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 Post subject: Remote racking issue
PostPosted: Sun May 03, 2015 10:39 am 
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When racking a circuit breaker remotely, there are instances where the Flash Protection Boundary (FPB) of the >Cat 4 exceeds the distance of the remote racking device umbilical cord.
• What level of AR clothing does the employee wear in this instance?
• Do we need to see if the umbilical cords can be extended?
• The AR gloves we’ve provided personnel are rated up to level 4. I’m not sure that a person could effectively operate the LCD screen with the gloves on. How do we ensure a person is protected and can effectively operate the remote control?

When at or just outside the FPB, what level of AR clothing should an employee wear when racking a breaker remotely?


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 Post subject: Re: Remote racking issue
PostPosted: Sun May 03, 2015 4:07 pm 
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How long is the umbilical? The Arc Flash Boundary is the distance where the incident energy drops to 1.2 cal/cm2.

The Category 4 (40 cal/cm2) PPE requirement is typically from incident energy calculations at a closer distance (working distance) which is normally 24 inches for low voltage switchgear and 36 inches for medium voltage switchgear. Although some just keep it at 18 inches.

What this means is the incident energy at the maximum length of the umbilical is probably somewhere between 40 and 1.2 cal/cm2.

Someone needs to determine this if you want to know what to wear. It could be that something like category 2 is sufficient but without details there is no way to know for sure.


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 Post subject: Re: Remote racking issue
PostPosted: Mon May 04, 2015 5:08 am 
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At most of our stations the operators controls cabinets are within the Arc Flash Boundary. We have calculated the Incident Energy at those distances and it drops considerably, even though it is still within the Arc Flash Boundary. Most of our operators can wear category 1 or even 0 when operating from the control desk.


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 Post subject: Re: Remote racking issue
PostPosted: Mon May 04, 2015 5:55 am 
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abk453 wrote:
When racking a circuit breaker remotely, there are instances where the Flash Protection Boundary (FPB) of the >Cat 4 exceeds the distance of the remote racking device umbilical cord.
• What level of AR clothing does the employee wear in this instance?


As others have stated, depends on the distance, IF REQUIRED.

70E-2015 requires a risk assessment to be done. 70E-2012 implies this but many people didn't use it. As per 70E-2012, "An arc flash hazard may exist when energized electrical conductors are exposed or circuit parts are exposed or when they are within equipment in a guarded, or enclosed condition, provided a person is interacting with the equipment in such a manner that could cause an electric arc". The last clause is critical...no interacting in a way that can cause an arc, no arc flash hazard. No further guidance was given in editions prior to 2015. Hence the reason for the new table in Article 130 under the 2015 edition gives a list of tasks and whether or not PPE is even required. Operation of circuit breakers is prominently on the list. The requirements are that:
1. Doors are closed and latched. This one may or may not apply as it sounds like you are talking about draw-out gear. Frankly, it's not really a necessary or sufficient condition so it probably needs to be replaced with something more clean such as being outside the restricted approach boundary which has the same net effect.
2. Equipment is properly installed and maintained. Note that the arc flash study is garbage if this is not true as you can't rely on overcurrent protection device opening times.
3. Not in eminent danger of failure. This latter is the only place where your procedures would apply.

Quote:
• Do we need to see if the umbilical cords can be extended?
• The AR gloves we’ve provided personnel are rated up to level 4. I’m not sure that a person could effectively operate the LCD screen with the gloves on. How do we ensure a person is protected and can effectively operate the remote control?


The problem here may be more fundamental. Most LCD touch panel things these days are capacitive in nature or resistive but depend on some sort conductive contact with the screen. Wearing a cotton multilayer glove totally disables this function. Rubber voltage rated gloves MAY work but my suspicion is that you simply can't do it.

Quote:
When at or just outside the FPB, what level of AR clothing should an employee wear when racking a breaker remotely?


No PPE needed under 2015 edition. Under 2012 edition the requirement generally was for nonmeltable fiber clothing (cotton, wool, silk, etc.). Clothing is not PPE. And as mentioned earlier, this is only assuming that the breaker or something around the cell has failed, or you aren't maintaining the equipment. And if you aren't maintaining it then the calculated incident energy value is garbage and the actual incident energy may be less or greater and is not predictable.

Either way, keep in mind what the basis behind 70E is. The calculated incident energy is the amount of thermal (heat) energy at the chest/face area for a typical working distance (hands in front of you, working on equipment) for the onset of second degree burn. The hands and arms are clearly exposed to much higher incident energy levels, and first degree burns are not addressed at all. The goal here is to avoid a fatality, NOT to avoid injury. This is also true of any PPE...all that it is designed to do is to reduce the injury, not eliminate a hazard.

I also question why you'd want to use a remote racking device rather than going in and addressing the incident energy in some other way such as using some scheme for faster tripping or reducing the current during an arcing event. The cost is frequently small and the effect is that although you may not be able to totally eliminate PPE altogether, you can at least bring it down to under 40 cal/cm^2 and frequently to under 10 cal/cm^2.


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 Post subject: Re: Remote racking issue
PostPosted: Mon May 04, 2015 6:36 am 
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We have several Medium Voltage lineups that are Cat 4 with a FPB of >100'. We have calculated the distance at which the incident energy is below 8 cal. When Remote Racking is used the operator must don Cat 2 PPE and initiate the process from the calculated distance or greater. The glove can be removed to activate the screen if needed, then put back on


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 Post subject: Re: Remote racking issue
PostPosted: Mon May 04, 2015 12:29 pm 
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generation safety adv wrote:
We have several Medium Voltage lineups that are Cat 4 with a FPB of >100'. We have calculated the distance at which the incident energy is below 8 cal. When Remote Racking is used the operator must don Cat 2 PPE and initiate the process from the calculated distance or greater. The glove can be removed to activate the screen if needed, then put back on


I am skeptical of incident energy high enough to warrant 40 cal/cm2 PPE and a AFB of >100 ft, especially for medium voltage. The last time I saw something like this was in a study that assumed the utility fault current was 40,000A at 34.5kV. The actual utility fault current was <5kA and Lee equations were used instead of ArcPro.

Would you be able to share the study that the values came from?

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 Post subject: Re: Remote racking issue
PostPosted: Mon May 04, 2015 1:47 pm 
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It's not unrealistic and usually the problem is excessively LOW short circuit current.

By way of example, my main substation is 23 kV and 21 kA. So of course Lee gives a silly result of 368 cal/cm^2. But going away from this location to a substation located about 5.5 miles away, I get 8.6 kA on the secondary side of a 10 MVA, 23:7.2 kV transformer. The substation uses cutout fuses on the primary side and a single breaker on the secondary side (just one large load) so the incident energy on the secondary bus is driven by the fuses. At that point it is 10 cal/cm^2 although half the contribution comes from the large (four 2000 HP) synchronous motors downstream. At the synchronous motors, about 1 mile away running on shielded power cable, the incident energy is up to 11 cal/cm^2. No amount of screwing with relaying will help either. As I get closer to the main substation with this smaller substation though the available fault current goes up and the relaying comes into its own so that I can get below 8 cal/cm^2. This is a portable mining application so having an "up to date" arc flash model is more of a moving target than it may be for some.

I really can't decrease the fuse and/or relay settings any more or I run into nuisance tripping with the synchronous motors, so this is about as good as it gets with this configuration. On another machine where I'm using AC drives instead of synchronous motors, incident energy is less than 1.2 cal/cm^2 with everything else approxiamtely the same. Although you can't compare these things identically I always run into the "40 cal/cm^2" line once the substation transformer gets over 1500 kVA with 480 V equipment. If everything scaled linearly (and no, it doesn't) I'd expect to hit this limit for medium voltage 4160 at around 13 MVA or 22.5 MVA for 7200 V. Due to the very high currents one typically goes up a voltage class at those size transformers for cost reasons so I'd expect to see 13.5 kV equipment by the time I would otherwise expect 22.5 MVA so it would seem that as you said, reaching 40+ cal/cm^2 is usually indicative of either relaying issues, invalid inputs into the power system model, or invalid results from Lee equation. On my site the largest 100 MVA transformer has a 22.9 kV secondary and the next largest with a 50 MVA transformer has a 13.5 kV secondary.


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 Post subject: Re: Remote racking issue
PostPosted: Mon May 04, 2015 3:05 pm 
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PaulEngr wrote:
It's not unrealistic and usually the problem is excessively LOW short circuit current.

By way of example, my main substation is 23 kV and 21 kA. So of course Lee gives a silly result of 368 cal/cm^2.


And since you are a installation with utility like equipment and voltages, per OSHA, using ArcPro, what are your results?

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 Post subject: Re: Remote racking issue
PostPosted: Mon May 04, 2015 4:55 pm 
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wbd wrote:
PaulEngr wrote:
It's not unrealistic and usually the problem is excessively LOW short circuit current.

By way of example, my main substation is 23 kV and 21 kA. So of course Lee gives a silly result of 368 cal/cm^2.


And since you are a installation with utility like equipment and voltages, per OSHA, using ArcPro, what are your results?


Utility like, huh? 70+ miles of power line and a 54 MW cogen is atypical? Although the main substation is definitely in OSHA jurisdiction, most of the previously mentioned power line and the 10 MVA substation and motor loads are actually in an MSHA jurisdiction. So we have shifting jurisdictions to deal with. And frankly, MSHA doesn't have the first clue. They're still hung up on ground faults and haven't moved past shock hazards yet.

Can't speak definitely but so far ArcPro gives something like 8-15 cal/cm^2 results for most of the 23 kV stuff. And if we put in the real world working distances for hot stick work with cutouts and similar pole-line equipment, we're down to <1.2 cal/cm^2. I don't have a copy to punch in data at my desk (going to be ordering it) but so far that seems to be about where we're at. Prior to 70E-2015, we used the "2 cal/cm^2" rule in 70E simply because there really wasn't any sort of "official" stance on using ArcPro. The last time we looked at it, we could prove that Lee was giving silly results. We couldn't prove anything about Duke Heat Flux or ArcPro because both of them output a result from closed source code with no test data to compare to. So in order to have a defendable position we used the tables in 70E-2012. Now that the tables stop at 15 kV, we're back to square one except that OSHA has looked at it and decided that although Lee obviously gives the highest results, they are provably grossly overly conservative. So they took the next highest results and thus ArcPro wins. And since OSHA has all but endorsed it, we can go with ArcPro.

But with over 2000 buses doing a 5 year update in house is prohibitively difficult. So we contracted it out. The contractor should be nearing completion in the next month or two. They've already done all the key punching and are now just reviewing all the results.


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