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ekstra   ara
 Post subject: Easy Power Transformer Arc Flash
PostPosted: Mon Jul 15, 2013 3:07 pm 

Joined: Mon Jul 15, 2013 9:47 am
Posts: 3
Hi, I am using Easy Power to model. In the facilities I am modeling, there are several transformers below 125 kVA at 208 V. I understand that IEEE 1584 well most likely change the standard (I heard drop down to 45 kVA), but currently we are using the 125 kVA standard.

Hence anything at the 208 VAC level is not being analyzed, the transformers are though. When I model the transformers, I have a bus before and after. The bus on the primary side is a hazard #0, but on the secondary side is hazard #2 or #3. For the circuit breakers you can say it will jump to the line side. Can you make that as an assumption for transformers? That a fault would jump to the line side, which in this case would decrease the hazard?

Any information would be appreciated!


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PostPosted: Tue Jul 16, 2013 5:17 am 
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tdra0804 wrote:
For the circuit breakers you can say it will jump to the line side. Can you make that as an assumption for transformers? That a fault would jump to the line side, which in this case would decrease the hazard?

The key is "it could", not "it will". So the assumed hazard is the worse of the two possibilities (stay on the load side, or jump to the line side), because you cannot say for sure it will stay on the less hazardous side. In the case of a circuit breaker, usually this means the hazard is greater on the line side. For a transformer, usually this means the hazard is greater on the secondary side (because the fault current will be much smaller as seen by the primary OCPD).


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PostPosted: Tue Jul 16, 2013 8:57 am 
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Location: Wisconsin
It depends on where you are defining your secondary location.

In most dry type transformer enclosures, it is is extremely likely that a fault on the secondary terminals will involve the primary terminals also. The industry standard construction of <600V primary transformers 150kVA and smaller is to intermingle the primary and secondary terminals.


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PostPosted: Wed Jul 17, 2013 9:37 am 

Joined: Mon Jul 15, 2013 9:47 am
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JBD wrote:
It depends on where you are defining your secondary location.

In most dry type transformer enclosures, it is is extremely likely that a fault on the secondary terminals will involve the primary terminals also. The industry standard construction of <600V primary transformers 150kVA and smaller is to intermingle the primary and secondary terminals.


For the transformers I am considering, they are all under 150kVA at 480V. Would it be better to be conservative and still use the higher hazard on the secondary side? Or for these lower transformers can the assumption be made it will jump?


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PostPosted: Wed Jul 17, 2013 10:15 am 
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Location: Wisconsin
Our standard is to not report the incident energy at the terminals of 'small' transformers.


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PostPosted: Tue Jul 23, 2013 7:36 am 
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JBD wrote:
Our standard is to not report the incident energy at the terminals of 'small' transformers.


Small is definitely a relative term. How small is small in this case?


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PostPosted: Tue Jul 23, 2013 10:50 am 
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Larry Stutts wrote:
Small is definitely a relative term. How small is small in this case?

The OP was specifically about transformers smaller than 150kVA.


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PostPosted: Fri Sep 06, 2013 6:37 pm 
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Location: Cincinnati, OH
JBD wrote:
Our standard is to not report the incident energy at the terminals of 'small' transformers.


Our standard is to include this incident energy in the report along with all of the other buses modeled in the software, but we typically do not print labels for any transformers. Some customers will request labels on the doors of transformer air terminal chambers, which are typically found on unit substation class transformers 1000 kVA or larger. I have not been asked for labels on pad mounted transformers or lighting style dry transformers, and if asked I would spend a considerable amount of time trying to convince folks not to open those enclosures while the units are energized.


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