It is currently Tue Dec 07, 2021 4:55 pm



Post new topic Reply to topic Go to page 1, 2  Next
Author Message
ekstra   ara
 Post subject: Transformer Secondary Work and PPE
PostPosted: Mon Aug 25, 2008 6:15 am 
User avatar

Joined: Mon Jul 21, 2008 5:00 pm
Posts: 14
When working on the secondary of a padmount transformer, the primary device usually takes a long time to clear and the incident energy on the secondary is often quite high because of it.

NESC Exception 2: states we can use clothing with a minimum rating of 4 cal/cm2. Is anyone going this low or are you calculating with IEEE 1584 or some other method and using a higher PPE? I know electrically safe is better but it is not always possible.
Thanks!


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Aug 25, 2008 7:13 am 
Plasma Level

Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
M. Ambrose wrote:
When working on the secondary of a padmount transformer, the primary device usually takes a long time to clear and the incident energy on the secondary is often quite high because of it.

NESC Exception 2: states we can use clothing with a minimum rating of 4 cal/cm2. Is anyone going this low or are you calculating with IEEE 1584 or some other method and using a higher PPE? I know electrically safe is better but it is not always possible.
Thanks!


You PPE needs to have an arc rating equal or greater than your calulated Ei.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Aug 25, 2008 7:30 am 
Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
M. Ambrose,

You are correct with the 4 cal exception in the NESC, however I still do not understand why it was included for exactly the reasons you stated. Clearing times will be higher as will the fault duties. That having been said you do have 2 options, use the 4 cal clothing or do the calcs. Once you do the calcs, and I think you should, your protection must be selected based on the IE results.

Alan


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Aug 26, 2008 12:10 pm 
Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
What are the other utilities doing?

As a thought, I am posting this to see what others in the utility business are doing with the pad mounted equipment. Doesn't seem to make sense to try to label all of the pad/ground mounted equipment just because of the number installed on a system, aside from the fact that it could be switched to a different circuit at any time. We all know that a distribution system is dynamic with respect to which circuit or substation a line may be fed from at any given time.

Presently we are planning to try to "bracket" the possible scenarios by trying to determine the best to worst cases on a system wide basis by assuming any piece of equipment could be installed anywhere on the system.

It appears that we are ok with 8 cal. PPE for most of the high voltage portions of the systems and are hoping to add FR coveralls for the low voltage portions.

Anyone using face shields or ear protection for high voltage work?
For lineworkers with ground mounted equipment?
Meter technicians, especially with self contained metering installations?

Any additional input/discussion would be appreciated.

Thanks,
Alan


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Dec 11, 2008 2:21 pm 

Joined: Tue Sep 23, 2008 7:39 am
Posts: 4
Arc Shields for Meter Techs

We too are discussing whether to require anyone replacing electric meters to wear an arc shield. Our calculations support the need for face protection, particularly when aligning a meter to the meter socket. However, this would not be a trivial requirement, particularly considering the array of people doing this work (meter techs, lineman, OSMs, gas crews etc.).

We'd be interested to know how other utilities are dealing with this.

Rob
[email protected]


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Dec 11, 2008 2:37 pm 
Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Secondary

The APPA webinar answered some questions today and raised more.....imagine that! Turns out that face shields will still not be a requirement in the NESC 2012 proposal. There are revised tables and more PPE requirements though. We are working hard to finish up our final recommendations, and the low voltage stuff is still the problem. Will post more as we figure out what the plan will be.

PG&E has posted some testing videos on youtube if you have not already seen them...you can search for pg&e arc flash and get them.

Alan


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Dec 15, 2008 8:40 am 

Joined: Tue Mar 25, 2008 1:00 pm
Posts: 32
What are you using for your clearing times when calculating the IE on these pad-mount transformers? I've run into some really ridiculous IE levels if I don't use some sort of cut-off.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Dec 15, 2008 9:20 am 
Plasma Level
User avatar

Joined: Wed May 07, 2008 5:00 pm
Posts: 862
Location: Rutland, VT
IMHO, I would use the 2 second cutoff time from IEEE 1584 based on that pad mount txf's are usually located in areas with enough clearance so that some one would not get trapped but be able to move or be blown clear of the fault.

_________________
Barry Donovan, P.E.
www.workplacesafetysolutions.com


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Dec 16, 2008 2:23 pm 

Joined: Thu Sep 13, 2007 9:28 am
Posts: 12
Location: Kenai, Alaska
Utility LV Concerns

The initial post has somewhat evolved into two different issues:

A. Working of the secondary side of the XFMR - ex. a meter audit of a nearby location where you have to wrap flexible CTs around the secondary bushings.

B. Pulling/inserting meters at a meter base.

(A.) The XFMR cabinet is a bigger "box", and likely has more spacing between the lugs. It also does not have the Z of the secondary wire.

(B.) The meter base is a smaller box, less lug spacing, but you benefit by having some additional Z being at the end of the secondary wire.

Now I wish I had answers .... it seems like every questions spawns AT LEAST one more question.

At my utility we are going to draw a line on our XFMR chart .... any PM XFMR rated higher than XXX kVA will be worked dead. XXX is to be determined by everyone based on their own system.

We are still debating the meter base issue. Some of the vid's are pretty scary. As a rural coop we run the gambit on secondary lengths .... meter box mounted on a high kVA XFMR cabinet to 100's of feet from a small OH can. Right now the best answer I have is 8 cal clothing, balaclava w/ arc-rated shield or goggles, gloves and meter puller. I think you have to do everything you can to reasonably create space b/t the meter base and the worker. Please note that I said "reasonable", and its got to be reasonable enough to the worker that they will actually use it.

If someone has any better solutions I would love to hear them.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Dec 16, 2008 5:28 pm 
Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Transformers and Meter Bases

I will try to address each one in order:
Working on the secondary of a "padmount": (Not pole mounted banks)
We are recommending 8 cal and optional face shields for 240 volts and below.
Above that we are recommending 20 cal and face shields with a note that they are not required by the NESC....just recommended. So actually they will still be optional.

Pulling/inserting meters at a meter base: (limited to self contained 3 phase type)
Again same as the transformer recommendation...don't see a difference between the locations, especially with the position of the face when installing a meter. And yes the videos are scary, but how often do we have a bolted 3 phase fault in a meter can....also it is the testing that is being heavily relied upon for the 2012 NESC.

We call for a 4 cal minimum on transformer rated meters and that is probably big overkill.

A. You are obviously correct that the transformer has a bigger box and no secondary wire. To raise another issue, the secondary spacings in most transformers are larger than the valid range in the IEEE 1584 calcs. There are those that will say just use the max in the tables. Well then I guess it just does not matter, but wait... they did go to the trouble to give us a range, so are we are left with but another assumption.

B. Also dead on with the remote meter can. I would submit that given the size of most of the services that there will be negligible Z in the wire with the assumptions already being made. The service would need to be so long that voltage drop would probably be a limiting issue for the wire size if the Z were much help, or hurt depending on change in clearing times.

On "typical" short muni systems with large primary conductor, we are seeing the issues start with 480 volts at and above 500 to 750 kVA pads.

We debated a 500 kVA limit but decided to make the PPE across the board for 480 volts. Simplifies administration and training.

Haven't recommended a size cutoff for hot work yet, but I do believe it will come.

I am becoming increasingly more concerned with the MVA available as it pertains to the initial blast energy and debris than the time allowed for the incident.

If you saw one of my earlier posts, I indicated that I was more concerned with the size of the charge than the time of the ride. Appears also that with the latest round of PG&E low voltage testing, 80 to 90 cycles was all they could manage to sustain the arc.

We all have to make our own decisions based on our systems, but pooling our resources and info here is sure a great help. Right, Wrong, or somewhere between, this is what we are recommending. As you are probably aware, you can do more or even less and still comply with the 2007 NESC. I feel that, if we can get it in place, it will be much more than is there now. Appears that the 2012 code will be more strict than the current version, so this will at least head them in that direction.

And you are exactly on point, every time I manage to answer one question, it seems to create at least one or more additional issues.

Hope it helps and look forward to your comments,
Alan


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Dec 17, 2008 7:09 am 
Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
Quote:
Working on the secondary of a "padmount": (Not pole mounted banks)
We are recommending 8 cal and optional face shields for 240 volts and below.
Above that we are recommending 20 cal and face shields with a note that they are not required by the NESC....just recommended. So actually they will still be optional.

Do you have similar requirements for pole mounted banks? On the one hand, these may have lower energy levels because you don't have an "arc in a box". On the other hand, you may want to have a higher time limit because working on a pole or in a bucket truck may make a 2 second limit inadequate.

Where in the NESC does in state that face shields are recommended but not required? Unlike NFPA 70E, the NESC does not go into detail on clothing systems. 410.A.3 basically states that "the empoloyer shall require employees to wear clothing or a clothing system that has an effective arc rating not less than the anticipated level of arc energy." NFPA 70E requires face shields or flash suit hoods for 4 cal/cm² and above. Why wouldn't a "clothing system that has an effective arc rating not less than the anticipated level of arc energy" include a face shield for 4 cal/cm² for utility work?
Quote:
We call for a 4 cal minimum on transformer rated meters and that is probably big overkill.

Transformer rated meters still have a voltage connection to the main circuit. Unless there is some fusing on the voltage lead, or if the impedance is enough to restrict the fault current (without increasing clearing times), I think the arc energy would be just as high for transformer rated meters.
Quote:
On "typical" short muni systems with large primary conductor, we are seeing the issues start with 480 volts at and above 500 to 750 kVA pads.

My calculations (based on IEEE-1584) show very high levels for smaller transformers. Considering a 2 second limit, I get incident energies of 25 cal/cm² for 150 kVA padmount, 40 cal/cm² for 300 kVA padmounts, and over 60 cal/cm² for 500 kVA padmounts at 277/480V. I get even higher energy levels at 120/208V, but I really question whether the arc can be sustained at 120/208V, at least for 2 seconds.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Dec 17, 2008 7:42 am 

Joined: Mon Mar 31, 2008 6:23 am
Posts: 2
Location: Charlotte, NC
jghrist wrote:
My calculations (based on IEEE-1584) show very high levels for smaller transformers. Considering a 2 second limit, I get incident energies of 25 cal/cm² for 150 kVA padmount, 40 cal/cm² for 300 kVA padmounts, and over 60 cal/cm² for 500 kVA padmounts at 277/480V. I get even higher energy levels at 120/208V, but I really question whether the arc can be sustained at 120/208V, at least for 2 seconds.

Can you describe what parameters you are using for your calculations? What is the arc gap, specifically? Bolted fault currents? Clearing times?

For "arc in a box" calculations the IEEE 1584 calcs assume a 32mm arc gap. However, ANSI standards show that the gap for padmounted transformers between bushings is much higher. Depending on the size of the transformer they can range from 8-10 inches at a minimum (or 203-254 mm). Manually accounting for this "discrepancy" will drastically effect your calculations. IEEE 1584 is only good for arc gaps up to 152mm, and even at these values the Incident Energy is much lower.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Dec 17, 2008 8:32 am 
Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Secondary

jghrist wrote:
Do you have similar requirements for pole mounted banks? On the one hand, these may have lower energy levels because you don't have an "arc in a box". On the other hand, you may want to have a higher time limit because working on a pole or in a bucket truck may make a 2 second limit inadequate.

Where in the NESC does in state that face shields are recommended but not required? Unlike NFPA 70E, the NESC does not go into detail on clothing systems. 410.A.3 basically states that "the empoloyer shall require employees to wear clothing or a clothing system that has an effective arc rating not less than the anticipated level of arc energy." NFPA 70E requires face shields or flash suit hoods for 4 cal/cm?? and above. Why wouldn't a "clothing system that has an effective arc rating not less than the anticipated level of arc energy" include a face shield for 4 cal/cm?? for utility work?

Transformer rated meters still have a voltage connection to the main circuit. Unless there is some fusing on the voltage lead, or if the impedance is enough to restrict the fault current (without increasing clearing times), I think the arc energy would be just as high for transformer rated meters.

My calculations (based on IEEE-1584) show very high levels for smaller transformers. Considering a 2 second limit, I get incident energies of 25 cal/cm?? for 150 kVA padmount, 40 cal/cm?? for 300 kVA padmounts, and over 60 cal/cm?? for 500 kVA padmounts at 277/480V. I get even higher energy levels at 120/208V, but I really question whether the arc can be sustained at 120/208V, at least for 2 seconds.


I still have problems trying to apply the equations to pole mounted banks because:
The IEEE model is for 3 phase fault. Phase spacings are usually so large on these that I don't see the fault escalating to 3 phase. Granted a pole mounted 3 phase transformer will be different, but it is in open air as you say.
Recent tests by PG&E demonstrated that 85 cycles was the max they could sustain the arc at 51 kA and 480 volts.

With regard to the face shield, the NESC does not mention them or require them....this is straight from the committee, not me. Why they don't is by me! What I was saying in my post was that we are recommending face shields in some cases, but they are not required by the NESC (and not proposed in the 2012 edition), hence since I ultimately don't make their rules they would still be optional.

When I say "transformer rated meters" I am speaking about 120 volt, 5 amp. This implies that with 277/480 there will be 2.4:1 PT's which are usually 750 to 1000 va in size, hence not much energy on the secondary at 120 volts, very high Z transformers. This is as compared to self contained units rated 600 volts where the utility transformer would be tapped on the secondary and brought directly to the meter can.

With the 208 volt stuff, one of the proposed notes to the 2012 code says that testing has demonstrated that the arc will not sustain for more than .5 cycles. Seems that they are saying it cannot restrike after the first zero crossing.

With respect to the energy levels and tx. sizes, I will be interested to see what you and Mr. Dawson determine.

Thanks for the input,
Alan


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Dec 17, 2008 11:03 am 
Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
madawson wrote:
Can you describe what parameters you are using for your calculations? What is the arc gap, specifically? Bolted fault currents? Clearing times?

For "arc in a box" calculations the IEEE 1584 calcs assume a 32mm arc gap. However, ANSI standards show that the gap for padmounted transformers between bushings is much higher. Depending on the size of the transformer they can range from 8-10 inches at a minimum (or 203-254 mm). Manually accounting for this "discrepancy" will drastically effect your calculations. IEEE 1584 is only good for arc gaps up to 152mm, and even at these values the Incident Energy is much lower.

I developed a series of graphs of incident energy vs available primary fault current. One graph for each transformer size with separate curves for different transformer impedances.
[ATTACH]18[/ATTACH]
I used a 32 mm gap, from IEEE 1584 Table 4 for switchgear. The bushing spacing in a padmount is larger than this, and larger gaps give larger incident energy, so 32 mm is unconservative. I justify this by:

This is a gap that is selected by the IEEE 1584 calculator.
I use arc durations that probably are conservative based on unpublished PG&E tests.
3Ø faults probably won't occur between bushings. They might occur somewhere on the secondary or service where connections are made and conductor spacings can be much smaller.

I used a working distance of 15 inches and a maximum exposure time of 2 sec. For the example shown on the attached curve, with 3kA available primary fault current and 2.5% transformer impedance, I use the following:

Equipment class 2
Transformer X/R = 2.14 (regression of typical values - not real critical)
12.5 kV system impedance at 300 kVA base = (0.1836 + j0.4238)%
Bolted secondary fault = 12,184A
Arcing current = 7,423A in secondary, 285A in primary
Primary fuse - Cooper bayonet dual current C10
Clearing time for arcing current - 0.843 sec
Clearing time for 85% arcing current - 1.1 sec
Incident energy at full arcing current - 25.7 cal/cm²
Incident energy at 85% arcing current - 28.3 cal/cm²

With a 4 inch gap, arcing current is 4,758A, arc duration is 1.85 sec, and incident energy is 41.5 cal/cm²

So much is unknown, and the available tools are inadequate, so you have to pick something and go with it. Although I ended up with a large number of graphs that cover a lot of situations, I realize that our clients will not be able to apply them accurately and that overall guidelines are needed. Our clients do not know what fuses are in the transformers for sure or even what the impedances are. To get this data, which is critical for an accurate calculation of arc energy, would require opening the transformer compartments and exposing linemen to arc hazards.

I used my approach to get an overall idea how much arc energy we could be dealing with. A lot.

What is the solution? How do you de-energize a pad-mounted transformer? Usually by opening switches or removing elbows in the primary compartment. You have to open the secondary compartment to get to the primary compartment. Anyone have a penta bolt wrench attachment for a hotstick?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Dec 17, 2008 1:40 pm 

Joined: Mon Mar 31, 2008 6:23 am
Posts: 2
Location: Charlotte, NC
jghrist wrote:
I developed a series of graphs of incident energy vs available primary fault current. One graph for each transformer size with separate curves for different transformer impedances.
[ATTACH]18[/ATTACH]
I used a 32 mm gap, from IEEE 1584 Table 4 for switchgear. The bushing spacing in a padmount is larger than this, and larger gaps give larger incident energy, so 32 mm is unconservative. I justify this by:

This is a gap that is selected by the IEEE 1584 calculator.
I use arc durations that probably are conservative based on unpublished PG&E tests.
3Ø faults probably won't occur between bushings. They might occur somewhere on the secondary or service where connections are made and conductor spacings can be much smaller.


Can you elaborate on this?

Quote:
I used a working distance of 15 inches and a maximum exposure time of 2 sec.

Why not 18 inches? In the IEEE guide 455mm seems to be the minimum working distance for just about everything. This may seem trivial but on the example below your IE would drop to 19.8 cal/cm² at 455mm.

Quote:
For the example shown on the attached curve, with 3kA available primary fault current and 2.5% transformer impedance, I use the following:

Equipment class 2
Transformer X/R = 2.14 (regression of typical values - not real critical)
12.5 kV system impedance at 300 kVA base = (0.1836 + j0.4238)%
Bolted secondary fault = 12,184A
Arcing current = 7,423A in secondary, 285A in primary
Primary fuse - Cooper bayonet dual current C10
Clearing time for arcing current - 0.843 sec
Clearing time for 85% arcing current - 1.1 sec
Incident energy at full arcing current - 25.7 cal/cm²
Incident energy at 85% arcing current - 28.3 cal/cm²



Why are you choosing equipment class 2 and not 3?
On the primary fuse have you evaluated if the fuse on the tap on the primary side is faster or slower than the bayonet fuse? What type of fuse is on the tap?

Quote:
With a 4 inch gap, arcing current is 4,758A, arc duration is 1.85 sec, and incident energy is 41.5 cal/cm²


Are you manually changing the arc gap or selecting a different equipment class to acieve the larger gap?
Also, the proposed NESC tables for 2012 have concluded that the maximum time that an air can be sustained is 85 cycles, 1.42 sec so that should help us some.
Quote:
So much is unknown, and the available tools are inadequate, so you have to pick something and go with it.


I agree. It is very frustrating!

Quote:
Although I ended up with a large number of graphs that cover a lot of situations, I realize that our clients will not be able to apply them accurately and that overall guidelines are needed. Our clients do not know what fuses are in the transformers for sure or even what the impedances are. To get this data, which is critical for an accurate calculation of arc energy, would require opening the transformer compartments and exposing linemen to arc hazards.


This is why I am using the typical fuse sizes we recommend for the transformer rather than the bayonet fuses. We too, are simply giving our clients guidelines based on voltage alone so that it is easier to implement. Down the road we may change that, but for now, we feel it is best.

Quote:
I used my approach to get an overall idea how much arc energy we could be dealing with. A lot.

The question is though, is that based on our present knowledge and tools is that accurate? According to the proposed tables for the 2012 NESC they recommend using a 20 cal clothing system for 251-500V self contained metering, pad mounted transformers, panels & cabinets regardless of size! Adn this was based on testing eprformed this summer, not from calculations.

Quote:
What is the solution? How do you de-energize a pad-mounted transformer? Usually by opening switches or removing elbows in the primary compartment. You have to open the secondary compartment to get to the primary compartment. Anyone have a penta bolt wrench attachment for a hotstick?


I think I can make some money if I design that tool. What do you think? :D


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Dec 17, 2008 3:53 pm 
Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Fault duty

jgrist,

I am looking at the data and you have a 300 kVA 480v tx with 2.5% z. The infinite source fault would be approx. 14,400 amps. You list the secondary bolted fault at about 12+k with a 3k primary fault duty at 12.5 kV. Seems to me that is a very "weak" system or area of the system and I would expect the z at 480 to be higher and less bolted fault current at 480.

Haven't run all of the numbers and backed through the tx to get the primary z, so please tell me what am I missing here?

Will also be interested in what you and Mr. Dawson find out! Want to make sure that we are using the 1584 calculator correctly.

Thanks for all of the info.....are we having fun yet?
Alan


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Dec 17, 2008 4:06 pm 
Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
Quote:
Can you elaborate on this?

The secondary cables are run with no spacing between phases. They are connected at least at the service, and maybe at intermediate points to splices or terminations with no particular spacing.
Quote:
Why not 18 inches?

To match the spacing used in the NESC tables for 1-15 kV. I figure if utility linemen are expected to work MV at 15", then they will not be further away for LV.
Quote:
Why are you choosing equipment class 2 and not 3?
On the primary fuse have you evaluated if the fuse on the tap on the primary side is faster or slower than the bayonet fuse? What type of fuse is on the tap?

I should have said class 3. I have looked at tap fuses as well as bayonet fuses. Sometimes the tap fuses are faster, sometimes slower. For instance, typically, a 300 kVA transformer would have a 20KS tap fuse. A 20KS fuse would clear in 0.91 sec with 285A, with an incident energy of 27.7 cal/cm².

Sometimes bayonet fuses are used without tap fuses and sometimes tap fuses are used without bayonet fuses.
Quote:
Are you manually changing the arc gap or selecting a different equipment class to acieve the larger gap?

I am using the IEEE-1584 equations in a Mathcad worksheet where I changed the gap independently of anything else.

I haven't yet decided how to make recommendations for transformer secondaries. We can't be unrealistically conservative or the clients will ignore the recommendations. On the other hand, we are reluctant to use unpublished test results to cut arc durations. We need to use industry accepted methods to avoid liability problems, but that's difficult when as far as we see, the industry has not truly accepted any methods. ;)


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Dec 17, 2008 9:26 pm 
Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
acobb wrote:
jgrist,

I am looking at the data and you have a 300 kVA 480v tx with 2.5% z. The infinite source fault would be approx. 14,400 amps. You list the secondary bolted fault at about 12+k with a 3k primary fault duty at 12.5 kV. Seems to me that is a very "weak" system or area of the system and I would expect the z at 480 to be higher and less bolted fault current at 480.

Haven't run all of the numbers and backed through the tx to get the primary z, so please tell me what am I missing here?

Will also be interested in what you and Mr. Dawson find out! Want to make sure that we are using the 1584 calculator correctly.

Thanks for all of the info.....are we having fun yet?
Alan

Base current at 300 kVA, 12.5 kV = Ib = 300/sqrt(3)/12.5 = 13.86A
12.5 kV system impedance = Zs = Ib/3000 = 0.00462 with an assumed angle of 66.6° = 0.00184 + j0.00424
Transformer impedance = Zt = 0.0106 + j0.0227
|Zs + Zt| = 0.0296
Ib/(Zt + Zs) = 468A at 12.5 kV = 12200A at 480V

Unless the fuse clearing time exceeds the 2 second cutoff, increasing the transformer impedance increases arc duration and increases incident energy. With Zt = 3%, incident energy is 31.5 cal/cm².

Of course, we're having fun - it's just that January is coming up all too soon. :eek:


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Dec 18, 2008 7:19 am 
Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
The numbers

Looks like it oughta work...and yes it is coming too fast, reckon we can get another extension?

Thanks,
Alan


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Jan 14, 2009 8:03 pm 

Joined: Thu Aug 14, 2008 5:00 pm
Posts: 10
Location: Wisconsin
acobb, you stated " We call for a 4 cal minimum on transformer rated meters and that is probably big overkill." I assume this installation uses aux pt's or a pt pack and the potentials are not direct-connected to the transformer secondaries? Please clarify?


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 21 posts ]  Go to page 1, 2  Next

All times are UTC - 7 hours


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
© 2019 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883