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 Post subject: arc flash survey proposal
PostPosted: Fri May 08, 2009 1:39 pm 
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I have been given the task of having an arc flash survey on our facility. Here is what has been given to us in a proposal --- does this sound sufficient or are their things missing? I really appreciate anyones help in this matter.

Short Circuit Current Analysis
The short circuit current analysis begins at the point where the local utility provides power and includes all alternative electrical power sources. It will continue down through the low voltage
terminals of the utility vault transformers and will continue downstream through the main switchgear and switchboards/panelboards/MCCs further downstream. It will also include pointing out where the available short-circuit current exceeds any overcurrent protective device’s interrupting rating; this can be a serious safety hazard.

Overcurrent Protective Devices Time Current Characteristics
In order to do the overall arc flash hazard study, each overcurrent protective device’s time current characteristic must be known: this includes relays, fuses and circuit breakers. The overcurrent protective device time current characteristic must be known from the main service entrance overcurrent protective device(s) at each service entrance equipment location and proceeding downstream to include all of the switchgear, switchboard, and panelboards that are 100 amps or greater.

Arc Flash Hazard Analysis
The Arc Flash Hazard analysis will be determined using the Short Circuit Current Analysis and Overcurrent Protective Device Time Current Characteristic results gathered in the previous steps. at each calculated point in the electrical system, the approach (shock) boundaries, arc flash incident energy levels, arc flash boundaries, hazard risk categories and level of PPE will be determined.

Excluded Electrical Equipment from Arc Flash Study: Examples of excluded electrical equipment: fused or non-fused switches smaller than 100A, combination motor starters smaller than 100A, junction boxes, tap boxes, motors less than 75HP, transformers with secondary voltage less than 440V or smaller and less than 125 kVA, lighting panels fed by transformers of less than 125 kVA, etc


Thank you again for your help in this matter.


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PostPosted: Fri May 08, 2009 4:42 pm 
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Looks pretty standard, do you have different modes of operation for your system?? If you do all modes need to be evaluated.

One thhing that concerns me is determining the HRC, you dont use HRC's when calculating the Ei, HRC;s are used when the tables are used.

I dont see anything there about providing labels.

I dont see anything about mitigation, the most important part IMO.


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PostPosted: Mon May 11, 2009 6:55 am 
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Thank you for your reply.
They do have in the proposal to model all modes of operation. They will also label according to the Incident Energy. And a section about mitigation. I just didn't want to add everything and overwhelm with my first question ever.
One of the things that caught my eye was in the Excluded Electrical Equipment - because in 70E - 130.3, i don't see where these things can be excluded.
Is there a significant difference in things under 100A or is this just what most people exclude when doing a survey? I don't find anything to support this.


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PostPosted: Mon May 11, 2009 1:19 pm 
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You might want to question the "excluded equipment" portion.

A couple of things jumped out at me.

The motor cut off from IEEE 1584 is less than 50HP so 50 HP and above should be included.

The voltage / transformer cut off is for circuits served from transformers less than 125 kVA at voltages less than 240 volts (i.e. 120/208) according to IEEE 1584. This is also referenced in NFPA 70E although they say 240V and below, it should have also been below 240V

Your right, there are not any other exclusions in 70E.

The 100 A starter / switch exclusion makes sense unless they are downstream from a large transformer with a lot of short circuit current. Is it likely to require servicing live? Is there an incident energy exposure above 1.2 cal/cm^2? No real way of knowing with out a little investigation.

Your in for a great learning experience!
Lots' of sharp people in this forum (like you've probably already noticed) that I'm sure will be willing to help give you a little direction.

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PostPosted: Thu May 14, 2009 1:23 pm 
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SKM offers this sample specification

http://www.skm.com/powersystemstudy_specs.shtml


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PostPosted: Wed Jan 05, 2011 7:36 pm 
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Some sample specifications for an Arc Flash Study can be found at

http://www.powerstudies.com/downloads/ArcFlashStudy-Specs.doc

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PostPosted: Wed Jan 05, 2011 8:06 pm 
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jas wrote:
Excluded Electrical Equipment from Arc Flash Study: Examples of excluded electrical equipment: fused or non-fused switches smaller than 100A, combination motor starters smaller than 100A, junction boxes, tap boxes, motors less than 75HP, .... .


Why exclude these?

If you test voltage or current at the disconnect switches and/or motors starters, they need to labeled.

Junction boxes and wireways can be excluded, unless testing at those locations.

Motors do not need to be labeled, unless testing is performed at the motors. IEEE standards suggest including motors, larger than 50HP, when performing short circuit studies.


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PostPosted: Thu Jan 06, 2011 8:40 am 
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Just try to point it out that 50hp motor can be only excluded from considering it as another source energy, you still need to provide ananlysis for motor controller, starters.


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PostPosted: Thu Jan 06, 2011 9:44 am 
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We have found that customers do not want to pay to have arc flash labels for low voltage non-fused disconnect switches. Including these radically drives up the cost of the arc flash study. Previous calculations we have performed result in most low voltage non fused disconnect switches having a HRC rating of 0 or 1. We recommend to our customers that if they are going to "interact" with a non fused disconnect switch that does not have an AF label on it, they should go upstream to the motor starter or panel (that is feeding the disconnect switch) and look at the AF label. Then assume that the AF energy at switch will be atleast the same. Use the PPE listed on the motor starter or panel before "interacting" with the switch.

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PostPosted: Wed Aug 31, 2011 6:52 am 
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Assumptions - '100 amps or less'

[QUOTE= We recommend to our customers that if they are going to "interact" with a non fused disconnect switch that does not have an AF label on it, they should go upstream to the motor starter or panel (that is feeding the disconnect switch) and look at the AF label. Then assume that the AF energy at switch will be atleast the same. Use the PPE listed on the motor starter or panel before "interacting" with the switch.[/QUOTE]

I dislike the statement above in reference to "assuming" a rating based on the source rating. If there is significant distance from the "rated" panel to the "non-rated" point, the reduced fault current can significantly increase the clearing time of the upstream protection, thus possibly resulting in an elevated rating.

In reference to 100 amp loads, tt is not necassary to "exclude" loads smaller than 100 amps, nor is it necassary to include all loads that fit this category. It is good practice to model a 'worst case' situation representative of these circuits to confirm indeed they are a low rating (<=1.2 cal/cm2). For exampe, an exceptionally long branch circuit run can increase the clearing time of the upstream protection, similar to the 'assumption' above.


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PostPosted: Wed Aug 31, 2011 7:28 am 
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[quote=" We recommend to our customers that if they are going to "]

[QUOTE=mdy, post: 8865"]I dislike the statement above in reference to "assuming" a rating based on the source rating. If there is significant distance from the "rated" panel to the "non-rated" point, the reduced fault current can significantly increase the clearing time of the upstream protection, thus possibly resulting in an elevated rating.

I do not like assuming either. However, our experience has been that 99% of the time, the arc flash energy will be lower at the downstream disconnect switch versus the line side of the upstream protective device as long as there is no transformer between these two locations. The reason is that the protective device protecting the circuit and disconnect switch is much smaller than the device protecting the line side (usually MCC main breaker) of the upstream protective device. Because the circuit/disconnect switch protective device is so small, it will trip instantaneously and the arc flash energy will be very low.

In the past, we have provided an option to our clients to include AF labels for their non fused disconnect switches. After completing 350 AF studies, no customer has wanted to pay the additional cost to label the disconnect switches. Depending upon the facility and design, including an AF label for the disconnect switches can double the cost of performing an AF Hazard analysis.

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PostPosted: Wed Aug 31, 2011 4:34 pm 
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"The motor cut off from IEEE 1584 is less than 50HP so 50 HP and above should be included."

Hey Jim, I was unaware the scope of IEEE 1584 excluded motors 50HP or less. In that case do you ignore the smaller contribution or just lump the smaller HP motors and attach to the bus?


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PostPosted: Wed Aug 31, 2011 5:31 pm 
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We always lump the smaller motors <50 HP as one motor on the MCC or Panelboard.

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PostPosted: Thu Sep 01, 2011 6:20 am 
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IEEE 1584 (4.2, p 6) says to include 50 HP motors and above as sources. It does not say to exclude any size from the arc hazard analysis. The 50 HP cutoff comes from IEEE Std C37.010 and C37.5, which are medium- and high-voltage breaker application procedures. For low-voltage or mixed systems, you can't neglect smaller motors for circuit breaker application, so you might as well include them for arc hazard analysis. You could lump motors smaller than 50 HP, but then you don't get an IE calculation at the smaller motor.

If electrical equipment is likely to require examination, adjustment, serviceng, or maintenance while energized, it needs a label according to the NEC. There are no size exclusions. If you have to test the voltage do assure de-energization, this is examination while energized because you have to assume it is energized until tested.


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PostPosted: Thu Sep 01, 2011 6:40 am 
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smaller motors

If electrical equipment is likely to require examination, adjustment, serviceng, or maintenance while energized, it needs a label according to the NEC. There are no size exclusions. If you have to test the voltage do assure de-energization, this is examination while energized because you have to assume it is energized until tested.[/QUOTE]

In that case, it sounds like you provide a label for motor disconnects at the motor? Wouldn't you just LOTO the upstream MCC bucket feeder?


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PostPosted: Thu Sep 01, 2011 7:47 am 
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JCV wrote:
"The motor cut off from IEEE 1584 is less than 50HP so 50 HP and above should be included."

Hey Jim, I was unaware the scope of IEEE 1584 excluded motors 50HP or less. In that case do you ignore the smaller contribution or just lump the smaller HP motors and attach to the bus?


Hey, great to see you out here on the forum!

Looks like Jghrist beat me to the resposne - great group of people here that are very willing to help!

The actual IEEE language is:

"The study must take into account all sources, including utilities, standby and power generators, and large motors—those 37 kW and larger that contribute energy to short circuits."

IEEE is mandated to use metric so hp is not allowed.

I always try to cover the bases and use 100% motor contribution and then try 50% and then maybe 10%. The whole idea is just to see if the change in short circuit current will affect the desired PPE arc rating. i.e. if you select an arc rating of 8 cal/cm^2, do any of the "what if" scenarios make the Ei go higher than the targed arc rating.

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