Science of Arc Flash & Engineering

mbrooke

Senior Member
Location
United States
In the US. Are other houses served by 100kVa pole transformer like the following?




For 100kVA pole transformer, assumed 2% impedance. 100,000VA / 240V / 0.02 = 20,083A short circuit current available.


Can 20kA initiate arc flash during shorting fault? Not by falling tools but by insects like lizards shorting the terminals?

I also want to point something out about this:

https://www.alabamapower.com/content/dam/alabamapower/Business/Services by Industry/Architects & Engineers/A-E-Fault-Currents-Tables-FINAL-8-2003.pdf


They assume 1/0 AL from the pole to the house, but it is very common for US POCOs to run #6, #4 or #2AL from the pole to a house, especially when they know that the home will never draw more then X number of amps. IE, an all gas home with a 125 amp service will probably never draw more then 50 amps.
 

kwired

Electron manager
Location
NE Nebraska
I also want to point something out about this:

https://www.alabamapower.com/content/dam/alabamapower/Business/Services by Industry/Architects & Engineers/A-E-Fault-Currents-Tables-FINAL-8-2003.pdf


They assume 1/0 AL from the pole to the house, but it is very common for US POCOs to run #6, #4 or #2AL from the pole to a house, especially when they know that the home will never draw more then X number of amps. IE, an all gas home with a 125 amp service will probably never draw more then 50 amps.
6, 4 and 2 AWG are common on service drops, if the house has electric heat it may actually have 1/0.

Service laterals anymore are 4/0 aluminum regardless of "service main rating" in many cases. This is usually POCO policy. I have had some cases where installing 100 amp main, yet needed a 200 amp meter socket just so it will accept the 4/0 incoming conductors.
 

mbrooke

Senior Member
Location
United States
6, 4 and 2 AWG are common on service drops, if the house has electric heat it may actually have 1/0.

Very true- electric heat does beef up the drop.


Service laterals anymore are 4/0 aluminum regardless of "service main rating" in many cases. This is usually POCO policy. I have had some cases where installing 100 amp main, yet needed a 200 amp meter socket just so it will accept the 4/0 incoming conductors.

With occasional complaints of voltage dips and electric vehicles/tankless heaters popping up I can see the reasoning behind not wanting to change the under ground or service drop latter.
 

tersh

Senior Member
Location
new york
Every application has it's own parameters. You can't really assume any kind of an average.

IMO there is an arc flash when you close a light switch, it just is low incident energy and not really destructive at all. The question you are maybe seeking is how high incident energy before it is considered a safety hazard to be around without any kind of PPE.
Yes, exactly. How high incident energy before it is considered a safety hazard to be around without any kind of PPE.

When you have a small transformer, like 500vA, the overcurrent can still reach 20 times or much higher than it before the winding heats up enough to be destroyed. So in a 100kVA utility pole transformer. Overcurrent can go much higher. Let's say its connected to a 12kV lines. And we eliminate all conductor impedance. And supposed the lineman (for sake of discussion) just shorted the secondary of the transformer. Would there be arc flash meters wide?

Or let's take the case of utility pole transformers where simply the terminals got shorted by accidents. Are there videos of these showing how big is the arc flash?
 

mbrooke

Senior Member
Location
United States
Yes, exactly. How high incident energy before it is considered a safety hazard to be around without any kind of PPE.

When you have a small transformer, like 500vA, the overcurrent can still reach 20 times or much higher than it before the winding heats up enough to be destroyed. So in a 100kVA utility pole transformer. Overcurrent can go much higher. Let's say its connected to a 12kV lines. And we eliminate all conductor impedance. And supposed the lineman (for sake of discussion) just shorted the secondary of the transformer. Would there be arc flash meters wide?

Or let's take the case of utility pole transformers where simply the terminals got shorted by accidents. Are there videos of these showing how big is the arc flash?

FWIW, here is a real world scenario where a 230 volt secondary faults in front of a supply pole transformer- I'm guessing 150kva-is:


https://www.youtube.com/watch?v=_m6l7Oy88E4
 

tersh

Senior Member
Location
new york
FWIW, here is a real world scenario where a 230 volt secondary faults in front of a supply pole transformer- I'm guessing 150kva-is:


https://www.youtube.com/watch?v=_m6l7Oy88E4
It's almost a continuous arc flash. During short, a transformer heats up and windings destroyed. Maybe it's partly the windings getting shorted and sparkling? Why didn't the pole breaker trip? In a facility. Arc flash is one time only because the breaker trips. Maybe the pole breaker was none functional in the video?
 

mbrooke

Senior Member
Location
United States
It's almost a continuous arc flash.
To me it looks more like sputtering- at least its not continuous through out the whole video.

During short, a transformer heats up and windings destroyed. Maybe it's partly the windings getting shorted and sparkling?
Only if the short persists long enough- a transformer will not fail the second its short circuited. It takes time, especially when you have oil acting as a thermal buffer.

There is an old video out there of a substation in Florida with a large power transformer experiencing a sustained secondary fault while the batteries in the substation were dead (no relaying). The fault persisted for about a whole minute before the oil finally boiled over in a ball of fire.


Why didn't the pole breaker trip? In a facility. Arc flash is one time only because the breaker trips. Maybe the pole breaker was none functional in the video?
Have a look at the recommend fuse size listed here vs the primary full load current


https://www.sandc.com/globalassets/s...in-350-110.pdf


To simplify the complex its not uncommon for pole pigs to have fuses 4-5 times their primary full load current rating, in addition to that most overhead fuse curves start when exceeding 200% of their listed rating.

4 reasons reasons for the way pole pigs are fused:

1. Due to the cyclic loading its not uncommon for 125%-300% short time (1-4hr) overloads on pole pigs- as such fuses must accommodate that.

2. Lightning protection. Its not uncommon for the surge arrestor to be right next to the trafo bushing as this provides the best protection, so accordingly the link must be strong enough to reduce the number of nuisance blows.

3. Inrush, which is very high (much higher then what you find in large 60MVA units relative to their FLA)- this due to the low Z and lightweight design of the core and coil taking precedence over all else.

4. Generally the philosophy is not to protect the transformer or clear secondary faults as much as removing a failed one from service. Some POCOs see a reason to provide some level of protection, others stick in a 50T link and will just let the unit boil over if the secondary does not "burn clear" during a fault.


In the industry there are many who call the secondary bushing of a transformer all the way to the main breaker a "blind spot" for this reason.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Ok. In industrial installations, most accidents involved falling tools that shorted the terminals which initiated arc flash. But what if an insect like a lizard cross the terminals, would this initiate arc flash too?
A squirrel across the primary certainly can. One day I heard what sounded like a shotgun blast and the lights went out. When the linemen came out to fix the problem they told me that there was squirrel shrapnel all over the top of the transformer.

I don't think they cause a flash, but our guys see "crispy critters" in residential MDP's all the time. Geckoes, usually.
 

kwired

Electron manager
Location
NE Nebraska
Very true- electric heat does beef up the drop.





With occasional complaints of voltage dips and electric vehicles/tankless heaters popping up I can see the reasoning behind not wanting to change the under ground or service drop latter.
POCO's around here that want 4/0 minimum for underground services, have had that policy for 25-30 years. EV and tankless water heaters weren't something they were seeing then. Electric heat was popular though, we have fairly low electric rates compared to rest of the country, plus winter months is low demand, so they used to promote electric heat with special rates during winter months if you had electric heat. Cost them money to have infrastructure even if it isn't loaded, so why not reduce rates a little to encourage adding some load during low demand months? was the reasoning behind it.

That can't be a 230V secondary fault. Title says 11kV which would be the primary.

-Hal
I agree, 230 volts isn't that likely to keep going like that unless someone is continuing to move conductors closer together as they burn up or feed other material into the arc to sustain it.
 

mbrooke

Senior Member
Location
United States
POCO's around here that want 4/0 minimum for underground services, have had that policy for 25-30 years. EV and tankless water heaters weren't something they were seeing then. Electric heat was popular though, we have fairly low electric rates compared to rest of the country, plus winter months is low demand, so they used to promote electric heat with special rates during winter months if you had electric heat. Cost them money to have infrastructure even if it isn't loaded, so why not reduce rates a little to encourage adding some load during low demand months? was the reasoning behind it.



I agree, 230 volts isn't that likely to keep going like that unless someone is continuing to move conductors closer together as they burn up or feed other material into the arc to sustain it.


It if the conductors keep touching each other due to other forces being involved the sputtering will continue. Not the best vid, but here is another example of an LV fire:


https://youtu.be/rpVyF6l2q9w?t=37


Lashing of conductors from short circuit forces can create perpetual self sustaining swing:


https://www.youtube.com/watch?v=7YX7CRWCGU4


https://youtu.be/4nLcjDmGr6g?t=180


If the arc was 11kv it would have be long and continuous (minus reclosing).
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
kwired said:
I agree, 230 volts isn't that likely to keep going like that unless someone is continuing to move conductors closer together as they burn up or feed other material into the arc to sustain it.
That first video is the worst of both worlds. A large 3 phase commercial service fed directly from large transformers. I can't tell if that is an underground service, but faults (and results) like that are common around here when an underground pipe service faults. Takes a long time to burn open. The RGS keeps the conductors together and as the burning progresses along the length new conductor is exposed.

-Hal
 

kwired

Electron manager
Location
NE Nebraska
That first video is the worst of both worlds. A large 3 phase commercial service fed directly from large transformers. I can't tell if that is an underground service, but faults (and results) like that are common around here when an underground pipe service faults. Takes a long time to burn open. The RGS keeps the conductors together and as the burning progresses along the length new conductor is exposed.

-Hal
I have to apologize a little as for some reason all I got was a blurry image and all I could get out of it was there was a lot of flashing. second time watching it - if I scrolled over the time marker in the video it did give me a mini still image of that point in video that was a clear image. I have noticed this happens occasionally watching video content and not sure why, doesn't seem to be limited to you tube either.
 

tersh

Senior Member
Location
new york
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?
 

kwired

Electron manager
Location
NE Nebraska
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?
There was an arc flash, it just didn't have same level of incident energy as an incident that does produce 3 meter wide effects. Apparently was enough energy to cause some burns to the person working on it though.

Guys that do welding wear protective clothing and face shield because they are also exposed to an arc flash - just happens to be fairly controlled and somewhat low level compared to what you might find available in a mishap at service/distribution equipment
 

tersh

Senior Member
Location
new york
There was an arc flash, it just didn't have same level of incident energy as an incident that does produce 3 meter wide effects. Apparently was enough energy to cause some burns to the person working on it though.

Guys that do welding wear protective clothing and face shield because they are also exposed to an arc flash - just happens to be fairly controlled and somewhat low level compared to what you might find available in a mishap at service/distribution equipment
Wow, that was really an arc flash? So I have witnessed one. When you have experienced witnessing one. You will never be the same. You will become paranoid. This was the reason why a simple GFCI question become tons of questions about others here because I don't trust the local electricians. Look what happened to them. Note my questions were not about DIY but just making sure I chose the right contractors and electricians by first quizzing them everything before letting them do anything like now.

Whatever, an arc flash is hotter than the sun.




For the arc flash I witnessed that happened to the contractor electrician in 2015. How hot do you think it is? How many fahrenheit? how do you compute or estimate it? Was it hotter than the sun at the arc plasma itself?

Lastly about the welder wearing protective gear and face shield. If you will see any Philippines welding contractor works. You will only see them putting towel over the hair and using shades only (like the following pic). I heard it could cause cataract? hmm.... The welding contractors don't have budget to give them protective gears and face shields, and our welders and electricians are so poor that if they don't have work for a week. They no longer have any money left.

 

kwired

Electron manager
Location
NE Nebraska
Extreme arc flash incident can be hotter that the sun, I don't think there is anything that says it must reach a certain temperature before you can call it an arc flash.


There are different levels of protection from different protective wear, so part of determining what to wear for protection involves knowing what maximum incident energy levels could be a possibility. Over certain levels, close up energized work isn't allowed, period no protective gear is rated high enough.
 

tersh

Senior Member
Location
new york
Extreme arc flash incident can be hotter that the sun, I don't think there is anything that says it must reach a certain temperature before you can call it an arc flash.


There are different levels of protection from different protective wear, so part of determining what to wear for protection involves knowing what maximum incident energy levels could be a possibility. Over certain levels, close up energized work isn't allowed, period no protective gear is rated high enough.






Let's compute the short circuit current available. The office building is served by an open delta 3 phase transformer using only two 75kVA transformers giving 3 phase. To compute the short circuit current available between two phase. Is it correct to do this as someone (cpickett) had done in message #25. Assumed 2% impedance. 75,000VA / 240V / 0.002 = 15,625A short circuit current. This means 15,625A short circuit current is enough to cause micro arc flash?

The office building service entrance is right below the pole so assume minimum load impedance. Most important. If a house is served by a 75kVA transformer. Is the available short circuit current the same as 2 phases of the 3 phases in the open delta pole?

Would the 3 phase transformer have higher impedance than the 2 phase transformer used in residential? If same, then assuming the load impedance is approx. identical. Then all homes served by 75kVA transformers in the Philippines can experience arc flash that US residential doesn't experience.

My electricians and contractor (electrical engineers) don't know how to compute it or aware of it. Hence as a former electronic engineer exposed to ac power too, i need to verify the safety of everything they are doing. I'm impressed if average US electricians know all about arc flash and safety.

For future projects. I may just hire international team of expert electricians schooled by Mike Holt. I don't trust the local electrician and contractor. They don't know what is arc flash.
 

mbrooke

Senior Member
Location
United States
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?
 
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