Can I take that question further.

Let me give an example to illustrate. Say you have fault current of 25KA and arcing current of 10KA. You have an adjustable trip breaker and you want to set the breaker to trip instantaneously upon an arcing fault. Where would you set the pickup on the instantaneous setting?

Setting it at 10KA may sound right but if something is even marginally wrong you move from a 2 cycle trip on instantaneous to a 7 cycle trip on short time, and IE doubles.
So do you set at 9KA for a 10% safety margin, or 8KA for a 20% safety margin.

I see this opportunity a lot on MCC's that are fed via switchgear, where the MCC has current limiting fuses in all the starters, and they are size 2 or under.

So what value do you use, straight arc current on the trip curve, or do you add in some safety factor.

If you are using the instantaneous setting as a "maintenance setting" to be used only while working on energized equipment, you don't have to worry about things like transformer inrush. I'd set the pickup at 200% of maximum load current, or above the largest motor starting current.

Brainfiller,

Thanks for the input. The poll question doesn't deal with breaker age, condition, or maintenance; but with an adder for a Close Open operation versus an Open only. You can expect this extra half or full cycle when closing into a fault with a brand new breaker in perfect condition. I imagine it must have something to do with overcoming the inertia of parts still moving from the close operation when called on to reverse.

No, I am not talking about Maintenance setting, but rather the permanent setting. In order to assure that the breaker trips on the instantaneous portion of the curve, to you want to have some safety margin. The breaker curve is exponential to the left of the instaneous portion, so clearing time could easily double or triple. Do you want to make sure you are solidly in the inst portion of the curve and if yes by how much. Disregard downstream coordination for purpuse of this question. Calculated arc fault current is 10KA. If the breaker inst is set at 10KA can you use the 2 cycle clearing time knowing that if you are off even a little and actual arcing current is 9.8KA then clearing time moves to 5 cycles.

What I am saying is that I would like a 10% minimum safety margin, having the inst set at 9KA to assure it clears at 2 cycle on a calculated 10KA arcing fault.

I understand what you are saying Haze. To allow for this, I run several scenarios based on the utility provided fault current. I run a scenario using 100% of their supplied current. I then run other scenarios at 125%, 70% and 40% of the supplied current. I label based on the worst case. Between doing this and calculating the worst case between the 100%/85% arcing currents, I should be covered.

No, that's not exactly what I am saying. Lets say the customer wants you to set his switchgear breakers with settings that provide the 'least' IE but 'maximum fault current. SO you have to select the instantaneous pickup value so that you can get a 2 cycle trip on the available arcing current. You've done what you said and solved for the highest and lowest theoretical arcing currents. Now you have to set the pickup to trip on the theoretical lowest arcing current. Do you set the pickup at 100%, 90% OR 80% OF THE ARCING CURRENT.

If you wanted to make sure that the calculations show the lower incident energy, you would have to use less than 85%. The calculation of IE includes a calculation assuming a 15% lower arc current to account for arc current variation.

If you run the arc flash calcs at the utility supplied fault current only (which I don’t recommend), then absolutely add a safety factor when adjusting the instantaneous. For example, if you estimated a cable length too short, the actual arcing current will be less than calculated. This safety factor would help to compensate.

In my case after running the different scenarios, setting the instantaneous at 100% of the arcing current should be OK. I have already taken into account the possibility of decreased arcing current. I don’t believe the utility will ever decrease their capacity to 40%. So I should be covered.

With that being said, if I can reduce the HRC from #2 to #0 on a panel by reducing the instantaneous of an upstream breaker from 8x to 6x, I do go ahead and reduce this to 5x in my recommendations for an added safety. But what about all of the other devices that calculated to be HRC 0 to start with? I don’t go back and check each one of these to see if they are at the breaking point on the curve. I am relying on my 70% and 40% scenarios to cover these.

I hope I said this in a way that makes sense.

My Vote was a no... if it matters lol

It matters. Any details on why are welcome too.

Perhaps other is the correct answer. Found the following in IEEE C37.010:

"For line-to-ground faults, the interrupting time is estimated to exceed the rated interrupting time by 0.1 cycle. For asymmetrical faults, it is estimated that the interrupting time may exceed rated time by an additional 0.2 cycle. Hence, for grounded asymmetrical faults, the last phase to clear is estimated to be 0.3 cycle slower than the rated interrupting time. Additionally, rated interrupting time may be exceeded during extreme cold weather or when the breaker has been closed for an extended period of time. Also, the breaker may be slower at the lower limits of control voltage and/or mechanism stored energy. These longer interrupting times are in the range of several milliseconds and may have system stability implications."

Seems we should add at least 0.3 cycles, and perhaps more.