## Max Fault Current or Amperage Interrupt Capacity(AIC) calculation

## Max Fault Current or Amperage Interrupt Capacity(AIC) calculation

(OP)

How to calculate Max Fault Current or Amperage Interrupt Capacity?

Given that it's a three phase load, consisting of 2 fuses(40 Amps, 200kA AIC), 2 Variable Fixed Drives(15 HP, 10 kA AIC) and two contacters( 5 kA AIC). I'm interested in finding the Max Fault Current or also known as the the total Amperage Interrupt Capacity(AIC). From looking online, I know that its (rated current/ &impedance)*100. Will I able to determine the AIC with these given data, or do I need more?

Given that it's a three phase load, consisting of 2 fuses(40 Amps, 200kA AIC), 2 Variable Fixed Drives(15 HP, 10 kA AIC) and two contacters( 5 kA AIC). I'm interested in finding the Max Fault Current or also known as the the total Amperage Interrupt Capacity(AIC). From looking online, I know that its (rated current/ &impedance)*100. Will I able to determine the AIC with these given data, or do I need more?

## RE: Max Fault Current or Amperage Interrupt Capacity(AIC) calculation

Available fault current is something you can calculate and is dependent on the source feeding the circuit, you also must account for motor contribution. There are several ways to do this calculation but not a simple forumla.

At each point in your system the available fault current may not exceed the AIC rating of the device.

## RE: Max Fault Current or Amperage Interrupt Capacity(AIC) calculation

This is also called the symmetrical short circuit current. These ASCC

values are used to select equipment with adequate fault interrupting capacity but they are not the total possible short circuit current.

Many fault currents are initially asymetrical. DC offset is a term that is often used to describe the onset of a fault. The amount of dc offset, or the actual peak current will depend on the X/R ratio of the source and the point on the waveform that the fault occurs.

The actual interupting capacity of a breaker rated at 10 kAIC will be a value that corresponds to the maximum peak current possible based on a representative value of the X/R ratio of the source.

Use the %impedance of the supply transformer to select equipment that will safely interrupt the actual possible fault currents.

Bill

--------------------

"Why not the best?"

Jimmy Carter

## RE: Max Fault Current or Amperage Interrupt Capacity(AIC) calculation

Better yet, you may calculate the symmetrical short circuit based on an MVA base and equivalent P.U. impedance at the fault point. If the calculated X/R at fault does not exceed the fuse or breaker X/R test, then you may directly compare the calculated symmetrical current against the KAIC rating.

## RE: Max Fault Current or Amperage Interrupt Capacity(AIC) calculation

Bill

--------------------

"Why not the best?"

Jimmy Carter

## RE: Max Fault Current or Amperage Interrupt Capacity(AIC) calculation

No offense, but it appears that you are in over your head here. You are not sure of the terms to use, let alone why this is important or how it all works. You are describing the process of determining the SCCR of a control system, not the AFC or the AIC. It's a very involved process that cannot be done justice to in a limited format such as this and it is definitely not something for amateurs to tackle.

You need the services of a qualified Electrical Engineer for this. If you are an EE, this is obviously outside of your level of expertise. We cannot substitute for that over the internet on something so involved. Please seek out the proper assistance from someone who can observe your entire system.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."— Kilgore Trout (via Kurt Vonnegut)

For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies