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 Post subject: Are risks for arc flash similar to explosions in hazardous locations?
PostPosted: Thu Apr 05, 2012 11:13 am 
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Hazardous locations as defined by the NEC have the possibility of explosion if three things are present, a fuel and oxygen in combustible concentrations and an ignition source. Mitigation for this hazard is to prevent or contain the explosion, not necessarily reduce the magnitude of the explosion or to assume that the explosion will occur and select PPE to tolerate it. In fact, with proper mitigations in place, workers do not wear PPE rated for the explosion hazard. Would the explosion hazard of hazardous locations be analogous to the potential arc flash hazard of electrical equipment as determined by the calculation of incident energy, and if so, could we utilize similar risk assessment and mitigation strategies for arc flash hazards as we do for hazardous locations?
For example, Class 1-Div 1 and Class 1-Div 2 are ways to classify the likelihood of the fuel being present based on normal operation or from accidental discharge. Wiring methods are dictated by the NEC based on the likelihood of the fuel being present. Explosion proof enclosures contain explosions near the ignition source, purged enclosures prevent explosive concentrations of fuel and oxygen, and intrinsically safe devices prevent ignition sources from being present where explosive concentrations of fuel and oxygen may be present. These are design and installation techniques that I would consider to be analogous to the use of arc rated switchgear, arc detecting relays, current-limiting fuses, differential relays for limiting or preventing the arc flash hazard and the need for arc rated PPE. These techniques are good at reducing the magnitude of the arc flash hazard to acceptable levels. Use of remote operation and remote racking is similar by using distance to reduce the hazard to acceptable levels.
Guards, insulated bus, touch safe terminals, etc, could also be considered as arc prevention techniques. Couldn’t these be considered similar to explosion prevention techniques noted above for hazardous locations in performing risk assessments for the arc flash hazard and the need for PPE?
There has been a lot of discussion about the use of the NFPA 70E task tables allowing a reduction of PPE based on the probability of the task having a low risk of initiating an arc flash. There have been similar discussions about whether it’s appropriate to reduce the PPE requirement for a calculated incident energy shown on an arc flash label because of the determination by risk assessment that a task has a low probability of initiating an arc flash. Is there any analogous risk assessment or mitigation for hazardous locations that considers the probability of a task initiating an explosion in lieu of explosion proof enclosures, intrinsically safe devices, purged enclosures, etc like we seem to be trying to do for arc flash hazards and could this shed light on how to evaluate arc flash risks associated with low risk tasks?


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PostPosted: Sun Apr 08, 2012 10:30 am 
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Understanding of hazardous location rankings also requires reading up on NFPA 499. There is not really any corrrelation at all. Hazardous location rules deal with designing equipment to contain or avoid a fire/explosion. There is no direct attempt to look at injuries or PPE. The only place I know of where hazardous location rules mention PPE is an interpretative paper from OSHA suggesting that EH boots should not be worn in hazardous locations.

There is general agreement that arcing faults do not happen in equipment under conditions of normal maintenance and operation. This would make "arc flash hazardous locations" automatically Division 2 rules.

The one exception that I can think of is intrinsically safe equipment. IS equipment is inherently so low energy that it cannot create a spark. It is the one case I can think of that is truly "arc proof" from the hazardous location rules.


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PostPosted: Tue Apr 10, 2012 9:36 am 
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I think that I know what jtinge is trying to say here. The principle that we follow in the United Kingdom is to reduce any risk to "as low as is resonably practicable" or "ALARP". A hierarchy of risk control measures must be followed and the use of PPE is very much last resort. I had the privelage of being asked to review an excellent paper at the IEEE ESW conference in Daytona Beach this year which was called "Risk Management of Electrical Hazards" by Daniel Roberts. Daniel used the principles of ALARP to describe the management of electrical risk to an acceptable level using internationally recognised health and safety standards. In summing up he said that the severity of exposure is used to select the value of PPE but the likelihood of exposure is used to determine when the deployment of PPE is warranted. A very worthwhile read.


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PostPosted: Tue Apr 10, 2012 5:03 pm 
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I agree with the intent. NFPA 499/hazardous location rules is not where you want to go to though. The reason is that the granularity of the hazardous location rules is too coarse. We only have 3 levels and virtually all electrical equipment falls into the middle category with rare exceptions that are in the minimum category.


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PostPosted: Tue Apr 10, 2012 6:30 pm 
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I would like to add another point. In battery room, around the battery vessel, there is a volume classified as Class 1 by hydrogen generated by recharge process (mainly at the final period of recharge). If someone initiates an arc flash during any task, it is possible start an explosion too. There is a risk associated with this situation and must be considered. It is difficult to think on desenergize the battery as each vessel has its energy associated. How can we mittigate the risk?


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PostPosted: Wed Apr 11, 2012 5:39 am 
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Mike Frain wrote:
....... In summing up he said that the severity of exposure is used to select the value of PPE but the likelihood of exposure is used to determine when the deployment of PPE is warranted. A very worthwhile read.


This is a point that have had mixed feedback on. I concur that the calculation is used to determine the maximum anticipated value of PPE and that likelihood of exposure could be used to determine if it needs to be used; however, should we be using likelihood of exposure to allow usage of lower value PPE rather than to determine whether or not to use the full value PPE like the 70E tables do?

We all have the same type of tasks that we do on electrical equipment, these are generally defined in the 70E tables, plus or minus specialty tasks that may be unique to each company. Has anyone actually done a proabability risk assessment on these tasks to determine when you need to use the calculated level of PPE? I hate to think that once I stick the label on a piece of equipment that the worker will have to suit up to that level for everyt task. I keep hearing about several methods on how to do PBA for low risk tasks, but I have yet to see any examples from someone that has used this approach. Is no one doing this yet or do we all have to figure this out for ourselves. I am perfectly happy to leverage off of someone else who is smarter that me.


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PostPosted: Wed Apr 11, 2012 6:41 pm 
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Dagoberto wrote:
I would like to add another point. In battery room, around the battery vessel, there is a volume classified as Class 1 by hydrogen generated by recharge process (mainly at the final period of recharge). If someone initiates an arc flash during any task, it is possible start an explosion too. There is a risk associated with this situation and must be considered. It is difficult to think on desenergize the battery as each vessel has its energy associated. How can we mittigate the risk?


You are clearly worried about battery rooms and you have a right to be. If you do not follow the safety standard, you can have big problems. NFPA 70E has an entire section dealing only with battery room safety. You must ventilate for cells that produce hydrogen gas. 70E covers how to do this and when to determine if it is adequate. If you do this, arcing faults don't matter.

Now, addressing the other issues. There is a huge amount of misinformation here. Time to switch hats from electrical guy to process guy. I hold a masters degree in process engineering. I've done the role for roughly 11 years. NEC is for sparkies to know how to properly install equipment in hazardous locations. It is not the code to use for determining how to classify an area. Almost the REST of the NFPA codes are meant for that.

First off, NO, it is not automatically Class 1. Hydrogen diffuses at a rate of roughly 10-20 times faster than other gases in air. It diffuses so rapidly that if adequate ventilation is available, you flat out are never going to get to the LEL (lower explosive limit), even though the LEL for hydrogen is fairly low (5%). You are also generating gas at a fairly small rate, even with large battery rooms. It becomes a problem ONLY if you don't provide adequate ventilation. Since you correctly surmised that you cannot ever achieve a safe condition in a battery room, you must ventilate. This is not optional. If you ventilate, it is unclassified, NOT class 1. If you don't ventilate, then it is not only Class 1 but you can't follow NEC in the first place since batteries which are big enough to initiate are not designed to meet Div. 1 or Div 2. requirements under Class 1 by themselves. At best you could pressurize and purge the cabinet to allow an unclassified device into a classified area but then this counts as ventilation so...QED, it's unclassified.

In addition, you said explosion. False. Frankly, impossible again unless you have a very closed in area and can achieve enough pressure, and have enough gas to burn that you can even get to the point where you rupture the building walls. An explosion means basically that you ruptured a vessel via deflagration or detonation. See NFPA 69. Explosions are very rare in the real world because very little equipment doesn't have a door or opening somewhere which prevents the pressure from rising to the point where you can get a deflagration, let alone honest to goodness detonation. On top of that, you have to have enough combustible material to get there. I'm not saying that you can't launch flames out doorways and otherwise clean the dust off the rafters, and that you can't occasionally hurt someone really bad with a flame of any kind. But an explosion is just not that common.

By far the biggest threat from a battery though especially a large lead acid cell is if you short it out and discharge so fast that the battery overheats and bursts from boiling acid. This makes a huge mess, will chemically burn you if you get sprayed, and can damage other batteries or equipment in the area. However, there is no fire for the most part involved.


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PostPosted: Wed Apr 11, 2012 7:04 pm 
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This is a point that have had mixed feedback on. I concur that the calculation is used to determine the maximum anticipated value of PPE and that likelihood of exposure could be used to determine if it needs to be used; however, should we be using likelihood of exposure to allow usage of lower value PPE rather than to determine whether or not to use the full value PPE like the 70E tables do?


Let's suppose for a minute that we do what you suggest...the likelihood is less, so we downgrade the PPE. If an arc flash occurs, then the employee is no longer adequately protected. Too little PPE is no different than none at all. However, I will grant you that if an increase in working distance is being taken into consideration, then this is perfectly legitimate.

Quote:
Has anyone actually done a proabability risk assessment on these tasks to determine when you need to use the calculated level of PPE? I hate to think that once I stick the label on a piece of equipment that the worker will have to suit up to that level for every task. I keep hearing about several methods on how to do PBA for low risk tasks, but I have yet to see any examples from someone that has used this approach. Is no one doing this yet or do we all have to figure this out for ourselves. I am perfectly happy to leverage off of someone else who is smarter that me.


In short, yes. It turns out to be easier than you think though. In the library on this forum, I just recently put together a paper detailing my results from doing this. I did not publish COMPLETE results because right now it's there as a paper tiger...I want to get it out in the public and let us get to consensus so that 70E will incorporate the idea on the next revision cycle.

If you look at what someone could be doing and how this might influence the risk for arc flash, you will find that you only get about 5 different categories of tasks such as "same as just walking by", or "same as opening a disconnect/breaker". If you then increment through each of the potential types of equipment that this applies to (which is a fairly short list, no more than a dozen types of equipment), and apply a few specific multipliers when they apply, you can manage to cover all tasks.

I did not publish a complete list because it would amount to heavily plagarizing IEEE 499.

Even without this approach, I currently have a draft form of this exact procedure that we are arguing about at work. It has about 30 specific tasks. For each task it lists "shock protection", "arc flash protection", and "EEWP required". As of right now since there is not consensus on applying probabilistic risk assessments to this procedure yet, it utilizes only the tables in 70E. This means that for instance even though the risk is minimal from opening/closing circuit breakers and disconnects regardless of the voltage it still lists "H/RC 2" for medium voltage equipment. As soon as I can get consensus on utilizing QRA, I'll move forward to the next step and update the task list.


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PostPosted: Thu Apr 12, 2012 8:03 am 
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Thank you for the valuable feedback. This is a great example of what I was looking for. I would be interested in looking at the draft version you mentioned if you are willing to share. Can give you contact info if offline is better.


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PostPosted: Thu Apr 12, 2012 10:23 am 
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I agree that using the probabilty of an arc flash could be used to determine when PPE should be used, not if reduced PPE can be used. ANSI/RIA R15.06 however adds exposure to the risk assessment. If exposure is considered with severity and probability during the risk assessement, could this be a factor then in determining if reduced PPE could be used? Just a thought.


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PostPosted: Thu Apr 12, 2012 6:30 pm 
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jtinge wrote:
Thank you for the valuable feedback. This is a great example of what I was looking for. I would be interested in looking at the draft version you mentioned if you are willing to share. Can give you contact info if offline is better.


Very shortly, yes.


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PostPosted: Thu Apr 12, 2012 6:43 pm 
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jtinge wrote:
I agree that using the probabilty of an arc flash could be used to determine when PPE should be used, not if reduced PPE can be used. ANSI/RIA R15.06 however adds exposure to the risk assessment. If exposure is considered with severity and probability during the risk assessement, could this be a factor then in determining if reduced PPE could be used? Just a thought.


Yes. I took that into account. IEC 61511 is concerned specifically with electrical safety systems (interlocks and the like) in process industries. In that system, there are two distinct types of systems that are considered. First, there are continuous systems. This would be for instance the valves used to regulate air/fuel mixture in a burner. The second type are "demand mode" systems. These would correspond not so much to the "high limit" switch which would be a normal part of the control system but the "high, high limit" that is supposed to trigger the safety system only in the event that normal controls have failed. The criteria for declaring it "demand mode" is if the trigger rate is less than once per year.

This works very well with electrical systems as well. An example of a "continuous mode" situation would be calculating the failure rates of equipment if you are "just walking by". Then the first concern is as you said exposure, and the second consideration is the chance that the equipment could just spontaneously have an arcing fault at the exact same time. Clearly the chance of both happening simultaneously is pretty small. Even without throwing in the exposure factor, the number I calculated is already a very low risk.

The second case would be for instance failure of a disconnect handle during use. The chance of it failing any other time is again very small. But there is a much greater risk of a failure occurring when the disconnect is being opened or closed, if it is going to fail. So this is a great example of an "on demand" situation.

What does not work that many risk assessments include is the likelihood of avoiding injury. This is usually meant for instance if the hazard is moving so slowly that there is plenty of time to recognize it and "run away". You can't run from an arcing fault. You might be avoid causing one when the failure is human-induced but there's already a factor taken into account for that.


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PostPosted: Thu Apr 12, 2012 10:09 pm 
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Sorry for my "amount of misinformation". I'm only an Electrical Engineer without masters degree, and a Junior Level on this forum. But looking in Annex B of the EN 50622-2:2001: "In close vicinity to the source of release of a cell or battery the dillution of explosive gases is not always ensured. Therefore a safety distance d extending through air must be observed within which flames, sparks, arcs or glowing devices ... are prohibited." By a simple calculation (by Annex B), for a 216 Ah battery, considering the boost period, and room ventilation with 12 air change per hour, the safety distance is approximately 200 mm. And we have to consider the presence of oxyhydrogen gas within the battery cell. I think this is a main issue regarding electrostatic discharges recommendations when handling batteries.


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PostPosted: Fri Apr 13, 2012 4:23 am 
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Sorry for my "amount of misinformation". I'm only an Electrical Engineer without masters degree, and a Junior Level on this forum. But looking in Annex B of the EN 50622-2:2001: "In close vicinity to the source of release of a cell or battery the dillution of explosive gases is not always ensured. Therefore a safety distance d extending through air must be observed within which flames, sparks, arcs or glowing devices ... are prohibited." By a simple calculation (by Annex B), for a 216 Ah battery, considering the boost period, and room ventilation with 12 air change per hour, the safety distance is approximately 200 mm. And we have to consider the presence of oxyhydrogen gas within the battery cell. I think this is a main issue regarding electrostatic discharges recommendations when handling batteries.


Within a lead acid cell, the oxygen and hydrogen that are liberated are almost immediately recombined into lead oxide and acid, which is how the thing works under normal conditions. You don't want either gas escaping. The only time this changes is in cases of overcharging, even if it's "trickle charging" which is just overcharging at a very slow rate in most cases. It's only during these conditions that hydrogen and to some degree, oxygen are liberated. This would be the definition of a Division 2 condition (occurs only under abnormal conditions). Under conditions of high discharge rates (e.g., we've shorted out the leads), there is rapid oxidation of the plates into lead oxide as well as rapid formation of lead sulfate. Both are consuming oxygen and hydrogen like mad. So the key parameter for gaseous release is the current generated by the charger, not discharging.

Within the cell itself there is no ignition source, so we don't have to worry about conditions inside the cell no matter what the current is. This would be considered "unclassified" using NFPA rules.

Outside the cell if we get 2H for every O, then we've got a mixture of 2/16 = 12.5% by weight, well within the LEL and UEL, assuming both gases are vented in the proportions that they are being generated, which is not true either because the maximum concentration of oxygen as a dissolved gas in solution is greater than hydrogen. I'll grant you that at high enough evolution rates, the gases are saturated in the liquid phase and we get a 1:1 release of gas.

Now, the oxygen/hydrogen mix is going to be at atmospheric pressure and temperature, so it's going to have very, very little energy due to the inherent low molecular weight of hydrogen (basically 1), unlike say natural gas (roughly 16). I haven't actually sat down to do the math assuming you 200 mm radius is correct for a hemispherical cloud of flammable gas, but it sounds like a trivial amount of energy. If all the energy is released, then we'd calculate the effect on a person 61.0 cm away...so we want to know the cal/cm^2 at that distance. The area is 4*pi*r^2/2 (hemisphere), or 23,300 cm^2. One need only calculate the number of calories in 838 cm^3 of gas and do the math to determine if it is less than 1.2 cal/cm^2. This works out to 33 cal/cm^3 of gas which seems like an awfully high number to me, but I haven't finished out the calculation yet. Given enough cells you might get there but then you are also diffusing the fire over a larger area so the relative amount of radiant heat won't increase that much due to increasing distances.

Even then, there is at least a modicum of a chance of having a fire. So we should at least address the risk of a small fire. And since we are unlikely to be able to manage such a small space adequately, we'll hang signs on the battery room that say "no open flames" and "no smoking". I'd even recommend using either nonsparking or at least static dissipative tools in the room. Perhaps we should recommend wearing acid resistant PPE and a face shield just in case one of the cells ruptures due to the pressures being generated. And we should restrict the room to authorized personnel only because we wany anyone entering the room to receive safety training regarding handling of acid cells. Does this make logical sense?

That is how we get to NFPA 70E and NEC's recommendations for battery rooms. You are not going to "blow up" the room (detonation, blow the doors off), unless you don't ventilate and something catastrophic happens in your charging system. It is certainly possible to rupture cells and spray acid all over. Safety toed boots would be highly recommended to avoid mangling your foot if you drop a battery. And it's possible to light up the hydrogen over a cell that is being overcharged for some reason and have a small fire singe the hair on your arms and face off. But you are not going to reach the level where hazardous location regulations take over.

The EN annex you quoted is just there to get to the point of estimating the proper amount of ventilation. If you undershoot the ventilation then certainly you convert the whole room into Class 1, division 2. I'm less familiar with the zone classification system so I don't recall the exact equivalent (zones are not typically used in the U.S., and have an absolutist approach to them for the marginal cases).


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PostPosted: Fri Apr 13, 2012 5:18 pm 
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With a 200 mm hemisphere of oxyhydrogen gas at standard temperature and pressure, we have 50 cal worth of heat. The incident energy even at the surface of the 200 mm sphere is less than 0.01 cal/cm^2. Ignition temperature is also 570 C (1065 F) so getting it lit is not easy, and even if it ignites, the effect is almost harmless.


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