How close? Like 4 inches or 8 inches? I just want to learn the differences between damage/injury by flying hot sparks or arc flash. Do you know the signs to look for or how to recognize which is which?
Science of Arc Flash & Engineering
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How close? Like 4 inches or 8 inches? I just want to learn the differences between damage/injury by flying hot sparks or arc flash. Do you know the signs to look for or how to recognize which is which?
mbrooke
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I'd say a few inches. But it can be feet with the hot sparks.
So the hot sparks can fly for feet without arc flash being formed? In my scorched chassis wall (pls see last page). Was it signature of arc flash or hot sparks flying. Are there tell tale sign for each?
About what good is digging into the physics of arc flash.
That's exactly how IEEE 1584-2002 erred. This was what occurred:
"
well at the time they were looking for repeatable tests that could be used for laboratory purposes so an artificial scenario was a necessity. And at the time the impact of things like horizontal barriers as well as phase barriers and many similar items was essentially unknown. The model they chose was substantial enough that it would survive more than a single test so it worked out pretty good and got a lot of good data in a short period of time. High current testing lab time is definitely not cheap and the entire effort for the most part came from voluntary donations of time and money, and still does. The barrier paper you have the reference for is now pretty old but prior to that we didn't even have a starting point...no IEEE 1584 empirical model so we didn't even know that it was worse than the "normal" test case. So it was very revolutionary at the time. But if you read the paper the conclusion is also that it's obvious that it's a problem but not enough data to know what to do about it."
This was the paper referenced:
https://drive.google.com/file/d/0B6mGRCG7wns_bHZIRDl5VzlDSWM/view
Bottom line is. With real world model used like barrier (representing breaker) inside an otherwise empty panel. The bolted short circuit current can be as low as 4kA and the 208v can still cause arc flash.
My electrician didn't wear PPE because of IEEE 1584-2002 recommendation that:
This was the paper referenced:
https://drive.google.com/file/d/0B6mGRCG7wns_bHZIRDl5VzlDSWM/view
Bottom line is. With real world model used like barrier (representing breaker) inside an otherwise empty panel. The bolted short circuit current can be as low as 4kA and the 208v can still cause arc flash.
My electrician didn't wear PPE because of IEEE 1584-2002 recommendation that:
“Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low-impedance transformer in its immediate power supply”"
Had IEEE hired real physicists in the experiments. It could have caught it earlier. But they were able to catch it a few years ago. That's why the above was deleted 4 months ago and in its place (IEEE 1584-2018) was:
“Sustainable arcs are possible but are less likely in three-phase systems operating at 240 V nominal or less with an available short circuit current below 2000A".
This is the actual nameplate of one transformer (plus a 2nd one that produce the open delta transformers).
The shorted circuit current was calculated at 6971 Amp.
Now if you will add conductor impedance, the bolted short circuit current was 4001A, enough to require PPE had they caught it in 2002. So you see. Digging into physics is what can nail it earlier and it could have saved scarring in my electrician arm. He was gay so he was bothered and now depressed. All because IEEE-1584-2002 didn't get physicists involved.
This is the actual nameplate of one transformer (plus a 2nd one that produce the open delta transformers).
The shorted circuit current was calculated at 6971 Amp.
Now if you will add conductor impedance, the bolted short circuit current was 4001A, enough to require PPE had they caught it in 2002. So you see. Digging into physics is what can nail it earlier and it could have saved scarring in my electrician arm. He was gay so he was bothered and now depressed. All because IEEE-1584-2002 didn't get physicists involved.
KISS it --> keep it stupidly simple, I was told.About what good is digging into the physics of arc flash.
That's exactly how IEEE 1584-2002 erred. This was what occurred:
"well at the time they were looking for repeatable tests that could be used for laboratory purposes so an artificial scenario was a necessity. And at the time the impact of things like horizontal barriers as well as phase barriers and many similar items was essentially unknown. The model they chose was substantial enough that it would survive more than a single test so it worked out pretty good and got a lot of good data in a short period of time. High current testing lab time is definitely not cheap and the entire effort for the most part came from voluntary donations of time and money, and still does. The barrier paper you have the reference for is now pretty old but prior to that we didn't even have a starting point...no IEEE 1584 empirical model so we didn't even know that it was worse than the "normal" test case. So it was very revolutionary at the time. But if you read the paper the conclusion is also that it's obvious that it's a problem but not enough data to know what to do about it."
This was the paper referenced:
https://drive.google.com/file/d/0B6mGRCG7wns_bHZIRDl5VzlDSWM/view
Bottom line is. With real world model used like barrier (representing breaker) inside an otherwise empty panel. The bolted short circuit current can be as low as 4kA and the 208v can still cause arc flash.
My electrician didn't wear PPE because of IEEE 1584-2002 recommendation that:
“Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low-impedance transformer in its immediate power supply”"Had IEEE hired real physicists in the experiments. It could have caught it earlier. But they were able to catch it a few years ago. That's why the above was deleted 4 months ago and in its place (IEEE 1584-2018) was:
“Sustainable arcs are possible but are less likely in three-phase systems operating at 240 V nominal or less with an available short circuit current below 2000A".
This is the actual nameplate of one transformer (plus a 2nd one that produce the open delta transformers).
The shorted circuit current was calculated at 6971 Amp.
Now if you will add conductor impedance, the bolted short circuit current was 4001A, enough to require PPE had they caught it in 2002. So you see. Digging into physics is what can nail it earlier and it could have saved scarring in my electrician arm. He was gay so he was bothered and now depressed. All because IEEE-1584-2002 didn't get physicists involved.
On difficult electrical situations where it is not advisable for personnel to do live work, the safest thing to do is not tinkering with the equipment when live. That way, you don't have to don the astronaut-like PPE. Or, remotely turn off the system, do voltage check (proximity voltage detectors), LOTO, do chaining to ground and then your good to go.
I just read the full paper called "Investigation of Factors Affecting the Sustainability of Arcs Below 250 V" by Michael J. Lang, Member, IEEE, and Kenneth Jones, Member, IEEE at Library Genesis.
The contents are horrifying. Here they used setups representative of real world equipments.
quoting their finding a bit:
"Tests performed with gaps of 12.7 and 50.8 mm were used to determine the effects of gap and X/R ratio on arc sustainability and incident energy with the barrier in place. Testing at progressively lower currents revealed the barrier configuration’s ability to reliably sustain arcs for more than 1 s with a 12.7-mm gap at 4 kA and 208 V. The 32-mm gap performed intermittently at the lower values."
Here is the abstract:
"Abstract—Recent testing with various electrode configurations and insulating barriers suggests that 250-V equipment omitted from arc flash hazard analyses has the potential for burn injury. Research into the sustainability of arcs at these voltages shows that assumptions about the magnitude of these hazards need to be revised. This research enhanced the work of previous efforts by focusing on the sustainability of arcs with fault currents lower than 10 kA. Gap lengths between electrodes, electrode shape, electrode material, and voltage variations are studied for their effects on arc sustainability. A modified barrier design representative of the space around panelboard bus bars is also studied."
The following is their conclusion:
"CONCLUSION
The testing discussed in this paper shows that sustained arcs are possible at 208 V even at relatively low fault currents but are dependent on several factors including voltage variations, conductor material, the configuration of conductors, and the presence of insulating barriers. The challenge to industry is to advance the research identified in the references, do additional testing on a variety of low-voltage equipment, and incorporate those findings into improved standards. These test strategies must consider all practical locations within the equipment where arcs may occur; within all equipment is the possibility for different electrode orientations.
Enhanced models for various equipment"
This is important for the electrical workers worldwide. In our country. What defined electricians is their willingness to work live. That's their trademark since we don't use any neutral or ground and there is no polarity in the red and black phase to phase. So their skills are defined by able to tape connections well and work live. We don't use any wire nuts or Polaris terminals. In fact the contractor electrician has connected a pair of AWG 4 wires into a room by just splicing them and taping them. Sometimes I got horrified imagining what if resistive heating would form in the splice. I may need to use Polaris terminals on them if there is a chance to service the unit which has tenant inside so can't just disturb them.
Anyway to those who have read the paper fully. Let's discuss the hard science and engineering aspect of it. But since this thread is already long. Anyone who wants to discuss them can start their own threads so as to get fresh inputs as many don't want to even browse any threads that are 10 pages long, lol!
mbrooke
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But remember, the service conductors limit the current.
I know that is why we are now computing the exact bolted short circuit current to see if it would at least be 3kA that can cause arc flash in the new IEEE findings or whether it is just 1kA or so.
See: https://forums.mikeholt.com/showthread.php?t=197011
mbrooke
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My apologies :ashamed1: I've been busy and following along on and off.
If you see there. The conductors impedances are almost insignificant to the decrease in the bolted short circuit current.
For example. The open delta transformers setup is calculated to have a short circuit current of 6971 amps.
Adding the 23 feet of AWG 1 conductors just decrease the bolted short circuit current to 6341 amps.
Not a significant difference considering the new IEEE 1584-2018 released just last November 2018 already considered 208v above 2ka as able to produce arc flash.
Where did you hear about the conductors impedance having big impact on the bolted short circuit current? Can you give an actual numeric example or configuration that made it possible?
mbrooke
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According to this, there is a good level of reduction.
https://www.alabamapower.com/conten...rs/A-E-Fault-Currents-Tables-FINAL-8-2003.pdf
https://www.alabamapower.com/conten...rs/A-E-Fault-Currents-Tables-FINAL-8-2003.pdf
Let's take page 12 for example:
I'm inputting the values in this Fault Current Calculation Spread I found
https://electrical-engineering-port...electrical-software/fault-current-calculation
What system should I enter based on the Alabama Power Poco info?
Whatever. The reduction may be useful for the old IEEE 1584-2002. But for the new IEEE 1584-2018 where they acknowledge arc flash could occur above 2kA then the reduction won't make the arc flash vanish because even the farthest distance already have 2399Amp.
The error in the old IEEE 1584-2002 was because empty panel were used.
They forgot to put breaker (or representative of it):
Then someone decided the put the breaker (or barrier representative of it) and they were surprised with the results:
Then all those 208v injuries in the arms were realized not to be due to flying sparks but arc flash with energy more than 1.2cal/cm2.
So they removed the following passage:
“Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low-impedance transformer in its immediate power supply”"
And replaced it just last November 2018 with:
"Sustainable arcs are possible but are less likely in three-phase systems operating at 240 V nominal or less with an available short circuit current below 2000A"
That means above 2000A, it's sustainable as proven in this paper:
"Investigation of Factors Affecting the Sustainability of Arcs Below 250 V" by Michael J. Lang, Member, IEEE, and Kenneth Jones, Member, IEEE at Library Genesis.
Single phase also implicated. Meaning even single phase with 4kA can produce arc flash at 12mm gap.
Btw what I found out from all the calculations is my residential has service entrance bolted short circuit current even greater than the office building due to the 100kVA transformer. Therefore from now on, any electrician will need to wear full my recently bought PPE 12cal/cm2 arc flash uniform.
And most important I realized is main breakers are important to limit it to just 3 cycles which can greatly cut the incident energy.
mbrooke
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208Y, 240 closed delta and 480Y for 3 phase. See if they match up.
mbrooke
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It doesn't come out correct because of the missing 480v.
Know any good bolted short circuit fault online or xcel calculation sheet?
mbrooke
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It doesn't come out correct because of the missing 480v.
Know any good bolted short circuit fault online or xcel calculation sheet?
:?
480 is another calculation and system all on its own.
Let us manually compute for them then:
For the single phase 240v at 0 feet result above (infinite bus assumption) = 15621amp,
computing using the short cut method
75,000/240/0.02=15625
or the long cut method
kVa = 75,000
voltage =240v
The impedance of a load that would draw 75 kVA is 240^2/75000 = 0.768 ohms.
Transformer impedance = 0.02x 0.768 = 0.01536Short circuit current = 240v/0.01536 = 15625
For the 3 phase
The table value is 13,528 for 480v
using infinite bus assumption short cut... sqrt(3)*75000/480/0.02= 13531 amp
at 240v = 13531 x (480/240) = 27063v
at 208v = 13531 x (480/208) = 31226v
which matched the above.
But I don't know the exact formula for getting impedance of wires. Any ideas?
What should be done is simply adding the impedance of the transformers and wires, then divide the voltage with the impedances to get the panel short circuit current. I want to manually compute everything and put in excel spreadsheet just to verify everything.
This is to make sure I get accurate results in getting the panel bolted short circuit current at office and residential (this is because I was alarmed and surprised today the arc flash potential at home is greater.. in case the main breaker fails to open. Remember IEEE 1584-2018 has now 2kA arcable threshold instead of 87kA).
mbrooke
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Its a bit more complicated, as I understand you have resistance plus reactance do deal with.
Yes. I've been reading a lot about it a while ago. It's time to master the concept before I move on from all this next week. Primary lessons learnt is never to let electrican work live and use PPE and put main breaker. The calculations solidified the concepts in the mind and strengthen the will.
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