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 Post subject: NFPA 70E and 50 Volt Standard?
PostPosted: Tue Jun 09, 2015 11:23 am 
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Our location uses a "50 volt" or above standard (30 volts or above on ships and at the shipyard) to set the bar when it comes to safety.

In regards to Arc Flash, isn't it still possible to have an arc flash explosion even at the defines "low voltage", < 600 volts?

We even had an arc from a crane power source, of 460 VAC and 5-6/10 amps. The guy was lucky he did not sustain injuries.

I'm in a disagreement with an engineer type who believes arc flash is solely dependent on voltage, current and timing. What are your thoughts?

We know arc flash can happen at close ranges, when conditions can cause electricity to divert from normal path and travel through the air. This release of energy instantly superheats the air and nearby components, causing an explosion.

High voltage isn't necessary to create a dangerous arc flash; it just makes it more likely. Arc flashes from relatively low voltage equipment can be just as deadly. Underestimating the danger of lower voltage equipment can result in workers not taking adequate protective measures. Always follow all of the protective measures at your workplace; Arc flashes pose several serious dangers.

The temperature of an arc flash can be as high as 35,000 degrees Fahrenheit, which is more than three times hotter than the surface of the sun. This intense heat can severely burn and even kill you.

An arc flash can also cause electrical components to explode, sending out a violent blast of molten metal. Bits of metal can penetrate skin and cause burns. The force from the blast can cause collapsed lungs, broken bones, and permanent hearing damage.

An arc flash can also produce intense light that can damage your eyes. Extremely bright or long-lasting arc flashes can even blind you.
Finally, even if an arc flash does not hurt someone directly, the blast could start another fire or explosion in the facility.


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 Post subject: Re: NFPA 70E and 50 Volt Standard?
PostPosted: Tue Jun 09, 2015 1:02 pm 
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torsiondrummer wrote:
Our location uses a "50 volt" or above standard (30 volts or above on ships and at the shipyard) to set the bar when it comes to safety.


That is the general rule for SHOCK. You can definitely strike an arc below that voltage...just ask any welder. But for power distribution purposes I haven't seen anything significant.

Quote:
In regards to Arc Flash, isn't it still possible to have an arc flash explosion even at the defines "low voltage", < 600 volts?


Yes, most definitely. Contrary to popular belief though it is kind of the opposite of what most people believe. Without getting into a lot of details, suffice to say that arc flash is approximately energy and is approximately volts X amps x time, as you would expect it to be. But I'm glossing over a LOT of details by stating this so the basic concept but not necessarily the math follows.

As voltage increases using a constant (ANSI standard) impedance for transformers, the arc flash hazard actually decreases using the same size transformer...or in other words, holding power constant. At 480 V, it is almost impossible to get below 40 cal/cm^2 with a 2500 kVA transformer using only overcurrent protection settings. At 4160 V, this level increass to around 20,000 kVA. So for most plants, the 480/600 V stuff is the most hazardous. By the time you get to medium voltage (2300, 4160, 7200, 13.5 kV) arc flash is still present but normally not as big of a hazard.

At a point below around 250 VAC, the arc itself becomes unstable and hard to sustain. The existing engineering only looks at stable arcs so we get into a sort of no-man's land below 250 VAC. Kinetrics has done some tests at 130 VDC and found that this is about the threshold using reasonable fault current levels (20 kA) for a 1.2 cal/cm^2 incident energy (threshold for 2nd degree burns) via a study commissioned by Duke Power. This is for DC...obviously for AC the threshold extends upward somewhat. Some studies have been done by EPRI as well and you can definitely get to a significant arc flash hazard with AC at 250 VAC, enough to wear an arc rated shirt and pants but that's about it. OSHA has documented a couple cases including one fatality with a temporary construction panel with a 120/240 V system in Georgia.

Quote:
We even had an arc from a crane power source, of 460 VAC and 5-6/10 amps. The guy was lucky he did not sustain injuries.


Hence as above...yep, very lucky. Right at around 480/600 V is the "sweet spot" in terms of arc flash energy...the highest you can normally get.

Quote:
I'm in a disagreement with an engineer type who believes arc flash is solely dependent on voltage, current and timing. What are your thoughts?


Mostly in terms of the HAZARD, this is true. It also depends on the shape/configuration of the equipment, the bus gap, X/R ratio, grounding system design, and distance from the equipment. It also misses the fact that grounding design, relaying, equipment venting (for arc resistant gear), maintenance, and design ("finger safe") play a role in determining the likelihood that an arc flash can happen in the first place.

One of the odd things that happens is how things interact. For instance if you were to reduce the current using say current limiting fuses or simply running longer cables, the general thought would be that arc flash should decrease. However that is not necessarily the case. A lower current also increases the trip time of the overcurrent protective device which typically increases the arc flash. Heavy/high currents that trip instantaneous protection produce very mild/low arc flash conditions.

Quote:
We know arc flash can happen at close ranges, when conditions can cause electricity to divert from normal path and travel through the air. This release of energy instantly superheats the air and nearby components, causing an explosion.

High voltage isn't necessary to create a dangerous arc flash; it just makes it more likely. Arc flashes from relatively low voltage equipment can be just as deadly. Underestimating the danger of lower voltage equipment can result in workers not taking adequate protective measures. Always follow all of the protective measures at your workplace; Arc flashes pose several serious dangers.


Partly true but again there's that pesky interaction issue. Higher voltages mean that bus space MUST increase and also often means using shielded cables. As bus gaps increase, the incident energy increases because there is simply a longer arc, releasing more heat. But wider bus gaps decrease the likelihood in the first place. The physics is that the core of the arc (usually very small) reaches a temperature of around 4000-6000 K. The air inside "disassociates" which means that the electrons aren't really attached to the atoms anymore. They simply float around and this is what we call plasma. It is HIGHLY conductive for obvious reasons (very little resistance). The effect is not instantaneous but it does happen in nanoseconds. And as the current (not voltage) goes through zero, the arc extinguishes. It then restrikes again once the voltage (not current) gets high enough to reignite the arc through the heated/plasma air. This continues until interrupted externally in a stable arc, and spontaneously disappears in an unstable (low voltage) one.

Quote:
The temperature of an arc flash can be as high as 35,000 degrees Fahrenheit, which is more than three times hotter than the surface of the sun. This intense heat can severely burn and even kill you.


I've seen this number thrown around before. Actual arc lab testing shows it gets to around 6000 K tops (Kelvin), which is roughly 10,000-12,000 F, or about has hot as an oxy-acetylene torch. Not nearly as hot as say thermite or a nuclear device.

Quote:
An arc flash can also cause electrical components to explode, sending out a violent blast of molten metal. Bits of metal can penetrate skin and cause burns. The force from the blast can cause collapsed lungs, broken bones, and permanent hearing damage. [qupte]

Copper vapor is about 86,000 times its orginal solid dimensions. I've found little bits of melted copper and some evidence (very little) of vapor. Launching parts is a real possibility. Not buying collapsed lungs though. The air pressure in the arc blast is much less than the pressure needed to do this although it will easily blow ear drums out.

Quote:
An arc flash can also produce intense light that can damage your eyes. Extremely bright or long-lasting arc flashes can even blind you.
Finally, even if an arc flash does not hurt someone directly, the blast could start another fire or explosion in the facility.


Not sure how it will start another one but damage your eyes...ask a welder what weld flash is.

Some true here, some not. I've seen a lot of this stuff thrown around before. Be careful about repeating it as it is not all based on factual information.


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 Post subject: Re: NFPA 70E and 50 Volt Standard?
PostPosted: Tue Jun 09, 2015 3:31 pm 
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PaulEngr wrote:
torsiondrummer wrote:
Our location uses a "50 volt" or above standard (30 volts or above on ships and at the shipyard) to set the bar when it comes to safety.


That is the general rule for SHOCK. You can definitely strike an arc below that voltage...just ask any welder. But for power distribution purposes I haven't seen anything significant.
Quote:



Great point, I was generalizing with this one, knowing you have basically three types of electrical hazards, electrical shock, arc flash and arc blast. In regards to the shiock, the NPFA goes into some distance calc's for 50volts+ to all the way to 800kV. I appreciate the validation on lower voltages.


Quote:
In regards to Arc Flash, isn't it still possible to have an arc flash explosion even at the defines "low voltage", < 600 volts?


Yes, most definitely. Contrary to popular belief though it is kind of the opposite of what most people believe. Without getting into a lot of details, suffice to say that arc flash is approximately energy and is approximately volts X amps x time, as you would expect it to be. But I'm glossing over a LOT of details by stating this so the basic concept but not necessarily the math follows.
Quote:
Does this derive from an IEEE or another reference?


As voltage increases using a constant (ANSI standard) impedance for transformers, the arc flash hazard actually decreases using the same size transformer...or in other words, holding power constant. At 480 V, it is almost impossible to get below 40 cal/cm^2 with a 2500 kVA transformer using only overcurrent protection settings. At 4160 V, this level increass to around 20,000 kVA. So for most plants, the 480/600 V stuff is the most hazardous. By the time you get to medium voltage (2300, 4160, 7200, 13.5 kV) arc flash is still present but normally not as big of a hazard.
Quote:
Ok, so the 40 cal/mc2 comes from the Haz Class 4 correct? Where does the high voltage of 4160 V come from? is that a lab rating or tested theory?


At a point below around 250 VAC, the arc itself becomes unstable and hard to sustain. The existing engineering only looks at stable arcs so we get into a sort of no-man's land below 250 VAC. Kinetrics has done some tests at 130 VDC and found that this is about the threshold using reasonable fault current levels (20 kA) for a 1.2 cal/cm^2 incident energy (threshold for 2nd degree burns) via a study commissioned by Duke Power. This is for DC...obviously for AC the threshold extends upward somewhat. Some studies have been done by EPRI as well and you can definitely get to a significant arc flash hazard with AC at 250 VAC, enough to wear an arc rated shirt and pants but that's about it. OSHA has documented a couple cases including one fatality with a temporary construction panel with a 120/240 V system in Georgia.
Quote:
brilloiant, I'd love to see the OSHA write up on that one


Quote:
We even had an arc from a crane power source, of 460 VAC and 5-6/10 amps. The guy was lucky he did not sustain injuries.
Quote:
He sure was


Hence as above...yep, very lucky. Right at around 480/600 V is the "sweet spot" in terms of arc flash energy...the highest you can normally get.

Quote:
I'm in a disagreement with an engineer type who believes arc flash is solely dependent on voltage, current and timing. What are your thoughts?


Mostly in terms of the HAZARD, this is true. It also depends on the shape/configuration of the equipment, the bus gap, X/R ratio, grounding system design, and distance from the equipment. It also misses the fact that grounding design, relaying, equipment venting (for arc resistant gear), maintenance, and design ("finger safe") play a role in determining the likelihood that an arc flash can happen in the first place.

One of the odd things that happens is how things interact. For instance if you were to reduce the current using say current limiting fuses or simply running longer cables, the general thought would be that arc flash should decrease. However that is not necessarily the case. A lower current also increases the trip time of the overcurrent protective device which typically increases the arc flash. Heavy/high currents that trip instantaneous protection produce very mild/low arc flash conditions.

Quote:
We know arc flash can happen at close ranges, when conditions can cause electricity to divert from normal path and travel through the air. This release of energy instantly superheats the air and nearby components, causing an explosion.

High voltage isn't necessary to create a dangerous arc flash; it just makes it more likely. Arc flashes from relatively low voltage equipment can be just as deadly. Underestimating the danger of lower voltage equipment can result in workers not taking adequate protective measures. Always follow all of the protective measures at your workplace; Arc flashes pose several serious dangers.


Partly true but again there's that pesky interaction issue. Higher voltages mean that bus space MUST increase and also often means using shielded cables. As bus gaps increase, the incident energy increases because there is simply a longer arc, releasing more heat. But wider bus gaps decrease the likelihood in the first place. The physics is that the core of the arc (usually very small) reaches a temperature of around 4000-6000 K. The air inside "disassociates" which means that the electrons aren't really attached to the atoms anymore. They simply float around and this is what we call plasma. It is HIGHLY conductive for obvious reasons (very little resistance). The effect is not instantaneous but it does happen in nanoseconds. And as the current (not voltage) goes through zero, the arc extinguishes. It then restrikes again once the voltage (not current) gets high enough to reignite the arc through the heated/plasma air. This continues until interrupted externally in a stable arc, and spontaneously disappears in an unstable (low voltage) one.

Quote:
The temperature of an arc flash can be as high as 35,000 degrees Fahrenheit, which is more than three times hotter than the surface of the sun. This intense heat can severely burn and even kill you.
Quote:
Interesting, I still see it quoted throughout OSHA and NFPA type training. I'd like the facts - thank you for pointing that out!


I've seen this number thrown around before. Actual arc lab testing shows it gets to around 6000 K tops (Kelvin), which is roughly 10,000-12,000 F, or about has hot as an oxy-acetylene torch. Not nearly as hot as say thermite or a nuclear device.

Quote:
An arc flash can also cause electrical components to explode, sending out a violent blast of molten metal. Bits of metal can penetrate skin and cause burns. The force from the blast can cause collapsed lungs, broken bones, and permanent hearing damage. [qupte]

Copper vapor is about 86,000 times its orginal solid dimensions. I've found little bits of melted copper and some evidence (very little) of vapor. Launching parts is a real possibility. Not buying collapsed lungs though. The air pressure in the arc blast is much less than the pressure needed to do this although it will easily blow ear drums out.

Quote:
An arc flash can also produce intense light that can damage your eyes. Extremely bright or long-lasting arc flashes can even blind you.
Finally, even if an arc flash does not hurt someone directly, the blast could start another fire or explosion in the facility.


Not sure how it will start another one but damage your eyes...ask a welder what weld flash is. Thats a good idea-

Some true here, some not. I've seen a lot of this stuff thrown around before. Be careful about repeating it as it is not all based on factual information.

Quote:
thank you for the informative and interesting replies, I really appreciate it.


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