Arc Flash Fault Clearing Time

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timm333

Senior Member
Location
Minneapolis, MN
Occupation
Electrical Design Engineer
In arc-flash calculations, the Fault-Clearing-Time of protectivedevices is found from TCC at a particular bolted-fault-current (Ibf). In case of fuse, wetake the total clearing time instead of the minimum melt time, this makessense.
Does Arcing-Fault-Current (Ia) also play a role in the calculationof Fault-Clearing-Time from TCC, or only bolted-fault-current is used tocalculate the Fault-Clearing-Time from TCC? Thanks
 

ron

Senior Member
The Ibf (bolted fault current) is used to calculate the Iaf (arcing fault current) with other variables such as voltage, space between conductors, etc.

Then Iaf is used with the TCC to figure how much time it takes to clear both breakers and fuses.
 

zog

Senior Member
Location
Charlotte, NC
You have to look at max and min clearing times for the fault, often the highest fault current will not result in the highest arc flash hazard.
 

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
Also, where are you getting the bolted fault current from? Is it the utility minimum available fault current? This will make a difference. Usually when you talk to the utility you first get customer service who will give you the maximum available fault current based on the transformer at the location. You need to get the actual values from engineering.

Also IEEE has you check the arc flash hazard using 85% of the arcing current.
 

timm333

Senior Member
Location
Minneapolis, MN
Occupation
Electrical Design Engineer
Some things are not clear in IEEE-1584 because they have only described the procedure of arc flash analysis in words, no solved example is provided.

Is arcing fault current (Ia) the current that passes through faulted buses of MCC, or is it the current that passes through the contacts of the breaker? Is arcing current (t) equal to time for which the arc fault current continues to pass?

So shall we calculate the arcing time (t) from TCC first by using 100% Ifa and then again by using 85% Ifa, and then select the lower value of arcing tie (t) for further calculations?


Yes I have the utility data, it is 415A @ 12.47kV, Z = 7.6 + j8.2 pu (X/R = 1.07).
 

timm333

Senior Member
Location
Minneapolis, MN
Occupation
Electrical Design Engineer
It looks that the arcing fault current (Iaf) is the current that passes through the circuit breaker on the primary side of the transfomer, is it correct?
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
It looks that the arcing fault current (Iaf) is the current that passes through the circuit breaker on the primary side of the transfomer, is it correct?

Iaf is basically the current that flows through your fault - its the current that causes the flash and heat, and all the other bad things. In most cases, its also the current that flows through the breaker or fuse that will clear the fault. So Iaf is used to calculate how long it will take a fault to open a breaker or fuse.

If there is a transformer between the fault and the clearing device, then you have to adjust Iaf per the transformer turns ratio to find the current through the clearing device..

Its good to understand how the equations work, but personally, I wouldn't try to calculate these by hand. Having a software program is much better. At the very least, I would program the equations into Excel and let it handle the heavy lifting. Finding Iaf isn't an exact science, so you want to try a couple of different values, and see where they land on the knee of the TCC curve.
 

templdl

Senior Member
Location
Wisconsin
Arc flash, to what degree? The magnitude as well as the length the time that the arc persists. It is imperative that the arc flash either persists long enough to be able to have the capability to cause a fuse to clear or a breaker to trip on thermal. Or the arcing must be of a high enough magnitude to cause the dust to clear or a breaker trip instantaneously. It is to be noted that both fuses and breakers are essentially stupid devices where both must be subjected to current and length of time to cause them to respond.
Breakers will clear in less than a cycle as I recall when tripped. With fuses depends upon the magnitude of the length of time which can affect the time that it takes a fuse to clear.
Finding an answer to the subject is often like shooting at shadows because arc faults/flashes differ in degree.
 
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