Arc Flash Reduction Methods On Secondary of Sub-unit
Arc Flash Reduction Methods On Secondary of Sub-unit
(OP)
I've seen this application time and time again, but I would appreciate any advice and comments on my findings for the following project I’m working on.
My project is a new 3MVA dry type sub-unit installation (step down 12.47kV to 600V) for a Grain Processing Facility.
Approximating the Impedance Data for the XFMR based on "Cutler-Hammer Reference Data" (p.1.5-11 of the 2006 Consulting Application Guide). The dry-type XFMR (80C rise) is modeled as having:
Z=5.75%
X/R=10.22
Note: The following is based on the secondary side.
The short circuit current is about 58 kVA because the Grain Processing load (on the secondary) is about 95% induction motors. (Largest motor is 400hp.) The load current is: 2700 A.
Modeling in Etap, the secondary of the XFMR is assumed to be a switchgear type equipment with an air gap between conductors of 32mm.) My simulations calculate an Arc Flash energy of 330cal/cm2 which (exceeds the Cat 4 level.) This is what I expect.
Now my design goal is to install a secondary protection device to bring the arc flash hazard down to a Cat 2. GOOD LUCK, because this is where I am having my difficulties:
Option 1) Secondary Protection Using Power CB
Power CBs (with ratings greater than 3000A) have typically an instantaneous 4 cycle clearing time. Provided that I coordinate the CB so that the arching current will trip in the instantaneous region, this brings the arc flash to a Category 3 (about 9.66 cal/cm2). (This is the best that a CB can do.) I've been looking at Schneider's Masterpact NW and NT Circuit Breakers with Arc Flash Protection that have faster clearing times, but they are available only up to 2000A. My CB options seem limited. :(
Option 2) Current Limiting Fuses
In the past, I’ve done research looking at the fastest fuses available for applications that require load currents between 1-5kA. The Bussman KTU fast acting fuse is the fastest available in a Class L format. Using a fuse rated above the XFMR’s secondary current, even with this protection device, the arc flash current flowing through the fuse lies in the clearing time’s tolerance range that using the worst case scenario, the arc flash is not significantly improved. I can always select a smaller current fuse, however, it will conflict with the XFMR’s load capacity.
Option 3) Increasing the Impedance of the XFMR.
I’ve simulated the XFMR as having an impedance of Z=7.5%. The arc flash reduced slightly, but was still in the Cat 3 region.
I’ve looked at the very interesting document “Distribution Equipment Modernization to Reduce Arc Flash Hazards” obtained from here: h ttp://www. eng-tips.c om/viewthr ead.cfm?qi d=203769&a mp;page=14
Their scenario is similar to mine. I have simulated their SLD (Figure 2 on page 29) and I’ve obtain similar result as them. However, their SLD has smaller secondary load then mine.
So now I'm stuck as I excercised my options. I would appreciate any comments.
Thanks,
Maj
My project is a new 3MVA dry type sub-unit installation (step down 12.47kV to 600V) for a Grain Processing Facility.
Approximating the Impedance Data for the XFMR based on "Cutler-Hammer Reference Data" (p.1.5-11 of the 2006 Consulting Application Guide). The dry-type XFMR (80C rise) is modeled as having:
Z=5.75%
X/R=10.22
Note: The following is based on the secondary side.
The short circuit current is about 58 kVA because the Grain Processing load (on the secondary) is about 95% induction motors. (Largest motor is 400hp.) The load current is: 2700 A.
Modeling in Etap, the secondary of the XFMR is assumed to be a switchgear type equipment with an air gap between conductors of 32mm.) My simulations calculate an Arc Flash energy of 330cal/cm2 which (exceeds the Cat 4 level.) This is what I expect.
Now my design goal is to install a secondary protection device to bring the arc flash hazard down to a Cat 2. GOOD LUCK, because this is where I am having my difficulties:
Option 1) Secondary Protection Using Power CB
Power CBs (with ratings greater than 3000A) have typically an instantaneous 4 cycle clearing time. Provided that I coordinate the CB so that the arching current will trip in the instantaneous region, this brings the arc flash to a Category 3 (about 9.66 cal/cm2). (This is the best that a CB can do.) I've been looking at Schneider's Masterpact NW and NT Circuit Breakers with Arc Flash Protection that have faster clearing times, but they are available only up to 2000A. My CB options seem limited. :(
Option 2) Current Limiting Fuses
In the past, I’ve done research looking at the fastest fuses available for applications that require load currents between 1-5kA. The Bussman KTU fast acting fuse is the fastest available in a Class L format. Using a fuse rated above the XFMR’s secondary current, even with this protection device, the arc flash current flowing through the fuse lies in the clearing time’s tolerance range that using the worst case scenario, the arc flash is not significantly improved. I can always select a smaller current fuse, however, it will conflict with the XFMR’s load capacity.
Option 3) Increasing the Impedance of the XFMR.
I’ve simulated the XFMR as having an impedance of Z=7.5%. The arc flash reduced slightly, but was still in the Cat 3 region.
I’ve looked at the very interesting document “Distribution Equipment Modernization to Reduce Arc Flash Hazards” obtained from here: h
Their scenario is similar to mine. I have simulated their SLD (Figure 2 on page 29) and I’ve obtain similar result as them. However, their SLD has smaller secondary load then mine.
So now I'm stuck as I excercised my options. I would appreciate any comments.
Thanks,
Maj






RE: Arc Flash Reduction Methods On Secondary of Sub-unit
It is typical to have high AF values at transformer secondaries, due to the impedance of the transformer. Increasing this impedance will usually make the AF worse, since it will limit the through-current to a point higher on the upstream device's TC curve, causing a longer clearing time.
Though I believe that Square D/Schneiders make NW/NT C/Bs up to the 4000A range, and the addition of this device will improve the AF result downstream from this C/B, you will still have the same AF on the line side of it.
Fuses are always the quickest-operating, and will provide the best possible AF result (typically), but as you said, it may not be the best device for coordination, and you still have the same problem on the line side.
Time to look at you options and decide what is best for the customer and the system. I personally don't like to sacrifice system coordination for lower AF values. I would instead insist that this particular equipment not be worked on when energized. After all, that's the safest alternative.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Current limiting fuses sized for the max transformer rating probably will not help much.
Getting down to 8 cal or less may not be achievable with that large a transformer.
If you have a local primary breaker on the transformer, another option is to install an overcurrent relay on the 480 V side of the transformer and trip the primary breaker.
So there is no existing main breaker?
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
I came to the exact same conclusion as you, but I wanted to test the waters to see what alternatives are out there.
A few things to note (just discussion, not arguing)
"Increasing this impedance will usually make the AF worse, since it will limit the through-current to a point higher on the upstream device's TC curve, causing a longer clearing time."
True, if you were using a fuse or (CB and going from Instantaneous to Short Delay), but if you were in the CB's instantaneous range for both situations like I purposely was, then the later decreased the AF (just a note on what I was doing.)
"Though I believe that Square D/Schneiders make NW/NT C/Bs up to the 4000A range, and the addition of this device will improve the AF result downstream from this C/B, you will still have the same AF on the line side of it."
YUP, you will only have protection upto the protective equipment. One thing, their AF CBs seem to be only rated to 2000A.
http:
Page 155
Maj
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
http://www.utilityrelay.com/QUICK-TRIP_Page.html
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Primary is GE Mutlin 575 at the service feed.
The story about this is that the existing XFMR is 3 single phase 1000kVA XFMRs built in 1923. They are massive. 10 ft high, liquid filled, self cooled. The incoming and secondary feeds have very interesting exposed bus work. I really like these XFMR because they are such antiques and been around well 3.5 times longer then myself.
The XFMRs still work, and have no PCB in their liquid. The client wants to remove them because these is no seconadary CB and they are old. Honestly, I'd leave them. They have been around for so long, why bother? If they aren't broken, why fix them?
But that's me.
Here is a photo of them.
http://i4
If I stand next to them, my head reaches a little higher than past half.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Pls correct me if I'm wrong but according to their manual:
THE AC-PRO is a state of art micro-controller based trip unit for use on 3 phase AC breakers."
ht
Basically it is a relay that offers different coordination setting for the CB without having to open the case.
If the ARC-flash is already in the the CB's instantaneous region, then this relay doesn't help. The quick-trip website shows a total clearing time of 50mS in their brochure. That is misleading, as those Curves aren't dependant on the QUICK-TRIPT product, but on the CB that is in use.
If you know of any Power CBs that have a clearing time less of 50ms or less (rated for 3000A or greater) I would love to know.
I heard ABB makes some and I shall investigate them after lunch. :)
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Best Regards.
Slava
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Do you have the option to split the load between transformers? If you can use 3 1000kVA Xfmrs, you will be much better off.
I'd suggest the ABB REA flash detector ( htt
), but I don't think you'll beat 4 cycles.
Comb your calculations and "sharpen your pencil" as my supervisor says. Look for anything in your data that may be off, or that you can adjust, even with a caveat. Look at scenarios, motor contributions, working distances, etc closely. Although 300 cal sounds about right for 3MVA. Good luck getting that down below 8 cal!
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
I have a very similar situation with approx.160 cal at main switchboard.Curves are very tight.This is a main data processing centre,that can't afford intermittent primary breaker tripouts to satisfy arc flash req'ts.
I am seriously looking at working on this switchboard in dead mode only.The site has lots of back-up diesel generation(not fed through this board) .
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Well like the article Distribution Equipment Modernization to Reduce Arc Flash Hazards that I mentioned in my first post says:
"At the mill's 470V Bus D we calculated energy values of hundreds of calories per square centimeter and flash hazard boundaries that put personnel completely out of the building!"
Comb your calculations and "sharpen your pencil" as my supervisor says. I LOVE THAT!
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Like DPC, I'd go with a relay on the 480 V side of the transformer and trip the primary breaker.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Yes they are still using them all over the plant on the 600V side. That one was just sitting there for god knows how long.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
What is the urgency of this particular system connection point to be <8cal? Is it just for oil sampling while the transformers are energized?
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
It turns out that the Arcflash Reduction Maintenance Switch from Cutler Hammer is what is needed. The odd thing I initially looked at the ARMS and I quickly assumed that the ARMS simply turns the instantaneous setting down to a lower value. Where I then went to the Magnum CB’s TC curves and look at their instantaneous total clearing times. Yup, it was approximately 60 mS.
However directly from the Arcflash Reduction Maintenance Switch brochure:
http:/
Myth: All it does is turn the instantaneous setting down
to a lower value."
FALSE: Unlike other products the Arcflash Reduction
Maintenance System involves a separate circuit that acts
faster than the standard "instantaneous" protection.
• Arcflash Reduction Maintenance System is an analog circuit
which provides faster interruption times versus the
digital "instantaneous" protection (~20ms faster).
• Faster interruptions = less fault current let through = less
arc flash energy.
The total clearing times is approx 40mS as shown here in the TC curves (Document 70C1446.PDF):
http://ww
PERFECT! THAT brang my system down to a Cat 2 :)
Which leads to a question (if anyone can answer it): why isn’t this analog method not incorporated into the standard protection scheme to give a faster instantaneous clearing times during normal operation? I’m guessing there is a compromise ie, lack of multiple settings that is possible with the current digital instantaneous system?
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Does this answer the question?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
No it doesn't. Basically what I'm asking:
Since this analog system improves the instantaneous clearing times of the circuit breakers, what are the draw backs preventing the ARMS technology to be designed for continuous applications?
Yeah that's my "hunch" too, anyone can confirm?
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Not sure why when I posted this way back in the begining of this thread it was dismissed and ruled out, now the C-H version is the saving grace. I smell a C-H salesman in the group.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
For the Incomer I have used a Masterpact NW32 with Micrologic trip unit 5.0A.
The results show that reducing the Short-Time-Time-Delay (STTD) on the Incomer from 0.2 sec to 0.0 sec result in a reduction of Incident Energy at a working distance of 610mm ( 24 inches ) from 63.8 J / cm² or 15.2 Cal/cm² ( PPE Cat 3 ) to 33.2 J / cm² or 7.94 Cal/cm² ( PPE 2 ).
However the big issue is the arc flash energy of 587.9 J / cm² or 140.5 Cal/cm² at the line side of the Incomer ACB. No PPE can protect against the fatal pressure blast produced by this level of incident energy.
An intertrip to the HV side breaker is required.
Arc Flash Detection equipment tripping the incomer and HV breaker would reduce trip times and consequently incident energy even further.
Please see attached files for the Arc Flash study results.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
From the ARMS manual:
The setting changes as shown by mnewman for the Masterpact reduce the arc flash permanently, but may sacrifice coordination. Either way, I agree with his statement about arc flash upstream of the main. The main breaker basically cannot be racked out without first de-energizing the system.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
I've never seen an adequate explanation of how something that can't safely be worked while energized can be made to be un-energized for safe work. Until then the concept of dead work to avoid high arc flash hazard is merely that, a concept that can not be applied in the field.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Well Zogzog I did bring up the C-H ARMS and I'm not a salesmen. I was just posting material to inform the community on the findings from my work.
As well, just to let you know I do enjoy your posts and feedback. I did reply back in posting #6 about your suggestion on the AC Pro. I had a hypothesis on how they work and I did look at AC Pro's Quick Trip Instantaneous Time Current Curve. According to page 29 of the "Instruction Manual: I-AC-PRO-QT"
ht
The trip time is shown as 50mS. However, I'm not clear if that is the total clearing time, or that the delay time for the relay to sense the fault? (Can you elaborate on that?)
While as in comparison with the C-H ARMS, it has a total clearing time of 40mS.
Now I'm saying go with manufacturer over another, it is up to you with what brand you prefer and whether those times mean much to your application. So please don't take it personally. I'm just in it for the discussion to learn.
Maj
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Isnt the same true about the C-H system?
I have installed and tested both systems, and the AC-PRO trip unit and arc flash reduction switch designs are superior. The trip unit has more programable settings, sturdier design, split core CT's, and security settings.
The quick trip has a padlock secured switch, leaving the switch in the "Reduction" mode sacrifices coordination, and 99% of the maintenance electricians do not understand that so having security for that switch is important.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
I like this feature. I am generally not a fan of the maintenance switches, because I think it puts the responsibility on the electrician, who may forget to switch it back when he's done. With the locked cover, it can become part of LOTO, and HOPEFULLY be less likely to be missed upon completion of the work.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
"But if it can't be safely approached while energized, how are the direct measurements to be made to confirm that it has been de-energized."
You suit up to the calculated values. Here at my facility, they have embraced the "40 cal limit" myth, but still allow suiting up to the displayed levels to perform checks to confirm de-energization.
I'd hate to see that 330 cal suit, though!
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
"A circuit breaker equipped with an Arcflash Reduction
Maintenance SystemTM can improve safety by providing a simple and reliable method to reduce fault clearing time. The Arcflash Reduction Maintenance System unit utilizes a separate analog trip circuit that provides faster interruption times than the standard dgital) "instantaneous" protection."
Mind you, the features with the quick trip product are great:
-more programable settings
-sturdier design
-split core CT's
-security settings
-padlock secured switch
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
I've seen this in digital relays as well. If you look at the specifications, there is a definite delay in the response time of the "instantaneous" element.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
One major problem to this system I see is it only appears to be for C-H breakers. AC-PRO can be put on just about anything made after WWII.
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
Have you considered remote operators and remote racking solutions?
Arc resistant switchgear?
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
In this case, perhaps you could isolate all sources and then test for potential and apply grounds on the high side rather than the low. It may be counterintuitive that 12.5 kV is safer to work hot than 600 V, but the arc flash hazard is usually much lower due to lower currents and faster clearing times. Would your work rules allow for the assumption that the state of the secondary follows that of the primary?
RE: Arc Flash Reduction Methods On Secondary of Sub-unit
That's an interesting solution. It is certainly the only thing I've seen that seems to address the issue without taking a head in the sand approach to "dead work".