NFPA 70E Annex D

[R.Immerkaer]

Hi.

For arc flash calculations in equation D.4.2 Arching current, bolted three-phase available short-circuit
current (symmetrical rms), is used.

Can anyone tell me the definition of "bolted three-phase available short-circuit
current"?

In my case I have 65kA short circuit current directly from the terminals of my transformer. But the location of my arc flash incident is downstream to CB limiting the current to 15kA.

What should I consider as input for the arching current calculations?

Thanks in advance

 

[jim dungar]

In my case I have 65kA short circuit current directly from the terminals of my transformer. But the location of my arc flash incident is downstream to CB limiting the current to 15kA.

How does your circuit breaker limit the current?
Or, are you saying that from the transformer terminals to the line side of the breaker, the circuit impedance limits the current?

Arc Fault calculations are based on the bolted fault current at the point of the fault. Most formula assume the current remains at this value for the duration of the arcing event.

 

[R.Immerkaer]

The point of fault, it is actually downstream to a fast acting fuse, so current is interrupted before first peak of short circuit current.
I doesn't count in any impedance from the circuit. I am only interested in knowing if any current limiting devices upstream to the fault can be considered as a reduction of the bolted fault current.
As I see it, bolted fault current could mean that we neglect the arc resistance, which of course is not present is the short is bolted, but limiting aspects are still counted in.

 

[jim dungar]

Current limiting devices should never be considered as lowering the available fault current for arcing fault calculations. The clearing speed of the device versus the fault current flowing through it are the important factors.

Why are you ignoring circuit impedance? That method is okay for short circuit and coordination studies, but is not appropriate for arc flash
The worst case arc flash incident energies are often found at relatively low available fault currents found at the end of feeder and branch circuits, which result in protective devices operating below their current limiting range.

 

[wbdvt]

Arc Fault calculations are based on the bolted fault current at the point of the fault. Most formula assume the current remains at this value for the duration of the arcing event.

More correctly, based on available bolted fault current and then the arcing current is calculated from that.

Ok, back to the OP's question. The 65kA fault current that you reference at the transformer terminals is the infinite bus fault current. Basically it is the transformers full load current divided by the transformer impedance. In the real world, you never see this value but it is used to spec the short circuit ratings of equipment.
The definition of available fault current from NFPA 70E-2018 is "The largest amount of current capable of being delivered at a point on the system during a short circuit condition."
So for you to find out what the incident energy (arc flash) value is at a point in your system, you need to model your system starting with the available short circuit current at the transformer. This value is obtained from the utility company. Most people doing arc flash studies will model the system by obtaining the utility available short circuit current at the utility's transformers primary protective device and modeling the system in from there.