Arc Flash Forum :: View topic - Multiple sites with low load ratings

Hi David,
I’m in a somewhat similar situation to you and use as a starting point clause 1.4.39 of AS/NZS3000 where reference is made to the “prospective short circuit current” or “bolted fault current”. Based on this reference I use an installation tester to measure the maximum possible fault current. This measurement takes away all assumptions, guesswork, and uncertainty as to what the maximum current could be. The measurement is made at each panel and provides reliable guidance on the maximum potential fault current at that point. I measure on the load side of the main switch, and then assume that measured value as the maximum fault current for the whole MCC.

I use this measured current as the known reference point and proceed with my calculations from using this as a starting point. Upon knowing the maximum currents i also consider my protection devices, tripping times etc (also bearing in mind that the possible arc current will be less than the bolted fault current)

Author:	David Sheldon [ Thu Jun 10, 2021 3:04 pm ]
Post subject:	Multiple sites with low load ratings
I am responsible for amongst a number of other things -175 aging Concrete batching plants around Australia - all are supplied from a pole mounted transformer via fuses at 400V. The sites range in size according to Load ratings from 150A to 700A at approximately 50 A increments with the majority of the sites being rated at 250A. All sites are basically of the same configuration; and typically consist of a main Switch feeding an MCC which feeds 2 conveyors (11kW ea) a mixer (45kW), Compressor(37kW), 3 Silos, Plant lighting and an admin office and a batching control room. We just cannot afford the $4000 per site for an Arc Flash Study! I was hoping to use the IEEE 1584 spreadsheet in order to perform generic AF calculations on typical sites ranging from 150A;200;250;300; ...700A) to determine the worst case scenario per rating and whether I could apply these results to all plants of the same load? Any advice would be most welcome.

Author:	mpparent [ Fri Jun 11, 2021 11:59 am ]
Post subject:	Re: Multiple sites with low load ratings
Perform generic studies at your own risk. Differing transformer impedances, available fault currents from the utility, etc. My opinion, but I don't think that is a good idea. Mike

Author:	Larry Stutts [ Sat Jun 12, 2021 1:51 am ]
Post subject:	Re: Multiple sites with low load ratings
mpparent wrote: Perform generic studies at your own risk. Differing transformer impedances, available fault currents from the utility, etc. My opinion, but I don't think that is a good idea. Mike Agreed. If you don't want to pay for a professional to do the study, you will need to do the study yourself (hopefully by someone who knows how to do it). There is software available - likely at a lower overall cost than paying for each individual site. There are too many possible variables to generalize given a plant's amp load or kilowatt rating.

Author:	David Sheldon [ Sun Jun 13, 2021 3:54 pm ]
Post subject:	Re: Multiple sites with low load ratings
Thanks very much for taking the time to respond I was hoping for someone who had been faced with the same dilemna - I will gather as much detailed info as possible and do the spreadsheet calcs, ask for a professional opinion on the results and then take action. Hard to motivate and justify a spend of over $700k when there have been no incidents on 175 sites spread around the country in over 30 years..

Author:	PaulN [ Mon Jun 21, 2021 2:47 pm ]
Post subject:	Re: Multiple sites with low load ratings
Hi David, I’m in a somewhat similar situation to you and use as a starting point clause 1.4.39 of AS/NZS3000 where reference is made to the “prospective short circuit current” or “bolted fault current”. Based on this reference I use an installation tester to measure the maximum possible fault current. This measurement takes away all assumptions, guesswork, and uncertainty as to what the maximum current could be. The measurement is made at each panel and provides reliable guidance on the maximum potential fault current at that point. I measure on the load side of the main switch, and then assume that measured value as the maximum fault current for the whole MCC. I use this measured current as the known reference point and proceed with my calculations from using this as a starting point. Upon knowing the maximum currents i also consider my protection devices, tripping times etc (also bearing in mind that the possible arc current will be less than the bolted fault current)

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Author:	RH@eace [ Mon Jun 21, 2021 4:35 pm ]
Post subject:	Re: Multiple sites with low load ratings
Hi David, I am available to discuss options and provide assistance to your problem. If you would like to discuss further give me a call on <personal contact info deleted> My website is eace.com.au for further detail.

Author:	mpparent [ Wed Jun 23, 2021 12:01 pm ]
Post subject:	Re: Multiple sites with low load ratings
PaulN wrote: Hi David, I’m in a somewhat similar situation to you and use as a starting point clause 1.4.39 of AS/NZS3000 where reference is made to the “prospective short circuit current” or “bolted fault current”. Based on this reference I use an installation tester to measure the maximum possible fault current. This measurement takes away all assumptions, guesswork, and uncertainty as to what the maximum current could be. The measurement is made at each panel and provides reliable guidance on the maximum potential fault current at that point. I measure on the load side of the main switch, and then assume that measured value as the maximum fault current for the whole MCC. I use this measured current as the known reference point and proceed with my calculations from using this as a starting point. Upon knowing the maximum currents i also consider my protection devices, tripping times etc (also bearing in mind that the possible arc current will be less than the bolted fault current) Not recommended (IMHO). The maximum fault current does not always produce the maximum incident energy. Just sayin'... Mike

Author:	PaulN [ Wed Jun 23, 2021 6:20 pm ]
Post subject:	Re: Multiple sites with low load ratings
Hi Mike, My comments related to measuring Ibf rather than calculating it. After all, which is more accurate - Ibf calculated or Ibf measured? From my perspective Ibf (measured) is faster, easier to do, and is more easily applicable to each installation's unique circumstances than Ibf (calculated). That is the starting point I use, and then proceed with calculations from that point. At the end of the day, whether Ibf is measured, or accurately calculated, the end result should be the same Incident Energy ...

Author:	David Sheldon [ Tue Jun 29, 2021 3:50 pm ]
Post subject:	Re: Multiple sites with low load ratings
Thanks Paul, at least that way as you say I will have the exact Ibf and continue from there. Is there a set procedure for this and what Tester do you use ?

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