Just wondering. Utilities can size their service laterals much smaller than the NEC would require for the premises wiring. They don't have to have the ampacity of the service disconnect. And they can deal with voltage drop by just upping a tap setting on the xfmr a little to make up for small conductors.

So, is it ever a possibility that a long service lateral run can result in enough resistance to cause available fault current to be too low to instantaneously trip the main or other breakers if there's a ground fault event? Anyone heard of any type of scenario such as this?

Yes before it even gets to the customer end of things. Forget about ground faults. That’s small potatoes.

Utilities use overcurrent protection at generating stations and locally in substations but outside substations it simply doesn’t work even after a couple miles. If a tree fell on a line or lightning hit and caused a dead short and they relied on overcurrent protection alone as we do at the loads, it would never trip.

What they actually use is called a distance relay. These work by measuring the voltage and current and calculating the ratio (Z=V/I, Ohms Law) and tripping if this ratio gets too low. Even distance relays have a distance limitation so they may program a recloser every few miles as the distance limit sets in.

Distance relay is designed to operate only for faults occurring between the relay location and the selected reach point, thus giving discrimination

electrical-engineering-portal.com

And grounding is pretty much a joke compared to what you expect. First off in a utility system they practice “peg” grounding. The distribution system is 3 wire. The poles are grounded and there is often a static line but it is in no way a 4 wire system, despite the fact that utilities most often run wye wye transformers. The reason is that the impedance on a cable is proportional to its length. But the impedance between two ground rods is proportional to the inverse of distance. This sounds nonsensical but you have to remember that the Earth is effectively a 2 dimensional sheet. It is slightly greater than 2D but over “short” distances we can ignore the curve. The resistance along any possible path through the Earth grows proportional to distance just as it does on a wire. The number of possible paths grows with the square of the distance. So we get d/d^2 for resistance. Two of the d’s cancel, so the resistance is proportional to 1/d, the inverse of distance. Engineers can read IEEE Green book to verify. Nonengineers will see this math equation in any 3 wire ground test which uses two grounds spaced a long distance apart. So grounding itself is very good but phase conductor impedance drives everything.

As far as the impact looking through a transformer though it’s not as bad as it sounds. We can model the transformer impedance as %Z. Similarly if we assume the transformer is the ideal model then we are looking at only the turns ratio N1/N2 which is called a. The secondary side voltage is V/a and the secondary side current is Ia. We can also transform primary side impedance. Z=V/I (Ohms Law) on the primary side. On the secondary side by substitution we get Z=(V/a)/(Ia) = (V/I) / a^2. So for example with 12,470 to 480 a=26 and a^2 = 626. To put this in perspective transformer %Z varies from a low of 1-3% on small transformers to 5-10% on large ones (1 MVA+). So utility losses are barely noticeable.

On the other hand on say a 1000 kVA transformer with 5% Z at 480 V is 0.0115 ohms. Or looking at short circuit current at the transformer and ignoring utility losses we are looking at 1000 / 480 / 1.732 / 0.05 = 24 kA. This is enough to trip a UL class B breaker curve (10x rating).

500 MCM has an AC impedance if about 0.0278 ohms per 1000 feet so with 4/C 500 MCM to give us 1200 A ampacity the impedance is about 0.007 ohms per 1000 feet. So to double the impedance we need about 16,000 feet. That’s not a long feeder…that’s operating our own utility! Generally it becomes a problem in longer distribution lines (hundreds of feet) with fairly high currents. Using this example going down to a 15 A branch circuit breaker feeding #14 the impedance is much higher, about 3 ohms per 1000 feet, but the instantaneous trip is now at only 150 A.