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 Post subject: Arc flash through ventilation holes?
PostPosted: Mon Oct 29, 2012 11:27 am 

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Attached is a picture of a switchboard with relatively large ventilation holes. The way I see it, even with the doors/covers shut this poses an arc flash risk. Although unlikely, a small rodent, conductive dust, etc. could cause an arc flash while someone in the facility is walking past. Has anyone else come across this situation, if so- how do you address it?

Just for reference, this panel is fed by a 1,500 KVA, 480 V, pad mounted utility transformer, the available fault current is 31,000 Amps. Using the 2 second guideline, incident energy is over 150 cal/cm^2 due to the fact that there is no upstream over-current protection.


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PostPosted: Mon Oct 29, 2012 7:01 pm 
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No credit is given anywhere for doors as far as incident energy due to the lack of testing data. So the arc flash hazard is the same whether there is an enclosure or not.

Risk is another matter entirely though. If the equipment is modified, things might be different. But otherwise it doesn't look any worse than any other NEMA 1 or most dry transformer enclosures.


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PostPosted: Tue Oct 30, 2012 2:04 pm 
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PaulEngr wrote:
Risk is another matter entirely though. If the equipment is modified, things might be different. But otherwise it doesn't look any worse than any other NEMA 1 or most dry transformer enclosures.


I agree with tim8282. I'm not all so sure this would be more in line with having no appreciable enclosure at all. While the vents allow air to circulate, they wouyld just as easily allow blast pressure and debris easy egress as well. The only protection this provides is accidental contact with the live circuitry. I think a NEMA 1 enclosure would likely offer more protection than large-bore screens across the front panel.


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PostPosted: Tue Oct 30, 2012 3:21 pm 
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Vents like this are the reason I have a beef with the whole "doors closed" issue, while all non arc rated enclosures should not be considered to offer any protection IMHO (And from experience) the 70E implies otherwise, with no mention of vented doors found often in switchgear. Should be added to the standard.


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PostPosted: Tue Oct 30, 2012 5:00 pm 
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Zog wrote:
Vents like this are the reason I have a beef with the whole "doors closed" issue, while all non arc rated enclosures should not be considered to offer any protection IMHO (And from experience) the 70E implies otherwise, with no mention of vented doors found often in switchgear. Should be added to the standard.


70E mentions this somewhere (I don't have a copy in front of me). It also gives credit for this in the tables. BUT there are two reasons for a potential PPE reduction in the tables. The first might be recognition of a reduced incident energy which would lend credibility to the idea that this "validates" the vent issue.

The second might be that with the doors closed and latched, even if it has holes so big you can stick your arm through them, that it is no longer "exposed" and thus it meets the notes under arc flash hazard...that normal operation with equipment that is properly installed and maintained does not pose a significant arc flash hazard. Thus the H/RC rating would be very low or most likely zero due to the low probability of an arcing fault in the first place.

I'm leaning more and more towards the "doors don't matter" camp and here's why. If we assume a simple 10 PSI pressure rise and we have a typical 20" x 20" MCC door, the pressure on the door is 20 x 20 x 10 = 4000 lbs. That's 2 tons of force on 20 gauge steel latches. In a low level arc flash where the pressure doesn't blow the doors off then we have the absorption of radiant heat but at the temperatures claimed, this is going to be fairly efficient. Heat transfer will be equal to (T1-T2)^4*emissivity where the emissivity for painted metal is usually around 0.7. Then it conducts across it (nearly instant) and then gets reradiated. I've done lots of work around kilns so I can honestly say that a cheap piece of aluminum corrogated roofing does a great job as a heat shield (emissivity=0.1) where two sheets is enough to knock radiated temperatures of 800-1000 degrees F or more down to a comfortable 100 F. But steel is a terrible heat shield. So I don't believe that the door offers much in the way of protection.

But given that P is proportional to T and assuming temperatures are going to go to 3000 K when room temperature is around 300 K, then pressure is going to increase to 100 atmospheres or else it's going to relieve itself. No door is going to withstand that kind of pressure if it is not heavily vented.

Now this is purely theoretical and speculative. But my speculation is that either the door is never going to stay in place or else it's not going to provide any realistic protection from the radiative heat transfer, or else the actual temperatures are not nearly as high as reported.

Hence the reason that I'm in the "doors don't matter" camp.


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PostPosted: Wed Oct 31, 2012 5:23 am 
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PaulEngr wrote:
even if it has holes so big you can stick your arm through them, that it is no longer "exposed" and thus it meets the notes under arc flash hazard...


Yes, it does keep people out, but not squirrels. I worked in a repair shop that did a lot of business with ski areas, and every fall they would begin starting the equipment up to get ready for ski season. I once got a softstart in for repair that a squirrel had made a nest in.

Unfortunately for the squirrel when the operator started up the tow lift up, he did not check for tresspassers first.


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PostPosted: Mon Nov 05, 2012 11:18 am 

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tim8282 wrote:
Attached is a picture of a switchboard with relatively large ventilation holes. The way I see it, even with the doors/covers shut this poses an arc flash risk. Although unlikely, a small rodent, conductive dust, etc. could cause an arc flash while someone in the facility is walking past. Has anyone else come across this situation, if so- how do you address it?

Just for reference, this panel is fed by a 1,500 KVA, 480 V, pad mounted utility transformer, the available fault current is 31,000 Amps. Using the 2 second guideline, incident energy is over 150 cal/cm^2 due to the fact that there is no upstream over-current protection.


For what it is worth to your site, we had a very similar setup (1500KVA and vented switchgear) blow up recently (twice within a month). If it were not for the "venting" in the switchgear, there would have been a whole lot more of the molten metal flying and perhaps a larger explosion. You believe it is more of a hazard with the venting, but as I have seen first hand, it helped when the blast occured. What I would like to recommend to you is that you should do a Hazard Analysis. This is in Annex F of NFPA70E. Your boundary information on your label is for doors open. Your risk assessment is for your "OWN" protection or "Tolerable risk" and the protection of your workers. If this system is in an area that people are walking by on a daily basis, then please guard it to at least the limited approach boundary where in my case is 3'6". Arc flash boundary is 42'. Our whole system is now guarded to keep people away.


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PostPosted: Mon Nov 05, 2012 11:46 am 
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gastoor wrote:
For what it is worth to your site, we had a very similar setup (1500KVA and vented switchgear) blow up recently (twice within a month). If it were not for the "venting" in the switchgear, there would have been a whole lot more of the molten metal flying and perhaps a larger explosion. You believe it is more of a hazard with the venting, but as I have seen first hand, it helped when the blast occured. What I would like to recommend to you is that you should do a Hazard Analysis. This is in Annex F of NFPA70E. Your boundary information on your label is for doors open. Your risk assessment is for your "OWN" protection or "Tolerable risk" and the protection of your workers. If this system is in an area that people are walking by on a daily basis, then please guard it to at least the limited approach boundary where in my case is 3'6". Arc flash boundary is 42'. Our whole system is now guarded to keep people away.


The venting does alleviate the pressure, and that is a good thing.

In fact some switchgear is built with arc-flash in mind, incorporating channels and blow-out panels to vent the pressure and reduce both the liklihood of molten metal flying and perhaps a larger explosion. This type of switchgear typically directs the blast away from the front of the switchgear or upwards to avoid any operator (or bystanders).

My observation was more focused on the increased possibility of something enterring the enclosure and inadvertantly triggerring an arc-flash incident - which is, of course, the obvious drawback to a more open design.


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PostPosted: Mon Nov 05, 2012 4:51 pm 
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I didn't read every comment, but I have to offer that we cannot over risk the potential for an arcing fualt occurring in the first place.

Energized electrical equipment is normally safe, an abnormal condition can increase the probability of an arcing fault that can become an arc flash event.

Vent holes in electrical equipment is normal and required to let heat out that can lead to creating an abnormal condition.

There is no arcing fault risk with walking in front of energized electrical equipment under normal condition whether is has vent holes or not.

If the electrical equipment is in an abnormal condition then only Qualified Electrical Workers should approach it and do work on it and it is at this time that the Arc Flash Boundary applies, NOT under normal conditions.

We all need to be careful we don't over risk the probabilty of an arcing fault. We cannot relocate lighting panels from 200 million homes, we cannot stop industry in North America because we now think that approved equipment installed to a very stringent installation code is inherently going to have arcing faults occur on it.

We need to use risk assessment procedures to establish risk and apply preventive and protective control measures if electrical equipment is in an abnormal condition to reduce the risk to as low as reasonably practicable when energized electrical work is justified to be completed on it. Use the tools in CSA Z462 and NFPA 70E as controls to reduce the risk.


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PostPosted: Tue Nov 06, 2012 3:56 am 

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There was a case near ready, where two workers were in an electrical room to work on a low voltage computer and the gear blew up and they were severely injured. Yes the navy has numerous cases and has documented that a ton of force can occur on a sheet metal door during an arc fault. This led to the development of the new Arc flash and noise sensors that some manufacturers have started to incorporated into large switchgear. This is another good reason why these electrical rooms should not be used for casual storage and other uses.


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PostPosted: Wed Nov 07, 2012 7:03 am 
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Terry Becker wrote:

Energized electrical equipment is normally safe, an abnormal condition can increase the probability of an arcing fault that can become an arc flash event.

Vent holes in electrical equipment is normal and required to let heat out that can lead to creating an abnormal condition.

There is no arcing fault risk with walking in front of energized electrical equipment under normal condition whether is has vent holes or not.


I am not disagreeing with you, and I am not advocating relocating electrical panels from 200 million homes, nor stopping industry in North America because approved equipment installed to a very stringent installation code is inherently going to have arcing faults occur on it.

I am just saying we can not neglect the situation. If we have grates on the front of electrical panels there is a chance, albeit a small chance, that something is going to get inside.

What can be done about it? In the situation I gave as an example (which happened back in the late 1980's), maintenance made it part of their start-up routine to physically inspect for vermin before powering up equipment that had been dormant in the off-season.


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PostPosted: Sat Nov 10, 2012 3:17 pm 
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What about a 1500 KVA transformer indoors in a dedicated electrical room with an expanded-metal cover on the front of the transformer? This is one of the main feeders for a small industrial facility, but the transformer sits just inside the main doorway to the room. You cannot get to any of the electrical panels without passing right by this almost open-transformer. IF this transformer were to arc, it was obviously be catastrophic. Any ideas on recommended PPE levels here? The secondary breaker is in a distribution panel about 5 feet beyond the doorway, right past the transformer. Their in-house arc-flash study put the unit at 48 cal/cm2. Obviously just walking by it does not produce an action that would directly cause an arcing event, but...

Thoughts?


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PostPosted: Sun Nov 11, 2012 5:00 am 
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Kenneth Sellars wrote:
What about a 1500 KVA transformer indoors in a dedicated electrical room with an expanded-metal cover on the front of the transformer? This is one of the main feeders for a small industrial facility, but the transformer sits just inside the main doorway to the room. You cannot get to any of the electrical panels without passing right by this almost open-transformer. IF this transformer were to arc, it was obviously be catastrophic. Any ideas on recommended PPE levels here? The secondary breaker is in a distribution panel about 5 feet beyond the doorway, right past the transformer. Their in-house arc-flash study put the unit at 48 cal/cm2. Obviously just walking by it does not produce an action that would directly cause an arcing event, but...

Thoughts?


70E Committee has repeatedly stated that just walking by is not a significant hazard.

The logic behind this is pretty simple. You've got a transformer just sitting there. In this case the most likely cause of a failure is going to be a loose connection. Probability of failure on a continuous basis is somewhere around 10^-12 according to numbers that the safety engineers typically use. Now on any given day lets just suppose that people take 20 trips through this area and are there for 1 minute each, or 20 minutes out of every day. So they are there 1.39% of the time. So on a continuous basis the chance of being int the wrong place at the wrong time is now 1.39 x 10^-14. Most quantitative risk analysis methods suggest that better than 10^-5 is a tolerable risk for one fatality and better than 10^-6 is toleratle for multiple fatalities.

Now the reality is that the most likely time for this thing to fail is under stress. So I'd put the risk while just walking by closer to 10^-12. Using IEEE Gold book numbers the risk is going to be closer to 10^-7 or 10^-8 and would happen every time during energizing or during clearing a dead short fault because that is when the magnetic force on the conductors is at a peak (proportional to the square of the current) and most likely to cause something to fly apart and arc. But still, we are still well below the fatality threshold.

Keep in mind that according to BLS data as compiled by the ESFI, fatalities due to arcing faults occur at a rate of about 0.1 per 10,000 workers per year or 10^-5 at the current time so unless you have some unusual circumstance that significantly elevates conditions above "normal operating conditions" the risk is going to be pretty insignificant. An example would be NOT doing routine inspections on the transformer every year to ensure that the connections are in good shape.


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PostPosted: Fri Jul 19, 2013 10:46 am 
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The issue we run into more is with Vented front switchgear and switchboards. For example what does the facility do while switching? If the line side of the breaker or bolted pressure switch is 150 cal/cm2 at 18" from the Utility (typical in downtown LA for multi-service high rises 3000-4000A 480V services) and the line side has vent holes (like common the 1990-2007 era GE Power Break switchboards), you have a significant level of risk. If the switch or breaker faults during operation, you could have a serious exposure issue. Currently there are a lot of people underestimating the risk with vented front switchgear, because the NFPA-70e chart indicates "with enclosure doors closed". Also their electrical engineers are not fully describing the risk, just providing stickers. More emphasis needs to be placed on proper education of the risks, AFTER, the Arc Flash Hazard Study is performed.


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PostPosted: Sat Jul 20, 2013 8:00 am 
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Van D Wilkins Jr wrote:
The issue we run into more is with Vented front switchgear and switchboards. For example what does the facility do while switching? If the line side of the breaker or bolted pressure switch is 150 cal/cm2 at 18" from the Utility (typical in downtown LA for multi-service high rises 3000-4000A 480V services) and the line side has vent holes (like common the 1990-2007 era GE Power Break switchboards), you have a significant level of risk. If the switch or breaker faults during operation, you could have a serious exposure issue. Currently there are a lot of people underestimating the risk with vented front switchgear, because the NFPA-70e chart indicates "with enclosure doors closed". Also their electrical engineers are not fully describing the risk, just providing stickers. More emphasis needs to be placed on proper education of the risks, AFTER, the Arc Flash Hazard Study is performed.


The 2015 edition of 70E will address this. the wording is changed from doing a HAZARD analysis (looking only at the consequence) to doing a RISK analysis (looking at both the consequence and likelihood). The tables are also similarly changing.

There are two issues at hand. First, there is the issue of the potential consequence if an arc flash occurs. This has been pretty well studied at this point. There are plenty of arguments to be had but we can now conservatively predict whether or not there is an issue IF an arc flash occurs. If the energy is sufficient, protection is pretty much an impossibility. This is the same situation that occurs for instance in the case of being struck by meteors.

Second, we need to consider whether or not an arc flash is LIKELY to occur. People walking around on the ground are pretty well protected from the atmosphere and people just walking by electrical equipment are similarly unlikely to be injured no matter what the consequence is. If you change the situation such as working in space or working on live electrical equipment, then the likelihood can get to be much greater. At the same time there are also things that you can do to reduce the consequence or likelihood depending on the task.

Hence the reason that closed doors merely reduce the risk of someone or something falling into a panel accidentally. There is some reduction in energy as well but it is unlikely to even matter in the cases that we are concerned with (where a significant injury potential exists).

So there are TWO approaches here. You can either reduce the hazard (if this is possible) to the point where likelihood does not matter, OR you can address the likelihood issue. 70E in Article 200 requires doing the latter (doing PM's), and the "EEWP" rule pretty much precludes the latter as well. These are both likelihood reduction techniques. Using PPE as your only solution is a clear violation of ALARP principles (also addressed in 2015 edition), and is fundamentally risky. It's like putting air bags on cars and then ignoring issues of reckless driving or driving too fast for conditions...not to say that air bags are not a good idea...just that it is not reducing accidents in the first place. Injuries can and still do occur. The best solution is engineering out or managing the likelihood of an injury in the first place.


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PostPosted: Mon Jul 22, 2013 9:43 am 
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Van D Wilkins Jr wrote:
The issue we run into more is with Vented front switchgear and switchboards. For example what does the facility do while switching? If the line side of the breaker or bolted pressure switch is 150 cal/cm2 at 18" from the Utility (typical in downtown LA for multi-service high rises 3000-4000A 480V services) and the line side has vent holes (like common the 1990-2007 era GE Power Break switchboards), you have a significant level of risk. If the switch or breaker faults during operation, you could have a serious exposure issue. Currently there are a lot of people underestimating the risk with vented front switchgear, because the NFPA-70e chart indicates "with enclosure doors closed". Also their electrical engineers are not fully describing the risk, just providing stickers. More emphasis needs to be placed on proper education of the risks, AFTER, the Arc Flash Hazard Study is performed.


Agreed, education is key.

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PostPosted: Sun Aug 04, 2013 2:03 pm 
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Not completely on topic but looking at the photo, it looks like a delta system with ground detection lights. It also looks like a ground is developing as one light looks dim and the other two look brighter. To me, this is more of a concern on the system as a whole, than worrying about the gear having an arc flash as I walk by.

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PostPosted: Mon Aug 19, 2013 9:31 am 

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wbd wrote:
Not completely on topic but looking at the photo, it looks like a delta system with ground detection lights. It also looks like a ground is developing as one light looks dim and the other two look brighter. To me, this is more of a concern on the system as a whole, than worrying about the gear having an arc flash as I walk by.


You are correct, this is a delta system with ground detection lights. I believe the issue was with the actual light bulb/socket connection and not a voltage imbalance. Several months ago I measured the phase to ground voltages and they were all within tolerance. I have asked our entire electrical department to monitor the indicator lights and I also check on a regular basis (they are currently equal brightness).

I'm not a huge fan of the ungrounded delta system and have no reason why it was ever installed as this feed is only used for testing equipment (not a production environment where you typically see them). Our machines typically include VFD's and servo drives, my fear is that one day we will have a ground fault and the MOVs inside of the drive will turn to toast. (Yes, you can typically remove a jumper or ground screw on these drives but since they are only used for a short time for testing it's hard for me to stay on top of the shop guys). Other than a Zig-Zag transformer I'm not sure if there are any other feasible fixes.


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PostPosted: Thu Aug 22, 2013 10:43 am 
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tim8282 wrote:
You are correct, this is a delta system with ground detection lights. I believe the issue was with the actual light bulb/socket connection and not a voltage imbalance. Several months ago I measured the phase to ground voltages and they were all within tolerance. I have asked our entire electrical department to monitor the indicator lights and I also check on a regular basis (they are currently equal brightness).

I'm not a huge fan of the ungrounded delta system and have no reason why it was ever installed as this feed is only used for testing equipment (not a production environment where you typically see them). Our machines typically include VFD's and servo drives, my fear is that one day we will have a ground fault and the MOVs inside of the drive will turn to toast. (Yes, you can typically remove a jumper or ground screw on these drives but since they are only used for a short time for testing it's hard for me to stay on top of the shop guys). Other than a Zig-Zag transformer I'm not sure if there are any other feasible fixes.


Zig-zag is ONE way. You can also use a standard delta-wye transformer to regenerate a ground reference. The zig-zag is just lower cost. All of this stuff is available pre-packaged with resistors from the typical companies that you'd buy resistors from (iGard, Post Glover come to mind).

The MOV's are fine as long as they are connected line-to-line or have full line-to-line MCOV. In my resistance grounded 23 kV system, the lightning arresters do just fine as long as MCOV is 23 kV and not 13.5 kV.


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PostPosted: Thu Aug 22, 2013 3:32 pm 

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PaulEngr wrote:
The MOV's are fine as long as they are connected line-to-line or have full line-to-line MCOV. In my resistance grounded 23 kV system, the lightning arresters do just fine as long as MCOV is 23 kV and not 13.5 kV.


Except these are Allen Bradley drives and the MOV's are connected to ground (along with filter caps). They actually spell out in their manuals that the MOV's need to be disconnected when used on a ungrounded or corner grounded delta. When you open the access panel they typically have a jumper wire to ground that you can remove.

A standard delta-wye would work great but like you say, expensive. It's a 3,000 amp main with 600 and 800 amp branches.


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