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Arc Flash and LV Transformers

Arc Flash and LV Transformers

Arc Flash and LV Transformers

(OP)
I'm having some trouble!
Can someone tell me if this is right? or point out the flaw in my calculations?
If I have a bolted fault current value on the secondary of a transformer, the closest upsptream protective device will be called on to interrupt the arc. If the closest device is on the primary (upstream in a piece of switchgear). In order to determine the trip time of the device, the arc current has to be reflected across the transformer using the LV/HV ratio, and then read from the TCC curve for the fuse. Using the fuse melt time, the NFPA arc flash calculations can be done. Check this out:

Here are the numbers I'm having trouble with:
Transformer Specs:
112.5kVA
600/220V (delta-wye)
5.0 %Z

Available Fault Current (from custom spreadsheet):
24.5kA at the switch gear
16.4kA at the primary (due to feeder impedance)
4.34kA on the secondary (due to transformer impedance)

Fuse Data:
C200HR English Electric (bought out by GEC)

My arcing current on the secondary is then 2.45kA. (using equation D.8.2(a) from the NFPA70E)

Do I then take the 2.45 * 220/600 = 0.898kA to get the primary side current. Reading from the TCC I get ~24 second melt time.
This gives a normalized incident engery of 0.6J/cm^2 and incident energy of 652 J/cm^2 at 24 inches (460mm). This number is WAAAAYYY bigger than I would have expected.

Where is the flaw in my procedure????

RE: Arc Flash and LV Transformers

I think you want the clearing time, not the minimum melt. Of course this ups your incident energy calculation. The other mistake I see is the assumption that the exposed individual remains bolted in place for the entire 24 + seconds. IEEE 1584 suggests using a maximum of 2 seconds, but longer if if a bucket truck is used, or if the individual has crawled into the equipment.

RE: Arc Flash and LV Transformers

Low fault currents can mean very nasty arc flash hazards due to extended tripping times.  Your math could be entirely correct.

RE: Arc Flash and LV Transformers

(OP)
So a good remedy would be in install a faster acting protective device closer to the load with the high arc flash hazard rating?

Am I right to say the high-side current is just the voltage-ratio times the low side arcing current?

RE: Arc Flash and LV Transformers

You are correct about the high side current.  Faster acting protection is a good solution, though in many cases it can be much easier said than done.

RE: Arc Flash and LV Transformers

You can install faster acting protection on the low side, but there will always be that bus section between the transformer and the low side protection to deal with.

RE: Arc Flash and LV Transformers

This is a common problem. I haven't checked your numbers, but you can generally count on the transformer secondary/main breaker area as having the highest arc-flash levels.  The reason is as you state - the primary device must clear a fault on the secondary side and the fault does not produce enough primary amps to clear quickly.

In these smaller transformers, the problem is even worse due to low fault current.

IEEE-1584 (in the Annex) does state that it is reasonable to use a maximum exposure time when the calculations show a very long arc duration.  They suggest 2 seconds as a maximum fault duration.  

From a practical standpoint, the probability of sustaining a 220V fault for 24 seconds is very low.  

RE: Arc Flash and LV Transformers

stevenal, absolutely correct.  A transformer diff zone that overlaps the low side protection will provide the necessary coverage.  But adding a diff relay to his installation would be a major undertaking as there is not a high side breaker to trip.  Maybe a transformer diff to trip a ground switch on the high side.  Once the ground switch closes, there is no more fault current on the low side and the fuses will clear much faster for a bolted fault on the high side than for a fault on the low side.

RE: Arc Flash and LV Transformers

(OP)
I don't recall reading anything about this "2 second rule" in the NFPA70E. Besides the IEEE1584 (I don't have a copy) is there somewhere with info on this rule-of-thumb?

RE: Arc Flash and LV Transformers

(OP)
How does clearing time relate to the melting time?

RE: Arc Flash and LV Transformers

Minimum melt is the lower line on a fuse curve, total clearing time is the upper line.  At minimum melt, a fuse at the fastest end of the manufacturing tolerance will begin to melt.  Melting takes time.  At total clearing time, a fuse at the slowest end of the manufacturing tolerance will have melted and the arc (in the fuse) will have extinguished.

RE: Arc Flash and LV Transformers

If you only have the melt curve:

"For fuses, the manufacturer’s time-current curves may include both melting and clearing time. If so, use the
clearing time. If they show only the average melt time, add to that time 15%, up to 0.03 seconds, and 10%
above 0.03 seconds to determine total clearing time. If the arcing fault current is above the total clearing time
at the bottom of the curve (0.01 seconds), use 0.01 seconds for the time.

RE: Arc Flash and LV Transformers

(OP)
I'm not quite sure I understand. The fuse charts I have been using usually only have one line per fuse size on the log-log grid.

For example, the Ferraz Shawmut AJT family of fuses:
http://www.ferrazshawmut.com/products/pdf_107/AJTSMartspot.pdf

All is says on the TCC graph is "Melting Time - Current Data, 600V Fuses". I've been assuming the lines represent an average melting time. I've been using my "Arcing Current" values to read off the melt time. I just read a website that claimed the clearing time is 10-15% more than the melting time. Does this sound right to anyone?

By this token, my 24 second melting time stated above would be inflated to 27.6 seconds, which is an even worse arc-hazard result!

RE: Arc Flash and LV Transformers

Re-read the info provided by David Beach and Stevenal.  The Shawmut curve is an "average melt" curve.  You need to adjust this per IEEE-1584 recommendations to get a conservative clearing time, and yes, this will cause longer arc-duration and higher calculated arc energy.  

  

RE: Arc Flash and LV Transformers

You can get minimum melt and total clear curves from the fuse manufacturers but it isn't easy.  Once you get to medium voltage they don't try to pull that average melt #*^% any more.

It would not be of any surprise that the worst arc flash hazard on a system would occur on the lowest voltage level used, behind a transformer just big enough that you have to evaluate the secondary of the transformer.  (I'm too lazy to look it up at the moment, but there is a transformer size below which you don't need to evaluate 240 or 208V systems.)

RE: Arc Flash and LV Transformers

(OP)
I'm only evaluating fault current and arc flash hazard for systems in the plant that can consume a max rating of 100kVA. So you're right, I'm not dealing with the 208V systems in the factory.

We use so many types of fuses here that I was hoping I wouldn't have to twist the arm of the fuse manufacturers to get the curves.

Do you think the add 15% to the average melt time is a valid method? It seems a bit loose and open ended to me.

Thanks for all the help, guys. I appreciated it.

I welcome any and all comments!

RE: Arc Flash and LV Transformers

Are you sure you worded your first paragraph correctly?

RE: Arc Flash and LV Transformers

(OP)
No, I got it completely backwards!

We're only evaluating the systems with potential power usage greater than 100kVA.

RE: Arc Flash and LV Transformers

IEEE 1584 actually distinguishes between 208 V and 240 V systems.  It recommends that arc energy calculations be done for **all** 240 V systems and for 208 V systems 125 kVA and larger.

Apparently this is based on the test results where arcs were more likely to be sustained at 240 V.  I think 208 V testing produced very few sustained arcs.   

RE: Arc Flash and LV Transformers

This may be a dumb question: have you considered using fuses with a lower rating?

RE: Arc Flash and LV Transformers

(OP)
I'll look in to hazard mitigation later. At this point I just wanted to make sure my analysis technique was correct. It seemed like I was finding a lot of "hot spots" in our factory, and I thought maybe I was doing it wrong.

RE: Arc Flash and LV Transformers

Likely a lower rated primary fuse would not survive a fully offset inrush current, or short term overloading. Usually there are not too many choices for a particular transformer. We are now trying to use system protection devices as PPE, something they were never intended or designed for.

RE: Arc Flash and LV Transformers

Interesting: for information.
(Inrush current)
http://www.sandc.com/webzine/012003_1.asp

Typically inrush is 12x FLC for 0.1s, and 25x FLC for 0.01s.
The 112KVA Tr with 200A fuses is easily too big for 12xFLC, but about right for 25xFLC.
150A fuses would be cutting it fine for inrush.

Are there any fuses with a slightly better curve?

RE: Arc Flash and LV Transformers

(OP)
Switching from a 200A fuse to a 150A fuse of the same type would reduce the melt time from ~24 seconds to about ~5.5 seconds.

This will reduce incident energy from 5.61 to 2.74 J/cm^2. This brings the PPE requirement down from >4 down to level 1.

The 112.5kVA transformer powers a 75hp grinder motor. Does this seem like a viable solution?

RE: Arc Flash and LV Transformers

Since the upstream fuse will clear the fault current, I believe that you need to calculate the arc flash fault current from the bolted fault current found at that fuse location in the distribution (24.5kA).  An arc fault at the secondary of the transformer will draw its calculated value, but the upstream fuse that will attempt to clear the fault during that event downstream, will carry the calculated value at the fuse location.
That is my understanding.  I generally use SKM software, so I haven't done it by hand in a while.

RE: Arc Flash and LV Transformers

Roughly speaking, you could stall the grinder for about 5 minutes, before the 150Amp fuses would blow.

RE: Arc Flash and LV Transformers

(OP)
"the upstream fuse that will attempt to clear the fault during that event downstream, will carry the calculated value at the fuse location."
--------------------------------
This idea is new to me. The calculated current value (bolted 3ph fault) at the fuse is much larger than the arcing current at the fault location on the transformer secondary.
I don't see how the fuse could possibly see the full bolted fault current based on a downstream fault. Or perhaps I'm misunderstanding?

I'm not sure how well the production line would respond to a stalled grinder... probably not a good thing.

RE: Arc Flash and LV Transformers

If the 150A fuse in question can carry the inrush, and short term overloading is not a factor; solution looks viable.

RE: Arc Flash and LV Transformers

Your energy values don't seem to match your PPE levels.

5.61 Joules = 1.34 cal - This is a low arc-flash level.

Are you missing some zeros somewhere?  


RE: Arc Flash and LV Transformers

(OP)
Sorry, dpc, I got my numbers mixed up. The 5.61 and 2.74 values are actually the Flash Protection Boundry distances (in meters). This is, for those who don't know, the distance at which a curable burn will be recieved in the even of an arc flash.

Here are the proper numbers:

@ 24 seconds, E = 652 J/cm^2  --> PPE level 4
@ 5.5 seconds, E = 155 J/cm^2 --> PPE level 3

These are clearly huge amounts of energy. Something I wouldn't expect to see on the low side of a 112.5kVA transformer at 220V.

RE: Arc Flash and LV Transformers

The fuse on the upstream side of the transformer will see the secondary fault current multiplied by the current ratio of the transformer.  This will be significantly less than the bolted fault current available on the primary of the transformer.  Kirchhoff wouldn't have it any other way.  Convert to per unit; the transformer becomes an impedance and amps in (in per unit) equals amps out (in per unit).  Fault on the secondary causes x per unit amps to flow, same x per unit amps are flowing on the primary side.  You'll find that the SKM software can be used to find the maximum fault current at each location, or you can use it to find the fault currents at any portion of the system for a specific fault.  Try that and you will find that the primary current for a secondary fault is considerably less than the current at the same location for a primary fault.

RE: Arc Flash and LV Transformers

Chapmeister,
Just to confirm your calculated results, in simulating your situation in SKM, I get approx 1050 J/cm^2 because it also calculates the worst case considering the arcing current low tolerance.

David,
Thanks for straightening me out.  I agree.

RE: Arc Flash and LV Transformers

I am not sure about the calculaitons, but per NFPA 70E and/or IEEE 1584 transformers rated 125kVA and below are assigned Category 0 for PPE and there is not need to do arc flash calcs.

RE: Arc Flash and LV Transformers

rbulsara,

The 125 kVA limitation in IEEE 1584 applies to 208V and lower.   Anything higher than 208 V is a different story.

RE: Arc Flash and LV Transformers

rbulsara,
The IEEE 1584 is that 208/120V systems should not be a concern.  Well, they shouldn't be, but what do I tell the judge when he says it was.  The text in the IEEE document is not shall or must, so I do the calc anyway.

RE: Arc Flash and LV Transformers

To clarify the voltage level: "Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance
transformer in its immediate power supply."

How does "need not" stack up against "should not"?

RE: Arc Flash and LV Transformers

ronshop/dpc:

I think stevenal answered it. As for the judge goes all bets are off. On the otherhand I am sure if you show them recommendations of IEEE any san jury will equitt you of negligence. After all, the person who decides to work live an equipment bears some responsibilty.

If the judge or a jury is insane, it would not matter any way. Do you seriously think that just because your calculation turns out to be techncally correct and you are involved in a lawsuit it will be of any more help than showing a published standard??

RE: Arc Flash and LV Transformers

The fact is arc flash calculation method is still evolving, in fact it is in embryonic stage, you do the best you can with available tehcniques.

The IEEE formulas are all imperical formulas or are curve fits. Meaning curves were plotted based on several test results and then mathematician were called in to find the equation that fits the curves. So there are infinite number of scenarios that are not yet covered. At the same time there is no sense in going your merry way to find arc flash energy based on some unfoudned formulas.

IEEE recommends ignoring below 125kVA and 240V because the tests proved so, I believe.




RE: Arc Flash and LV Transformers

(OP)
RonShap,
The values I gave were for 100% arcing current/100% bolted fault current. I have been performing my calculations at 100% AND 38% bolted fault current. 38% is the lowest fault level at which an arc can sustain itself on a 480V system (note: "industry accepted level", see: Annex D.6(1) NFPA70E).

Also, from these two BFC values I'm performing my calculations at 100% and 85% arcing current (as stated in Annex D.8.2(c)), to "account for variations in the arcing current and the time for the overcurrent device to open."

Does this 85% idea cover the addition of the 15% to the melting time as discussed above??? Do you think double counting this 15% might make the results too conservative?

Addressing the other comments,
If you do what the standards say then you will be deemed as having performed your due diligence to the best of your ability and by industry accepted ideals. This will essentially clear you of liability, barring strange circumstances.

rbulsara,
It doesn't matter if I NEED to do the calc, I'm doing it anyway while I'm at it. I'm working my way though about 200 loads in our system on 600V feeders. I am not generally dealing with the 208V loads in the plant. The example I gave just happened to step down to 220V. We have lots of step downs to 415V, 460V, 480V too.

RE: Arc Flash and LV Transformers

The 85% is to account for fault impedance. You want to look at clearing time (not melting time) at each current level.

RE: Arc Flash and LV Transformers

Chapmeister:

You can do the calcs, but the calculation method you may be following has not been backed up by tests and your results could be misleading.

The calculation methods mentioned in IEEE 1584 are for limited scenarios and they are not to be used for systems they do not support.

RE: Arc Flash and LV Transformers

(OP)
Great, thanks for all the tips, everyone.
I'm fairly comfortable with my method now.
In the end of all things, I'll probably have an electrical firm which specializes in arc flash analysis double check my numbers.
As was said earlier, arc flash is still in its infancy. Here at our facility we're just getting the ball rolling.
Thanks again for all the help~!

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