Arc flash hazards and the 2 second rule ? 3

[bdn2004]

There’s a part of the arc flash calculation method that bothers me some: the 2-second rule. We rely on this sometimes to lower the reported Incident energy by saying the max time of exposure is 2 seconds: that your natural reaction time getting away from a flash is maximum 2 seconds. It does not seem very scientific. It also discourages finding a real mitigation method.

Anyone else have any thoughts on this ?

 

[dik]

Not being a sparky... the two second rule seems silly and is a means of not addressing a problem.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[davidbeach]

I don't use the 2 second "rule", it's asinine. The only way it could possibly matter is if you get blown out of the danger zone in less than 2 seconds.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[dik]

Heheehhehee

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Fischstabchen]

Davidbeach,

As they say in Game of Thrones, "what is dead may never die". Anyone blown out of the danger zone no longer needs to worry about arc flash.

Dik,

I think that most of the time someone knows enough to duck out of the way in under 5 seconds, maybe 2. I think 2 seconds is in the standard to create fewer situations where the arc flash rating exceeds 100 cal/cm^2 without a bunch of upgrades. I suspect the fear was that industry would push completely against the standard if it mandated too many upgrades all at once.

 

[dik]

I didn't think you had even close to 2 seconds... let alone 5... thanks.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Fischstabchen]

This was added into IEEE 1584 2018 edition:

"If the total protective device clearing time is longer than two seconds, consider how long a person is likely to remain in the location of the arc flash. It is likely that a person exposed to an arc flash will move away quickly if it is physically possible, and 2 s usually is a reasonable assumption for the arc duration to determine the incident energy. However, this also depends on the specific task. A worker in a bucket truck, or inside an equipment enclosure, could need more time to move away. Use engineering judgment when applying any maximum arc duration time for incident energy exposure calculations, because there may be circumstances where a person’s egress may be blocked."

Dik,

Two seconds sounds worse than it should. The amount of energy released is going to equal to I^2*(arc impedance)*(time). 2 seconds is a long time for a protective scheme to operate and if the scheme is tight or you have maintenance switches to miscoordination the system while it is being worked on or something else like light sensing arc flash fiber, a high current fault will get cleared out quickly. Normally, a fault that exist for 2 seconds is lower current and will release energy more slowly. The easiest thing to clear out is a fault with a lot of current because it is easy to distinguish it from load current. For a high current fault, I suspect the bus would be gone in two seconds if the protective relaying didn't clear it.

 

[dik]

Way back when... when I first encountered 'arc flash', I asked an electrical engineer, "How hot is arc flash?" He replied, "Hotter than the sun." I was surprised, and a little doubtful, and asked another electrical engineer (remote and distinctly different), and got the same reply basically. For that sort of occurrance you have to be pretty quick on your 'hoofies'...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Fischstabchen]

Dik,

Read the second part of my edit on the speed of clearing a fault. It though is hotter than the sun and the copper and aluminium instantly turns to a a plasma and expands 3,000+ times creating a arc blast. There are two components to an arc event, an arc flash and the arc blast. The arc flash is intense light, which will burn everything around it, from the arc. The arc blast is like I said early, the physical explosion due to metal changing states into gas and plasma. Once you get above 100 cal/cm^2, the physicalness of the arc blast will likely kill or badly injury the worker in spite of the gear they have on. No one is walking away from a blast that throws them 30 feet across the room.

 

[waross]

Years ago, I was in the room when there was an arc flash/blast event about 30 feet away.
I was sitting at a print table when an electrician either grounded or shorted phase to phase, a 600 Volt bus in an MCC.
I looked up and saw two men standing frozen in place, out on their feet and one man down with his coveralls smouldering.
I stood up, ran over and slapped out the embers on his coveralls. I thought that he was dead, but as I was slapping his leg to extinguish the embers, he started to moan and groan. About that time, the other two men, who had been about three or four feet from the blast center, started to respond.
While my response time could be measured in seconds, it was well past two seconds before anyone but myself moved.
While the MCC contained most of the flash, we found several blank covers from the MCC across the room. Thankfully no-one was hit by them.
Had the arc persisted rather than being cleared instantaneously, no-one was going anywhere under their own power for something more than 2 seconds.
Anecdotal, yes, but you had to be there.

I don't use the 2 second "rule", it's asinine. The only way it could possibly matter is if you get blown out of the danger zone in less than 2 seconds.

I agree completely.
The worker had been kneeling in front of the MCC. After the blast, he was stretched out on the floor. His feet were close to the MCC but his torso was farther from the MCC than the two "frozen in place" workers who had been standing beside him.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jghrist]

How long would you stand in front of an arc? Not a huge arc that burns you badly right away, but one that is hot enough to burn bare skin in 2 seconds.

 

[Fischstabchen]

Jghrist,

I posted the addition above that was added as a note to 1584. I don't beleive that a tight protection system will allow typically a high power arc flash to persist out to 2 seconds. You would start having cal/cm^2 energies that would require enough ppe that it would be hard to work. The examples you guys gave I believe were quickly extinguished in spite of the injuries and it was not a roaring plasma fire ball of aluminium and copper for a full 2 seconds.

 

[stevenal]

You need some sort of limit. Consider a secondary fault ahead of the main protection that can only be cleared by a primary fuse. Also consider a battery terminal fault. The IE adds up if you assume someone simply stands there slowly cooking. Engineering judgement is required in these situations.

 

[bdn2004]

Stevenal,

That is the situation I'm looking at that made me ask the question. We've open bus on the secondary side of a large transformer that is protected by the primary side relay only. They want to know what is the arc flash at the open bus? It depends, default to 2-seconds it's still high, but not off the charts like it would be if it were based on the relay settings.

 

[dik]

No one is walking away from a blast that throws them 30 feet across the room.

No doubt in my mind about that, and I didn't dispute it... my only concern is the time involved... 2 sec seems like an awful long time.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Fischstabchen]

bdn2004,

Why are you working on live bus on the secondary side of a transformer instead of just taking the transformer out of service? It feels like you are trying to create a scenario where a long arc could be sustained rather than how the system would be designed so that it could be worked on live safely or if it shouldn't ever be worked on live because you couldn't reduce the energies.

 

[bdn2004]

Fischstabchen,

We have two identical lineups 12' apart: Transformer -> open ac bus -> open AC to DC conversion equipment. Neither is ever worked hot. But when one lineup is down for maintenance - the other one is still hot. The question is - what is the arc flash hazard boundary due to the hot equipment?

I approached this as an arc flash at the hot equipment and tried to determine that and the decreasing intensities the further away from the open bus you get - up to about 8'. That might not be the right way to think about this. And this situation might not be an arc flash problem. It would take a screw up or a catastrophic event to cause an arc flash on one of these open skids. They are surrounded by Plexiglas.

 

[bdn2004]

There is another identical skid 12' to the right of this skid. This skid for example would be dead during maintenance - but the next one over will not be.

 

[waross]

Expect the unexpected. Most accidents are unexpected. Many are due to unforeseen events.
I was with a crew installing a new switch section in an underground sub-station.
There were open 12kV bus everywhere across the ceiling and on the walls, but the work area was well protected by plexiglass screens, (we thought).
The crew was pulling a3,600,00 CM cable into an underground duct with a tugger when the chain drive on the tugger broke.
The pull rope tension spun the capstain backwards and the chain doubled up and smashed the aluminum chain guard.
There were pieces of aluminum and broken chain flying everywhere, not just in the protected area.
Fortunately none of the open buses were shorted.
It could easily have been very different.
It is possible that some sort of accident or breakage at the worksite may cause material to be projected into the hor area.
I applaud your due diligence in considering the possible arc flash of an adjacent section.
Thinks will probably never go wrong, but if they do the crew will be safer due to your foresight.

Tugger, Similar

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

I'm late to this, but I can't resist a few comments:

1. It's not a RULE. Simply a suggestion in IEEE 1584 as to a reasonable limit.
2. What's the alternative? If you compute the arcing current and determine that the arc time is 15 seconds or 150 seconds is that how you think that the incident energy should be calculated? That would not be reasonable or practical. That has to be a reasonable upper limit on the exposure time. If you don't like 2 seconds, pick your own maximum time. The top of the TCC curves is typically 1000 seconds, so you can always use that.
3. Arc flash calculations are a rough estimate intended to reduce the risk of arc-flash burn injuries.
4. Not all arc-flash events are the same. Some are extremely high energy, other much less so. The situations that appear to have extremely long arc times are normally where the fault current is low and the fault would have to be cleared by an inverse time or thermal element. This is much different that an explosive event the blows the door of a switchgear cubicle.