Science of Arc Flash & Engineering

[tersh]

Question- is the office service open delta midpoint grounded? Is there a neutral to the service? Do you have pics of the service panel inside?

There is a neutral wire to the service panel in the office building. No problem about that. Generally the power utility has neutral or ground wire even to service entrance in residential as well as commercial. Only problem is the contractor. In residential, they omit adding the neutral (ground) wire to the service panel because none of our appliances have any EGC. In the office building. The contractor added ground wire to the service panel because the cost in commercial electrical contractor is more expensive. So they are more willing to share a wire or two. But then the ground wire is only up to the panels. All outlets in the wall don't have additional third wire, again because none of our appliances have any EGC.

Bottom line is. In my parents home, there is no third ground wire. In the office building, there is additional third ground wire (neutral).

It's an open delta because one of the leg is 207.84V which the electrician has measured and confirmed. I have discussed it previously with multiple US electricial engineers late last year.

What we didn't discuss was about the arc flash. At that time I didn't know the meaning of arc flash. I thought it was just flash from arc. I didn't know it was 6 times hotter than the surface of the sun. That is why renaming to arc explosion is better than using innocent sounding term like arc flash.

 

[mbrooke]

There is a neutral wire to the service panel in the office building. No problem about that. Generally the power utility has neutral or ground wire even to service entrance in residential as well as commercial. Only problem is the contractor. In residential, they omit adding the neutral (ground) wire to the service panel because none of our appliances have any EGC. In the office building. The contractor added ground wire to the service panel because the cost in commercial electrical contractor is more expensive. So they are more willing to share a wire or two. But then the ground wire is only up to the panels. All outlets in the wall don't have additional third wire, again because none of our appliances have any EGC.

Bottom line is. In my parents home, there is no third ground wire. In the office building, there is additional third ground wire (neutral).

It's an open delta because one of the leg is 207.84V which the electrician has measured and confirmed. I have discussed it previously with multiple US electricial engineers late last year.

What we didn't discuss was about the arc flash. At that time I didn't know the meaning of arc flash. I thought it was just flash from arc. I didn't know it was 6 times hotter than the surface of the sun. That is why renaming to arc explosion is better than using innocent sounding term like arc flash.

Thank you for clarifying.


If you really want to see something ugly, Google arc blast.

 

[mbrooke]

The reasons I inquired about arc flash and asked a lot of questions all these months was because 4 years ago in year 2015. I witnessed an electrical accident by the contractor electrician in our office building. This was the reason I kept asking US electricians/engineers to learn why our local electricians were so inept. So consider me an engineer scrutinizing the safety of our contractor and electrician and this will be my last installation related question as I know I have reached the limit of what is possible.

In the above which occurred in year 2015. The contractor was asked to replace a breaker. He sent two electrician who convinced my boss that it's normal for them to replace a breaker live. All they did was put a lot of cardboard around it, then insulate each live wire. The electrician showed us his slippers and told us as long as his feet don't touch ground. He won't get electrocuted. When he inserted the third wire. There was a big flash which melted the screws and even affected the panel wall chassis. He later got second degree burn in his arms and some spark even reached his chest.

When the event occurred. We weren't exactly sure what caused it. The electrical engineer who was head of the contractor company didn't know either. Our electrical engineers only mostly signed and sealed plans and double check them. They don't have much field experience. Other become electrical engineers to sell products. So it was still bit of a mystery what occurred.

Until I read about arc flash weeks ago. What happened was initially when the his electrician companion tried to connect them live. He caused a short which deposited carbon all over the breaker. When the second electrician tried to connect them live again. The carbon caused an micro arc flash that burnt his arms and melted the screws and deposited the flash to the chassis wall?

Or was it not arc flash and just regular short which can do that? Arc flash should cause 3 meter wide effects. I was standing 1 meter from the panel and beside the two electricians so since it's only localized. It was not arc flash and just short? The breaker in the transformer didn't trip and there was no main breaker before the breaker in picture above.

After that incident. I was so afraid of electricity. Not only that. I don't trust the local electricians and even the contractor. Hence this was the reason I was asking advice from international forum like this. It's not for DIY (Do It Yourself) but just to scrutinize the local practice of electrician and engineers so I'd make sure I hire skilled ones.

Genuinely carbon initiated arc flash in the industry also exists. Remember the case of Donnie? In high power installation, it doesn't occur just from dropping tool but from carbonized particles from previous short or burn. So warning to all beginner electricians to be careful about carbonized initiated arc flash.

Also warning to all DIY. Don't do it yourself. Hire skilled technicians, engineers and electrician. And if they insist to do it live. Don't be near them. After the incident. I never allowed any live replacement or live works. And I don't go near them but via binocular. So don't worry about me DIY because I won't do that because I was scared already.

This will be my last inquiry about practices questions (I'd no longer bother you with any questions like this.. ok hbiss?) My last question now (to emphasize) is whether the above was caused by regular shorts or arc flash (with greater incident energy and source and load impedance that can cause arc flash). During bolted short in the main panel with your regular 120/240v split phase (not the 480V industrial power), can it also do the above and burn the hands of the electrician by the energy of the short (not directly getting shock) or is it really a micro arc flash that can deposit those energy and reached the arms of the electrician and caused second degree burn?

What caused the first short?

 

[tersh]

What caused the first short?

I wasn't sure. It was long ago around year 2015. Maybe the breaker itself was bought already in carbonized state? I remembered they isolated it right by putting the cardboard to avoid short to the chassis. Then when they tried to initially connect the wire to the terminal. It shorted. But the strength was low and the electrician wasn't hurted. When the second electrician tried to put the wire back into the terminal. It arc flashed. His arms blistered. The architect and the electrical contractor had long meeting after that.

This experience was the reason I'm nervous when hiring electrician now and asking him a lot of questions before he installs anything, and why I'm asking you guys many questions to make sure it won't happen again.
​

Maybe before the first short, the breaker had some quantity of carbon or conductive particles? which caused a small short just enough to deposit huge amount of carbon in the terminal. In the second short, it made a more robush arc flash? But then during the first short, if it was really carbon and few quantity. As long as the arc had established, the intensity should be independent of the quantity of the carbon? What do you think?

 

[mbrooke]

I wasn't sure. It was long ago around year 2015. Maybe the breaker itself was bought already in carbonized state? I remembered they isolated it right by putting the cardboard to avoid short to the chassis. Then when they tried to initially connect the wire to the terminal. It shorted. But the strength was low and the electrician wasn't hurted. When the second electrician tried to put the wire back into the terminal. It arc flashed. His arms blistered. The architect and the electrical contractor had long meeting after that.

This experience was the reason I'm nervous when hiring electrician now and asking him a lot of questions before he installs anything, and why I'm asking you guys many questions to make sure it won't happen again.
​

Maybe before the first short, the breaker had some quantity of carbon or conductive particles? which caused a small short just enough to deposit huge amount of carbon in the terminal. In the second short, it made a more robush arc flash? But then during the first short, if it was really carbon and few quantity. As long as the arc had established, the intensity should be independent of the quantity of the carbon? What do you think?

Was the breaker new? Why was it being replaced to begin with?


Ideally, never work live. Actually, never work live period.

 

[tersh]

Was the breaker new? Why was it being replaced to begin with?


Ideally, never work live. Actually, never work live period.

Most contractor electricians work live. But of course I won't allow it ever especially after the scary experience witnessing an arc flash happened to them.

The breaker was new and wasn't exactly replaced (I am recalling the exact events that transpired now. It was replaced after the arc flash event). The contractor was slow in applying for electricity. So the line side was tapped from the neighbors. When finally there was ac power to the building already. The electrician (by the architect and electrical contractor instructions) tried to tap the main lines to the breaker (after removing temporary power from neighbor). It means the breaker was working before it. Maybe the first electrician caused a short to the chassis? But there was cardboard all over it. So we still didn't know what caused the initial short. Is there a minimum distance between terminals when spontaneous arc can occur? What is that distance? Maybe he reached the minimum distance threshold.

 

[tersh]

Most contractor electricians work live. But of course I won't allow it ever especially after the scary experience witnessing an arc flash happened to them.

The breaker was new and wasn't exactly replaced (I am recalling the exact events that transpired now. It was replaced after the arc flash event). The contractor was slow in applying for electricity. So the line side was tapped from the neighbors. When finally there was ac power to the building already. The electrician (by the architect and electrical contractor instructions) tried to tap the main lines to the breaker (after removing temporary power from neighbor). It means the breaker was working before it. Maybe the first electrician caused a short to the chassis? But there was cardboard all over it. So we still didn't know what caused the initial short. Is there a minimum distance between terminals when spontaneous arc can occur? What is that distance? Maybe he reached the minimum distance threshold.

This was zoom of the screws.

Have you ever witnessed a micro arc flash or any arc flash before? See how the plastic between the screws didn't melt, only the screws. If you were electrical forensics.. what do you think happened or can you describe the dynamics of the arc flash?

 

[tersh]

This was zoom of the screws.

Have you ever witnessed a micro arc flash or any arc flash before? See how the plastic between the screws didn't melt, only the screws. If you were electrical forensics.. what do you think happened or can you describe the dynamics of the arc flash?

Initially the contractor thought the carbonized particles was inside, and he still did. But is it not in breaker teardown the internals were isolated internally with only the plastic common trip piece between them? So could the flash form outside enough not to touch the middle plastic like this:

The arc flashed breaker is still with me. Tomorrow I'll open it up to see if there is arc flash damage within the breaker itself. If none, then it arced like the above? If there was internal damage, then the arc flash was within? What is your guess?

This is the last thing I wanna know before moving on from all this. Lessons from this to respect electricity (and avoid it as much as possible and only let real professionals handle it). I was glad I witnessed it because after the event. I avoid live breakers and live panels at all costs.

 

[kwired]

After the first incident, I would have never attempted to connect it again while it is energized.

Have any pictures of what it looked like after the first attempt? If byproducts of the first attempt were coating things that is a definite no no to connect it live the next time, and even reason to at least clean it up before ever energizing again.

 

[mbrooke]

Initially the contractor thought the carbonized particles was inside, and he still did. But is it not in breaker teardown the internals were isolated internally with only the plastic common trip piece between them? So could the flash form outside enough not to touch the middle plastic like this:

The arc flashed breaker is still with me. Tomorrow I'll open it up to see if there is arc flash damage within the breaker itself. If none, then it arced like the above? If there was internal damage, then the arc flash was within? What is your guess?

This is the last thing I wanna know before moving on from all this. Lessons from this to respect electricity (and avoid it as much as possible and only let real professionals handle it). I was glad I witnessed it because after the event. I avoid live breakers and live panels at all costs.

Thats not a "real" arc flash. Just line charging current- probably a few amps at most at several hundred thousand volts

 

[mbrooke]

This was zoom of the screws.

Have you ever witnessed a micro arc flash or any arc flash before? See how the plastic between the screws didn't melt, only the screws. If you were electrical forensics.. what do you think happened or can you describe the dynamics of the arc flash?

Honestly, it look like one of the conductors hit the wrong screw. Dead short.

 

[tersh]

Honestly, it look like one of the conductors hit the wrong screw. Dead short.

When the electrician installed it in 2015. I clearly remembered him carefully trying to connect the live wire to the right terminal. The moment he made contact (which I clearly saw). I saw huge spark. So I was certain it was not the conductors hitting the wrong screw.

If it did. Why were both screws fried?

I sure hope you were right though because I can't imagine arc flashes occurring in houses with 75-100kVA transformers. Note din rail breakers even have closer spacing between terminals so it's horror to think that if bolt on breakers with larger spacing can do that. What more din rail with closer spacing. In industrial setting, they don't use din rail but maybe use breakers with larger spacing?

Here's the puzzling mystery. If the carbon deposited themselves to the breaker plastic case above. How come the arc flash didn't conduct via the surface and damage the plastic in between but jumped higher avoiding the plastic. The video was just illustration of the possible path.

Can dead short also send flash that can cause blisters in the electrican hands? What are the differences in the damage and injuries caused by exposing to dead short (not getting shock) in contrast to arc flash?

 

[tersh]

When the electrician installed it in 2015. I clearly remembered him carefully trying to connect the live wire to the right terminal. The moment he made contact (which I clearly saw). I saw huge spark. So I was certain it was not the conductors hitting the wrong screw.

If it did. Why were both screws fried?

I sure hope you were right though because I can't imagine arc flashes occurring in houses with 75-100kVA transformers. Note din rail breakers even have closer spacing between terminals so it's horror to think that if bolt on breakers with larger spacing can do that. What more din rail with closer spacing. In industrial setting, they don't use din rail but maybe use breakers with larger spacing?

Here's the puzzling mystery. If the carbon deposited themselves to the breaker plastic case above. How come the arc flash didn't conduct via the surface and damage the plastic in between but jumped higher avoiding the plastic. The video was just illustration of the possible path.

Can dead short also send flash that can cause blisters in the electrican hands? What are the differences in the damage and injuries caused by exposing to dead short (not getting shock) in contrast to arc flash?

mbrooke and other arc flash experts,

After 20 minute search in the attic. I found the breaker that suffered mysterious disorder back in 2015.
Here it is:

I can swear I saw the electrician connect the live wire to the right most terminal with the two already in place. The moment he touched the live wire to the 3 phase breaker right most terminal. It flashed a bright light and his arms got second degree burn with blisters that need patches. A tiny spark got to his chest too.

Was it an arc flash (as kwired commented) or just a dead short (as mbrooke commented). Both can produce flashes and sparks. But he didn't touch the two live wires. Just the last remaining live wire to the right most terminal of the new breakers in OFF position. If it is arc flash. How could it affect only the middle screw (melted it) with major damage in the right most terminal. Anyone got any idea? Usually what path in the air does arc flash take?

I'd like to hear comments before I tear it down to see the internal components if there is melting inside. I want the experts to see first the original external form before I open it. Thanks.

 

[tersh]

Here are more information in addition to above picture so you can help determine if it is really caused by non-touched arc flash or touched dead short.

Finally after 4 years I was able to teardown the breaker in question after mbrooke got me so familiar with the internal mechanisms of breakers.

The 3 pole breakers were separately cased and only connected by the common trip mechanism (shown in white).

Here the flash heat also got in between the two surfaces, but the initial short didn't put any particles inside the unit via the common trip which is still clean. The wire in middle breaker still there because the screw melted and I couldn't remove the wire.

This is the surface of the right most breaker with most damaged lugs:

The internal mechanism of the right most breaker (the one with most lugs damage) is still normal and the mechanism still functions ok:

So any ideas what really happened?

I was certain the live wire was not connected to another live wire. The electrician in 2015 clearly connected the live wire to the right most breaker/terminal without existing wire. I saw it clearly. There was no way for the internal mechanism to cause any short between the middle pole and right pole (it was OFF). So the initial short must have deposited carbon on the outside, but why did the arc flash (if it is such) didn't affect the middle plastic but kinda jumped, does it happen?

This is my last inquiry in this forum so kindly share what you think? If it is really arc flash. My concern is, can din rail MCB be used in sensitive panels that can arc flash. Note din rail MCB breakers have very close spacing between the terminals. Does it mean for arc flashable residential panels. Plug-in breakers with more distance terminals is safer? What do you think?

 

[romex jockey]

Here are more information in addition to above picture so you can help determine if it is really caused by non-touched arc flash or touched dead short.

Finally after 4 years I was able to teardown the breaker in question after mbrooke got me so familiar with the internal mechanisms of breakers.

The 3 pole breakers were separately cased and only connected by the common trip mechanism (shown in white).

<snip>

So any ideas what really happened?

The only thought i have is, the breaker was defective, and not really off tersh, although the internals in the pix above do not look badly abused, is the rear (exit) terminal there showing signs of heat??

~RJ~

 

[tersh]

Here are more information in addition to above picture so you can help determine if it is really caused by non-touched arc flash or touched dead short.

Finally after 4 years I was able to teardown the breaker in question after mbrooke got me so familiar with the internal mechanisms of breakers.

The 3 pole breakers were separately cased and only connected by the common trip mechanism (shown in white).

<snip>

So any ideas what really happened?

The only thought i have is, the breaker was defective, and not really off tersh, although the internals in the pix above do not look badly abused, is the rear (exit) terminal there showing signs of heat??

~RJ~

It was brand new breaker, not even been used. The internal with broken plastic was because I had a hard time opening it this morning so I just cracked the bolt holder. I used drill to open the bolted ends. But the end metal kept getting wider and I still couldn't open the case, so I saw the thin bolt wire and fried it open with many tools. So it looked abuse because of the way I opened it today.

About it being brand new. The following was the sequence of events:

In early 2015. The building had no power yet so the line side of the original 3 pole breakers were tapped from the neighbor (first picture.. the red numbers showed the non live wires yet).

Then when power came late 2015. The original breaker was moved to front to avoid loss of power to the tenant (second picture). Then a new 100A 3 pole breaker was put (the one I "teardown" today|). It was tested ok by continuity tests in the 2015. The contractor didn't want to ask the power utility to cut the power because they said they were used to connecting live wires. So the live wires starting from left were connected. When it was turn for the third and final live wire to be connected to the right most terminal of the breakers. It banged with bright flash and sound.

Was it arc flash? I didn't take photo of the initial short because there was no damage (I remembered). Maybe the breakers had been exposed to some conductive particles in the supplier warehouse? Then during the first short, it deposited carbon to the plastic material. Then during the second short, it established a path for the arc flash? Why it didn't vaporize the plastic material maybe the the carbon was vaporized in microseconds then the arc forms inches away from it between the terminals explaining why the middle plastic was not scorched? What do you think?

Btw. The building has no 3 phase equipments. The architect just put 3 phase aircondioners.. but the tenants all use 2 phase airconditioners only so there was no 3 phase loads ever since. The electrical plans were based on architect specification.
​

 

[kwired]

When the electrician installed it in 2015. I clearly remembered him carefully trying to connect the live wire to the right terminal. The moment he made contact (which I clearly saw). I saw huge spark. So I was certain it was not the conductors hitting the wrong screw.

If it did. Why were both screws fried?

I sure hope you were right though because I can't imagine arc flashes occurring in houses with 75-100kVA transformers. Note din rail breakers even have closer spacing between terminals so it's horror to think that if bolt on breakers with larger spacing can do that. What more din rail with closer spacing. In industrial setting, they don't use din rail but maybe use breakers with larger spacing?

Here's the puzzling mystery. If the carbon deposited themselves to the breaker plastic case above. How come the arc flash didn't conduct via the surface and damage the plastic in between but jumped higher avoiding the plastic. The video was just illustration of the possible path.

Can dead short also send flash that can cause blisters in the electrican hands? What are the differences in the damage and injuries caused by exposing to dead short (not getting shock) in contrast to arc flash?

Why you think you can't have arc flash with a "dead short"? Any arcing situation has "arc flash" but common arcing in a switch contact releases a very insignificant amount of energy to outside the current path. If it released no energy at all you wouldn't even see the light produced by the arc. It is when the amount of energy released to outside the current path becomes a hazard to bystanders that it becomes a hazardous situation.

Just because you have a 100 kVA transformer outside doesn't mean you have same incident energy level at the furthest outlet inside as you have at the terminals of that transformer either. Conductors between the two points have resistance as well as overcurrent devices are typically installed somewhere and that will change characteristics at each end. Arc flash incident energy is not only amount of energy released by fault current but is also impacted by how long that current can be delivered. A faster acting overcurrent device will give a lower limit to the energy released at the fault point. A lower current but for longer time can release more energy than a higher current for shorter time.

I keep telling you that the amount of incident energy that can potentially be released is complex and just because you have a large source doesn't automatically mean you have more hazard than you do with a smaller source there are other things that contribute to what can happen.

 

[tersh]

Why you think you can't have arc flash with a "dead short"? Any arcing situation has "arc flash" but common arcing in a switch contact releases a very insignificant amount of energy to outside the current path. If it released no energy at all you wouldn't even see the light produced by the arc. It is when the amount of energy released to outside the current path becomes a hazard to bystanders that it becomes a hazardous situation.

Just because you have a 100 kVA transformer outside doesn't mean you have same incident energy level at the furthest outlet inside as you have at the terminals of that transformer either. Conductors between the two points have resistance as well as overcurrent devices are typically installed somewhere and that will change characteristics at each end. Arc flash incident energy is not only amount of energy released by fault current but is also impacted by how long that current can be delivered. A faster acting overcurrent device will give a lower limit to the energy released at the fault point. A lower current but for longer time can release more energy than a higher current for shorter time.

I keep telling you that the amount of incident energy that can potentially be released is complex and just because you have a large source doesn't automatically mean you have more hazard than you do with a smaller source there are other things that contribute to what can happen.

Is there any textbooks about arc flash or even software? I want to know how exactly they compute everything as I'll enter every variable and combinations possible to see what would happen as I don't want a repeat of that ever. And to understand whether an initiating factor as small as few carbon particles in the industrial setting panel breaker can initiate a big arc flash like the following given sufficient incident energy and source/load impedance and short circuit current:

 

[Eddie_T]

The negative resistance characteristic of an arc in plasma is interesting. Scientists do some crazy experiments, in the 60s a group of engineers and scientists at Chrysler charged a number of capacitors in series then switched them to parallel to create a one farad charged capacitor. They were connected with a switched circuit to a wire filament located at the focal point of a parabolic dish. They waited for a thunder storm, aimed the dish at a cloud with lightning activity then flipped the switch. The sacrificial filament established the arc and end of story, the arc flash had no impact on the storm. An engineer that worked in the group said they just wanted to see what would happen. I think he was fearful that it might establish a leader for a lightning strike to the dish.

I don't understand what this linemen was doing, it looks scary, I don't even see gloves.

 

[mbrooke]

The negative resistance characteristic of an arc in plasma is interesting. Scientists do some crazy experiments, in the 60s a group of engineers and scientists at Chrysler charged a number of capacitors in series then switched them to parallel to create a one farad charged capacitor. They were connected with a switched circuit to a wire filament located at the focal point of a parabolic dish. They waited for a thunder storm, aimed the dish at a cloud with lightning activity then flipped the switch. The sacrificial filament established the arc and end of story, the arc flash had no impact on the storm. An engineer that worked in the group said they just wanted to see what would happen. I think he was fearful that it might establish a leader for a lightning strike to the dish.

I don't understand what this linemen was doing, it looks scary, I don't even see gloves.

Le sigh. Going to sum the few replies I left on that vid combating the army of arm chair experts.


It is not scary nor dangerous. The sticks do all the insulation and the bucket itself is insulated from ground. Gloves for 115kv do not exist as they would have to be at least several inches thick.

They know the arc will break without going phase to phase or phase to ground. Group operated switches on poles and within substations have much smaller operating distances yet can safely interrupt higher currents (bigger arcs) without flashing over.


By the looks of it they are isolating a section of unloaded loaded line that is no longer needed. The arc is from the charging (capacitive) current being broken. It is absolutely nothing dramatic compared to what it would look like carrying load let alone a short circuit. They start from the bottom and work there way up, that way in case something went terribly wrong a live jumper would not come crashing into the phase below it.


Live line work is typical, especially at 34.5kv and below as those voltages serve distribution transformers directly and would result in customers being turned off. At 115kv you usually have multiple sources to the load serving substations, however in some cases the feed can be radial or you might have other reasons (such as pre scheduled outages elsewhere) and need to do the work live.


All in all its a typical days work.