[UCANSEE2]

I don't know if the fuel pumps are electrical or mechanical however.

Mostly mechanical. But there are multiple pumps besides the ones on the engines.

In this incident, the 'short' centers on the wires to sensors for the #4 engine.

[UCANSEE2]

In the illustration, it shows a short occurring far away from the CWT. The idea is that high voltage ran down the sensor wires to the tank, and the sensors created a spark.

Since the sensors are electrically isolated, how did they produce a spark ?

The independent test (I can find the link to the video again, if you wish) just used a spark gap and kept increasing the output until the tank exploded. Of course a spark gap creates a spark when you apply enough current.