Arc flash

[Microwatt]

I'm trying to do a little research myself on arc flash. I've found the neumerous calculators on-line to help find the hazard catagories for specific work enviroments. However, they all ask for the size of the OCPD ahead of the fault. What if there isn't one? What if it's the secondary side of a 1500kVA, 23.9kv primary/480v secondary, distribution transformer where your only protection in the primary fuses. Then what?

 

[ron]

Depedning on how the calculator wants the data input, you enter the primary fuses are the overcurrent protection.
You will likely get very high arc fault currect and high PPE requirements.

 

[mayanees]

HRC Dangerous!~

HRC Dangerous!~

Ron is being gentle!!!

.. pretty much guaranteed that the incident energy on the secondary of that transformer is over 100 cal/sq cm, for a Dangerous Hazard Risk Category classification.

John M

 

[zog]

I'm trying to do a little research myself on arc flash. I've found the neumerous calculators on-line to help find the hazard catagories for specific work enviroments. However, they all ask for the size of the OCPD ahead of the fault. What if there isn't one? What if it's the secondary side of a 1500kVA, 23.9kv primary/480v secondary, distribution transformer where your only protection in the primary fuses. Then what?

Then you use a max clearing time of 2 seconds, and as mentioned already your arc flash boundary will be about an acre.

Be careful using those free online calculators, lots of assumptions, some are dated, not good for modeling, etc.

SKM, easypower, etc.. those are the ones to use, you will save $$ in the long run hiring a firm that specializes in arc flash analysis and mitigation (Which it sounds like you will need).

 

[jim dungar]

If the instructions for the 'program' you are using do not explain how to handle routine situations such as this, then I would question its results.

As Zog said estimate a device clearing time of 2 seconds.

As the others said, it is all but unlikely that any transformer primary protective device will result in anything less than an incident energy of +40cal/cm? on the secondary, regardless of voltages for TX larger than 125KVA.

I wish the task tables of NFPA70E, Table 130.7(C)(9), included an FPN concerning this condition rather than simply relying on footnotes 1 and 2.

 

[zog]

I wish the task tables of NFPA70E, Table 130.7(C)(9), included an FPN concerning this condition rather than simply relying on footnotes 1 and 2.

You still have a few days to get your ROP submitted.

 

[ron]

I don't agree with the concept of limiting the fault to 2 seconds unless it is obvious to the user that a blast will blow a worker clear so they can escape. I work in too many small electrical/SWGR Rooms where a blast will pin me against the wall and I will not be able to get away in 2 sec as my head will slap against the back wall or another piece of gear.
The 2 second limitation is not part of the calculation standard, just in the appendix.

 

[jim dungar]

I don't agree with the concept of limiting the fault to 2 seconds unless it is obvious to the user that a blast will blow a worker clear so they can escape. I work in too many small electrical/SWGR Rooms where a blast will pin me against the wall and I will not be able to get away in 2 sec as my head will slap against the back wall or another piece of gear.
The 2 second limitation is not part of the calculation standard, just in the appendix.

2 secs may not a bad general estimate when you consider the probability of any arc maintaining 100% strength forever is remote. Most, if not all, software packages assume that the arc does not weaken in strength, due to material being evaporated causing the arc to lengthen, until the arc is cleared.

Also 2 secs will most often result in more than 40cal/cm?, which means there is no acceptable PPE for those locations.

But, there absolutely situations where a 2 sec cutoff is not appropriate.
My company's standards are to use 2 secs, then revaluate those few locations where 'live work' may still be permitted (<40 cal/cm?) to see if this cutoff time is appropriate.

 

[jghrist]

The 2 second limit is not so much based on a blast blowing the worker away from the arc as in the worker being able to move away from the arc in less than 2 seconds. If the current is low enough to have less than 40 cal/cm? IE with 2 seconds arcing time, then the blast energy will not be high.

 

[Microwatt]

Ron is being gentle!!!

.. pretty much guaranteed that the incident energy on the secondary of that transformer is over 100 cal/sq cm, for a Dangerous Hazard Risk Category classification.

John M

I kind of figured it would be a pretty big number and it probably won't be much different at the transition cabinet 15' away. I do have 100cal. suits, but they are worthless if the blast concussion alone will kill ya.

Thanks to everyone for their input.

 

[mayanees]

I kind of figured it would be a pretty big number and it probably won't be much different at the transition cabinet 15' away. I do have 100cal. suits, but they are worthless if the blast concussion alone will kill ya.

Thanks to everyone for their input.

Micro,
I just did a real quick look at a 1500kVA secondary, 300 MVA primary contribution, fused primary but timeout at 2 seconds.
18" Incident Energy = 112 cal/sqcm
180" Incident Energy = 2.56 cal/sqcm

So if the transition cabinet is 15" from the transformer and physically separated, the Hazard Risk Category drops significantly to HRC 2 at 2.56 cal/sqcm.
John M

 

[Microwatt]

Micro,
I just did a real quick look at a 1500kVA secondary, 300 MVA primary contribution, fused primary but timeout at 2 seconds.
18" Incident Energy = 112 cal/sqcm
180" Incident Energy = 2.56 cal/sqcm

So if the transition cabinet is 15" from the transformer and physically separated, the Hazard Risk Category drops significantly to HRC 2 at 2.56 cal/sqcm.
John M

I may have not been clear. What I meant by the TC being 15' away was there is only roughly 15' of conductor from the secondary terminations to the terminations in the TC. The Xfmr and TC are only about 2' appart. So, I'm guessing the 15' of conductor will offer very little additional impedance to the calculation.

What I'm basicly getting at is, working in the TC would require just as much PPE as the Xfmr. Correct?

 

[mayanees]

I may have not been clear. What I meant by the TC being 15' away was there is only roughly 15' of conductor from the secondary terminations to the terminations in the TC. The Xfmr and TC are only about 2' appart. So, I'm guessing the 15' of conductor will offer very little additional impedance to the calculation.

What I'm basicly getting at is, working in the TC would require just as much PPE as the Xfmr. Correct?

Yes you are correct micro.

I mistook that to be 15 feet from the transformer secondary, as in no cabling.

John M