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 Post subject: Recommendations for lowering incident energy
PostPosted: Fri Dec 11, 2009 2:05 pm 
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All of the main distribution boards in my model have high incident energies thus having a "Dangerous" arc flash label associated. There is not one main overcurrent protection on any of these boards. Obviously adding a main OCPD would lower my incident energy.

All my OCPD's are fuses down stream. Short of putting in main OCPD's is there anything that I can do to lower the incident energy? I know I am not providing much information, but any suggestions would be greatly appreciated.

Thanks in advance!


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PostPosted: Fri Dec 11, 2009 3:00 pm 
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Put in a breaker at the source with an instantaneous trip and a maintenance switch to enable the instantaneous during maintenance.


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PostPosted: Fri Dec 11, 2009 5:02 pm 
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bruinfan wrote:
All of the main distribution boards in my model have high incident energies thus having a "Dangerous" arc flash label associated. There is not one main overcurrent protection on any of these boards. Obviously adding a main OCPD would lower my incident energy.

All my OCPD's are fuses down stream. Short of putting in main OCPD's is there anything that I can do to lower the incident energy? I know I am not providing much information, but any suggestions would be greatly appreciated.

Thanks in advance!


My suggestion is add a main breaker (Which by the way I can help you with) or provide us with more information.


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PostPosted: Fri Dec 11, 2009 6:28 pm 
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lowering incident energy

Depending on the voltage and use, you can consider high resistance grounding (HRG) if you are not using line to neutral loads. HRG is one of methods recommended in NFPA 70E to reduce arc flash incident energy.
Even with the main breaker, you still have substantial incident energy exposure between transformer secondary and the main breaker. A HRG system connected to the transformer terminals would reduce incident energy since ground fault would be about 5 A instead of thousand of ampere in case of a first fault. HRG can also allow you to alrm on the first fault condition thereby increasing the sytem reliability,
Depending upon the system 85- 90% faults start as phase to ground faults. So in 85- 90% of the cases you will be able to essentially eliminate arc flash concerns,Of course you may have to alarm the HRG system and remove the first fault before second fault happens. Second fault, if first one is not removed, will be 87% of the bolted fault level.


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PostPosted: Sat Dec 12, 2009 5:56 pm 
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ElectricalSafetyman wrote:
Depending on the voltage and use, you can consider high resistance grounding (HRG) if you are not using line to neutral loads. HRG is one of methods recommended in NFPA 70E to reduce arc flash incident energy.
Even with the main breaker, you still have substantial incident energy exposure between transformer secondary and the main breaker. A HRG system connected to the transformer terminals would reduce incident energy since ground fault would be about 5 A instead of thousand of ampere in case of a first fault. HRG can also allow you to alrm on the first fault condition thereby increasing the sytem reliability,
Depending upon the system 85- 90% faults start as phase to ground faults. So in 85- 90% of the cases you will be able to essentially eliminate arc flash concerns,Of course you may have to alarm the HRG system and remove the first fault before second fault happens. Second fault, if first one is not removed, will be 87% of the bolted fault level.


While high resistance grounding does make it less likely you will have an arc flash event, it will raise the energy levels if you do have a three phase fault, which is what the calculations are based on. This will raise the calculated energy levels by ~30%, and so will really make the problem of high energy levels worse!


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PostPosted: Thu Dec 17, 2009 6:24 pm 
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I am having a problem regarding setting instantaneous trip circuit breakers inbetween the source and downstream bus. That downstream bus is a MCC. If I set an instantaneous circuit breaker and more than a couple motors start at the sametime than that breaker will trip everytime.

Any suggestions?


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PostPosted: Thu Dec 17, 2009 9:11 pm 
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bruinfan wrote:
I am having a problem regarding setting instantaneous trip circuit breakers inbetween the source and downstream bus. That downstream bus is a MCC. If I set an instantaneous circuit breaker and more than a couple motors start at the sametime than that breaker will trip everytime.

Any suggestions?


Retorfit your breaker swith a trip unit that has maintenance settings, basically a INST trip you turn on when working on the gear and disable for normal operations. URC's Quick Trip is the best one on the market for most applications.

Typically you can go from a Dangerous level or HRC 4 to a HRC 0-1. My shop does these retorfits everyday for plants nationwide for this specific purpose. PM me and I can send you all the details,


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PostPosted: Fri Dec 18, 2009 7:30 am 
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bruinfan wrote:
I am having a problem regarding setting instantaneous trip circuit breakers inbetween the source and downstream bus. That downstream bus is a MCC. If I set an instantaneous circuit breaker and more than a couple motors start at the sametime than that breaker will trip everytime.

Any suggestions?


I've been able to achieve pretty good results by lowering the short time pickup and delay on breakers where an instantaneous setting is undesirable. You won't get the levels down as low, but can usually work on them dressed out.

Do you have good enough data to plot the starting curves on these motors? If so, be sure and put this into your model.


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PostPosted: Thu Dec 24, 2009 11:29 pm 
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bruinfan wrote:
All of the main distribution boards in my model have high incident energies thus having a "Dangerous" arc flash label associated. There is not one main overcurrent protection on any of these boards. Obviously adding a main OCPD would lower my incident energy.

All my OCPD's are fuses down stream. Short of putting in main OCPD's is there anything that I can do to lower the incident energy? I know I am not providing much information, but any suggestions would be greatly appreciated.

Thanks in advance!


Adding a main ocpd to a panel will not reduce the incident energy calculation. The only type of equipment that is technically barrier-ed (stops fault propagation) is metal-clad switchgear. Try and get a manufacturer to agree to your scheme (on a panelboard) and have them put it in writing.

The HRG is a great idea.


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PostPosted: Thu Dec 24, 2009 11:39 pm 
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bruinfan wrote:
All of the main distribution boards in my model have high incident energies thus having a "Dangerous" arc flash label associated. There is not one main overcurrent protection on any of these boards. Obviously adding a main OCPD would lower my incident energy.

All my OCPD's are fuses down stream. Short of putting in main OCPD's is there anything that I can do to lower the incident energy? I know I am not providing much information, but any suggestions would be greatly appreciated.

Thanks in advance!


Adding a main ocpd to a panel will not reduce the incident energy calculation. The only type of equipment that is technically barrier-ed (stops fault propagation) is metal-clad switchgear. Try and get a manufacturer to agree to your scheme (on a panelboard) and have them put it in writing.

The HRG is a great idea. An easier scheme would be to insert a disconnect on the line side. You would then use Class J non-time-delay (preferable) or Class RK1. Those fuse type will typically get you down to a HRC#0 if you do not exceed 400A.

At higher fault currents you could go as high as 800A and generally drop below an extreme danger. Once you exceed 600A, that fuse would be a Class L. Ferraz has one of best arc flash mitigation Class L fuses in their type A4BQ.


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