temporary protective grounds - rmel

RMEL Safety Conference

March 28‐29, 2018Gary Zevenbergen, Western Area Power Administration, Electrical Engineer

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Subject, Office or event

Temporary Protective Grounding

in High‐Fault Current Locations

29‐March‐2018

RMEL 2018 Safety Conference

Lone Tree, Colorado


Temporary Protective Grounds

• Temporary Protective Ground cables (TPGs)• Primary Purpose: Protect the worker from electric shock hazards while working on de‐energized lines or equipment.

• Are the most effective means of protecting against electric shock if:

• Properly designed

• Correctly installed

2RMEL 2018 Safety Conference



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Regulations

• OSHA 1910.269(n)• Protective grounding equipment shall be capable of conducting the maximum fault current that could flow at the point of grounding for the time necessary to clear the fault.

• Note: ASTM F855–09, contains guidelines for protective grounding equipment.

• ANSI C2‐2017: National Electric Safety Code• The grounding device shall be of such size as to carry the induced current and anticipated fault current that could flow at the point of grounding for the time necessary to clear the line.

• Note: Refer to ASTM F 855‐04 [B22] for specifications for protective grounding equipment.



Specification: ASTM F 855

• Recognizes ground cables as an assembly(clamps, ferrules, cable)

• Provides standardized materials(size, material strength, & mechanical properties)

• Provides short‐time current ratings of all components

• Provides material testing requirements




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Application Guides


IEEE Std 1048 IEEE Std 1246


Technical Papers




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• Two Primary Objectives1. TPGs must be capable of carrying the maximum

available fault current for the required clearing time.• Applies to both thermal and mechanical energy.

2. TPGs must limit the exposure voltage at the work site to safe levels.• Proper grounding equipment in conjunction with proper work practices must protect the worker from hazardous exposure voltage levels.


It cannot be assumed that accomplishing one objective automatically accomplishes both objectives




• Objective 1: Short‐time current rating• ASTM F 855 provides short‐time current ratings: Table 1

DUltimate values are based upon application of Onderdonk’s equation to 98% of nominal circular mil area allowed

by Specification B172 and B173

AWithstand and ultimate short circuit properties are based on performance with surges not exceeding 20% asymmetry

factor.20% Asymmetry ≈ (X/R ≤ 1.8)



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Fault Current Characteristics

• A short circuit (fault) in an electrical circuit is a sudden change in circuit impedance (usually much lower) causing a sudden increase in current.




• Short Circuit Programs • Used to predict the maximum available fault current using a computer model of the power system.


Ifmax = 20,000 Arms



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• How does the predicted fault current compare with actual fault current.


Offset

Decay Time



• Fault Current Offset (DC Component)• Determined by the point on the voltage sine wave when fault occurs.

• Random, Unpredictable (Worst case occurs @ 90°)

• Decay Time:• Determined by the X/R ratio of the system as seen from the fault location.

• High X/R ratios – Long Decay Times• Increases the thermal energy the cable must absorb• Imposes greater mechanical forces on the ground cable


EquivalentSource

Fault LocationREQ XEQ



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Effect of X/R on Decay Time


Decay Time

Decay Time

Decay TimeDecay Time



• Objective 1: Short‐time current rating for High X/R• ASTM F 855 provides short‐time current ratings: Table 2


Correlate Grade Size to Conductor Size

1H ‐> #22H ‐> 1/03H ‐> 2/04H ‐> 3/05H ‐> 4/06H ‐> 250 MCM or 2‐2/0 7H ‐> 350 MCM or 2‐4/0



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• What is the basis for ASTM F 855 Table 2?

• ASTM F 855• 1.6.1 Currents presented in Table 2 are based upon the values from EPRI Project RP2446 Computer Program RTGC “A Desktop Computer Program for Calculating Rating of Temporary Grounding Cables” and are to be used in situations involving an asymmetry value greater then 20 % (X/R > 1.8), see Appendix X4.


Ratings account for the mechanical energy/stress as well thermal energy.



• Based on the results of EPRI Project RP2446,

• WAPA computed 15 & 30 cycle Ultimate cable ratings based on the X/R ratio

• (0 – 50).

• Created Withstand cable ratings based on the new Ultimate cable ratings.


15 30 15 30 15 30

X/RUltimate

(A)

Ultimate

(A)

Ultimate

(A)

Ultimate

(A)

Ultimate

(A)

Ultimate

(A)

0 30,549 21,605 38,522 27,239 61,253 43,314

1 30,083 21,438 37,923 27,025 60,303 42,973

2 29,990 21,404 37,807 26,983 60,118 42,906

3 29,899 21,371 37,691 26,941 59,934 42,839

4 29,787 21,330 37,550 26,889 59,710 42,757

5 29,663 21,284 37,394 26,832 59,462 42,666

6 29,534 21,236 37,231 26,771 59,202 42,570

7 29,401 21,187 37,064 26,709 58,937 42,470

8 29,267 21,137 36,896 26,646 58,669 42,370

9 29,133 21,086 36,727 26,582 58,400 42,268

10 29,000 21,035 36,558 26,518 58,133 42,167

11 28,867 20,984 36,391 26,454 57,867 42,065

12 28,736 20,994 36,225 26,390 57,603 41,963

13 28,605 20,883 36,061 26,326 57,341 41,862

14 28,476 20,833 35,898 26,262 57,082 41,760

15 28,348 20,782 35,736 26,199 56,826 41,660

Duration (cycles)Duration (cycles)

Cable Size

Cross ‐Sec (Sq‐mm) (98%)

Initial Temp (C)

Final Temp (C)

Initiation Angle (deg)

1/0

52.41

90

50

1,083

Duration (cycles)

2/0

66.07

50

1,083

90

4/0

105.06

50

1,083

90



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• What about work sites with fault current that exceeds the single cable rating?

• Use a larger cable• Works ok until you get larger than 4/0

• Use more than one cable

• Generally speaking for fault currents > 40,000 Amps• Multiple ground cable assemblies per phase may be needed.


Conductor Weights (lbs./ft.)– 2/0 AWG – 0.424– 4/0 AWG – 0.676 (+59%)– 250 MCM – 0.802 (+19%)– 350 MCM – 1.120 (+40%)


Multi‐cable Considerations

• IEEE Standard 1048‐2016 • Identical Cable Assemblies

• Apply De‐rating factor to single‐cable rating

• Install cables close together

• Only full current testing can determine the actual short circuit capability

• In the absence of actual test data, de‐rate the TPGs by at least 10% when multiple cables are required.




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• IEEE Standard 1246‐2011 • To be sure that balanced current flows through each TPG, the following items can be made equal:

• Size and type of stirrups

• Size and type of clamp

• Length and ampacity of each conductor

• Similar connection of each conductor in the clamp

• Torque applied to each clamp

• Install cables close together

• To account for unequal current division, reduce the thermal current rating by at least 10% of each TPG used in the multiple assembly set




• ESMOL Technical Paper: “Factors in sizing Protective Grounds”

• When it is essential that more than one cable be employed, the following requirements should be observed:

• The cables shall be of equal length

• The cables shall be of equal cross‐section

• The cables shall be from the same material, stranding, and lay distance.

• The same connecting parts and components shall be employed

• The cables shall be installed next to each other.

• De‐rate the cable ratings by

• 10% if the cables are restrained

• 20% if the cables are not restrained




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• ASTM F 855‐2015• To account for unequal current sharing, it is recommended that the subsequent multi‐TPG per phase rating be reduced by at least 10 %.

• Users seeking applications of multiple assemblies should perform their own tests to determine the ratings for multiple TPGs per phase for both low and high asymmetry.



High‐Current Multi‐Cable Test


The Desired Outcome



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• Successful deployment of multi‐cable grounding for high‐current locations requires more than just properly rating the cable assemblies.

• Special attention must be paid to:• Cable Assembly Construction

• Cable Assembly Installation



Cable Assembly Construction




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• Cable pulled out of the ferrule




• Crimping a ferrule onto the cable must be done such that maximum strength is achieved.

• It may require a different crimping process.

• Any crimping specification applies to a specific cable/ferrulecombination.

• Perform pull tests on conductor/ferrule to validate strength of crimping process.


Burndy Crimp Finn Power



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Cable Assembly Installation




• Clamp Spacing and Current Distribution• The more spacebetween the clamps, the more divergent the current distribution becomes.

• More than 3 inches, usually resulted in cablefailure.

28RMEL 2018 Safety ConferenceCourtesy of Duke Energy



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• Just because a clamp, ferrule, and cable is rated 5H (47,000 Amps) as a single assembly

• Does not guarantee that two such assemblies will survive when installed in parallel at 80,000 Amps.







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• Clamp Spacing:• Mechanical Torque on the Clamps.

• Placing clamps close together reduces the rotational torque dueto the attractive forceapplied to the cables.

• Best option: install the clamps so they touch each other.


ForceForce ForceForce



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• Multiple Ground Cables & Current Density• The more TPGs that are paralleled,

• The current carried by the last TPG incrementally decreases

• The overall performance uncertainty of the parallel TPGs increases.

• Minimizing the number of parallel cables increases the probability of successful performance

33RMEL 2018 Safety ConferenceCourtesy of Duke Energy


Cable Attachment Points

• IEEE Std 1246‐2011• Fixed‐point protective grounding terminals attached to the bus conductors, equipment terminals, or structures have been gaining acceptance in the utility industry.

• These terminals provide an attachment point for protective grounds that lends itself to the adaptability of standard clamps.

• This avoids forcing these clamps to conform to a wide range of conductor sizes and configurations.

• These fixed attachments (studs and stirrups) need to be able to withstand, mechanically and electrically, the available short‐circuit current.

• The corona protection of the attachment points needs to be considered.




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Custom Attachment Designed by DMC Power for WAPA



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Summary

• Insure that ground cable assemblies are rated properly.

• Most HV and EHV substations and lines exceed ASTM F 855 Table 1, X/R ≤ 1.8

• Near power plants or large EHV transformers• X/R ratios can exceed 30.

• If cable assemblies are not properly designed• High‐current tests have validated that ground cable assembly failure occurs within the first few cycles with high X/R locations



Summary

• Insure that ground cable assemblies are constructed properly.

• Insure that the cable‐to‐ferrule crimping process provides the necessary mechanical strength.

• Have a QA process in place to insure that the cable assemblies meet the specification standards

• Only use clamps that have proven to survive multi‐cable installations.




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Summary

• Insure that ground cable assemblies are installed properly.

• Pay attention to the spacing between the clamps.

• If possible include restraints to limit cable movement.

• Provide designated attachment hardware.



Questions?


temporary protective grounds - rmel

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