Because EMD Es have a prime mover/generator dedicated one to each truck a cutout switch would be superfluous. In the case of a ground fault the particular engine/generator/ truck-with-traction-motors would simply fail to load, leaving the other "end" of the unit still working (independent, remember). Thus an E locomotive was it's own protection because virtually ANY breakdown left the other half of the locomotive 100% functional. A beautiful system, simply a take-off on the concept of MU'd multiple locomotive "units" whereby with Es two "units" share a common frame.
On edit: I am repeating an already given explanation 
