Science of Arc Flash & Engineering

[mbrooke]

Understood. If you must start from scratch, a relay will give you the most protection from arc flash. However, you must perform a short-circuit and arc flash study in accordance with IEEE guidelines to determine how to set the relay. A relay will also allow you to have a maintenance setting that can be enabled when personnel are working on live parts. This setting will push the relay's trip curve as low as possible to pick up a fault almost instantaneously.

Remember, time is your enemy. Incident energy is a variable created by the power industry, not physicists or mathematicians. You need to think more like a specifying engineer to solve this problem. An academic approach will not provide a real world solution to mitigate arc flash at a meter can.

I am not sure what other methods exist to decrease arc flash energy. At the end of the day, you can only control how fast a protective device opens. Messing around with air gap distances, materials, ppe, etc. only slightly mitigate an arc flash that will always be present when two conductive bodies are separated.

Relay? Why not a current limiting fuse?

 

[tersh]

Understood. If you must start from scratch, a relay will give you the most protection from arc flash. However, you must perform a short-circuit and arc flash study in accordance with IEEE guidelines to determine how to set the relay. A relay will also allow you to have a maintenance setting that can be enabled when personnel are working on live parts. This setting will push the relay's trip curve as low as possible to pick up a fault almost instantaneously.

Remember, time is your enemy. Incident energy is a variable created by the power industry, not physicists or mathematicians. You need to think more like a specifying engineer to solve this problem. An academic approach will not provide a real world solution to mitigate arc flash at a meter can.

I am not sure what other methods exist to decrease arc flash energy. At the end of the day, you can only control how fast a protective device opens. Messing around with air gap distances, materials, ppe, etc. only slightly mitigate an arc flash that will always be present when two conductive bodies are separated.

Jeremy. Do you have any schematic of how the relay works as it interacts with other devices in the service panels? We never use it here but just want to have idea how it all works.

Relay? Why not a current limiting fuse? ​

Mbrooke. We don't use relay or fuses here. The contractors are not familiar with them. They only use breakers. Also what if the fuses blow out. Then to replace them, you have to request the POCO to pull out the service entrance wires and this can interrupt the power for days especially as it requires city hall electrical permit to fix anything in the panels.

Originally there was supposed to be this main breaker to be installed (upstream of the arc flashed breaker/disconnect and 3 other disconnects downstream of the main breaker). The main breaker is supposed to be a GE General Electric TQD32225 225A Circuit Breaker 3 Pole 10 kA @ 240 VAC 225 Amp. However the contractor didn't install it to save money because he argued the budget was used up elsewhere in the building. What would be wrong if I'd go ahead and and use this breaker instead as main breaker? The contractor is not used to install fuses or relays. Is 10kA interrupting current not enough to reduce the incident energy? What values must it be before it can significantly reduce it? Won't this breaker significantly reduce the incident energy? Many thanks.
​

​

 

[romex jockey]

Do US POCO electricians use PPE when installing meters in sockets as GoldDigger emphasized? .

I would say most of us preach more than practice safety tersh.....Myself , i have donned the entire suit,face shield ,nomex hood, gloves, etc for a few of my more risky ventures.

That said, isolative designs are greatly appreciated

~RJ~

 

[tersh]

Understood. If you must start from scratch, a relay will give you the most protection from arc flash. However, you must perform a short-circuit and arc flash study in accordance with IEEE guidelines to determine how to set the relay. A relay will also allow you to have a maintenance setting that can be enabled when personnel are working on live parts. This setting will push the relay's trip curve as low as possible to pick up a fault almost instantaneously.

Remember, time is your enemy. Incident energy is a variable created by the power industry, not physicists or mathematicians. You need to think more like a specifying engineer to solve this problem. An academic approach will not provide a real world solution to mitigate arc flash at a meter can.

I am not sure what other methods exist to decrease arc flash energy. At the end of the day, you can only control how fast a protective device opens. Messing around with air gap distances, materials, ppe, etc. only slightly mitigate an arc flash that will always be present when two conductive bodies are separated.

The reason I need the physicists help is because of the following facts.

First. There has been a bonafide arc flash case at 240/120 VAC in 2009 in Georgia documented by OSHA in which someone died from the injuries and not much cases like it. That's in the USA. In my country. I am one of the few being direct witness of a 240/120 vac arc flash.

Second. Is it an arc flash initiated by carbonized surface or dead short.

In the above 2015 incident. A 1st short (probably between live wire and chassis scorched the enclosure plastic and made the surface conductive). During the 2nd short where the electrician just lowered the live wire to the right most terminal. He didn't connect the live wire to another live wire, just to their respective terminals. Can it cause that kind of damage?

Or imagine you have a normal breaker with terminals connected to the wires already . Then someone blow a dust of carbon. Is it enough to initiate an arc flash?

The key is to understand the physics of flashover.

Someone suggested me the following intro paper written in 1981.

http://www.electrician2.com/html/elec_haz.pdf

And to clarify on a paragraph inside it:"Electric arcing is the term applied to the passage of substantial electric currents through what had previously been air. But air is not the conductor; current passage is through the vapor of the arc terminal material, usually a conductor metal or carbon.".

If the carbon material is used up. Can the electric arc still sustain itself? Can normal air get hot enough during the initial carbon flashover to be used as the vapor for the current passage? And how does the carbon gets heat up so much the air ionized? This is why fully understanding how the arc plasma form is key to know if the above is caused by carbon initiated flashover or just dead short of some freak nature. This is what I need to learn today. Can you share the physics of arc plasma in detail?​

 

[tersh]

I plan to buy these at amazon (cheapest price at $284) to let my electrician wear it when he would access the service panel to turn off or on a breaker.

It has protection up to 8cal/cm2 and the cheapest yet in amazon.

https://www.amazon.com/HRC2-Series-...1551498296&s=gateway&sr=8-7-spell-spons&psc=1

But how come no gloves are included. What do you use that is reasonably priced?

If you just wear any long sleeve cotton shirt, what usually is the calorie resisting protection it can give you?

 

[mbrooke]

Jeremy. Do you have any schematic of how the relay works as it interacts with other devices in the service panels? We never use it here but just want to have idea how it all works.

Mbrooke. We don't use relay or fuses here. The contractors are not familiar with them. They only use breakers. Also what if the fuses blow out. Then to replace them, you have to request the POCO to pull out the service entrance wires and this can interrupt the power for days especially as it requires city hall electrical permit to fix anything in the panels.

Originally there was supposed to be this main breaker to be installed (upstream of the arc flashed breaker/disconnect and 3 other disconnects downstream of the main breaker). The main breaker is supposed to be a GE General Electric TQD32225 225A Circuit Breaker 3 Pole 10 kA @ 240 VAC 225 Amp. However the contractor didn't install it to save money because he argued the budget was used up elsewhere in the building. What would be wrong if I'd go ahead and and use this breaker instead as main breaker? The contractor is not used to install fuses or relays. Is 10kA interrupting current not enough to reduce the incident energy? What values must it be before it can significantly reduce it? Won't this breaker significantly reduce the incident energy? Many thanks.
​

​

10ka should be enough if your breaker is rated to interrupt 208 volts.

Edit: It will work, 10ka is listed at 240 volts instead of 120/240 (as is with US 3 phase breakers)- so no problem with the high leg.


https://www.widespreadsales.com/Products/Circuit-Breakers-General-Electric/TQD32225

 

[kwired]

I plan to buy these at amazon (cheapest price at $284) to let my electrician wear it when he would access the service panel to turn off or on a breaker.

It has protection up to 8cal/cm2 and the cheapest yet in amazon.

https://www.amazon.com/HRC2-Series-...1551498296&s=gateway&sr=8-7-spell-spons&psc=1

But how come no gloves are included. What do you use that is reasonably priced?

If you just wear any long sleeve cotton shirt, what usually is the calorie resisting protection it can give you?

Quite possibly because there are different voltage ratings as well as different dexterity ability needs for some tasks, though the higher the protection level needed the less likely you will find a glove suitable for those higher dexterity tasks.

Inserting the conductor on the breaker (and using a screwdriver to tighten the set screw) that has been your main topic of discussion can be difficult to do with a really heavy glove, but if that glove is the only one rated high enough for the potential incident energy available, you still end up considering what it will take to disable the supply to do this task.

 

[tersh]

Quite possibly because there are different voltage ratings as well as different dexterity ability needs for some tasks, though the higher the protection level needed the less likely you will find a glove suitable for those higher dexterity tasks.

Inserting the conductor on the breaker (and using a screwdriver to tighten the set screw) that has been your main topic of discussion can be difficult to do with a really heavy glove, but if that glove is the only one rated high enough for the potential incident energy available, you still end up considering what it will take to disable the supply to do this task.

Browsing at amazon (Jeff Bezos got so rich from international customers who have no other choices to order). This seems to be the best arc flash gloves they have.

https://www.amazon.com/gp/product/B00BHLUFIS/ref=ox_sc_act_title_2?smid=A25SRWVYK5VL1R&psc=1

"ARC FLASH TESTED: Arc-flash glove testing placed these cut proof gloves at level 3, with a rating of ATPV = 36 cal/cm². (Note: these work gloves are designed for spark and flame resistance, not thermal contact.) "​

Anyone can recommend a better one? Very thin and same 36cal/cm2 or better protection?

 

[tersh]

Browsing at amazon (Jeff Bezos got so rich from international customers who have no other choices to order). This seems to be the best arc flash gloves they have.

https://www.amazon.com/gp/product/B00BHLUFIS/ref=ox_sc_act_title_2?smid=A25SRWVYK5VL1R&psc=1

"ARC FLASH TESTED: Arc-flash glove testing placed these cut proof gloves at level 3, with a rating of ATPV = 36 cal/cm². (Note: these work gloves are designed for spark and flame resistance, not thermal contact.) "​

Anyone can recommend a better one? Very thin and same 36cal/cm2 or better protection?

I found out these are not electrical gloves. Yet it has arc flash protection. What characterize electrical gloves? How do they differ to non electrical gloves?

 

[tersh]

I was discussing with a physicist about arc plasma and I'd like to share how thin carbon sooth can initiate flashover or arc flash.

I asked him what if the carbon sooth can only take 20% of the current. He said that it'll carry a lot less than that, probably just thousandths of an amp prior to flashover. Once the arc is struck electrons accelerating in the electric field colliding with molecules knock loose more electrons, as described in the link.
https://web.stanford.edu/~ajlucas/Breakdown, Atmospheric Electricity and Lightning.pdf

That's what goes on inside a Geiger Counter detector tube, where we call it "gas amplification"
and along a lightning bolt as it grows. It's often described as 'avalanche'. Something akin to this:

​

I asked him how ionization started and how voltage triggered it. He gave me this link:

http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/ionize.html

Also told me to watch these videos, explaining they used salt water not graphite (carbon sooth), but the principle was the same.

In that video the current was limited so the flashover was benign. When it happens on a line with real current capability it sounds like a dynamite blast..

https://www.youtube.com/watch?v=MvOGaBkFt6E

Conclusion seems to be even thin carbon sooth that takes just thousandths of an amp can initiate a flashover or arc flash even when the carbon sooth get vaporized due to the avalanche principle explained above.

Now I wonder what kinds of common dusts are conductors. What do you think? Most panels in homes were not cleaned, you have thick dusts lying there for decades. Maybe residential panels don't arc flash because of the low incident energy, but not commercial building, isn't it.

So I'm convinced more than ever to get PPE Cat 2 and arc flash rated gloves to give to electrician even when he only opens the panel and switch off or on any breakers.

 

[mbrooke]

Here is a real world example:


https://www.youtube.com/watch?v=ZPe0Ucy0wMY

 

[kwired]

I found out these are not electrical gloves. Yet it has arc flash protection. What characterize electrical gloves? How do they differ to non electrical gloves?

For low voltages (under 1000 volts) they are probably still pretty effective though they may not have an actual rating.

Higher voltage work often you wear rubber gloves rated for the voltage but those are vulnerable to puncture and other damage so they have a leather glove worn over top of them. The combination is bulky enough you won't be using them for landing the wire in your breaker example with these on.

 

[milldrone]

I found out these are not electrical gloves. Yet it has arc flash protection. What characterize electrical gloves? How do they differ to non electrical gloves?

This link below is for a company selling arc flash gear so keep that in mind when you view the content. They also describe the different zones. Only the person entering the prohibited zone needs both arc flash and electrical shock hazard protection.

https://www.superiorglove.com/blog/arc-flash-rated-gloves

This link below is also from the same company.

https://www.superiorglove.com/blog/expert-interview-arc-flash-safety

Also note that they suggest the only way to be safe is to de energize the equipment

 

[tersh]

10ka should be enough if your breaker is rated to interrupt 208 volts.

Edit: It will work, 10ka is listed at 240 volts instead of 120/240 (as is with US 3 phase breakers)- so no problem with the high leg.


https://www.widespreadsales.com/Products/Circuit-Breakers-General-Electric/TQD32225

When one discusses with arc flash folks. They always mentioned about breakers having inverse time curves. But for normal breakers with magnetic instantaneous trip function (the typical bi-metal/ magnetic strip mechanism). They don't have inverse time curves at all, isn't it? The following is example of their description "If for instance we cut the short circuit current in half, the obvious assumption is that arc flash should be cut in half (Arc power cut in half). This is close to accurate as far as the arc thermal power goes. BUT the breaker timing is also increased but since breakers operate on an inverse time curve most of the time, the increase in opening time more than doubles. So the resulting incident energy often actually goes up rather than down." If the normal breakers are not really inverse time curves. Do you have sample of breakers with inverse time curves features? Are they mechanical or electronic? Perhaps the minimum ampacity is more than 200A?

How fast are breakers with inverse time curves compared to the typical instantaneous tripping magnetic strip based breakers?

Also you said 10kA Interrupting Current is enough. But using the infinite bus assumption.

75,000/240v/0.02 = 15,625A

15K is more than 10K. How can 10K be enough?​

​

 

[kwired]

When one discusses with arc flash folks. They always mentioned about breakers having inverse time curves. But for normal breakers with magnetic instantaneous trip function (the typical bi-metal/ magnetic strip mechanism). They don't have inverse time curves at all, isn't it? The following is example of their description "If for instance we cut the short circuit current in half, the obvious assumption is that arc flash should be cut in half (Arc power cut in half). This is close to accurate as far as the arc thermal power goes. BUT the breaker timing is also increased but since breakers operate on an inverse time curve most of the time, the increase in opening time more than doubles. So the resulting incident energy often actually goes up rather than down." If the normal breakers are not really inverse time curves. Do you have sample of breakers with inverse time curves features? Are they mechanical or electronic? Perhaps the minimum ampacity is more than 200A?

How fast are breakers with inverse time curves compared to the typical instantaneous tripping magnetic strip based breakers?

Also you said 10kA Interrupting Current is enough. But using the infinite bus assumption.

75,000/240v/0.02 = 15,625A

15K is more than 10K. How can 10K be enough?​

​

Typical general purpose circuit breakers in the US are thermal-magnetic and have both the inverse time as well as an instantaneous trip component.

Mechanical action of a breaker is still slower acting than melting of a fuse link, so certain fuses can limit current/or at least duration of the fault easier than a breaker can.

 

[tersh]

Typical general purpose circuit breakers in the US are thermal-magnetic and have both the inverse time as well as an instantaneous trip component.

Mechanical action of a breaker is still slower acting than melting of a fuse link, so certain fuses can limit current/or at least duration of the fault easier than a breaker can.

The bi-metallic strip is inverse time, the magnetic strip is instantaneous trip.

Arc flash folks only talk of magnetic strip or equivalent with inverse time curve. They ignore the bi-metallic strip because in arc flash, it's a short circuit so only the magnetic strip or equivalent is the important factor.

 

[kwired]

The bi-metallic strip is inverse time, the magnetic strip is instantaneous trip.

Arc flash folks only talk of magnetic strip or equivalent with inverse time curve. They ignore the bi-metallic strip because in arc flash, it's a short circuit so only the magnetic strip or equivalent is the important factor.

Correct, so unless you have a thermal only unit, you have a quicker acting breaker on short circuits and ground faults, but as I said they still must mechanically respond and one can still have faster interrupting time out of certain fuses because they don't have to rely on mechanical response.

 

[tersh]

Correct, so unless you have a thermal only unit, you have a quicker acting breaker on short circuits and ground faults, but as I said they still must mechanically respond and one can still have faster interrupting time out of certain fuses because they don't have to rely on mechanical response.

Problem with fuses taking the function of main breaker is if it fails. It will take time to request the POCO to remove the service entrance wires and re tap after replacing the fuses which tenants won't allow. Do you have sample of how 250A fuses look as main disconnect anyway?

And I wonder how breakers with inverse time magnetic function look like. Hope mbrooke can share something in his collections of breakers.

 

[kwired]

Problem with fuses taking the function of main breaker is if it fails. It will take time to request the POCO to remove the service entrance wires and re tap after replacing the fuses which tenants won't allow. Do you have sample of how 250A fuses look as main disconnect anyway?

And I wonder how breakers with inverse time magnetic function look like. Hope mbrooke can share something in his collections of breakers.

Your 100 amp GE three pole breaker you have posted pictures of has both thermal and magnetic trip features.

250 amp fuses fit into 400 amp frame size fuseholders - if used as the service disconnect they would typically go in a "safety switch" which is a switch assembly with fuseholders on the load side so the supply circuit is opened by the switch so you can safely remove/replace fuses.

400 amp fused safety switch - normally would be turned 90 degrees clockwise from what is shown - best image that I could find that shows both the switching mechanism and the fuses installed.

 

[tersh]

Your 100 amp GE three pole breaker you have posted pictures of has both thermal and magnetic trip features.

The 100A GE has

thermal trip = inverse time curve
magnetic trip = instantaneous trip

The Arc Flash folks were referring to:

thermal trip = inverse time curve
magnetic trip = inverse time curve

I was asking what kind of breakers have magnetic strip with inverse time curve?

250 amp fuses fit into 400 amp frame size fuseholders - if used as the service disconnect they would typically go in a "safety switch" which is a switch assembly with fuseholders on the load side so the supply circuit is opened by the switch so you can safely remove/replace fuses.

400 amp fused safety switch - normally would be turned 90 degrees clockwise from what is shown - best image that I could find that shows both the switching mechanism and the fuses installed.

View attachment 22466

How many percentage of homes or office installations do you use fuses?