Sizing personal protective grounds on a 15kV distribution system 1

[seanl]

How do I determine the correct size for personal protective grounds on an electrical distribution system (#6, 1/0, 2/0 cu)which range for 35kV delivery points to 115 kV substation? Can oversizing protective grounds cause problems?

 

[dpc]

The ground conductors and the connections must be able to withstand the mechanical forces and thermal load imposed by the worst-case short circuit that could occur. You will need to know that maximum short circuit current and worst-case duration for that particular location.

Oversizing is not a problem - the larger sets just get more difficult to handle.

I'd recommend purchasing grounding sets specifically designed and tested for this type of use. There are cases of serious injuries occuring when makeshift grounding conductors were accidentally left on when the circuit was re-energized. The connectors can fail, causing the grounding conductors to fly off in all directions, creating a massive arcing fault in the process.

It's good to assign a tracking number to each ground and make sure all are accounted for before you re-energize.

 

[pablo02]

Following up on dpc's comment for tracking grounds -- we included them on our cleance procedures, assigning them a clearance tag (equivalent to a formal lock-out / tag-out procedure) so that the "ground" had to be physically removed and accounted for prior to energizing the equipment.

 

[rempman]

When it comes to Personal Protective grounds you are treading on a sensitive area.

Refer to ANSI Standard F855 for sizing of conductor and clamps.

You will need to know what your available fault current is in order to size the components properly along with a good idea as to how quickly your protection system functions.

When looking for components make sure they have been tested to meet the ANSI F855 and associated testing standards. A good start on suppliers is Chance, Hastings, White / Safety Line, & Salisbury. We currently specify Hastings 2/0 copper cable with Yellow jacket in that the jacket does not stiffen up significantly in cold waether unlike other "welding" cables. I would resist specifying clear jacket material in that it will cloud over in a relativley short period of time and may give a false sense of security from a inspection prespective.

We test our grounds once a year with a Hastings ground tester. The vast majority of test failures are from corroded clamps, not broken strands. We then clean the clamps with a ceramic bead blaster and they come out better than new.

As for oversizing grounds, that is not problem electrically. However physically it can be a big deal. The workers will want to use the lightest combination possible because it will be easier to apply.

You will also want the cables to be as short as they can practically be. Not only from a weight consideration but also from a possible injury situation. When fault current flows through the cable the resulting magnetic fields generated will cause the cable to move with great force.

As for grounding application standards I would recommend contacting the Western Area Power Administration (WAPA). They have done extensive testing on the application of personal protective grounds and thier work practices are quite detailed and complete. You can also look at Pacific Gas and Electric standards as a great refenece for distribution applications. I would imagine Virginia Power has good standards for 35kV applications, however grounding is not really voltage determined.

There is a great deal more to know about grounding. I recommend you and others of your staff attending the next IEEE Power Engineering Society meetings and sit in on working group discussions.

 

[jbartos]

Suggestion: Visit

http://www.technicaldiagnostic.com/training/TDS Training - OSHA Electrical Safety.htm

http://search.netscape.com/ns/boomf...2B+%22cfr%22&clickedItemURN=http%3A%2F%2F

http://www.osha.gov%2Fpls%2Foshaweb...roup%3Fp_toc_level%3D1%26p_part_number%3D1926

for: OSHA rules.
etc. for more info

 

[busbar]

A suggested reference is IEEE Std 1048-2003 Guide for Protective Grounding of Power Lines

Grounding knobs {Safety Line} and grounding balls {Chance} have through-fault ratings consistent with correctly sized, rated and assembled grounding clusters. For use in metal-enclosed gear, BIL-rated insulating covers are also available.

 

[jbartos]

Suggestion: Beside safety aspects, e.g.
Use of personal protective grounds (29 CFR 1926.954(e)),

http://www.maintenanceresources.com/productsshowcase/industrial.html

Part 6: Installing and Removing Protective Grounds,
contact the manufacturer for technical data sheets, and recommendations.

 

[busbar]

Another reference is IEEE Std 1246-2002 Guide for Temporary Protective Grounding Systems Used in Substations