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 Post subject: Arc Flash Scenarios
PostPosted: Mon Apr 04, 2016 2:22 pm 

Joined: Mon Jul 06, 2015 8:42 am
Posts: 2
Location: MN
Hello All:

I am working with a Government Client to perform an Arc Flash Hazard Risk Evaluation. For this project they provided a design guide with a few requirements they have for this study. I was curious if anyone has recognized similar requirement and perhaps a suggestion on where/how they came up these. I would like to review these references and determine if these are items I should be considering for every evaulation project.


#1) Run a seperate scenario for each utility primary side contribution:
a) Max of 10,000A
b) Min of 3,000A


Does IEEE or other publication suggest to do something like this?

#2) When under generator power assume that N-1 motors are in service.

Any idea why N-1 and not all motors. (I am also assuming they are referring N-1 motors of the same function and HP)

#3) The following assumption shall be made for plotting raw sewage pumps on TCC and sizing motor circuit prtective devices (for a lift station):
a) FVNR Istart=FLA x NEMA Code Letter Mulitplier
b) VFD Istart=2.5xFLA
c) SSRV Istart=3.5xFLA


Is there a standard publication that gives these suggestions?

#4) Run utility and generator scenarios with each of the following sewage pumps subtransient reactance (Xd")
a)Xd"=0.167 (NEMA Code "G" - Xd"=FLA/LRA=0.167)
b) Xd"=0.167x3=0.5

Again, is there a standard publication which suggests doing so with these motor values?


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 Post subject: Re: Arc Flash Scenarios
PostPosted: Wed Apr 06, 2016 10:42 pm 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
AaronM wrote:
Hello All:

I am working with a Government Client to perform an Arc Flash Hazard Risk Evaluation. For this project they provided a design guide with a few requirements they have for this study. I was curious if anyone has recognized similar requirement and perhaps a suggestion on where/how they came up these. I would like to review these references and determine if these are items I should be considering for every evaulation project.


#1) Run a seperate scenario for each utility primary side contribution:
a) Max of 10,000A
b) Min of 3,000A


Does IEEE or other publication suggest to do something like this?

#2) When under generator power assume that N-1 motors are in service.

Any idea why N-1 and not all motors. (I am also assuming they are referring N-1 motors of the same function and HP)

#3) The following assumption shall be made for plotting raw sewage pumps on TCC and sizing motor circuit prtective devices (for a lift station):
a) FVNR Istart=FLA x NEMA Code Letter Mulitplier
b) VFD Istart=2.5xFLA
c) SSRV Istart=3.5xFLA


Is there a standard publication that gives these suggestions?

#4) Run utility and generator scenarios with each of the following sewage pumps subtransient reactance (Xd")
a)Xd"=0.167 (NEMA Code "G" - Xd"=FLA/LRA=0.167)
b) Xd"=0.167x3=0.5

Again, is there a standard publication which suggests doing so with these motor values?


The utility max/min values probably came from the utility. With double ended setups usually you check combinations of one or both feeders because low current results in higher incident energy. I haven't seen the guide for arc flash calculations (IEEE 1584.1) but this is probably in there since only arc flash is concerned with low fault currents.

There is a slight variation with motors in terms of locked rotor amps. Smaller ones are higher and they tend towards code G (6xFLA) at higher ratings as per NEMA MG-1 designs.

The transient/subtransient values are typical for ANSI short circuit calculations.

Only surprise here is the N-1 notation which suggests they have one online spare. Also handling of small motors (<150 HP) is that you sum them in two groups which wasn't mentioned.


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