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 Post subject: Arc Flash IE calcs?
PostPosted: Sun Nov 09, 2014 6:35 am 

Joined: Sun Nov 09, 2014 6:26 am
Posts: 3
Hello,

While reviewing a past study in our facility, I noticed the IE at our main GE switchboard is in the 350+cal/cm2. Looking further the switchboard fault is cleared by the utility company xfrm primary fuse (SM-20, 150E fuse) instead of the switchboard main. The IE is the result of long fault clear time. Transformer is 2500KVA, 12.47KV, 5.82%. Transformer is 25ft from switchboard, underground conduits.

Is this range of IE is typical for this size transformer.?
if the IE is correct, when we accessing this room to reset breaker or just take reading, what PPE is needed? or even when doing IR study.?

Thank you.


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 Post subject: Re: Arc Flash IE calcs?
PostPosted: Mon Nov 10, 2014 8:36 am 
Sparks Level

Joined: Fri Apr 15, 2011 7:43 am
Posts: 177
Location: Colorado
There really is not enough information to completely determine if it is correct. I have seen very high IE values at the service but it is usually a case of using infinite bus and over sized fuses (best available but probably incorrect info from the utility).

As far as the PPE, technically there is none unless you look at having qualified people and look at the job task (risk assessment)


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 Post subject: Re: Arc Flash IE calcs?
PostPosted: Mon Nov 10, 2014 9:00 am 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 822
Location: Rutland, VT
pdn6420 wrote:
Hello,

While reviewing a past study in our facility, I noticed the IE at our main GE switchboard is in the 350+cal/cm2. Looking further the switchboard fault is cleared by the utility company xfrm primary fuse (SM-20, 150E fuse) instead of the switchboard main. The IE is the result of long fault clear time. Transformer is 2500KVA, 12.47KV, 5.82%. Transformer is 25ft from switchboard, underground conduits.

Is this range of IE is typical for this size transformer.?
if the IE is correct, when we accessing this room to reset breaker or just take reading, what PPE is needed? or even when doing IR study.?

Thank you.


The study should have what the available fault current is from the utility. I did a quick look and assuming 480V secondary, 500mcm Cu (8 per phase) and 200 ft of 1/0 AL riser cable from fuses to txf, and got ~88 cal/cm^2 for an infinitie bus. Utilitzing a utility fault current of 10kA, I got ~105 cal.cm^2.

More information would be helpful but I find it doubtful about 350 cal/cm^2 but I just noticed your statement about a long clearing time. What is it? Maybe if situation allows it you could use a max of 2 sec clearing time.

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


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 Post subject: Re: Arc Flash IE calcs?
PostPosted: Mon Nov 10, 2014 1:48 pm 
Sparks Level

Joined: Tue Jan 19, 2010 2:35 pm
Posts: 137
You've provided most of the information needed for someone to model the system (with certain assumptions). Do you have information on the main breaker? Could you post a photo of the main plus the trip pack and settings? How about the secondary wire sizes, number of wires per phase, the insulation, the wire material and the conduit type? If you can provide these, it's not difficult to model. Also, the arc flash report ought to give the trip time for the main breaker. It's possible it's longer than 2 seconds and that the 2 second rule could apply and be used.


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 Post subject: Re: Arc Flash IE calcs?
PostPosted: Thu Nov 13, 2014 8:02 am 

Joined: Sun Nov 09, 2014 6:26 am
Posts: 3
Hi,

Thank you all for your feedback. Additional Infos.

2500KVA, 12.47KV Prim/480VAC, 5.8%Z, X/R 10.67 secondary xfmr.
Primary fuse is S&C, SM-20, 150E slow. Max fault 3 phase current is 6.065KA on high side at 9.67X/R, single phase 6.5KA high side
Secondary feeder is 13 conductors per phase, 350MCM ran in PVC buried, 25ft distance.
GE Spectra panelboard distribution with 4000A main CB, powerbreak II, intelliguard trip LSIG function. Largest loads on this side totaling around 1600HP (VFDs non regen, chillers). Instaneous setting is set at max.
Panelboard is also parrallel fed with same setup with a tie-breaker in between (has never been closed). I am not sure why.
Maximum available fault at sec terminals is around 52KA.
Study assumed that the main is not isolated from the panelboard (per IEEE1584) and the tripped device would be the fuse which take almost 7+ sec to clear. Even with a 2 sec rule, the energy at secondary terminal still be in the 90s+ ( guessing from the fuse curve).


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 Post subject: Re: Arc Flash IE calcs?
PostPosted: Sat Nov 15, 2014 4:03 pm 
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Joined: Tue Jan 19, 2010 2:35 pm
Posts: 137
Hi,
I assume the wiring is 350 MCM Copper? I used your figures for the primary fault current and get 39,000 amps at the transformer secondary terminals. From there it drops to 38,653 amps at the main breaker line side. However, if I bump the max primary current up a lot, I get 52 K amps on the secondary of the transformer. Also, I assumed the transformer was oil filled naturally cooled and with a rise of 55C? It's not forced air cooled?
I stopped the trip at 2 seconds and got 132 calories @ 18" with a 437 inch arc flash boundary. The actual trip was more than 47 seconds.
I'm not sure I have the same breaker as you have specified. I selected a GE ICCB Power Break II SH-40 (that's a 4000 amp frame with a 4000 amp sensor).
I set the LT pick up at 1 or 4000 amps and LT delay at 4, the maximum for this trip unit.
I set the ST pick up 9, or 36000 amps and Delay at Max.
Again, those are the choices for that trip pack and this breaker. If you can send me your email, I'll send you PDF's of everything.
Hope this helps some.


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