I have recently been playing with SKM to see what effects differenct cable arrangments had on fault currents. I noticed the following and was interested in hearing comments, or explanations to some of the questions that resulted from my modeling.
I first noticed that a 3/c tray cable had a lower overall impedance when compared to 3 single conductor cables in a conduit, the reason being that the reactance of the 3/c cable is less with the resistances being the same. Does anyone know why the reactances are lower for a 3/c cable? From what I can tell this can impact fault currents from a modeling perspective however mostly in the higher cable sizes? Do others agree?
I also noticed that cable reactances are lower when installed in PVC conduit vs rigid steel conduit. What causes these reactances to be lower?
Also when doing data collection in the field I was wondering how close one needed to be when measuring cable distances and how this distance accuracy would effect calcuated fault currents. From playing around with a few models I noticed that for larger cable sizes around 500MCM a 10% difference in cable lengths resulted in about a 5% differnece in fault currents and this seemed to be linear. For smaller cable sizes a 10% difference seemed to have a more dramatic effect on fault currents that didn't really change much between different 10% differences. Does anyone know from experience or a rule of thumb what % accuracy one should have when determining cable lenghts for fault studies, or what the relationship is for differences in lenghts?
I first noticed that a 3/c tray cable had a lower overall impedance when compared to 3 single conductor cables in a conduit, the reason being that the reactance of the 3/c cable is less with the resistances being the same. Does anyone know why the reactances are lower for a 3/c cable? From what I can tell this can impact fault currents from a modeling perspective however mostly in the higher cable sizes? Do others agree?
I also noticed that cable reactances are lower when installed in PVC conduit vs rigid steel conduit. What causes these reactances to be lower?
Also when doing data collection in the field I was wondering how close one needed to be when measuring cable distances and how this distance accuracy would effect calcuated fault currents. From playing around with a few models I noticed that for larger cable sizes around 500MCM a 10% difference in cable lengths resulted in about a 5% differnece in fault currents and this seemed to be linear. For smaller cable sizes a 10% difference seemed to have a more dramatic effect on fault currents that didn't really change much between different 10% differences. Does anyone know from experience or a rule of thumb what % accuracy one should have when determining cable lenghts for fault studies, or what the relationship is for differences in lenghts?