AFHA Update

Status
Not open for further replies.

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Suppose you had a five year report done on a large commercial building, and later wanted to add a couple panels to the system and report. Does the study have to be completed for the whole building again, generally? Can the first report be used as a base and just calculate values for the additions?

Does it make sense that adding two panels to a two-year-old report costs almost exactly the same as the initial report of dozens of panels?

Asking for a friend
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Suppose you had a five year report done on a large commercial building, and later wanted to add a couple panels to the system and report. Does the study have to be completed for the whole building again, generally? Can the first report be used as a base and just calculate values for the additions?

Does it make sense that adding two panels to a two-year-old report costs almost exactly the same as the initial report of dozens of panels?

Asking for a friend

No, yes, and maybe.

AFHA are updated all of the time. I have one customer that sends me his 'additions' every quarter.
I work off of the base model, and the new equipment simply gets added. A complete new report is not produced. The customer gets a revised table. Once a year, or so, a complete new report is generated.

There are times I am asked to update a report produced by a different firm. In these cases I almost have to start from scratch by creating a new model, so it comes close to the original study especially if the original report input data is suspect and needs to be verified.

It is also possible that the amount of 'overhead cost' is the same for 2 panels as it is for 24 panels.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Adding a panel to an existing distribution system will not alter the results of the AFHA for any of the existing panels. You can, if you wish, assign the new panel the same arc flash category as its upstream panel - it certainly won't be worse. Two things would alter these statements:
1. If the new panel powers large motors that would contribute fault current to the rest of the system.
2. If the new panel is fed via a step-down transformer.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
charlie b;1937176[SIZE=4 said:
.. it certainly won't be worse. ..[/SIZE]

Charlie, what about when the conductor length actually its impedance) is great enough to lower the arcing fault current and therefore slow down the clearing time of the protective device?
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Good question, Jim. My answer is that that extra impedance would impact the fault current (and arc flash energy) at the location of the new panel. So if you choose to simply assign the upstream panel's AFHA results to the new, downstream panel, you might be giving unnecessarily high numbers to the new panel. Your choice: revise the analysis or use the previous results.

But that extra impedance won't alter the fault current (or arc flash energy) anywhere else. The analysis looks at a fault at one location at a time, and determines the fault current (and arc flash energy) at that location. The only impedances that enter into that analysis are those between the fault point and any energy sources in the system (i.e., utility and motor contributions). So if the new panel does not power any large motors, it will not have an influence on the arc flash energy felt at any other location.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
So if the new panel does not power any large motors, it will not have an influence on the arc flash energy felt at any other location.

Absolutely.
But other than that, the incident energy at the 'added' equipment is just an educated guess without doing the calculations.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I agree, Jim. But if the upstream panel has a category 1 rating, you don't need a calculation to assign category 1 to the new panel. Also, if the upstream panel has a category 2 rating, and you are willing to live with a category 2 for the new panel, once again you don't need a calculation. On the other hand, if the upstream panel has a category 2 rating, but you would prefer to assign category 1 to the new panel, then you will need a calculation. That is all I am saying.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
I agree, Jim. But if the upstream panel has a category 1 rating, you don't need a calculation to assign category 1 to the new panel. Also, if the upstream panel has a category 2 rating, and you are willing to live with a category 2 for the new panel, once again you don't need a calculation. On the other hand, if the upstream panel has a category 2 rating, but you would prefer to assign category 1 to the new panel, then you will need a calculation. That is all I am saying.

Charlie,
This is just your experience, mine is different.

I have made calculations where simply adding 50' more of conductor raised the downstream to over 4 cal/cm^2 while the upstream was still less than 4. It seems to happen most often using 200A Class RK1 fuses with a starting bolted fault current of less than 5kA.
 

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
I agree with Jim as I have seen where fault currents are low to begin with at a panel and then downstream of that panel, another panel or disconnect switch has a higher incident energy level due to the low fault current and longer clearing time.
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Thanks for the replies. When I passed the original report to a competitor, I got a quote for 30% of the original update quote; I don't make it a habit to shop prices, but felt that the original company was pushing their luck. The competitor was apologizing for his "high price" (the 70% cheaper one) because they would have to build that portion of the one line from scratch using the one line in the original report...
 
Status
Not open for further replies.
Top