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 Post subject: The 208 V vs. 240 V less than 125 kVA transformer debate
PostPosted: Wed Feb 04, 2009 6:05 am 
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I just received clarification at the IEEE 1584 meeting from an NFPA 70E committee member about the conflict between IEEE 1584 and NFPA 70E's version of statements about calculations not being necessary downstream of transformers less than 125 kVA and circuits less than 240 Volts as in IEEE 1584 or circuits 240 Volts or less as in NFPA 70E

The question:
IEEE 1584 states less than 240 Volts which implies 208 Volts and less
NFPA 70E says 240 Volts or less implying 240 Volts and less

Which one do you use?

What I heard from the NFPA 70E committee member was the committee had one fall through the cracks. It was a mistake. :eek: They had hundreds of proposals to review in a short period of time and it looked like this was just going to be a copy of IEEE 1584 language except the difference between "less than" and "or less" slid by them.

It should be: less than 240 Volts i.e. 208 Volts like IEEE 1584.

They are presently considering a Tentative Interim Amendment (TIA) to straighten this out. Arcs at 208 Volts are difficult to sustain with low arcing current and the small transformer limits the current. 240 Volt arcs are more likely to be sustained. Also many people in the industry are using a cut off of 75 kVA to be safer. Arcs at higher currents at 208 Volts can be sustained and can still be dangerous.

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PostPosted: Wed Feb 04, 2009 6:48 am 
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Thanks for the very valuable post. Good to have some clarification there. A buddy of mine that does a lot of arc flash testing claims they have generated plenty of self sustaining arcs at 208V and low currents. I think IEEE needs to revisit this someday and do some further testing, your post was the first I have heard about the 75kVA cutoff, can you give some more details on where this comes from?


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PostPosted: Wed Feb 04, 2009 7:33 am 
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Right now it is still somewhat unclear. It has a lot to do with bus spacing and bus orientation i.e. horizontal vs. verticle etc. In talks yesterday with various people at the meeting / conference, it seems depending on the configuration of the bus and barriers, arcs around 2 kA at 208V are difficult to sustain but when you go up from there it sounds like it becomes a little more questionable. Other tests with different configurations indicate currents can be much greater and still be sustained. I [url="http://www.brainfiller.com/documents/PGETestingbrainfillerposting.pdf"]posted some tests by PG and E[/url]that showed the actual currents that were on the higher side and extinction times that were short but that was one series of tests and a lot more testing is still needed.

Someone went me a photo of an FR shirt (was told it was 6 to 8 cal fabric) worn by a utility worker on the 208V secondary of a padmount xfmr and it blew through the sleeve. Not sure of the kVA rating of the padmount but I'm sure it was larger.

The 75 kVA cut off was nothing more than table talk with several people. They chose to cut it off at 75 kVA on their own because of the uncertainty.

This is one of the major test areas that is being looked at but as I think everyone might be aware, "committee time" is a lot slower than "real time" so it will take a while.

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PostPosted: Fri Apr 10, 2009 8:15 am 

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Is current the major player in sustaining an arc?

Correct me where I’m wrong, but as I see it:

Current is not the major initiator of an arc nor is it a major player in sustaining an arc. For an arc to start, the major players are voltage across the gap, the size of the gap, the properties of the air (resistance/impedance) and contaminants with in the air (moisture, atmospheric pressure, dust, conductors, insulators, etc.). For an arc to be present the air must be ionized.

The voltage across the arc gap must be greater than or equal to the voltage required to ionize the air for that gap length. Once the air is ionized, to sustain the arc the voltage must be greater than the minimum required voltage to sustain ionization for that gap length. The voltage to establish an arc (ionize the air) across a given gap, is much greater than the voltage required to sustain ionization. Keep in mind ionization is not instantaneous (zero time), it takes time (small maybe, but none the less time) to ionize air when a voltage is present and to de-ionize when the voltage decreases below the level to sustain ionization for the given gap. Think 60Hz and zero crossings. 60x2 times a second the voltage is below the level to sustain an arc and the air starts to de-ionize until the voltage level gets back above the level to sustain ionization (the rise and fall transition times, relatively speaking, are quite slow).

Current is a secondary result (consequence of voltage and impedance), based on the source voltage and all the impedances (source impedance, transformer impedances, conductor impedances, arc impedance/resistance which is dependent on the gap size and the properties of the air and any contaminants within the air), think ohms law here.

The voltage across the arc gap (to sustain ionization) is then dependant on the voltage drop across all the impedances/resistances in the fault circuit subtracted from the source voltage (Who's law is this?? Remember?? This is a test).
Current is source voltage divided by the sum of all the impedances (assuming all impedances, including source impedance, are in series). The radiated energy is based on the current through the arc (arc current) over a period of time.
Anyone who has done any arc welding is quite familiar with establishing and maintaining an arc at low voltage and low current. You first establish a bolted fault current by 'striking' the rod to the metal, then pulling the rod back to maintain an arc which melts the metal and rod to form a welded joint (This is harder than it seems, but once mastered is a beautiful thing to see). Obviously, the arc gap is quite small, but still a gap. You can pull the rod away from the metal quite a distance before the arc extinguishes). The ArcFlash we're talking about is no different, really. It is just that the powers to be have set up equations and and standards based on gaps commonly found in distribution panels 25mm 9about 1 inch and 33mm about 1.3 inch, then assumed that the gap remains relatively constant during the arc time.


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PostPosted: Sun Apr 12, 2009 8:42 pm 
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Where available fault currents are high, the impedance except for the arc is low. The arc impedance is a larger fraction of the total impedance and the voltage across the arc is higher.


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PostPosted: Mon Apr 13, 2009 5:08 am 
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DL,
You are correct about the dielectric breakdown but the arc sustainablity is something we are still exploring. Voltage does have a lot to do with it. There have been many "surprises" in the laboratory. One of the roles the current plays is how large is the conducting plasma. JL is also correct about the low vs. high source impedance vs. the arcing impedance. Whether the arc is in a box or not also impacts the concentration of the plasma as well as many other factors.

Arc flash research is taking this to a whole new level and I encourage you to dig as deeply as you can into the subject, it's pretty amazing stuff! I'll be in the lab in June looking at some new ideas and watching the "fireworks" (can't discuss it yet) but if I learn anything new that I can pass along, you'll see it here.

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PostPosted: Mon Aug 31, 2009 10:57 pm 
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Oh grand one,
Wow, great info but now I have to rethink some things. Thanks for keeps us informed.


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PostPosted: Mon Aug 31, 2009 10:58 pm 
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For keeping us informed. Sorry.


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