Arc flash study

[anbm]

Is it necessary to do coordination study before arc flash study? i.e. with software like SKM, Etap, can we do short circuit study, then skip coordination study and go straight to arc flash study for result, arc flash labels, etc? Thank you!

 

[ron]

Arc flash is based on short circuit calculations and OCPD settings. If you don't want to coordinate the OCPD and make them as selective as practical, then you could theoretically just run the arc flash calculations after you put the existing OCPD settings into the software.

 

[PE (always learning)]

Is it necessary to do coordination study before arc flash study? i.e. with software like SKM, Etap, can we do short circuit study, then skip coordination study and go straight to arc flash study for result, arc flash labels, etc? Thank you!

I don't know why you'd skip the coordination study unless none of your breakers have any sort of adjustability. It is always recommended to coordinate any overcurrent protective devices as optimal as possible. Just imagine the nightmare scenario when a main circuit breaker for a hospital opens due to a miscoordination with down stream overcurrent protection. This would be a worst case scenario, but I'm just trying to make a point.

Your arc flash values are directly related to your coordination study so, yes it is necessary to do coordination before arc flash.

Now, selective coordination, while always desirable, is not required by the NEC except in certain situations:

• In health-care facilities, per NEC 517.17(C): “Ground-fault protection for
operation of the service and feeder disconnecting means shall be fully selective
such that the feeder device, but not the service device, shall open on ground
faults on the load side of the feeder device.”
• In elevator circuits when more than one elevator motor is fed by a single feeder.
See NEC 620.62.
• In emergency and legally-required standby power systems (including those in
hospitals and other health-care facilities where so required), per NEC 700.28 and
NEC 701.27.
• In critical operations power systems (COPS) such as, but not limited to, power
systems, HVAC, fire alarm, security, communications, and signaling; refer NEC
708.54. Generally, these are facilities or parts of facilities that require continuous
operation for reasons of public safety, emergency management, national
security, or business continuity.

 

[xptpcrewx]

Arc-Flash Hazard analysis requires protective device ratings/settings. You can either develop settings yourself as done in Protective Device Coordination analysis or have them be provided by others (if all you care about is an illustration of as-found settings). Either way, protective device ratings/settings must be input into the model. The benefit of doing Protective Device Coordination analysis first is that it gives you an opportunity to lower incident energy by adjusting settings as low as possible while still providing some degree of selectivity. Keep in mind Protective Device Coordination isn’t just about selectively coordinating devices; it also provides a means to analyze the level of protection achieved by device ratings/settings and can identify where both adjustable and non-adjustable equipment currently miscoordinate. Overall, I recommend doing the Protective Device Coordination analysis. You’d be surprised how many times I’ve caught a manufacturer providing non compliant devices which are out of spec for a particular design... only thing that caught it was doing the Protective Device Coordination analysis.


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[wbdvt]

Since you have all the information for a coordination study, why would you not do one? This can be very useful as you may be able to reduce incident energy levels by lowering instantaneous settings on OCPDs that are adjustable. You only know if this can be done without affecting coordination by performing a coordination study.

I am concerned that you may not also not be doing an equipment duty study also. This will tell if any protective devices AIC ratings are being exceeded.

You may want to read IEEE 1584.1-2013 which is the IEEE guide for the specification of scope and deliverables for an arc flash study IAW IEEE 1584. Another worthwhile read is Jim Phillips' book, Complete Guide to Arc Flash Hazard Calculation Studies.

 

[PE (always learning)]

The benefit of doing Protective Device Coordination analysis first is that it gives you an opportunity to lower incident energy by adjusting settings as low as possible while still providing some degree of selectivity. Keep in mind Protective Device Coordination isn’t just about selectively coordinating devices; it also provides a means to analyze the level of protection achieved by device ratings/settings and can identify where both adjustable and non-adjustable equipment currently miscoordinate.

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Solid advice, I should have mentioned that the coordination study also gives you a chance to reduce arc flash incident energies, but I kind of figured that's always the goal except for a few select scenarios where setting breakers low is not always feasible such as selective coordination, motor inrush, and transformer inrush.

 

[anbm]

Thanks, all... the reason I said skip coordination study as this is existing facility and they want to keep the settings for all electronic breakers as is.
they only want to uptade fault current, print arc flash labels and do not want to spend extra money for coordination study task. I am not sure if this makes sense...

 

[ron]

Thanks, all... the reason I said skip coordination study as this is existing facility and they want to keep the settings for all electronic breakers as is.
they only want to uptade fault current, print arc flash labels and do not want to spend extra money for coordination study task. I am not sure if this makes sense...

Sure, they can just document the existing settings and perform the arc flash study and provide no value add with the coordination study.

 

[jim dungar]

... this is existing facility and they want to keep the settings for all electronic breakers as is.

Then the coordination portion has already been performed. In the NEC, selective coordination is optional except for some hospital and emergency loads.

 

[petersonra]

It is always about money. Since the software will do the coordination numbers for you at no extra cost, I would be inclined to run it and see what it says. if it comes up with some situation that badly needs to be addressed for some reason, you have hard numbers to back up the need to do something. But whether or not anything gets done is always up to the guy paying the bills.

 

[xptpcrewx]

Thanks, all... the reason I said skip coordination study as this is existing facility and they want to keep the settings for all electronic breakers as is.
they only want to uptade fault current, print arc flash labels and do not want to spend extra money for coordination study task. I am not sure if this makes sense...

If this is the case I would recommend Protective Device Coordination analysis only to confirm existing settings and to review protection/coordination adequacy. If the fault current has changed, you should review: (i) how this impacts device coordination (due to altered TCC truncation), (ii) whether instantaneous elements are still appropriate for the new fault magnitudes expected, (iii) impact to other protection settings (if any).

Note: If the previous study was conducted before 2018, there is a good chance you will see elevated incident energy levels, which will probably require going back to reevaluating existing protective device settings. Also, depending on new additions/removals you are going to want to comply with the latest NEC arc-flash reduction requirements, which again takes you back to Protective Device Coordination analysis. No escaping this if you want to do it right!


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[ron]

Since the software will do the coordination numbers for you at no extra cost,

The software does not. Although the existing settings are put into the software, there is time required to setup the TCC's and the associates partial onelines and the commentary that goes with the TCC's for it to make sense to the almost-lay-person, nevermind if the engineer were to provide recommended setting adjustments.

 

[confused-engineer81]

The order of conducting a study (for new work) is 1. Collect and Input data into software 2. Run the Short Circuit calculation to ensure devices aren't overdutied 3. Coordinate Breakers with motor and transformer inrush and each other.

In this case, i would do the study AS IS and provide the study, it sounds like youre not being paid to recoordinate anything. If the client wants to explore coordination then do so and get them to pay for it.