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 Post subject: Arc Flash in Manholes
PostPosted: Fri May 11, 2012 10:40 am 

Joined: Thu Mar 08, 2012 6:47 am
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Location: Massachusetts
Hello everyone,
I was wondering if you could give me some ideas or show me in the right direction on how to calculate the Arc Flash incident energies inside manholes. The manhole we are trying to analyze only has 480V and 120V cables and no exposed conductors, that we know of, although being in there for more than 40 years they may have been degraded to the point the insulation is broken.
I guess my specific question would be what parameters for box configuration, short circuit current available and clearing time for the upstream breaker should I take into account. Should I just calculate the current for a fault at that point from what information I have available of the short circuit in the upstream MCC and the cable data and distance? Or are there special considerations when performing arc flash calculations in manholes?

Thank you for your help!


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PostPosted: Fri May 11, 2012 1:46 pm 
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One thing to keep in mind is that the "2-second rule" definitely does not apply. IEEE 1584 tells you to consider how long a person is likely to remain exposed to the arc flash, which in a manhole is longer than 2 seconds.

If your software cuts off an uninterrupted arcing current at 2 seconds by default, turn that option off and make sure that settings for protective devices trip reliably on lowest-case arcing currents.


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PostPosted: Sat May 12, 2012 3:56 am 
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Second problem is that IEEE 1584 equations assume that there is sufficient volume. If the area where the worker is standing is pretty small, then many of the effects of the arc flash will be enhanced. There have been some tests for naval ships done. I'm not sure where the test data is but the end result is that it increases the arc flash hazard over the results given by IEEE !584. IEEE 1584 refers to the test data but since it does not foot note it, I can't tell which of the references contains the data. There have also been some tests done on arc flash blankets where as I understand it Kinectrics has a "simulated vault" set up.

If you want to know for sure what the effect is in your situation, your best answer is to get in touch with Kinectrics and pay for some simulations to be done of your specific situation. Vaults are one of those situations where there are no "easy" answers. Otherwise, you are basically making guesses.

The 120VAC stuff is most likely a nonissue because 120 VAC equipment has relatively quick tripping characteristics and the available arcing energy is pretty minimal. In addition, as the voltage goes down, it is hard to get an arc to be self sustaining in the first place (it likely will go out on it's own). However, none of this is true with 480 VAC. 480 VAC arcs are almost always self sustaining and only clear via the overcurrent protection. If using a calculation method such as IEEE 1584 it appears that the incident energy is really low (<<1.2 cal/cm^2), you might feel comfortable that arc flash is not a major concern.


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PostPosted: Mon May 14, 2012 1:10 pm 

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So from what I gather, I can use IEEE 1584 calculation methods assuming that there actually is an exposed source (broken isulation in the conductors). That should give me at least a ballpark value.
I understand that in the manholes there is no escaping through the 2 second rule, and also the effects of the arc flash will be enhanced. So any amount of incident energy will still cause more injuries than in an open space at the same energy level.
Would you say its fair to assume that the risk is minimal if the activities being performed in the manhole do not disturb or touch the 480V cables? The activities will be only for evidence gathering (pictures) and accounting for the condition and amount of cables in it (visual inspection). Hopefully these activities can be performed without disturbing the actual installation.
Thanks again!


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PostPosted: Mon May 14, 2012 5:28 pm 
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SirLearnALot wrote:
So from what I gather, I can use IEEE 1584 calculation methods assuming that there actually is an exposed source (broken isulation in the conductors). That should give me at least a ballpark value.


Arcing faults can occur regardless of whether there is exposed insulation or not. In editions of 70E prior to 2009 this area was rather ill defined in the fact that shock hazards and arc hazards are separate and distinct. That is no longer the case in the 2012 version. However the initiator is also different. Generally if you don't do anything to cause equipment to change state, the risk of an arcing fault is fairly low.

Quote:
I understand that in the manholes there is no escaping through the 2 second rule, and also the effects of the arc flash will be enhanced. So any amount of incident energy will still cause more injuries than in an open space at the same energy level.


So far so good. However if the incident energy is <1.2 cal/cm^2 then obviously the arc flash boundary is less than the working distance and so chances are that the "enhancement" problem is probably minimal. I just did a poor job of describing this. However as soon as incident energy gets close to being a significant concern, the problem may be worse than IEEE 1584 calculates.

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Would you say its fair to assume that the risk is minimal if the activities being performed in the manhole do not disturb or touch the 480V cables? The activities will be only for evidence gathering (pictures) and accounting for the condition and amount of cables in it (visual inspection). Hopefully these activities can be performed without disturbing the actual installation.
Thanks again!


Hmm...are you going to ask me again in a court room? Because I may not want to answer.

Seriously...I'll give you an indirect answer. Look at the definition of arc flash hazard in 70E. The key words are that under conditions where the electrical gear is going to readily change state, the risk of an arcing fault is pretty good. Many times the 70E Committee has rejected various proposals because the likelihood while "just walking by" is remote. However my answer would change drastically if there is already insulation cracked and fallen off and there is a possibility of stumbling into it or knocking something loose just by crawling down into the manhole.

Even if the equipment was in good condition if for instance you have non-round manholes, my answer would change. In this case you would have somewhere between a 4% and 16% chance (based on HEART data) of an electrician accidentally dropping the manhole, and some chance of it landing the wrong way. I don't have good statistics on this last part because it would depend on your vaults so I'd have to give you a worst case analysis of around a 10% chance of an arc flash from removing the manhole in a non-round scenario. This used to be a big problem with vaults before round became the standard.

Another consideration...what's the chance that even if you try to impress on your workers that they could be killed by poking/prodding at the wiring but then you ask them to go do a thorough inspection that they absolutely can't resist the temptation to poke at the wiring?

I really recommend that for this kind of answer where there is no textbook answer, you really need to do a risk assessment using a formal risk assessment methodology. There are several that are an ANSI standard. IEC has a couple. There is also the RIA standard (my favorite for manufacturing plants), and ISA S88 (IEC 61511, my favorite for process plants). Any one of them will work as long as you are consistent in how you do it, include all the players, and document it.


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PostPosted: Tue May 15, 2012 6:37 am 
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PaulEngr wrote:
Arcing faults can occur regardless of whether there is exposed insulation or not. In editions of 70E prior to 2009 this area was rather ill defined in the fact that shock hazards and arc hazards are separate and distinct. That is no longer the case in the 2012 version. However the initiator is also different. Generally if you don't do anything to cause equipment to change state, the risk of an arcing fault is fairly low.



So far so good. However if the incident energy is <1.2 cal/cm^2 then obviously the arc flash boundary is less than the working distance and so chances are that the "enhancement" problem is probably minimal. I just did a poor job of describing this. However as soon as incident energy gets close to being a significant concern, the problem may be worse than IEEE 1584 calculates.



Hmm...are you going to ask me again in a court room? Because I may not want to answer.

Seriously...I'll give you an indirect answer. Look at the definition of arc flash hazard in 70E. The key words are that under conditions where the electrical gear is going to readily change state, the risk of an arcing fault is pretty good. Many times the 70E Committee has rejected various proposals because the likelihood while "just walking by" is remote. However my answer would change drastically if there is already insulation cracked and fallen off and there is a possibility of stumbling into it or knocking something loose just by crawling down into the manhole.

Even if the equipment was in good condition if for instance you have non-round manholes, my answer would change. In this case you would have somewhere between a 4% and 16% chance (based on HEART data) of an electrician accidentally dropping the manhole, and some chance of it landing the wrong way. I don't have good statistics on this last part because it would depend on your vaults so I'd have to give you a worst case analysis of around a 10% chance of an arc flash from removing the manhole in a non-round scenario. This used to be a big problem with vaults before round became the standard.

Another consideration...what's the chance that even if you try to impress on your workers that they could be killed by poking/prodding at the wiring but then you ask them to go do a thorough inspection that they absolutely can't resist the temptation to poke at the wiring?

I really recommend that for this kind of answer where there is no textbook answer, you really need to do a risk assessment using a formal risk assessment methodology. There are several that are an ANSI standard. IEC has a couple. There is also the RIA standard (my favorite for manufacturing plants), and ISA S88 (IEC 61511, my favorite for process plants). Any one of them will work as long as you are consistent in how you do it, include all the players, and document it.


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PostPosted: Tue May 15, 2012 6:38 am 
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Are the manholes flooded with water before you enter? If they are, I seriously doubt you have any live exposed conductors.


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PostPosted: Tue May 15, 2012 10:07 am 

Joined: Thu Mar 08, 2012 6:47 am
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Well Paul, thank you for the very comprehensive answer. I will get together with the industrial safety supervisor to work on a risk assessment to perform the inspections. But you are totally right, there seems to be no one answer to solve this issue. So we'll try to do something that errs on the safe side always.

Haze: The manholes were not found flooded. However in the past it seems that this may have ocurred and the degradation of the cables, we think could exist, would be due to flooding after the snow melts and brings down all the salt spread during the season.

In any case, thanks again for all your input. Any further comments and insight is appreciated, but we are going to work something out together with our industrial safety department.


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PostPosted: Thu May 17, 2012 10:50 am 

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We would all be curious to know what you and your industrial safety department end up deciding.


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PostPosted: Thu May 17, 2012 2:37 pm 

Joined: Fri Oct 30, 2009 9:20 am
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It's not exactly an electrical engineering answer, but at the last plant at which I worked, we hired a company to lower a video camera with lights into a full water tank for internal inspection. I would think there would be a firm out there that could also do a camera inspection on the conductors in your manholes without actually putting personnel into them. Amazing things are now done with digital images.


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PostPosted: Thu May 31, 2012 11:27 am 

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Like the last response about keeping personnel out of the manhole, and using remote video technology. Hey what about thermal imaging the cables for hot spots using an IR camera? The risk assessment indeed for your case is a must. What about an outage to these cables, is such feasible prior to entry? Setting the trip relays to instantaneous (especially ground fault) is a good idea. Realize the IEEE 1584 Incident Energy calculations may be challenging since they are three phase equations and the likelihood of such a fault occurring is minimal. Instead, your team would more likely experience a single phase fault to ground. If you’re questioning the condition of these cables, then maybe you should consider IR or DWV testing each cable one at a time, minimizing your outage impact. Just be prepared to repair them soon after bad results. Quite frankly if the condition of your cables are that suspect, your risk assessment should indicate no entry for shock and arc flash hazards. NOTE: cable splices tend to be your weakest link in the manhole areas.

Some thing I learned from a Kinetrics customer that had manhole arc flash testing performed at the MV range. Arc Flash blanketing does protect from blast even if they’re just draped over the cabling/splices. Typically based on relay instantaneous settings engaged Cat II PPE appears to be ample, but let your risk assessment drive the proper PPE to be worn. Prevent from wearing a shield. The circulation of heat and gases during an arc blast tend to get caught under the shield and can cause severe injury to face. All pants and shirt wear should be tucked in for same circulating effects.

Hope this info helps…


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PostPosted: Fri Jun 01, 2012 3:16 pm 
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IR is great for detecting joint issues in conductors. It doesn't work so hot for insulation issues. PD testing does a great job detecting problems with joints but only works when your system voltage is over 2 kV, so starting typically at 4160 V for most folks.


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PostPosted: Fri Jun 01, 2012 3:16 pm 
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IR is great for detecting joint issues in conductors. It doesn't work so hot for insulation issues. PD testing does a great job detecting problems with joints but only works when your system voltage is over 2 kV, so starting typically at 4160 V for most folks.


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PostPosted: Tue Jun 05, 2012 4:43 am 

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Hey guys, great input! thank you!
After doing more research we found out that only two cables could have high energy issues, and one case was bound by the other. To get an approximate we calculated the incident energy on both as if it was a 480V enclosure. And we got positive results, to go over 20 cal/cm2 we have to be 3 inches away from it which would make it impossible to work or even look at them for inspection. Also, while being this close there is a risk associated with shock rather than an arc.
However, to keep it on the conservative side, the PPE to be used will be level 3 and with an emphasis on ear protection because the sound will get amplified greatly in the manhole.
If we run into one of these mistery cables cases in our manholes again I will definitely suggest using IR cameras and corona detection to get some previous idea of what is going on in it.

Thanks again, everyone!


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