We have some old Westinghouse 8000 amp DC rectifiers in our plant that supply 250 VDC to overhead cranes. These rectifiers have 18 parallel diodes in the positive leg and 18 parallel diodes in the negative leg.

We believe that we need to calculate the forward resistance of the 18 diodes in parallel in order to calculate the DC arc resistance. The data sheet on the diode shows a curve of "forward current vs forward voltage" And I believe we can calculate the forward resistance based on this curve. Are we on the right track?

Has anyone performed DC arc calculations on a similar system? Appreciate any input on this.

Not that easy. If you had DC then this would work since you have volts and amps which gives you resistance (ohms law). Howevef in power electronics the knee is around 0.7 Volts which is inconsequential at 250 V. There is some small resistance further up the curve but again, almost inconsequential.

You really need to model the AC side as a voltage source and a resistance where the resistance is the wiring and transformer as usual, plus the resistance of the wiring on the DC side. You then need to know the opening time of the overcurrent device and that should do it. I know that ETAP and SKM both have DC modelling capabilities now but DC systems are usually small and fairly well isolated so that doing a paper model is easy.

By the way, the values in the tables in 70E-2012 are so large that even very large systems hardly ever approach them. I have a large MG set in a Ward Leonard Loop (motor-generator tied to a large motor so that excitation applied to the generator controls current on the DC motor's armature with about a 1000:1 amplification). The motor rated voltage is 90 V and the rated current at stall is around 5000 A. Even projecting full instantaneous output of the control system, I can't get above 4 cal/cm^2. In another system (electrostatic precipitator) the output is 55,000 VDC with milliamp current. Another example is large 125 VDC battery banks. They can draw a heck of a spark but not much energy. In your system since you just have free wheeling diodes you might get higher but in my experience I doubt it.