High voltage distribution line fault current

high voltagemainspower

On a conventional 7,200 volt distribution line, what would be the average fault current in a scenario like the one shown in the video below?


Specifically, I'm wondering what the power dissipation is at the fault and why it does not trip an inline fuse farther up the line. Is the line resistance itself limiting the fault current? (What would the resistance of this fault be anyway? Approaching zero ohms? 1 ohm? Thousands?) Does this fault current have any significant effect on the overall load for the area or is it "negligible"? The only reason I ask is that it does not seem to heat the rest of the line significantly or trip the circuit protection upstream.

Best Answer

To begin with the potential of the upstream system needs to be known and the distance and transformers (impedance) that exists.

Once that is known and the closest inline fuse or circuit breaker device is known - some modeling and calculations can be performed.

One metric which is modeled is ISC or Instantaneous Fault Current. This is also referred to as a bolted fault - as in if the phases were bolted together creating a line to line short - what threshold would the current reach and for how long before the upstream overcurrent device could clear the fault. ISC usually has the quickest reaction time IF the fault current gets that high.

Another Metric is LSI or Long term Fault current. I think of this as the long time overload trip on a motor. You don't want this to trip during a short duration starting current but want it to trip if this value is sustained for a period of 5 or 10 seconds.

Depending on the system (which I doubt the utility overhead lines have) it may have LSIG or Line to ground fault monitoring.

The video your refer to was most likely a Line to ground fault which results in a lower current threshold for LSI and ISC detection.

You can see from the video there are arcs that are sustained for a few seconds and then stop for a few seconds.

To answer your question IF the fault currents and interrupt times for the utility were modeled - the event probably had a lower current threshold to pickup on ISC and a shorter time duration than can be detected through LSI. IF the system had LSIG the fault may have been cleared, but again I doubt if the utility has that fault detection.

Fault clearing devices usually have little to do with power dissipation and more to do with sensing current over a time duration.