Arc flash calculation for 208V System fed from 125KVA transformer
Arc flash calculation for 208V System fed from 125KVA transformer
(OP)
I am doing an arc flash study for a 480/208V system. There are some small transformer, 15KVA, 75KVA. But ETAP gave me results that indicates Incident energy levels are above Cat.2, within which I would have to do some adjustment to reduce the available arc flash levels.
But this aslo is confusing me. Becasue as per IEEE 1584, Equipment under 240V need not be considered unless it involves at least one 125KVA or larger low-impedance transformer in its immediate power supply.
The client engineer indicated the above point either, but as an engineer, we have to take any possible injuries into the consideration. Even though it amy be over conservative.
What would you recommend in this situation?
But this aslo is confusing me. Becasue as per IEEE 1584, Equipment under 240V need not be considered unless it involves at least one 125KVA or larger low-impedance transformer in its immediate power supply.
The client engineer indicated the above point either, but as an engineer, we have to take any possible injuries into the consideration. Even though it amy be over conservative.
What would you recommend in this situation?






RE: Arc flash calculation for 208V System fed from 125KVA transformer
NFPA 70E has a task-based table that can be applied to the small 208 V systems.
RE: Arc flash calculation for 208V System fed from 125KVA transformer
Etap will calculate an Arc Flash on a 3 phase bus where ever it can determine the 3phase bolt short circuit regardless of the XFMR's size. However, as there is exceptions to everthing, this is pasted directly from ETAP's help file:
Range of Operation
These calculations follow the methodology described in IEEE 1584-2002. The same limitations of this method apply to the quick incident energy calculator.
1. If any of the following: the Bus Nominal kV, Bolted Fault Current or Fault Clearing Time are set to zero the calculation is not triggered, and there are no displayed results. This applies to either set of parameters (User-Defined and Calculated).
2. If the bus nominal kV is less than 0.208 kV, this message is displayed: "The Bus kV is outside the range allowed by the Empirical IEEE Method" and there are no displayed results. This applies to either set of parameters (User-Defined and Calculated).
3. If the bolted fault current is outside the range of 0.7 kA to 106 kA and the bus nominal kV is between 0.208 and 15 kV the following message is displayed: "The fault current is outside the range allowed by the Empirical IEEE Method" and there are no displayed results. This applies to either set of parameters (User-Defined and Calculated).
Now that being said, in your model, have you included all the upstream overcurrent protections before these XFMRs? What is protecting the XFMR from short circuit?
RE: Arc flash calculation for 208V System fed from 125KVA transformer
RE: Arc flash calculation for 208V System fed from 125KVA transformer
If you link a snap shot of the SLD photo, I'll simulate it myself and see what I'm getting.
Maj
RE: Arc flash calculation for 208V System fed from 125KVA transformer
The breaker size is 150A (Transformer primary side). The transformer is 75KVA(480V/208V).Ignore the cable impedance. Fault clearing time is 6.083 Second. Grounded. The incident energy level by ETAP is 45.401 Cal/cm2,which exceeds the Max PPE rating.
Let's see what you are getting.
Thanks
RE: Arc flash calculation for 208V System fed from 125KVA transformer
Now I have discussed this with some 70E members and some people that have done thousands of arc flash lab tests that discagree with the 1584 rule, so I am not sure what to belive.
If I were you, I would use the 70E tables for the system below the 1584 rule as dpc mentioned.
RE: Arc flash calculation for 208V System fed from 125KVA transformer
RE: Arc flash calculation for 208V System fed from 125KVA transformer
(now I just have to remember)
Maj
RE: Arc flash calculation for 208V System fed from 125KVA transformer
RE: Arc flash calculation for 208V System fed from 125KVA transformer
Try applying the 2 second rule, if you insist on calculating it.
Also "The client engineer indicated the above point"
I would say do what the client says. If you feel like you must go further, then do so with a footnote, and spell out all of your assumptions, and why you felt the need to present additional concerns.
"But for the safety of personnel, it is better to be conservative than sorry."
There are times when the additional PPE will create more of a hazard than necessary. You have to also consider this in you deliberations. If you continually provide overly conservative results, the people in the field will begin to disregard your calculations, and may end up under dressed. I try to find out if any events have happened in the past, and compare those events against what I am finding in the course of my study.
RE: Arc flash calculation for 208V System fed from 125KVA transformer
RE: Arc flash calculation for 208V System fed from 125KVA transformer
I simulated it at work on my copy of Etap, and it seems that the Arcing Current is in the overlap of the CB's Magnetic and Thermal tripping region.
I did do several runs, with various XFMR's impedance. If I use say Z=2.6%, then the CB trips in its instantaneous region. If XFMR settin is at z=5%, then it can be a long trip time. Here are the results:
This is for Z=5%
ht
And this is for Z=2.6%
h
As DPC mentioned, I'd go with what IEEE recommends.
A good read if you haven't seen it yet is this post:
http://www
RE: Arc flash calculation for 208V System fed from 125KVA transformer
How do you calculate the short circuit current with ETAP where you have to select either 1/2 cycle or 5 cycle option. Namely, this should depend on the protection device.
I also appreciated that you read the help lines of the software provider to see who does what.
thanks
RE: Arc flash calculation for 208V System fed from 125KVA transformer
RE: Arc flash calculation for 208V System fed from 125KVA transformer
Most of the transformers 75 kVA or less that I have seen usually have an impedance of around 5-6 % thus with the source impedance your not likely to have a very high bolted fault. However, if you have a low impedance transformers 75 or 45 kVA transformer you will get a enough fault current that you should go with the calculated values.
Bob
RE: Arc flash calculation for 208V System fed from 125KVA transformer
Arcing can continue until the fault clears (think welding). It is not an instantaneous event.
RE: Arc flash calculation for 208V System fed from 125KVA transformer
RE: Arc flash calculation for 208V System fed from 125KVA transformer
Gave you a star! Sharp and quick thinking does it! I could be the fastest man in an event like that!
RE: Arc flash calculation for 208V System fed from 125KVA transformer