordination, testing of overvoltage-limiting devices, and verification that reclosing is disabled for the duration of the job.
• If you cannot ensure the assumed controls, you cannot rely on the reduced transient value from the analysis and must use the conservative assumptions in Table V‑8 or other controls.

An employer may reduce MAD by following the four-step approach in the appendix: (1) determine the maximum voltage for the energized part, (2) choose a control technique to limit transient overvoltage and determine the new maximum transient (with 3σ confidence), (3) require employees to implement procedures that keep that control in effect during the work, and (4) recalculate the MAD using the reduced transient value. This procedure is described in the appendix guidance to 1926.960.

• Control techniques include disabling reclosing, changing breaker operation, installing surge arresters, or portable protective gaps.
• Any calculation that relies on these controls must be backed by procedures ensuring they remain active while employees are exposed.

You must increase the minimum approach distance by about 3 percent per 300 meters (1,000 feet) of altitude above 900 meters (3,000 feet) because reduced air pressure lowers air dielectric strength, as explained in the appendix guidance to 1926.960.

• Use the altitude correction factor specified in the appendix (Table V‑4) when calculating MAD at high elevations.
• Remember that atmospheric conditions (temperature, humidity, pressure) also influence electrical breakdown and the table equations assume standard atmospheric conditions.

Employers must treat broken or damaged insulator units as having no insulating value unless testing proves otherwise; broken units may reduce an insulator string's withstand capacity by up to 70 percent. This is stated in the appendix guidance to 1926.960.

• Because you usually cannot determine the remaining insulating capability without testing, assume damaged units provide zero insulation for MAD calculations.
• Also consider that using a live-line tool next to an insulator string with broken units can further reduce overall insulating strength.

Typical transient overvoltage magnitudes (per unit) include examples such as 3.5 for an energized 200‑mile line without closing resistors, 2.1–2.5 for various restrained switching or fault initiation scenarios, and 1.7–1.9 for fault clearing—these examples are provided in Table 5 of the appendix guidance to 1926.960.

• Use these typical per‑unit values as reference points in an engineering analysis, but rely on site‑specific analysis when possible.
• If you perform an engineering analysis that results in a lower T, ensure any operating constraints or devices that support that lower T are actually in place during the work.

Yes — selection of a proper working position must be addressed in the qualified-employee training required by 1926.950 and in the job planning and briefing required by 1926.952, as referenced in the appendix guidance.

• Training and briefings should cover how working position, tool choice (e.g., rubber gloves vs. hot sticks), and work methods affect the ergonomic component and the ability to maintain MAD.
• Figure 1 in the appendix illustrates maintaining the MAD through proper working positions and methods.

Use T = T_L-G (the phase‑to‑ground maximum transient per unit) for phase‑to‑ground exposures and use T = T_L-L (the phase‑to‑phase transient) for phase‑to‑phase exposures, as required by 1926.960(c)(1)(ii).

• If you start from a phase‑to‑ground T_L-G and need a phase‑to‑phase value, compute T_L-L using the formula T_L-L = 1.35×T_L-G + 0.45 before applying Equation 1.
• Choose the value consistent with the exposure geometry you are evaluating (phase‑to‑ground or phase‑to‑phase).

The response time–distance analysis estimates the time it takes a worker to perceive they are entering a danger zone, react, decelerate, and stop movement toward an energized part, and converts that time into the distance traveled; that distance becomes the ergonomic component of MAD as explained in the appendix to 1926.960.

• The analysis accounts for human reaction time and stopping distance for typical working motions.
• The appendix uses this method to justify the fixed ergonomic distances in Table 4 for the different voltage ranges.

If your analysis relies on surge arresters or portable protective gaps to limit transient overvoltage, you must ensure the devices are properly installed, operable, and remain in effect during the work and, if necessary, document procedures that require their use; see the discussion of surge arresters and controls in the appendix guidance to 1926.960.

• Verify device ratings, installation integrity, and operation before exposure begins.
• Incorporate device use and verification into job briefings and live‑work procedures so employees and system operators understand the requirements.

Your engineering analysis must account for coupling from adjacent lines—especially in double‑circuit construction—because transients on an adjacent line can create significant overvoltages on the line where employees work, as stated in the appendix guidance to 1926.960.

• Model or conservatively estimate the maximum possible coupling effects when determining the maximum transient overvoltage.
• If adjacent-line coupling increases the transient, you must use that higher value when calculating MAD or implement controls to mitigate the effect.

Using a '3σ' withstand voltage means you set the allowable transient level to a value three standard deviations below the critical sparkover voltage so that the probability of sparkover is about 1 in 1,000; the appendix requires using this 3σ confidence level when you determine a reduced transient for MAD calculations. See the appendix guidance to 1926.960.

• When you select a control technique and calculate the new maximum transient, you must use the 3σ withstand voltage as the basis for the MAD tables.
• You must also put procedures in place to ensure the control remains in effect during work (see the four-step reduction procedure in the appendix).

The electrical component can be quite small (about 0.3 m / 1 ft at roughly 72.5 kV), but that small electrical distance alone will not protect body parts like the face, so the ergonomic component (e.g., 0.61 m / 2 ft for 0.751–72.5 kV) must be added to ensure worker safety as described in the appendix to 1926.960.

• Even when the electrical calculation yields a small value, you still must add the ergonomic component appropriate to the voltage range.
• Employers should evaluate the work method (rubber gloves vs. hot sticks) and ensure working positions protect exposed parts not covered by insulating PPE.

The employer must determine the maximum anticipated per-unit transient overvoltage (T) at the worksite through an engineering analysis or, if appropriate, use the default values in Table V-8. This is required by 1926.960(c)(1)(ii), and reiterated in the guidance on minimum approach distances in Appendix B to Subpart V.

• Do an engineering analysis that evaluates system switching, fault behavior, surge sources, and protective devices to estimate the phase-to-ground transient overvoltage at the specific worksite.
• If using default values from the tables, document why those defaults apply.
• Keep the analysis on file and use the calculated T when consulting the minimum approach distance tables (Tables 7–14) or the equations in Table V-2.

(See the employer duty described in 1926.960 and the Appendix guidance in Appendix B to Subpart V.)

Yes — properly installed portable protective gaps can be used to control transient overvoltage at the worksite and thereby reduce the minimum approach distance, when the employer follows the Appendix B calculation steps. Appendix B describes a 5-step method for calculating T (the ratio of peak transient to nominal voltage) and then using T to get the MAD from the appropriate table. See Appendix B to Subpart V and the procedure referenced in 1926.960(c).

• Step 1: Select a protective-gap withstand voltage based on system requirements and acceptable probability of sparkover.
• Step 2: Choose a physical gap spacing that tests to a critical sparkover voltage ≥ the withstand voltage.
• Step 3: Take 110% of the gap's critical sparkover voltage to get the phase-to-ground peak voltage at sparkover (VPPG Peak).
• Step 4: Compute T using the formula in Appendix B (T = VPPG Peak / (VL‑G√2)).
• Step 5: Use T in the equation in Table V-2 or look up the MAD in Tables 7–14.

Also note: all rounding must be upward (round up to the next higher value), and the worksite elevation must be ≤ 900 meters (3,000 ft) to use Tables 7–14 directly. See Appendix B to Subpart V and 1926.960(c).

An employer may install a protective gap on an adjacent structure, at a terminal station, or at the worksite, but each location has limits and conditions the employer must consider. Appendix B explains these placement options and their effects; see Appendix B to Subpart V.

• Adjacent structures: Installing the gap on a structure adjacent to the worksite is permissible and usually does not significantly reduce protection.
• Terminal stations: Gaps at terminal stations can provide protection but may not protect the entire line to the worksite because surges can originate at both ends and add together. Appendix B notes that a gap installed within 0.8 km (0.5 mile) of the worksite will protect against intersecting waves; engineering studies may justify more distant placements.
• Worksite: Installing the gap at the worksite sets the local impulse insulation strength; even distant lightning (up to about 6 miles) can cause sparkover, so employers must protect employees from hazards resulting from any sparkover that could occur.

Always document the rationale and, unless using default T values, determine T at the worksite as required by 1926.960(c)(1)(ii) and Appendix B.

Yes — the employer must protect employees from hazards that could result from any protective-gap sparkover. Appendix B explicitly states the employer must provide protection from hazards due to gap sparkover; see Appendix B to Subpart V.

In practice this means employers should:

• Engineer the gap setting and placement so sparkover probability and resulting energy are acceptable for worker safety and system reliability.
• Provide appropriate work controls, barriers, insulating equipment, or PPE to prevent injury if a sparkover occurs.
• Consider disabling automatic reclosers, implementing switching restrictions, and other administrative controls to reduce the chance and impact of reenergization or switching transients (see Appendix B guidance on disabling automatic reclosing and switching restrictions).

These actions are part of the employer’s obligation under 1926.960 and the Appendix B guidance in Appendix B to Subpart V.

Disabling automatic reclosing is recommended to prevent reenergizing a circuit faulted during work and to avoid switching transients that could create hazardous overvoltages, but it may not always be feasible for system-stability reasons. Appendix B lists these two primary reasons and acknowledges practical limits; see Appendix B to Subpart V.

• Primary safety reasons: (1) prevent unexpected reenergization of a fault created during the work; (2) avoid transient overvoltages caused by reenergization.
• Practical constraint: In some power systems, disabling automatic reclosing may not be feasible because of reliability or stability requirements; where it cannot be disabled, employers must use other protective measures and document the analysis and controls used.

See 1926.960 and Appendix B for the guidance employers should follow.

Employers must always round up to the next higher value when using calculated values to determine the minimum approach distance. Appendix B explicitly states that all rounding must be to the next higher value (always round up); see Appendix B to Subpart V.

• This conservative rounding applies when converting computed values (for example, T or MAD) to the tabulated entries in Tables 7–14 or when applying formulas in Table V-2.
• Also confirm that the worksite elevation is at or below 900 meters (3,000 ft) before relying on Tables 7–14 directly, as Appendix B limits their use to that elevation range.

See 1926.960(c) and Appendix B to Subpart V for these rules.

For phase-to-phase exposures, the employer must demonstrate that no insulated tool spans the gap and that no large conductive object is in the gap before relying on the alternative phase-to-phase minimum approach distances in Tables 7–14. Appendix B includes this requirement in the notes to the tables; see Appendix B to Subpart V.

• This demonstration is necessary because objects or tools across the phase-to-phase gap could create a conduction path or change the flashover behavior, invalidating the table assumptions.
• Keep documentation of the demonstration and the conditions present at the worksite as part of the work plan.

See 1926.960(c) and the notes in Appendix B to Subpart V for details.

Employers may use switching restrictions, often called "hold-off" or "restriction" procedures, to limit circuit operations during live-line work; these do not physically prevent switching but modify how and when equipment may be operated. Appendix B describes the use of a tagging or hold-off system similar to a permit and notes that the restriction modifies operation rather than absolutely preventing it; see Appendix B to Subpart V.

• Use a clear tagging/hold-off protocol that identifies required system conditions before a circuit breaker can be operated.
• Communicate restrictions to operations staff and control-room personnel and document the restriction in the work permit or job plan.
• Coordinate with system operators to ensure that switching restrictions are enforceable and do not conflict with reliability needs.

See 1926.960 and Appendix B for guidance on implementing switching restrictions.

The sample follows the Appendix B 5-step method: it selected a gap withstand voltage equal to 1.25 times the crest phase‑to‑ground voltage, picked a gap with a tested critical sparkover voltage that met that withstand requirement, used 110% of the tested critical sparkover to get VPPG Peak, computed T = VPPG Peak ÷ VL‑G√2, and then used T in Table V-2/Tables 7–14 to find MAD = 2.29 m. The Appendix B worked example is shown in Appendix B to Subpart V.

Summary of the appendix example steps:

• Step 1: Smallest practical max transient overvoltage chosen as 1.25 × crest phase‑to‑ground voltage for a 550 kV line (resulting in ~561 kV).
• Step 2: Select a gap tested to a critical sparkover ≥ 561 kV; the example used a gap with 665 kV tested critical sparkover.
• Step 3: VPPG Peak = 110% of 665 kV = 732 kV.
• Step 4: T = 732 ÷ 564 ≈ 1.7 p.u.
• Step 5: Use T = 1.7 in the appropriate table (or equation) to obtain MAD = 2.29 m (7.6 ft).

See the worked example in Appendix B to Subpart V and the rule requirements in 1926.960.

Use a withstand voltage that is at least 1.25 times the maximum crest operating voltage as a practical target, and calculate it from the gap manufacturer’s critical sparkover voltage (withstand voltage = 85% of the critical sparkover voltage).

• The Appendix guidance in 29 CFR 1926 Subpart V, Appendix B recommends using a withstand voltage of no lower than 1.25 per unit (p.u.) of the system maximum crest operating voltage to reduce sparkovers from minor disturbances. See also the related rule text in 1926.960(c).

• How to compute it in practice:

  • Obtain the gap’s critical sparkover voltage from the manufacturer (example in Appendix B: critical sparkover = 665 kV for a 1.2 m gap).
  • Calculate the withstand voltage as 85% of that critical sparkover voltage (example: 0.85 × 665 kV = 565.25 kV).
  • Verify that this withstand voltage equals or exceeds 1.25 × the system’s maximum crest operating voltage; if it doesn’t, select a larger gap or other protection so the 1.25 p.u. practical target is met.

• Additional practical notes from the guidance:

  • If you already know the protective gap length, you may reverse the steps (use the known length and manufacturer data to determine critical sparkover, then take 85% for withstand).
  • IEEE Std 516-2009 practice is often used as an extra safety step (employers commonly add 0.2 to the calculated value of T as an additional safety factor).
  • The Appendix clarifies that the 1.25 p.u. recommendation is an operational/practical consideration; in some situations an employer may be able to justify using a lower withstand value, but this should be based on acceptable system performance and documented engineering judgment.

For the authoritative guidance, see 29 CFR 1926 Subpart V, Appendix B and the related provision at 1926.960(c)