Increased Ground Fault - NGR

I wouldn't jump straight to the conclusion that the impedance dropped. I'd check the CT readings and where the current was measured first. Was the 86 A recorded on the NGR itself or on the feeder? That could make a big difference.

The system charging current is usually pretty high for medium voltage systems, so high resistance grounding is not normally used. To reduce over voltages and stress, the resistor is sized to pass the charging current, to be effective at reducing transient over voltages.

Past practice has been to employ medium resistance grounding, which detects the ground fault and sectionalizes and removes the ground fault, not allowing it to persist.

The current you measured is likely a combination of charging current and fault current, to explain your question. You possibly could have 3rd harmonic current circulating, if you have rotating generation on the system.

I am assuming your medium voltage system would be 2.3kV or higher.

Also, you stated “of course, the breaker tripped”. The advantage of a high resistance grounding system is that it does not trip for a single line to ground fault.

If your breaker tripped, it was likely due to line to line fault, or typically a double line to ground fault. In this case, a low grade line to line fault, tripping the breaker, assuming the CTs are sensitive enough to pick up that low a current.