Arc Flash Categories 3

[rockman7892]

I am reviewing an Arc Flash study which was done for our plant. Each of the pieces of equipment (Switchgar, MCC's, etc..) are labeled with their corrosponding Arc Flash rating category 1-4.

Some equipment has the category level listed as "Dangerous" I was not sure what this dangerous level exactly meant. Does this mean that this equipment was above all Arc Flash categories and could not be worked on hot? Is there a standard understanding of this term "dangerous" or is this user defined by the individual performing the study?

 

[Zogzog]

Equipment that has an Ei at the assumed working distance of >40cal/cm2 is labeled as dangerous, that is because there is no PPE that can protect a worker from the pressures created by this level of arc flash.

And to be clear, you cant work on anything energized with out filling out an Energized Electrical Work permit and having a justifiable reason to do energized work as defined by article 130.1 of the NFPA 70E.

There are methods to reduce those dangerous levels to acceptable levels, remote racking and remote breaker/switch operation, arc flash reduction systems, and other mitigation solutions.

 

[rockman7892]

Zogzog

Thanks for the response. If I understand correctly the hightest category level that I have seen is a category 4 which corrosponds to the 40cal/cm2 that you referenced.

Here at the plant we have a 100Cal Arc Flash suit. What is this suit good for or what can it protect against if you are saying that above 40Cal/cm2 there is no PPE that can protect a worker.

What you are saying in that anything above a category 4 is considered dangerous and cannot be work on or operated when live no matter what the amount of PPE used.

 

[DanDel]

The 100 or 103-cal suit was originally used for locations where the cal/cm^2 was above 40, however, recent changes in the arc flash recommendations limit work to 40cal or below locations, because of the force of the blast as Zogzog mentions. You can use the suit for <40 cal locations if you want.

 

[Eleceng01]

I always use the 'open casket' or 'closed casket' analagy when asked about this in the classes I teach. You won't get burned in the 100-cal suit, but you will still be dead (from collapsed lungs, damage to internal organs, etc.).

Basically you have to come up with a solution to reduce the level into the Cat. 0-4 per NFPA-70E. Remember that 65-cal/cm^2 @ 18" may be significantly reduced at 36" or 50". Distance is your friend when it comes to arc flash! There are many other solutions though as well as ZogZog mentions.

 

[Zogzog]

" If I understand correctly the hightest category level that I have seen is a category 4 which corrosponds to the 40cal/cm2 that you referenced."

HRC 4 is for Ei's from 25-40cal/cm2

"Here at the plant we have a 100Cal Arc Flash suit. What is this suit good for or what can it protect against if you are saying that above 40Cal/cm2 there is no PPE that can protect a worker."

Yep, eleceng01 hit ot on the head, open casket vs closed casket, same terms I used in my training classes.

"What you are saying in that anything above a category 4 is considered dangerous and cannot be work on or operated when live no matter what the amount of PPE used. "

That is right, either it needs to be de-energized or the haxard needs to be reduced.

 

[stevenal]

Since incident energy exposure is related to current, distance and clearing time; it cannot be related to blast pressure which is a function of only current and distance. A class I attended recently showed a video of a free standing dummy receiving 80 cal/cm^2 without even getting knocked over. If your HRC > 4 is due to long clearing time, it may be possible you can delay the open versus closed casket decision if you select the right PPE.

Deenergizing is not the cure all. The process of isolating, testing, and grounding are likely to put someone inside the boundary before the circuit can be assumed to be dead.

 

[rockman7892]

I understand that an individual cannot work on an energized piece of equipment unless they are wearing the proper PPE for the rated category of the equipment. This includes taking readings, changing fuses, etc...

What about during the process of energizing equipment? In other words do you have to have the propper PPE for racking in or closing a breaker? What about if the equipment is enclosed and has a deadfront, do you still need this PPE when closing breakers etc....

 

[Zogzog]

"What about during the process of energizing equipment? In other words do you have to have the propper PPE for racking in or closing a breaker?"

Absoultly, if fact this is one of the most common tasks where an arc flash can occur. Have you seen this video yet?

http://www.metacafe.com/watch/1555440/arc_flash_while_racking_a_breaker/

"What about if the equipment is enclosed and has a deadfront, do you still need this PPE when closing breakers etc.... "

Unless the switchgear is arc rated, yes you do, there is no way to know if switchgear will contain an arc flash without the proper ANSI testing and design.

 

[rockman7892]

Wow that video really strikes home the importance of personel protection!

In going through our Arc Flash Report it suprised me that many of our downstream devices at LV(480V) had a higher incidnet engergy than our upstream MV (4.16kV) equipment. An example is a secondary 480V breaker on the secondary of a 4.16kV/480V transformer is listed as a category level of dangerous while the upstream 4.16kV switchgear feeding this transformer only has a category 2 level. Although the Arcing fault current is higher at the 4.16kV gear, the trip delay time at the 480V equipment is much longer. I guess this delay time can make all the difference. There are different working distnaces listed for each as well.

I dont think many people here realize the fact that this LV (480V) equipment can be even more hazardous than the 4.16kV switchgear in terms of Arc Flash, and dont give it the necessary respect. I see many people here use the utmost caution when around 4.16kV gear but then get somewhat lackadasical when around 480V gear thinking it does not pose as much of a threat. The results I am finding in this study are very eye-opening to the fact that the 480V gear can be just as dangerous if not more dangerous than the 4.16kV gear.

I'm assuming that all incident energy levels listed in an Arc Flash report our with the deadfront of the gear intact? I would think that the IE and therefore category would go up with the deadfronts removed, but are only listed in report with deadfronts on.

 

[Zogzog]

"In going through our Arc Flash Report it suprised me that many of our downstream devices at LV(480V) had a higher incidnet engergy than our upstream MV (4.16kV) equipment. An example is a secondary 480V breaker on the secondary of a 4.16kV/480V transformer is listed as a category level of dangerous while the upstream 4.16kV switchgear feeding this transformer only has a category 2 level."

That is very typical, both the result of the study and the reaction of the client.

"Although the Arcing fault current is higher at the 4.16kV gear, the trip delay time at the 480V equipment is much longer"

Do you understand why? Not sure if this was a statement or a question.

"I'm assuming that all incident energy levels listed in an Arc Flash report our with the deadfront of the gear intact? I would think that the IE and therefore category would go up with the deadfronts removed, but are only listed in report with deadfronts on. "

unless the gear is arc rated the covers are assumed to fail, so your Ei's are actually assuming doors open/covers removed. There is no way to know if the gear will contain the arc and how much energy it would contain, even if you did you would need 2 HRC's for every point for covers on/covers off and that would be confusing to most industrial electricians and/or equipment operators.

 

[davidbeach]

In many ways you'd be better off with the cover off to begin with, so it doesn't hit you and knock you to the floor (or out) when it flies off.

 

[rockman7892]

"Although the Arcing fault current is higher at the 4.16kV gear, the trip delay time at the 480V equipment is much longer"

"Do you understand why? Not sure if this was a statement or a question"

I believe I understand why. I belive that it is due to the delay time on the protection device allowing the fault current to persist for longer and thus allowing it to contribute more to the incident energy.

 

[Zogzog]

"I believe I understand why. I belive that it is due to the delay time on the protection device allowing the fault current to persist for longer and thus allowing it to contribute more to the incident energy. "

Right, and with the protective device being on the primary side of the transformer, the fault on the seconday side is more or less an overload as seen by the OCPD on the primary. Hence, the long delay times. What we have done here is replace the fuses in the MV switch with a mini VCB that has a relay sensing the secondary side of the tansformer, so a fault is cleared quickly, we have been able to go from HRC 4 to HRC 0 in some cases. Not to pratical for most plants but paper mills for some reason dont usually have a 480V main breaker so the dangerous levels are seen for all of the feeders in that sub. Here is a white paper discussing this problem and solutions.

 

[rockman7892]

Zog

That was a very interesting article that you attached. After I shared the results of the Arc-Flash study with others here their suprised reaction was the same as mine towards the high levels at the secondary of the transformer. This paper definitely gives us some ideas for dealing with this issue.

A couple of follow up questions:

You mentioned that a fault on the secondary bus of the transformer will only show up as an overload at the primary fuses. Is this due to the impedence of the transformer?

The secondary busses of our transformers have a breaker which then feeds an MCC with the breker being located on a LV section connected to the transformer. Some transformers have multiple breakers feeding multiple MCC's. What differentiates thse feeder breakers (same as in paper) from a main breaker on the secondary of the transformer? Is it the settings?

The paper talked about installing a mini circuit breaker at the existing 15kV switch located near the transformer and being able to put the CB in maintenance mode to reduce Arc Flash levels. What if instead of adding a new CB in place of the primary fuses I add a maintance mode to a breaker that is upstrem of the transformer and primary switch and seperated by a bus or two? Will this have the same effect in reducing these Arc Flash levels by using an upstream breaker in maintance mode?

When condisering Arc Flash levels, is closing a swich considered the same as racking in a breaker? In other words I'm assuming that the propper PPE levels have to be followed when closing a switch on an energized piece of equipment?

 

[Zogzog]

"You mentioned that a fault on the secondary bus of the transformer will only show up as an overload at the primary fuses. Is this due to the impedence of the transformer?"

That and the different voltages.

"The secondary busses of our transformers have a breaker which then feeds an MCC with the breker being located on a LV section connected to the transformer. Some transformers have multiple breakers feeding multiple MCC's. What differentiates thse feeder breakers (same as in paper) from a main breaker on the secondary of the transformer? Is it the settings?"

Well a main breaker would feed all of the feeder breakers and provide protection for the switchgear, some paper mills dont have a main breaker (Cost cutting).

"The paper talked about installing a mini circuit breaker at the existing 15kV switch located near the transformer and being able to put the CB in maintenance mode to reduce Arc Flash levels. What if instead of adding a new CB in place of the primary fuses I add a maintance mode to a breaker that is upstrem of the transformer and primary switch and seperated by a bus or two? Will this have the same effect in reducing these Arc Flash levels by using an upstream breaker in maintance mode?"

The breaker would have to use relays that monitor current on the secondary side of the transformer. either CT's or light sensing.

"When condisering Arc Flash levels, is closing a swich considered the same as racking in a breaker? In other words I'm assuming that the propper PPE levels have to be followed when closing a switch on an energized piece of equipment? "

Yep, same PPE required, however, greater chance of arc flash when racking. Remote operators are available for switch operation, breaker operations, and breaker racking. This seems to be the direction most plants are going.

 

[rockman7892]

"That and the different voltages."

I understand the impedance however even with different voltages the currents are proportional to that voltage ratio and therefore the fuses on the primary I would think are sized for this proportional current. Therefore ignoring impedance I would think that a fault on the secondary would produce a proportional current on the primary that would activate and OCPD

"Well a main breaker would feed all of the feeder breakers and provide protection for the switchgear, some paper mills dont have a main breaker (Cost cutting)."

OK. The output of our transformers are directly coupled to a switchgear bus which then has breakers feeding from it. This means that these are feeder breakers. If there was a breaker connected after the trasnformer secondary but before the secondary switchgear bus (not direcly coupled) then this would constitue a secondary main breaker and therefore protect the bus.

 

[WDeanN]

zog is going to want to shoot me, but here it goes:

There is NO restriction that says you SHALL NOT work on equipment above 40 cal/cm. It is a Fine Print Note recommendation in NFPA 70E - 130.7(A). It is a recommendation, not a requirement.

That being said, I would recommend adopting the recommendation. You should keep your 100 cal suit, however, because you will still need it for actions such as voltage testing, etc.

Now, to continue the discussion:
"If there was a breaker connected after the trasnformer secondary but before the secondary switchgear bus (not direcly coupled) then this would constitue a secondary main breaker and therefore protect the bus."

Yes, but remember now you have just moved the hazard from the whole bus to the main breaker. The high energy level is still there between the main breaker and the transformer output, and it should be lower on the LV bus section (provided correct coordination exists). You will still need to adopt another method (which Zog's paper discusses) to totally eliminate the hazard if you wish to operate the main breaker. The other method, of operating the Xfmr high side disconnect to de-energize the bus may not be possible if this is not a load break switch, without first opening all of the load breakers.

 

[Zogzog]

"zog is going to want to shoot me, but here it goes:

There is NO restriction that says you SHALL NOT work on equipment above 40 cal/cm. It is a Fine Print Note recommendation in NFPA 70E - 130.7(A). It is a recommendation, not a requirement."

No arguement here, you are correct, but if it was my decision I wouldnt want to go against that "recomendation"


"Yes, but remember now you have just moved the hazard from the whole bus to the main breaker. The high energy level is still there between the main breaker and the transformer output, and it should be lower on the LV bus section (provided correct coordination exists). You will still need to adopt another method (which Zog's paper discusses) to totally eliminate the hazard if you wish to operate the main breaker. The other method, of operating the Xfmr high side disconnect to de-energize the bus may not be possible if this is not a load break switch, without first opening all of the load breakers."

Right but with the Main breaker you can lower the HRC on the feeders in that switchgear lineup easily with an arc flash reduction switch (E.G. Quick trip)