Hi everyone,

I did some research on internet about estimation cable length when you don't know this parameter, but I didn't find anything. I found this forum and it seems to have a good community and knowledge that I need.

Did you ever have made some tests if you change the length by about more or less 10%, what was the effect on the energy ? I know that the effects is more important with low voltage side. I.E that we need to have the best estimation for 600V equipment than 13,8kV for example.

Does someone have more explanations, ideas or papers that will help me with this thing.

Thanks a lot for the help.

cable length would have an impact on predicted fault current in the equipment hence on the calculated IE and AFB. Typical software for short circuit analysis would allow you to make changes to cable length and observe its impact on available fault current.

Yeah I understand, but rule of thumbs if you have no idea how far the equipment is. Do we put 100', 300' or 500' for example?

I know that if I put 10' I will have the worst case for short-circuit. But for IE and AFB it may not...

So know, i don't know how to approach it.

It depends on what type of disconnector you have (breaker or fuse)

With breakers you usually have the greatest IEs with high short-circuit currents (so short cables) unless your current gets so low that you hit the lower relay settings (short or even long time pickup instead of instantaneous). Also keep in mind that the arcing current is lower than the calculated short-circuit current when you look at your trip settings!

For fuses there is a tipping point resulting from the fuse diagrams. As long as you stay in the "instantaneous" range (usually when the short-circuit current is greater than 20-30 x nominal fuse current) then highest currents again translate to highest IE. But once you get in the thermal region of the fuse, then lower currents usually mean higher IEs as the increase in fuse melting time is greater than the decrease of short-circuit current (I hope that makes sense). A short example based on DGUV 203-078 calculations but it should translate similarly to IEEE 1584 calculations:

1000 kVA 6% 400 V Transformer, Alu 95 mm² cable, 200 amps fuse
Length - Current - Fuse Time - Arc Energy
50 m - 11.0 kA - 10 ms - 19 kJ
140 m - 4.8 kA - 125 ms - 95 kJ
150 m - 4.5 kA - 180 ms - 128 kJ
160 m - 4.2 kA - 298 ms - 200 kJ
220 m - 3.2 kA - 1950 ms - 978 kJ

As you can see the length difference and resulting short-circuit current difference is small but the Arc Energy is doubled between 140 m and 160 m.

The energy will continue to rise with longer cable lengths. If you apply the 2 second rule it will hit a maximum at that point because the current will go down further but your calculation time wont.

Long story short- without meaningful data you can't really do a good study. You will most likely end up overprotecting if you go for the worst-case approach.

If you cannot walk-down the feeder length and measure it, you are forced to use different lengths and evaluate the results. Use your engineering judgement.

Mike

Thanks MarkusK for the great answer.

It seems to be good and it makes sens ! So in general, if we know that we are close to the long time region for a fuse protection.
It should be good to perform a double check of the length cable(If we were not sure). And with that, we will ensure that our results reflect the "real life".

And do you have any recommendations/suggestions for higher voltage ?

thanks mike for the answer !

I normally know the general length, i.e.: it is 40 ft from transformer to switchboard with 8 ft of vertical. I would model it with 40 ft, 50 ft. and 60 ft. to see the effect and choose the worst case Arc flash not worst case short circuit current. I have many cases with long lead lengths to a pump in the depths of a quarry that create a high arc flash potential. I opt for field gathering the general length and then running multiple scenarios.

If the conduit is installed underground, it's our practice to figure the original contractor installed the run in the shortest way possible. This is just logical as the longer the run, the higher the cost for the contractor. However, if you are in doubt about the length of the wire, do what some of the others have recommended, change the length of the wire to see how much difference an adjustment of 10 to 15% makes. Then, use worst case.

thanks wilhendrix & tish53

thanks for both answer ! It seems very logical ! I will apply those recommendations in my next studies.