# Thread: Arc flash CL fuse series with breaker

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## Arc flash CL fuse series with breaker

In my opinion the IEEE formulas do not consider the current limiting capability of devices in series with one another. So for example if there is a breaker which has calculated 50kA may only see 30kA and may take longer to trip and may be a higher or lower incident energy based on that. This invalidates every single study that has been ever done because none of them take this into consideration.

Arc flash engineering calculations are the biggest farse and should stop immediately. I might start writing a paper about how bad it actually is because there is so much depth that it's not possible to do it accurately.

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2. As you probably know, the current limiting fuse contribution to reducing fault current is considered when it is part of a tested combination and determining SCCR of the breaker / equipment where it resides or series listed.

The protection methodology is inverse time (less current means slower acting OCPD) and thus results in higher PPE most of the time when lower current is considered.

Dynamic impedance downstream of a current limiting fuse makes it hard to reliably use the fuse curves for let through because it may bring the fuse into its non-limiting range by the time downstream impedance dynamically grows.

http://www.ecmweb.com/content/fault-...turer-response

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Originally Posted by 310 BLAZE IT
In my opinion the IEEE formulas do not consider the current limiting capability of devices in series with one another. So for example if there is a breaker which has calculated 50kA may only see 30kA and may take longer to trip and may be a higher or lower incident energy based on that. This invalidates every single study that has been ever done because none of them take this into consideration.

Arc flash engineering calculations are the biggest farse and should stop immediately. I might start writing a paper about how bad it actually is because there is so much depth that it's not possible to do it accurately.

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For more in depth discussion of arc flash related topics I highly recommend https://brainfiller.com/arcflashforum.

I would like to see you post there and see what Mr. Phillips and the members' responses would be.

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I would also suggest that you find the IEEE paper written by Doan, et. al. concerning the results of a DuPont study of arc flash incidents at their facilities. In a nutshell, the results were:

• Facilities where no arc rated PPE was worn, every incident resulted in an injury 100% of the time
• Facilities where arc rated PPE was based on the tables in NFPA 70E, the injury rate was 50%
• Facilities where arc rated PPE was based on an arc flash study following the methodology of IEEE 1584, there were no injuries as the correct PPE was being worn for the hazard, a 0% injury occurrence.

So you tell me which facility you would like to work at and wear the PPE.

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Originally Posted by dkidd
For more in depth discussion of arc flash related topics I highly recommend https://brainfiller.com/arcflashforum.

I would like to see you post there and see what Mr. Phillips and the members' responses would be.
I will be much more tame and reasonable if I post, I guess I'm just frustrated with having to perform the studies and not believing the results I produce.

Series rating is a test and ieee arc flash was also created through tests... Maybe someone should do some empirical calculations In parallel with the existing tests and then add some variables to the existing formulas.

Is there a new ieee paper due out soon?

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Originally Posted by wbdvt
I would also suggest that you find the IEEE paper written by Doan, et. al. concerning the results of a DuPont study of arc flash incidents at their facilities. In a nutshell, the results were:

• Facilities where no arc rated PPE was worn, every incident resulted in an injury 100% of the time
• Facilities where arc rated PPE was based on the tables in NFPA 70E, the injury rate was 50%
• Facilities where arc rated PPE was based on an arc flash study following the methodology of IEEE 1584, there were no injuries as the correct PPE was being worn for the hazard, a 0% injury occurrence.

So you tell me which facility you would like to work at and wear the PPE.
Sorry not talking about existence of PPE but rather calculated incident energy. Good paper to read though thanks for the reference. I understand what arc flash is doing is good to save people and lives.

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As far CL fuses and incident energy, there are formulas in IEEE 1584 for calculating incident energy where CL fuses are in the circuit.

So maybe I am unclear what your question is.

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Originally Posted by wbdvt
As far CL fuses and incident energy, there are formulas in IEEE 1584 for calculating incident energy where CL fuses are in the circuit.

So maybe I am unclear what your question is.
I'm referring to downstream devices to CL fuses during the fault that don't see the current limitation of the upstream fused switch. The cl fuse calculation also only uses bolted fault current as an Input.. I'm skeptical of its simplicity but that's another topic.

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The CL fuse equations are the result of testing and developing a formula based on test results, really no different than the rest of the equations in IEEE 1584. Even though the software program I use has the ability to use the CL equations for arc flash results, I don't use it.

Per IEEE 1584 on CL fuses: Actual field results could be different for various reasons, as follows:
a) Different system voltage
b) Different closing angle on the voltage wave
c) Different distance from the arc

Note that the formulae were developed based on testing at 600V whereas in my world the voltage is 480V so that fits in with a) above.

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The cl fuse calculation may or may not be precise in scope but my issue is that CL calculations were not given enough attention for more than incident energy Calculations.

I'll state my thoughts one last time in a different way:
As a fault occurs, the impedance of the current limiting fuse increases. This added impedance reduces the fault current and in turn would impact the clearing time of overcurrent devices downstream of the current limiting fuse. If it drops Below the short time pickup you will have a much longer duration arc and therefore higher incident energy.

While this is true another issue is that utility fault currents are given as maximums and there's no way to know when a fault occurs what this will actually be seen as in a system. This issue compounds with the CL fuses, creating a much lower fault current than is expected.

Arc flash studies are fundamentally reliant on fault current and if it can be as low as 50% of the calculated value what's the point of doing all of this work labeling with calorie values which might be completely different from reality.

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