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 Post subject: Are Large Clearing Time Values Realistic?
PostPosted: Wed Nov 02, 2011 5:39 am 
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I have been performing Arc Flash Analysis for about 4 years now and have been doing switchgear maintainence for over 10 years now. One thing that has allways irked me is the incredibly large values on equipment that is being feed directly by a transformer. For example; a customer has a 2000 KVA transformer, 5.7%Z, 4160Y-480Y/277 feeding a molded case panelboard with a 2500 Amp main buss. The calculations I did after calculating for the X/R values and such, with a 2 second clearing time (actual clearing time was larger, but used the 2 second kinda-rule) and the values came out to be, Ibf=69.165kA, Ia=43.917kA, iE=292.797cal, and DB(empirical)=25.381'. Can this panel realistically produce that ammount of energy? (would it blast/melt away the concrete wall that is behind it and be able to give people a second-degree burn at a distance of 25'? I've had a 480V, 2000A Main arc flash on me, phase-phase-phase, line side, totally exposed and even though I did recieve burns and had to get life flighted to the hospital, it was because I had NO PPE (not even safety glasses) other than class 2 gloves. I had a plain t-shirt from walmart on and it didn't even catch on fire and only burnt through in one section about 4" diameter. Of course I happened to be lucky that the blast saved me from molten shrapnel, but even if I had cat 1 on, I would have walked away. These values indicate that this panel is like 7 times what a 40 cal suit could handle?! Way to dangerous for the to even operate (so, what do they do when one of the breakers in this panel trip...call the power company and have them open the pole jack while all the other breakers in the panel are closed (dangerous) and shut down the whole plant just to be able to close that breaker?
Another example (with some standard values used, not off an actual case), say I had a 150 KVA transformer, 480-240V feeding a 250A panel. The values come out to be like 13.4 cal and 5.4'. So on that small panel they need to wear cat 3? Can the small wires coming into that panel realistically support that ammount of energy? And even to just operate the breakers they would need to wear like cat 2 or 1 at the least??
It just seems that the values are Way above what can actually be produced by the equipment. What I normally tell customers is that it is a extreemely high value, but it represents the magnitude of hazard that it has. But isn't it to extreme.
Working on equipment I would expect to see more like,
208 or 240V panels, 250A or so...cat 0 to operate and cat 1 or 2 inside
480V, 600-800A range...cat 1 to operate and cat 2 or 3 inside
480V, 1200-1600A range...cat 2 to operate, cat 3 or 4 inside
480V, 2000A and above...cat 3 to operate, cat 4 or "Dangerous! No PPE" inside

Wouldn't this be more realistic and still a safe level for the workers to do what they need to at times. Isn't that what OSHA is concerned with and not the derived formulas that IEEE got from certain tests?


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PostPosted: Wed Nov 02, 2011 7:51 am 
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The new low voltage table in the NESC puts exposure at the transformer at 4 cal/cm^2 based on:

Eblen, M. L., and Short, T. A., Arc Flash Testing of Typical 480V Utility Equipment, IEEE Industry
Applications Society-Electrical Safety Workshop Paper No. ESW2010-05.


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PostPosted: Wed Nov 02, 2011 11:40 am 
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stevenal wrote:
The new low voltage table in the NESC puts exposure at the transformer at 4 cal/cm^2 based on:

Eblen, M. L., and Short, T. A., Arc Flash Testing of Typical 480V Utility Equipment, IEEE Industry
Applications Society-Electrical Safety Workshop Paper No. ESW2010-05.

But that is at the transformer only, based on typical pad-mounted transformer secondary spacings. NESC incident energies for switchgear and MCCs is 8 cal/cm┬▓ for 50-250 V, 40 cal/cm┬▓ for 251-600 V, and 60 cal/cm┬▓ for 601-1000 V. These are based on IEEE-1584 formulas with some assumed values.


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PostPosted: Wed Nov 02, 2011 1:44 pm 
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Why is it just voltage they classify it at when there is such a difference from other things? For example, a 480V substation with a 4000A main breaker being fed by a 10,000 KVA transformer is a little different than a 480V substation with a 2500A main breaker being fed by a 2000 KVA transformer. I understand simplifying things, and my original question was about the 300-500 cal ratings I see on such equipment, but I would be much happier seeing something like 60 cal and 35 cal on my two examples, respectively. And smaller (240 and less) should be a safe value (cat 1, perhaps even cat 2 if fed by say something like a 500KVA 480-240V transformer), but saying a 240V, 150A panel requires cat 3 or 4 seems extreeme to me.
Another thing I was wondering about on clearing time is that I know from doing circuit breaker testing that the normal clearing time for a breaker is 1 cycle, but the 70E mentions about high volt breakers having a clearing time of 6 cycles. I do not like this because 1. people think that clearing time of LV breakers is 6 cycles rather than the actual 1 cycle and 2. HV breakers don't have trip units...they will have relay overcurrent protection (which most common ones such as GE IFCs and GE CO8's and CO9's have a lot less than 6 cycle clearing time too).
I just think that the real world application to provide a safe work practice for tasks that need to be done still needs a lot of improvement. If it's conservative due to lack of understanding or detail I can understand that. But I'd just like to get a better reasoning and understanding for the values I often get.
Also I think the aspect of doing regular testing of switchgear should be more emphasised due to the dramatic difference it makes if the upstream protective devices fails to trip. And not only testing but also even more frequent cleaning and operating. The condition I see equipment in at plants really posses a high level hazard (disconnects in service that have been smashed in by fork lifts up to 1/2 their size, electrical panels and switches just roped off to columns, places that have cutting oil dripping out of their substations (HV switch, transformer and the 480V side) to the degree of having to mop the floor before we could even setup, opening 400 A disconnects at textile plants and just seeing wool (could not see blades or nothing, just wool out to the door), opening up the backs of substations and can tell it hasn't been opened in years due to the cobwebs and dust, etc., etc.)...The list goes on.


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PostPosted: Mon Nov 07, 2011 11:02 am 
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BruceJAX wrote:
I have been performing Arc Flash Analysis for about 4 years now and have been doing switchgear maintainence for over 10 years now. One thing that has allways irked me is the incredibly large values on equipment that is being feed directly by a transformer. For example; a customer has a 2000 KVA transformer, 5.7%Z, 4160Y-480Y/277 feeding a molded case panelboard with a 2500 Amp main buss. The calculations I did after calculating for the X/R values and such, with a 2 second clearing time (actual clearing time was larger, but used the 2 second kinda-rule) and the values came out to be, Ibf=69.165kA, Ia=43.917kA, iE=292.797cal, and DB(empirical)=25.381'. Can this panel realistically produce that ammount of energy?


It appears that you are assuming infinite upstream current available?

Quote:
(would it blast/melt away the concrete wall that is behind it and be able to give people a second-degree burn at a distance of 25'?


The calculation is for THERMAL (radiant heat) effects only. It does not consider arc blast because at this time there isn't enough research available to do that. Ralph Lee did do a calculation for it but experience (and the experiments performed) suggest that it doesn't work all that well.

The 2 second rule is under the assumption that you either flee or are blown away from the panel. It doesn't consider the possibility that the arc may not be sustained, especially at the levels you are talking about.

The calculation also assumes you are standing directly in front of the panel and is for WORST CASE. Depending on for instance downstream effects (which motors are running) as well as phase angle (affects the X/R value), it may not be that bad.

In addition, it depends strongly on panel design. Recent data from IEEE/NFPA joint tests suggests that the incident energies might be twice as big as calculated depending on equipment configuration. However at the same time, they might also eject plasma in which case the ASTM tests are invalidated and your 40 cal/cm^2 might not protect you after all.

Also, distance has a huge impact. The incident energy is roughly proportional to the inverse of the square of the distance (more or less). The assumed distances are for bus bars in typical panels at the back of the panel.

Also, the calculation is for a phase-to-phase fault. Granted within milliseconds everything is engulfed in plasma/vapor from copper so it's the same thing but those are at much longer arc distances so the arc energy is less than calculated. Over 90% of arcing faults are phase-to-ground. Based on this alone, it would be silly not to install high impedance grounding in most systems, and you really need to install CBCT's even on high current systems like you described because it may not trip on a ground fault with those kinds of settings in most cases. I've seen countless cases of panels being nearly vaporized and the circuit breaker or fuse did not trip.

Also, impedance has a huge effect. You might get to the calculated level IF you had a phase-to-phase fault across.

Quote:
Another example (with some standard values used, not off an actual case), say I had a 150 KVA transformer, 480-240V feeding a 250A panel. The values come out to be like 13.4 cal and 5.4'. So on that small panel they need to wear cat 3?


Depends on if everything is calculated properly.

If the hangup was just with opening/closing breakers, and you are putting in new equipment, you can spend 25% extra and get arc resistant gear. This only applies to medium voltage switch gear according to the standard but many companies are now selling arc resistant 480 VAC switchgear that meets the medium voltage testing spec. It does nothing for the electricians though, and adds a lot of money to the cost. I'd rather work on reducing arc flash incident energy for everyone, especially electricians and not worry about small amounts of discomfort for operators for short periods of time.

Quote:
Can the small wires coming into that panel realistically support that ammount of energy?


Yes, for a short period of time before the copper vapor that WAS the wire dissipates enough not to sustain an arc.

Quote:
And even to just operate the breakers they would need to wear like cat 2 or 1 at the least??


NFPA 70E, 2012 edition is much more clear on this subject than the previous editions. The consensus of the NFPA 70E Technical Committee appears to be that with doors latched and secured under normal operating conditions and under the minimum required maintenance recommended, there is not an appreciable arc flash hazard. That's your hint...the arc flash calculation you are doing shows the consequence. You need to do the full risk assessment though to consider the probability of that happening. If it is below the risk tolerance of the company, then there's no NEED for PPE (they accept the risk). Hence the reason for the "zeroes" in the tables.

Risk assessments done for pretty much every other safety system today use a probabilistic approach to safety. Essentially the whole idea is that for instance if a particular incident happens and the consequence is that it is fatal, but that the probability of it occurring is very remote, then it would be treated as an acceptable risk. Conversely even if the chance of it happening is more frequent, but the consequence is little more than a first aid case, then this would also be considered acceptable. Other cases involving higher frequencies of occurrence though would fall under the category of unacceptable risk. So we'd have to either re-engineer the system to eliminate or reduce the hazard (preferred but often hard to do with arc flash), redesign the task (de-energize upstream or using remote trip/close breakers for instance), or wear appropriate PPE.

There is general agreement that closed panels significantly reduce the incident energy. The disagreement is that there is no experimental data to show how much of a reduction.

Quote:
Working on equipment I would expect to see more like,
208 or 240V panels, 250A or so...cat 0 to operate and cat 1 or 2 inside
480V, 600-800A range...cat 1 to operate and cat 2 or 3 inside
480V, 1200-1600A range...cat 2 to operate, cat 3 or 4 inside
480V, 2000A and above...cat 3 to operate, cat 4 or "Dangerous! No PPE" inside

Wouldn't this be more realistic and still a safe level for the workers to do what they need to at times. Isn't that what OSHA is concerned with and not the derived formulas that IEEE got from certain tests?


No. Arc flash incident energy is a (not quite linear) product of volts, current, and time. You did not consider time. Newer, very fast breakers with instantaneous trips set below the arcing fault current frequently produce incredibly low arc flash ratings. For example a brand new MCC I just put in has an incident energy of around 0.6 cal/cm^2 on an 800 A breaker as long as the instantaneous trip is below 6 kA. Although I know the rule will eventually change because some arcs below 240 VAC can be sustained, I also just keep (or convert) my lighting panel transformers to 112.5 kVA or less. That converts your first 2 groups to a zero.

As a general rule, the first step to dealing with the AF levels is to first see if you can change fuses or adjust circuit breakers to get below the arcing fault current with either instantaneous or short time settings without screwing up coordination. This generally reduces arc flash values to very low levels. Newer breakers with electronic trip units are very flexible in this regard. Sometimes you can set up "switchable" settings on larger breakers so that when maintenance is going on, the circuit breaker instantaneous trip can be either turned on or reduced. Coordination is messed up but the time factor mentioned above is at a minimum. Once maintenance is completed, the switch can be turned back on.

At the higher power levels even with VCB's you are going to hit a brick wall around 60 ms as a general rule. At that point there are only two devices I know of . You can use "smart fuses" from S&C (fuse speeds, relay adjustability), or the arc terminator from Square D.

Upstream of that circuit breaker however, everything is over 40 cal/cm^2. Typically you will have a very hard time maintaining less than even 40 cal/cm^2 for 480 V transformers below about around 1500-2500 kVA due to the large incident energies available. The solutions that I've used are:
1. Live with it. Not OK with panelboards but in the case of MCC's having a main that cannot be worked on without shutting down the upstream bus isn't usually that constraining.
2. Put a fused or circuit breaker switch UPSTREAM of the panelboard or MCC. This contains the arc flash energy in a much smaller area. Since that switch/circuit breaker is only used for working on the transformer and/or bus going to the MCC, you generally don't need to bother using it and it can just sit there silently doing it's job with little impact on maintenance except for cleaning/maintenance annually on circuit breakers, or 3-5 years for fused switches.
3. Sometimes (but usually not) you can dial down the primary side protection on the transformer to the point where you can trip this side fast enough to provide some reduction in arc flash on the secondary side.
4. Install cable protectors. These are fuses that are intended to protect individual cables when you need to run multiple lines per phase in parallel. In practical reality they are not very effective for their intended use but they work well for arc flash purposes. The fuses are so small that usually you can bolt them directly to the lugs in the transformer, thus providing secondary fuse protection right inside the transformer air termination enclosure.
5. If it's just breaker operation that is a problem, consider a breaker with remote trip/close control.


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PostPosted: Mon Nov 07, 2011 11:18 am 
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BruceJAX wrote:
Why is it just voltage they classify it at when there is such a difference from other things?


You are comparing NESC to other cases. NESC is specific to the utility industry where there are large numbers of other assumptions that you can make.

Quote:
Another thing I was wondering about on clearing time is that I know from doing circuit breaker testing that the normal clearing time for a breaker is 1 cycle, but the 70E mentions about high volt breakers having a clearing time of 6 cycles. I do not like this because 1. people think that clearing time of LV breakers is 6 cycles rather than the actual 1 cycle and 2. HV breakers don't have trip units...they will have relay overcurrent protection (which most common ones such as GE IFCs and GE CO8's and CO9's have a lot less than 6 cycle clearing time too).


Older breakers can get as high as 12 cycles, such as the nasty GE Limitamps. Generally all the older 1960's-1970's vintage air and oil circuit breakers are and were incredibly slow. Current model vaccuum interruptors can generally hit 2-3 cycles (3 cycles is typical). Only very small air break units are tripping at 1 cycle or less. At low (under 1 kV) voltage, there is not a significant restrike chance and the critical flash overvoltage for air makes the sustainable arc length very short. This results in it being very easy to mechanically separate the contacts far enough to clear the arc very quickly. At medium voltage and higher, a lot more space is needed to do this except in mediums other than air (oil, SF6, or vacuum). Vacuum bottles in particular have the neat property that the arc is more or less instantaneously quenched at the zero crossing. SF6 is so fast that it tends to cause problems with high dv/dt's causing transients in the system, but it definitely approaches the speeds you are talking about. Above a certain voltage, it becomes necessary to add resistors before closing the main contacts just to keep the charging current under control.

Quote:
Also I think the aspect of doing regular testing of switchgear should be more emphasised due to the dramatic difference it makes if the upstream protective devices fails to trip. And not only testing but also even more frequent cleaning and operating..


Agreed. Thus the reason that even though they have an entire chapter dedicated to it in the 2009 edition, When the commentary in the arc flash hazard sections comes right out and says that the arc flash study is worthless if you don't do proper maintenance, I'm not sure how much more clear it could be. 70E in the 2012 edition adds even more commentary.

The only remaining problem I see is that the arc flash hazard section as well as the maintenance chapter don't come right out and recommend either NETA MTS or NFPA 70B in the same way that NEC says "arc flash hazard labels needed" and then immediately recommends NFPA 70E. Instead, one has to read the rest of NFPA 70E and the recommended maintenance standard is only mentioned at the introductory paragraphs of that section. So a casual user would read that and say, "well golly gee, we already do PM's and we fix our stuff when it breaks. We got all the drawings filed away in the vault. So we are doing what we're supposed to."

It would definitely help if 70B was cleaned up and polished like NEC & NFPA 70E. The editting on it is awful.

It would help too if OSHA would fine a few major manufacturers for lack of maintenance. It got attention when they did it using 70E as the standard.


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