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 Post subject: Does the spanner melt?
PostPosted: Wed Sep 25, 2013 10:39 pm 

Joined: Mon Oct 08, 2012 2:59 am
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Location: India
Lets just say I dropped an uninsulated spanner onto a bus bar. The spanner has initiated a bolted fault. Now does the spanner melt to initiate an arcing current or does the spanner slip down (due to gravity) while leaving the plasma for an arc to strike?
Your views please


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PostPosted: Thu Sep 26, 2013 3:45 am 
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Option 3. Busbars become rail gun and spanner is thrown off magnetically, while initiating and arc that is magnetically propelled away from the power source until it reaches enough of a barrier for the plasma to pool. Likely spanner is damaged as well.


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PostPosted: Thu Sep 26, 2013 5:19 am 

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PaulEngr wrote:
Option 3. Busbars become rail gun and spanner is thrown off magnetically, while initiating and arc that is magnetically propelled away from the power source until it reaches enough of a barrier for the plasma to pool. Likely spanner is damaged as well.


For initiating an arc it needs a conducting media. Is it wrong to assume that a part of spanner is melted to give enough enough conducting media for the arc to strike and then sustain? Then would come the rail gunning of spanner. How good is this?


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PostPosted: Thu Sep 26, 2013 3:29 pm 
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Arc is initiated by spanner through conduction. I am assuming its made of steel. No melting necessary because after that as air temperature increases, conductivity increases, and the critical flashover potential decreases. This is the reason that arcs are self sustaining in the first place because every time we reach a current zero, the arc extinguishes. Then as the voltage reaches the critical flashover potential, the arc restrikes. This repeats until something makes it no longer possible to restrike.


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PostPosted: Thu Sep 26, 2013 3:33 pm 
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You are assuming that air is always an insulator and that metals are always conductors. This is temperature dependent. As with phase changes, it is both material and temperature dependent. There are just a few oddities. Carbon conductivity decreases with temperature. it is this property that makes smelting (reducing iron, copper, zinc ores to their pure metal forms) possible.


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PostPosted: Fri Sep 27, 2013 10:10 am 
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PaulEngr wrote:
Option 3. Busbars become rail gun and spanner is thrown off magnetically, while initiating and arc that is magnetically propelled away from the power source until it reaches enough of a barrier for the plasma to pool. Likely spanner is damaged as well.

Yep! I did this in the lab by laying a a short piece of bus bar across the blades of a pad mount transformer. (It was the end of the day and the end of testing so we were amusing ourselves) There was the expected loud boom followed immediately by two loud "pings" as the bus bar bounced off the adjacent wall and hit the floor. I would not want to be in it's path. :eek:

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PostPosted: Tue Oct 01, 2013 4:36 am 

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Thank You PaulEngr and Jim. Seems like there has been a good amount of research on the causes and physics of arc flash. Got to read a lot.
Thanks again
Regards
Girish


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PostPosted: Tue Oct 01, 2013 5:46 am 

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Definately get an arc flash. It destroyed the whole 480V section of one of our subs many years ago. Wrench was left inside the gear during installation and about 2 weeks later it finally vibrated loose and fell on the bus.


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PostPosted: Fri Oct 04, 2013 1:57 pm 
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Jim Phillips (brainfiller) wrote:
Yep! I did this in the lab by laying a a short piece of bus bar across the blades of a pad mount transformer. (It was the end of the day and the end of testing so we were amusing ourselves) There was the expected loud boom followed immediately by two loud "pings" as the bus bar bounced off the adjacent wall and hit the floor. I would not want to be in it's path. :eek:

Wow - is that a job? :) . Just curious, has anyone experienced (or know someone that has experienced) a screwdriver, wrench or other tool being thrown from an arc flash? All I have heard of in a real world experience is melting the end off a screwdriver and just having a stub left.


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PostPosted: Sat Oct 05, 2013 3:47 am 
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Google "rail gun". Second, mostly I have seen the same screwdrivers but they do get ejected, not melted in place unless there is something to hold them there. Also look up AIC and what is important because if everything is not fastened tightly in place, magnetic force is proportional to I^2 so even small changes in current become the difference between bus bars flying apart and staying together.


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PostPosted: Mon Oct 07, 2013 8:14 am 

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This happened to an un-insulated crescent wrench being used by a buddy of mine when it slipped on him while bolting a cable onto a live bus bar. As near as I could determine, the points of contact were relatively high in resistance, so the greatest amound of heating occured there. Enough to vaporize a pea sized chunk of the copper bus bar, a dime sized hole in the aluminum splitter and small chunks of the cresent wrench. The wrench flew approximately 100 feet and left quite a dent in the wall where it hit. I would assume that the vapourization of the metal and possibly the combination of vaporized metal with oxygen in the air created the explosive force to propel the wrench as well as providing an initial path for the ensuing arc flash. Buddy was lucky enough to survive but was flash blinded for several days and still has burn scars on his hand.


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PostPosted: Mon Oct 07, 2013 9:19 pm 
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David Chief wrote:
This happened to an un-insulated crescent wrench being used by a buddy of mine when it slipped on him while bolting a cable onto a live bus bar. As near as I could determine, the points of contact were relatively high in resistance, so the greatest amound of heating occured there. Enough to vaporize a pea sized chunk of the copper bus bar, a dime sized hole in the aluminum splitter and small chunks of the cresent wrench. The wrench flew approximately 100 feet and left quite a dent in the wall where it hit. I would assume that the vapourization of the metal and possibly the combination of vaporized metal with oxygen in the air created the explosive force to propel the wrench as well as providing an initial path for the ensuing arc flash. Buddy was lucky enough to survive but was flash blinded for several days and still has burn scars on his hand.


No. It was simple magnetism. If you had an oxy fuel propellant, you would also have to confine it to direct it (think rocket nozzle). The force generated in a bolted fault is huge. You get .85x.85 =
72% of the bolted force in an arcing fault.


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PostPosted: Mon Oct 07, 2013 10:16 pm 

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I totally agree with you PaulEngr that the magnetism is enormous and the conductor gets rail gunned. But what Mr. David Chief is saying, adds to the discussion in a way that, for the vaporization of metal Copper may aid the arc to strike.


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PostPosted: Thu Oct 10, 2013 8:26 am 

Joined: Wed May 06, 2009 12:12 pm
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Girish,
An EPRI video on youtube shows exactly what you're looking for. [media=youtube]fZP47mlELSc[/media]. I show portions of this video in my electrical safety classes. At 2:40 in you will see a tool blown away from a fault. At 3:40 in they indicate that 480 volts is some of the most dangerous voltages available for arc flash and at 4:40 in they indicate that lower current faults are worse for arc flash than higher current faults. This ALWAYS gets the attention of the electricians and plant operators that are my audience. Hope this helps.


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PostPosted: Mon Oct 28, 2013 9:50 am 

Joined: Wed Nov 07, 2012 5:34 pm
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Thank you very much for posting this video


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