# Science of Arc Flash & Engineering

#### tersh

##### Senior Member
125VAC cannot sustain an arc. Arc faults are caused by FAULTS not loose connections. Generally, transformers serving residential services do not provide substantial fault currents.

Do you stay up at night dreaming up things to ask here?

-Hal
How about the 240v red and black phase to phase, can it sustain an arc?

Also during short circuit. isn't it the current can go to over 20X. What did you mean the
transformers serving residential services do not provide substantial fault currents?

Last edited:

#### tersh

##### Senior Member
190204-1016 EST

tersh:

Impedance external to the arc is what determines the arc current. The source voltage, transformer, and wires, or anything else in series with the arc determines the current. The arc itself may not be a major factor.

All devices such as arc lamps, LEDs, fluorescent bulbs, Zener diodes, etc., require some form of external current limiting.

.
gar, in typical US home like yours, or the
typical black and red 240v main feeder, and the white neutral and green ground wires. And let's say the main breaker is 125A with 1/0 going to the typical residential utility pole split phase transformers. If the electrician accidentally drop a wrench that shorts the bus bar of the 240v black and red lines. Can the 240v short sustain a arc enough to cause arc flash.

Let's actually compute. What is the maximum fault current in a typical residential utility pole transformers? What is the typical impedance of such utility pole? What units is the arc produced and what do you estimate is the value given typical US home residence? Thanks.

#### hbiss

##### EC, Westchester, New York NEC: 2014
Let's actually compute. What is the maximum fault current in a typical residential utility pole transformers? What is the typical impedance of such utility pole? What units is the arc produced and what do you estimate is the value given typical US home residence?
How about you compute? There is plenty of information out there for you to either come to a determination or see how it can't be done. You seem to be really good at trawling the internet.

-Hal

#### tersh

##### Senior Member
I've been searching for one hour in the net but couldn't find any sample computations. But I found this stuff and have a question:

https://www.banner-day.com/index.php/technical-briefs/bakery-technical-briefs/23-arc-flash/131-arc-flash-general-overview.html

[h=2]"What is Arc Flash? [/h]
An arc flash is a voltage breakdown of the resistance of air resulting in an arc which can occur where there is sufficient voltage in an electrical system and a path to ground or lower voltage. An arc flash with 1000 amps or more can cause substantial damage, fire or injury."

Question: It says a path to ground or lower voltage. How about the split phase 240v phase to phase (black and red wire) in US ac system. When there is phase to phase contact and arc, there is no path to ground or lower voltage. Does this mean purely phase to phase (120v) can't produce an arc or arc flash even if the source current is thousands of amps?

Second related question:

"
In general, arc flash incidents are highly improbable on systems operating at less than 208 volts phase to phase (120V to ground) when fed by less than a 125 KVA transformer (very typical of most office and home environments). 120 volts does not provide sufficient energy to cause an arc flash hazard. Most 480V electrical services have sufficient capacity to cause an arc flash hazard. Medium-voltage equipment (above 600V) is higher energy and therefore a higher potential for an arc flash hazard. "

It doesn't mention 240v (phase to phase without path to ground), does this mean phase to phase arc is immune to arc flash even if the source current or utility pole transformer is high?

#### cpickett

##### Member
What did you mean the transformers serving residential services do not provide substantial fault currents?
My house is served by a 10kVA pole transformer, assumed 2% impedance. 10,000VA / 240V / 0.02 = 2,083A short circuit current available.

While that's a lot of current, compared to some industrial installations that have 50kA available, I'd say 2kA is not substantial as far as arc fault risk is concerned. I'm still not going to put myself in a situation where I could come in contact with live busbars or drop tools or hardware on them.

#### tersh

##### Senior Member
My house is served by a 10kVA pole transformer, assumed 2% impedance. 10,000VA / 240V / 0.02 = 2,083A short circuit current available.

While that's a lot of current, compared to some industrial installations that have 50kA available, I'd say 2kA is not substantial as far as arc fault risk is concerned. I'm still not going to put myself in a situation where I could come in contact with live busbars or drop tools or hardware on them.
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?

#### tersh

##### Senior Member
My house is served by a 10kVA pole transformer, assumed 2% impedance. 10,000VA / 240V / 0.02 = 2,083A short circuit current available.

While that's a lot of current, compared to some industrial installations that have 50kA available, I'd say 2kA is not substantial as far as arc fault risk is concerned. I'm still not going to put myself in a situation where I could come in contact with live busbars or drop tools or hardware on them.

For US 480V 3 Phase industrial power, what is the pole transformer kVA? Here I read 2 electrical workers dead from arc flash at a mall.

#### tersh

##### Senior Member
My house is served by a 10kVA pole transformer, assumed 2% impedance. 10,000VA / 240V / 0.02 = 2,083A short circuit current available.

While that's a lot of current, compared to some industrial installations that have 50kA available, I'd say 2kA is not substantial as far as arc fault risk is concerned. I'm still not going to put myself in a situation where I could come in contact with live busbars or drop tools or hardware on them.
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?

#### jim dungar

##### Moderator
Staff member
I've been searching for one hour in the net but couldn't find any sample computations.
Then you have not looked at the industry standard for arc flash - NFPA70E.

If you have not read this standard, or the Canadian equivalent, any further discussion is going to be futile.

#### gar

##### Senior Member
190205-0859 EST

tersh:

You have an EE degree from many years ago. There are basics from that background that you should be able to use to answer some of your questions. However, it seems to me that you don't go back to those basics, and you ask some questions like a person with none of this background might ask. Try to look at your problems or questions using basic electrical theory.

On circuit protection there is a lot of good information at
http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/technical-literature/bus-ele-an-3002-spd-selective-coord.pdf . This is some 100 pages long.

Whether a short is line to line, or to ground, or whatever the fault current is determined by the equivalent circuit source voltage divided by the shorted equivalent circuit impedance. cpickett gave you an example.

.

#### kwired

##### Electron manager
Or to be realistic. Let's take the issue of loose breakers and bus bar or any loose connections in the main panel in typical US home.

When the breaker or wiring got loose. Arc would be formed. For typical 125A main breaker and 1/0 wire to the service entrance and the utility pole tap in US home. Assume it is the main breaker lugs that is loose. Can this form a self sustaining plasma that can grow in size as it feeds on the current enough to explode the panels and the room it is put on?

I'd like to hear reports of this. Worse-case scenario.
loose breaker to bus bar isn't same thing as short circuit or ground fault. Sure some arcing can occur but current is limited by the load that is in series with the arc. You have an arc every time you make or break a switch contacts, but for normal intended operation you have a load in series with the contacts as well, which limits the effects of that arcing. A load that draws 1 amp will have less arcing than a load that draws 1000 amps, but the switch for the 1000 amp load is going to be designed to better withstand this also.

How about the 240v red and black phase to phase, can it sustain an arc?

Also during short circuit. isn't it the current can go to over 20X. What did you mean the
transformers serving residential services do not provide substantial fault currents?
How high short circuit current can reach is dependent on source capacity, source impedance and circuit impedance between the source and the fault. You can have exact same constructed houses, short main terminals together and have different amount of current flow in each because of different sized transformers, different conductor sizes or type supplying them, different lengths of supply conductors, or differences in combinations of all those things. Every install has it's own circumstances. Then taking things a step further, a condition that maybe is not so much present at a dwelling, is that any running motors on the system can introduce even more energy to a fault.

gar, in typical US home like yours, or the
typical black and red 240v main feeder, and the white neutral and green ground wires. And let's say the main breaker is 125A with 1/0 going to the typical residential utility pole split phase transformers. If the electrician accidentally drop a wrench that shorts the bus bar of the 240v black and red lines. Can the 240v short sustain a arc enough to cause arc flash.

Let's actually compute. What is the maximum fault current in a typical residential utility pole transformers? What is the typical impedance of such utility pole? What units is the arc produced and what do you estimate is the value given typical US home residence? Thanks.
One might be able to say an average residential pole transformer has a certain fault current level, but that is at the transformer terminals. Thing is just 25 feet of conductor gives enough impedance to lower the maximum available fault current at the load end of said conductors. Now as already been mentioned just because the maximum fault current has been lowered, you still have the amount of time it takes to open the circuit (or before something burns open) involved in your total incident energy. Higher current for shorter time can result in less incident energy than lower current for longer time.

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?
The lizard likely has high enough resistance that current is pretty limited, for under 600 volts anyway. Higher voltages, that lizard is still same resistance initially, but as it gets "cooked" could change things, carbon pathways can develop and arcing that continues on from there has little to do with initial resistance of the critter.

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?
Houses with a lot of load may be individually served by a large transformer. Other times a large transformer may serve multiple customers. You can even find 4 or 5 customers served from only a 25 kVA transformer - all depends on demand from the customers. If they all have gas heat, water heating, and not much for other electric loads they get away with small transformers. Just because one has a 100 or 200 amp main breaker doesn't mean the current normally runs that high. A lot of homes with a 200 amp supply never see much more than 70 or 80 amps, and some may never even see that much - just depends on what loads there are an how they get used.

#### gar

##### Senior Member
190205-0920 EST

tersh:

You do present some interesting questions.

I see in post #28 you did the calculation. The lizard will vaporize. After 1/2 cycle, 8 mS, will the arc reignite? I don't know.

.

#### kwired

##### Electron manager
190205-0920 EST

tersh:

You do present some interesting questions.

I see in post #28 you did the calculation. The lizard will vaporize. After 1/2 cycle, 8 mS, will the arc reignite? I don't know.

.
Won't that depend on skin resistance. If comparable to human skin, it probably doesn't vaporize other than some limited burning at contact point. From there on it depends on whether current path changes, resistance changes etc. Damage to cells within the body is likely, but conditions will vary from one incident to another. If tough enough skinned critter might even be possible they don't even feel anything on say 120 volts contact. Higher the voltage the more likely something happens though.

#### gar

##### Senior Member
190205-1108 EST

kwired:

I was assuming that the critter created a really good arc.

.

#### kwired

##### Electron manager
190205-1108 EST

kwired:

I was assuming that the critter created a really good arc.

.
I thought so. Not saying it can't happen, but for 120/240 volt systems that probably doesn't happen that often. If it crawled into some medium voltage gear it might, but isn't exactly a direct correlation to resistance of it's skin, but rather resulting from what happens to it as it gets "cooked".

#### tersh

##### Senior Member
loose breaker to bus bar isn't same thing as short circuit or ground fault. Sure some arcing can occur but current is limited by the load that is in series with the arc. You have an arc every time you make or break a switch contacts, but for normal intended operation you have a load in series with the contacts as well, which limits the effects of that arcing. A load that draws 1 amp will have less arcing than a load that draws 1000 amps, but the switch for the 1000 amp load is going to be designed to better withstand this also.

How high short circuit current can reach is dependent on source capacity, source impedance and circuit impedance between the source and the fault. You can have exact same constructed houses, short main terminals together and have different amount of current flow in each because of different sized transformers, different conductor sizes or type supplying them, different lengths of supply conductors, or differences in combinations of all those things. Every install has it's own circumstances. Then taking things a step further, a condition that maybe is not so much present at a dwelling, is that any running motors on the system can introduce even more energy to a fault.
What brands and models of breakers are the most advance in the world that are even faster than spring and thermal magnetic strip combo used to prevent specifically arc flashes? Maybe electronic?

One might be able to say an average residential pole transformer has a certain fault current level, but that is at the transformer terminals. Thing is just 25 feet of conductor gives enough impedance to lower the maximum available fault current at the load end of said conductors. Now as already been mentioned just because the maximum fault current has been lowered, you still have the amount of time it takes to open the circuit (or before something burns open) involved in your total incident energy. Higher current for shorter time can result in less incident energy than lower current for longer time.

The lizard likely has high enough resistance that current is pretty limited, for under 600 volts anyway. Higher voltages, that lizard is still same resistance initially, but as it gets "cooked" could change things, carbon pathways can develop and arcing that continues on from there has little to do with initial resistance of the critter.

Is there actually experiment where lizards are made to cross terminals of typical residential service entrance. They have high enough resistance they are immune to getting shock from 120/240 volt system?

Houses with a lot of load may be individually served by a large transformer. Other times a large transformer may serve multiple customers. You can even find 4 or 5 customers served from only a 25 kVA transformer - all depends on demand from the customers. If they all have gas heat, water heating, and not much for other electric loads they get away with small transformers. Just because one has a 100 or 200 amp main breaker doesn't mean the current normally runs that high. A lot of homes with a 200 amp supply never see much more than 70 or 80 amps, and some may never even see that much - just depends on what loads there are an how they get used.
Let's say the 100 kVA transformer serves 10 houses, and most of their appliances are off during the day. Then one house has an accidental short in the terminal right below the power line (say the lineman drop a tool). Do you think this would produce an arc flash? At least we can tell based on US experience by the rating of the transformers, whether 10 KVA, 50 kVA, 100 kVA or 2000 kVA used in 480 3 phase system. Perhaps the threshold is 1000 kVA or something? What is the estimate assuming source and conductor impedance is average or typical.

#### junkhound

##### Senior Member
Have done lots of arc extinguish tests over the years.

Depends on materials and initial gaps, 120V, 12000 amps fault current can keep an arc going over a 1/2" gap if aluminum, will continue until the aluminum melt away to a couple of inch gap.

Have done sets on 1/8" spacing on bus bars with worms and insects getting into a panel. Even a wet spider web can initiate a 480Vac arc that does not extinguish at 400 Hz or so. Under 2 kA, arc nearly always extinguishes at 1st or second zero crossing.

A 900 Hz 240Vac arc is great for welding, easy to keep an arc going. E.g try to weld at 60 Hz with 7018 rod, you have to have a very steady hand. At 900Hz and above, even smoother than dc.

Lots of variables.

#### tersh

##### Senior Member
25 years ago in engineering in college. I was not taught about arc flash.. or didn't pay attention. When someone mentioned to me arc flash lately. I thought it was just flash from arc. But it is much more. In fact, a better description is "arc grenade" so it can get the average electrician attention. In my country. Most electricians never heard of arc flash or ignore it. They thought it was just flash from arc.

Consider this:

Arc flash is 4 times hotter than sun, it's brighter than the sun, louder than a jumbo jet flying meters away from you, can expand 67,000 times its solid form. It's really an arc grenade!

https://www.geindustrial.com/sites/geis/files/ultimatearcflashguide.pdf

When I told the electrician to research about "arc flash". He said "ah, flash from arc like welding". When I told another electrician to research about "arc grenade". He paid more attention.

#### kwired

##### Electron manager
What brands and models of breakers are the most advance in the world that are even faster than spring and thermal magnetic strip combo used to prevent specifically arc flashes? Maybe electronic? Don't know. Electronic current sensing models still have mechanically operated contact mechanisms. I don't know of any device that is rated as branch circuit protection device that would be entirely electronic in nature, something that is supplemental protection could maybe be like that.

Is there actually experiment where lizards are made to cross terminals of typical residential service entrance. They have high enough resistance they are immune to getting shock from 120/240 volt system? Don't know again. Can tell you I have seen critters in enclosures many times, obviously had contact conductors in some cases, yet you only find evidence they were there and seldom find a dead critter. Most common is evidence rodents have been in an enclosure and have nested, deficated, etc. inside and chewed insulation off of conductors that were live. All the waste and urine they left behind certainly has to help increase conductivity and you would think make them even more subject to electrocution. Over 30 years or so I bet I only actually seen maybe half a dozen rodents max that got electrocuted. Did find a dead snake one time laying across input leads on a air conditioner contactor. Seen several others with shedded snake skins in them, so you know they were in there.

Let's say the 100 kVA transformer serves 10 houses, and most of their appliances are off during the day. Then one house has an accidental short in the terminal right below the power line (say the lineman drop a tool). Do you think this would produce an arc flash? At least we can tell based on US experience by the rating of the transformers, whether 10 KVA, 50 kVA, 100 kVA or 2000 kVA used in 480 3 phase system. Perhaps the threshold is 1000 kVA or something? What is the estimate assuming source and conductor impedance is average or typical.
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.

#### mbrooke

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

Typically homes in the US are served by a 25kva pole transformer which feed on average 5 homes; 37.5kva and 50kva for more homes or several which are all electric homes. 10 or 15kva feeding a single home.

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?

Have a look at this. Save it to your computer:

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

It factors in the service drop size and the distance for a given kva. Transformer impedances assumed are typical.