| Arc Flash Forum https://brainfiller.com/arcflashforum/ |
|
| Grounding MV in High Calorie Environments https://brainfiller.com/arcflashforum/viewtopic.php?f=5&t=5637 |
Page 1 of 1 |
| Author: | Kenneth Sellars [ Mon Dec 05, 2022 7:02 am ] |
| Post subject: | Grounding MV in High Calorie Environments |
I do a lot of consulting with large power generation sites, of course, mainly utilities. In doing so, I see very large arc flash levels, some as high as 140 cal/cm2, and rarely but sometimes over 200 cal/cm2. Obviously, we get into a situation at these levels of "what is the proper PPE?" question. Often this equipment is older, so no options like grounding switches are installed. The procedure involves remote switching when available (often not an option), verifying lack of nominal voltage, and then installing grounds via a live line tool (shotgun stick). The arc flash boundaries, of course, at these high fault current levels can be as high as 50 feet, and are usually calculated at a 36" working distance at the 15 KV range. I am discussing specifically indoor metal-clad switchgear with internal grounding ball studs in most cases. As far as I know, there are arc flash suits available at 140 cal/cm2 (one brand I know of for sure). The suits are obviously very thick, and the "gloves" are actually mittens with no individual fingers. The electricians have to do Live-Dead-Live with a proximity detector, and then hang grounds to establish the required electrically safe work condition. Obviously, once the switch is open, and the cabinet door is opened, the arc blast issue is not nearly as serious (no ability for compression forces), and the grounds are typically hung with 6 foot hot sticks - never at the 3 foot working distance. This obviously lowers the incident energy significantly using the inverse square of distance concept. I have a couple of companies that demand the workers hang grounds in 140 cal suits, no matter the hot stick distance. I cannot seem to talk logic with them about lowering the required PPE as long as the workers stay back a certain distance. One example is a site with a 108 cal/cm2 rating, but with a 6 foot hotstick, the levels are significantly under 40 cal/cm2. Just trying to get a feel how others are handling similar situations. |
|
| Author: | stevenal [ Mon Dec 12, 2022 8:40 am ] |
| Post subject: | Re: Grounding MV in High Calorie Environments |
A few thoughts: Sounds like you are using NFPA rather than the NESC. NFPA specifically excludes utilities. NESC uses tabulated numbers generated by ArcPro, and include distances based on hot sticking. These distances may be appropriate to use in a study if you choose to not use the tables. (MAD-2*arc gap length) The NESC rule is for working on or near energized stuff. Once the voltage test has been made and the circuit is proven deenergized, are there still nearby energized parts? If not, encumbering arc flash PPE should be allowed to be removed following the test. An exception in NESC allows employers to assess whether the clothing causes a greater hazard than the exposure. A second exception allows the use of insulating gloves with leather protectors. |
|
| Author: | wbd [ Thu Dec 15, 2022 9:37 am ] |
| Post subject: | Re: Grounding MV in High Calorie Environments |
Actually, OSHA 1910.269 should be used which does list IEEE 1584 as suitable for the voltages discussed. The NESC seems to focused alot on line work and substations but not in the area of generation. Much of the equipment utilized in generation stations is the same equipment used in industrial facilities so the analysis should be the same. As far as the OP's situation, if the safety people don't want to acknowledge that the arc flash hazard can be reduced by increasing the distance from the hazard by use of hotsticks or other methods, not sure there is much you can do. Typically the utility companies have safety committees with union representation and I would think the union would be able to have some say as with that type of PPE, the job may not be any safer and could be less safe. |
|
| Author: | Kenneth Sellars [ Tue Dec 20, 2022 8:45 am ] |
| Post subject: | Re: Grounding MV in High Calorie Environments |
I suppose I should have specified, but yes - definitely in a generation environment. The sites are using 1584 in the calculations as Barry expressed. And again, Barry, you are correct. The union is actually the ones questioning WHY they can’t lower their PPE with being back further from the equipment. |
|
| Author: | stevenal [ Wed Dec 21, 2022 8:50 am ] |
| Post subject: | Re: Grounding MV in High Calorie Environments |
The same (MAD-2*arc gap) length can be found in 1910.269. See the second note in table 7 of appendix E. The table is for phase to ground faults in open air, but may be a reasonable way to increase distance conservatively for this situation. |
|
| Page 1 of 1 | All times are UTC - 7 hours |
| Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group http://www.phpbb.com/ |
|