IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 16, NO. 3, JULY 2001 429

Estimating the Ignition Hazard of 100% CottonClothing Worn by Transmission and Distribution

Line WorkersTask Force 15.07.04.02 ESMOL SUBCOMMITTEE

AbstractThis paper describes a method of estimating the faultcurrent magnitude necessary to ignite 100% cotton clothing wornby transmission and distribution line workers.

I. OVERVIEWA. Scope

THIS paper provides technical evaluation criteria for esti-mating the magnitude of the electric arc current requiredto ignite 100% cotton fabric worn by transmission and distribu-tion line workers inadvertently exposed to the thermal hazardsof an electric arc.

This paper is intended to suggest methods for initial evalua-tion of the ignition hazard assessment of clothing normally wornby transmission and distribution line workers when workingon overhead lines operating at 4160 volts, phase to phase, andabove. Once the hazard has been identified a further risk anal-ysis is required.

The paper does not consider the effects of three-phase arcs, orarcs involving reflected energy, as can be found inside enclosedspaces or equipment. These effects are being studied and maybe included when they are quantified. In any event, these typesof arcs are not commonly found in overhead work.

The paper does not address effects other than electric arcs.An employers hazard assessment must take other factors intoconsideration when determining whether or not 100% cottonclothing is acceptable.

ASTM standards provided methods used to test fabrics forignition and the thermal performance of material.

B. ObjectiveThe objective of this paper is to present sufficient details for

the performance of a technical evaluation of clothing that willnot ignite during transmission and distribution overhead linework.

C. Regulatory RequirementsOSHA 1910.269(l)(6)(iii) [1] contains requirements that

The employer shall assure that each employee who is exposedto the hazard of flames or an electric arcs, could increase theinjury that would be sustained by the employee."

OSHA enforcement criteria [2] regarding the use of 100%cotton fabrics given in OSHA draft Appendix C indicates that

Manuscript received September 24, 1999.Publisher Item Identifier S 0885-8977(01)04679-9.

TABLE I [3]IGNITION THRESHOLD FOR COTTON FABRICS

NOTE: Fleece worn as outside clothing isextremely flammable.

To be considered reasonable, an employers estimate of themaximum heat energy to which an employee would be exposedmust be based on the assumption that;

The electric energy consumed by the arc would gen-erate the maximum available power and, that for hot stickwork that places the employee to the side or above the ex-posed energized parts, the employer may determine thatthe arc will occur at a distance greater from the employee.However for any work method, the employer must considerthe different types of mechanical failures and human errorsin determining the distance (which in any case will be lessthan the actual approach distance).Appendix C further states that If the maximum heat energy

estimated by the employer does not exceed the value in Table Ifor a particular weight and color of a cotton material, then anouter layer of that material would not be expected to ignite andwould be permitted under paragraph 1910.269(l)(6)(iii).

II. DEFINITIONS

A. Arc EnergyThe total energy liberated to the surrounding space by an elec-

tric arc.

B. Arc GapThe distance between the electrodes.

C. Arc VoltageThe rms. value of the voltage drop across the arc column

caused by the arc resistance and the electrode drops.

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430 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 16, NO. 3, JULY 2001

D. Arc CurrentThe rms. value of the current in the arc.

E. DurationThe time span of the arc from initiation to extinction in cycles

of the 60 Hz current.

F. Hazard ZoneThe line length where the heat energy exceeds the ignition

threshold of the fabric for the assumed parameters.

G. Heat FluxThe thermal intensity indicated by the amount of energy

transmitted per unit area and time. (cal/cm /s or W/cm ).

H. IgnitionThe property of a material involving the ignition of com-

bustion accompanied by heat and light, and either continuedburning for at least 2 seconds or consumption of at least 25%by area of the cotton fabric.

I. Ignition ThresholdThe minimum value of heat energy at which the material will

reach ignition temperature and start to burn. In this guide thatvalue is taken as the 10% probability of ignition with a 95%confidence level.

J. Incident HeatThe value of energy that is impinged on the receiving surface.

K. OPSY (Ounce per Square Yard)Fabric weight in ounce per square yard of the material.

L. Separation DistanceThe distance from the center line of the arc to the receiving

surface at the worker.

M. Trigger CurrentThe value of arc current required to reach the ignition

threshold heat flux for the parameters of that utility. Arc currentmagnitude above this value will trigger a risk analysis.

N. Minimum Approach Distance (MAD)The distance required to withstand the voltage stress plus

an adder for inadvertent movement. The MAD is specified inregulations.

III. CRITERIA FOR DETERMINING IGNITION THRESHOLDSA procedure to establish the system arc current magnitude

that will NOT ignite 100% natural cotton fabrics normally wornby electric utility line workers is given.

Step 1: Determine the minimum weight and color of cottonclothing normally worn by the line workers.

The normal clothing worn will probably be different forwinter and summer wear. It will be different in different regions

TABLE IIINSULATED RUBBER GLOVES

of the country. The selection for this initial evaluation shouldbe the minimum weight and darkest color clothing worn by lineworkers who may be exposed to the thermal hazard of an electricarc.

As indicated in Table I, fabric color may have a significantaffect on the ignition threshold. Fabric weave and weight alsoaffect ignition. The worker normal color and weight combi-nation should be selected and used in the analysis.

Clothing tests have indicated that a cotton fabric has a 50%probability of ignition in the range of 0.8 to 1.2 cal/cm /oz withthe average about 1. If the characteristics of the fabric is notknown, a conservative estimate of the ignition threshold can bemade by taking the 50% ignition level as 0.8 cal/cm /oz for thefabric and multiplying by 0.8 to estimate nonignition.

Step 2: Establish the ignition threshold heat flux for thenormal weight and color clothing selected in Step 1.

Step 3: Determine the fault current magnitude that representsthe ignition threshold for the clothing worn by the worker.

Calculate the fault current magnitude that will result in theignition threshold level for the weight and color of the clothingworn by the worker.

Computer programs [4], [5] are used to determine the faultcurrent magnitude that will produce the threshold energy thatwill reach the workers clothing. The programs will not produceidentical results but can be used to establish threshold currentlevels for additional calculations. The incident energy levels cal-culated in this guide were calculated using Arc Pro [4].

The following input is required for the programs.a) Operating Voltage: The maximum system operating

voltage should be used.For distribution and transmission voltages, the operating

voltage generally will not be a limiting parameter. The arcdrop voltage will be in the order of 50 volts per inch so thesystem voltage will support arcing distances much longer thananticipated. However the maximum operating voltage is usedat each of the system voltage levels.

b) Arc Duration: Arc duration depends upon the utilityprotection schemes and can vary quite widely between utili-ties. Each utility therefore must establish representative faultclearing times for the fault magnitudes anticipated.

For high current magnitude faults on most transmission anddistribution systems, the primary clearing time of 0.1 second(6 cycles) is reasonable. At EHV voltages 3 cycles can be used.For low magnitude faults, clearing times depend upon protectivedevice coordination and will vary widely between utilities. Eachutility must establish the normal clearing times for the systemfault involved and calculate the incident heat flux resulting fromthose fault magnitudes and clearing times.

TASK FORCE 15.07.04.02: ESTIMATING THE IGNITION HAZARD OF 100% COTTON CLOTHING 431

TABLE IIIGLOVING METHOD

TABLE IVHOT STICK METHOD

Maximum Fault Current not to exceed 4.6 cal/ cm2 incident energy

c) Arc Gap Distance: It is assumed that, as a worker isperforming a task, the initial arc is struck when the worker takesa conductive article within the 60 Hz breakdown distance in air.The dielectric strength of air is taken at 10 kVrms/inch.

d) Separation Distance: At distribution voltages, wheregloving work is used, the length of the glove and, if used,the length of the sleeve represents the separation distance toclothing. For hot stick and barehand work methods, NESC1997 [6] Table IV41-1 minimum approach distance, minustwice the sparkover distance in air for that operating voltage,represents a reasonable allowance for mechanical failure or aviolation of the ergonomic distance.

With these parameters, the maximum arc current that willgenerate the allowable incident heat flux, determined in Step 2can be calculated.

Step 4: Compare the station line-to-ground fault capabilityto the ignition threshold trigger current.

System fault calculations normally do not include any arc orground resistance and are therefore conservative for estimating

the number of stations that could have faults that might exceedthe ignition threshold.

Step 5: Determine the zone of hazard for utility line workersby adding the line impedance, which will limit the arc currentto the trigger magnitude.

Step 6: Review the parameters used in the evaluation, workpractices and procedures and evaluate the options available toreduce the risk of ignition for these locations and tasks.

1. Review the Fault Current Magnitudes.The calculations are somewhat conservative, as they do not

include the arc and earth resistance. For stations with high mag-nitude fault currents there may be ways to sectionalize and re-duce fault current during the time that work is in progress. Sincethe current is directly proportional to the heat, reducing the cur-rent level will reduce the heat energy.

2. Review the Clearing Times.For high magnitude fault current stations, faster clearing

times than used in the evaluation may be available. Note thatthe fault duration is directly proportional to the incident energy.

432 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 16, NO. 3, JULY 2001

Cutting the clearing time in half will result in 1/2 the incidentenergy.

3. Increase the Separation Distance.Since the incident energy is a function of the separation dis-

tance to the 2.2 power, an increase in that distance by a changein work practice will reduce the incident energy.

4. Change the Weight of Clothing Worn by the Worker.

IV. WORK METHODS

A. GlovingAt distribution voltages, live line work is done using dielectric

gloves. Table II gives the maximum use voltage and the lengthof the glove from the crotch of the thumb to the cuff for the min-imum length glove in that class. The minimum separation dis-tance is determined by using the glove length, stated in ASTMD120, minus 4 inches for the distance from the fingertip to thethumb crotch and minus twice the 60 Hz sparkover distance.

B. Hot SticksFor hot stick work methods, the MAD, less 2 times the 60 Hz

sparkover distance is used.

C. BarehandWhen barehand work methods are used, the separation dis-

tance is determined by the work practice. However the workerwill be wearing a conductive suit made from FR fabrics and notfrom 100% cotton.

D. ExampleWork Methods by VoltageTables III and IV are examples of the maximum current mag-

nitudes calculated by this method for gloving from 4.16 through36 kV and from 46 to 800 kV based on a 5.2 OPSY blue twillnatural cotton fabric.

REFERENCESThis guide should be used in conjunction with the following publications.[1] Electric Power Generation, Transmission and Distribution,, 29 CFR

Part 1910.269.[2] OSHA Draft Appendix C, Clothing, to CPL 2-1.18A Guidelines for the

Enforcement of 29 CFR 1910.269,.[3] N. Doughty and D. Bingham, Testing update on protective clothing and

equipment for electric arc exposure,, Paper PCIC 97-35.[4] Arc ProComputer Program, Ontario Hydro Technologies.[5] A. Privette, Heat Flux Calculator: Duke Power.[6] National Electric Safety Code 1997, ANSI C2-1997.[7] Standard Specifications for Rubber Insulating Gloves, ASTM D-120-87.[8] Standard Performance Specification for Textile Materials for Wearing

Apparel for Use by Electrical Workers Exposed to Momentary ElectricArc and Related Thermal Hazards, ASTM F 1506-94.

[9] Standard Test Method for Determining the Ignitability of Clothing by theElectric Arc Exposure Method Using a Mannequin, ASTM PS 57-97.

[10] Standard Test Method for Determining the Arc Thermal Performance(Value) of Textile Materials for Clothing by the Electric Arc ExposureMethod Using Instrumental Sensor Panels, ASTM PS 58-97.

[11] Standard Specification for Arc and Flame Resistance Rainwear, ASTMF 18.40 D9.

[12] Standard for Electrical Safety Requirements for Employee Workplaces1995 Edition, NFPA 70E.

[13] Protective Clothing for Structural Fire Fighting SF 1971-91, NFPA1991.

[14] Guide for Care and Maintenance of Flame Retardant and Thermal Pro-tective Clothing, ASTM F 1449-92.

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