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 Post subject: What Fault Current(bolted or Arcing)is used for CoordinationPosted: Mon Mar 09, 2020 12:15 pm
 Sparks Level

Joined: Tue Oct 09, 2012 5:00 pm
Posts: 53
Hi Arc Flash Gurus,
Hope you all are doing great. I have a question on protective device coordination. Per IEEE 1584-2018, we gather utility information, calculate bolted fault current for short circuit analysis which is vital in equipment evaluation. Then we use the bolted fault current to calculate the arcing fault current and perform the Protective Device Coordination and determine its clearing time using Arcing Fault Current. But we have a situation where protective device coordinates when we use arcing fault current but mis-coordinates when bolted fault current is plotted.
Therefore, my question is:
What Fault Current (bolted or Arcing) is used for Protective device Coordination?

Thanks,
N. Tshering

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 Post subject: Re: What Fault Current(bolted or Arcing)is used for CoordinaPosted: Tue Mar 10, 2020 8:32 am
 Sparks Level

Joined: Fri Apr 15, 2011 7:43 am
Posts: 177
Location: Colorado
I always use the bolted fault for coordination but then look at the arc flash and adjust the settings to reduce the arc flash where possible. Sometimes it is a fine balance to reduce the IE and keep the plant running. I can always reduce the IE but if I have to reduce the capability of my equipment then I should reduce the size of the equipment.

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 Post subject: Re: What Fault Current(bolted or Arcing)is used for CoordinaPosted: Mon Mar 16, 2020 8:47 am
 Sparks Level

Joined: Tue Oct 26, 2010 5:00 pm
Posts: 184
Location: Maple Valley, WA.
Without getting into too much detail..... Most software programs will use the bolted fault current and then calculate the arcing fault current. The program then looks to see how long it takes the device to operate using the arcing fault current. As the previous person stated, you can then lower your device settings down until the arc flash energy drops to a more comfortable level of energy. However, you are most likely sacrificing selective coordinate between your breaker and downstream protective devices.

_________________
Robert Fuhr, P.E.; P.Eng.
PowerStudies

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 Post subject: Re: What Fault Current(bolted or Arcing)is used for CoordinaPosted: Tue Mar 17, 2020 4:09 pm
 Sparks Level

Joined: Tue Jan 19, 2010 2:35 pm
Posts: 132
You'll need to answer a few things first. What is the goal of your coordination study? Do you want to greatly limit any possible damage to the electrical system during a short circuit situation? Do you want to maintain power because the cost of an outage would be greater than any possible damage to the electrical system? Or do you want the best of both worlds where you protect the electrical system and also keep power on as long as possible? Once you answer that question, you can perform you coordination study. But I wonder if it's possible you are conflating arc flash protection with coordination? A short circuit doe not necessarily mean there is an arc flash incident. If there were an arcing fault at a device, it is likely the closest upstream ground fault device would open along with the nearest OCD to the fault. Ground fault systems often clear a fault and the OCD nearest the fault also opens. The trick is to give the ground fault enough delay so the OCD opens first. But that is often not an easy task. The very nature of an arcing fault makes it difficult to predict how long and how high the amperage of the fault will be. If I were you, I'd consider bolted fault currents for your coordination study.

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 Post subject: Re: What Fault Current(bolted or Arcing)is used for CoordinaPosted: Wed Mar 18, 2020 12:10 pm
 Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 524
In order to keep outaged areas to a minimum, the goal of selective coordination is to coordinate for all faults. Start with bolted, then look to the left on the curves to see if there are other problem areas.

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 Post subject: Re: What Fault Current(bolted or Arcing)is used for CoordinaPosted: Mon Mar 23, 2020 10:55 am

Joined: Mon Aug 20, 2018 6:03 am
Posts: 2
This is the question!

If I may make some observations and suggestions:

1) Realize any and all calculations you make are, almost by definition, wrong... there are lots of assumptions, lots of built in errors. All of them traditionally accepted and ignored. Generally this results in the theoretical short circuit currents being calculated being higher than, in reality it is. And this is fine as it allows for desirable margin when equipment ratings are decided based on that calculated value.

2) Traditionally that same fault current is then used for coordination study purposes. And in a world were arc flash is not a consideration and delays are acceptable, that also allows the coordination to be very conservatively correct.

3) In 2005 selectivity requirements were added to the NEC requiring certain occupancies to implement full selectivity. Hospitals being a segment very much affected by that requirement. it was interesting to note, at least for me, that in my 30+ years in the industry I had never heard that type of user complain about lack of selectivity, except for ground faults where lack of selectivity was mentioned from time to time. leads me to believe that commercial systems, notoriously under-coordinated on paper, where, in actuality much more coordinated than they seemed... This is very anecdotal, I realize.

4) Now, via our much improved understanding of the arc flash phenomena we understand that arcing currents can be quite low. Particularly in lower V systems such as 208V, or even 480V... Keep in mind the IEEE models ignore asymmetry and may tend to understate the arcing current for various reasons in some cases, but not in others.

5) So I suggest that you identify the mission of the system... optimize reliability or protection or try to do both as best as possible. What is acceptable performance in light of the more probable situations, not theoretical situations that will, probably, never happen

6) Accept that there are probabilities involved... the probability of a full bolted fault at the theoretical maximum fault value is relatively small at LV... May happen if you close a CB on forgotten grounding conductors, but most CBs will have a making current release making protection settings for selectivity moot at that point. Would you care about selectivity in that situation? Even in MV, a bolted fault will be difficult to support, but arcing currents are much closer to Ibf values.

7) Faults at arcing current levels are much more probable.. but Iarc may be higher or lower than IEEE 1584 predicts for various reasons.

8) In LV most manufacturers have improved their technology and their CB are MUCH more selective than they used to be, or their real selectivity has been identified, so if you are doing a coordination study with LV CBs purchased last year the way you did a coordination study 20 years ago you may be leaving a lot of coordination on the table!... Talk to the manufacturer and make sure you are using the technology available to its fullest!

9) In LV, solidly grounded systems the most common fault, by far (per conventional wisdom), are ground faults... try to coordinate those as best as possible... which does not mean add GF functions everywhere! GF detection and protection in CB <400-600A can be problematic from the selectivity perspective if there are are many 1P lighting ckts below, which by definition are GF detectors.

10) I would not advice adding delay to get selectivity at 95% of maximum theoretical bolted fault current... Seems like a sacrifice for something that will never happen.

11) Understand the compromises and the impact of error in you decisions and try to bias the error in the direction that is more acceptable to the organization.

Hopefully this makes sense.

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