In US locomotives built prior to the latest generation incorporating dc-ac inverters, a diesel engine drives an alternator the field winding of which is used to control alternator output, hence, engine loading. Locomotives with which I'm familiar (EMD, GE, English Electric to a lesser extent) included in the alternator two transformers -one to sample potential and another to sample current. Matched, semiconductor rectifiers are used to convert ac to dc for the series wound, traction motors. Output from the sampling transformers is used by the control system to limit current voltage over the range of locomotive speed. Horsepower is limited by the load regulator in the hydromechanical governor. Traction motor current from (typically) motor #2 is passed through a 75 mV shunt and presented to an ammeter on the operator's control stand. Ground relay circuits provide for immediate shutdown of alternator field excitation and engine fuel supply in the event of a ground fault.

As EMD and GE supply engines, alternators and motors to the mining industry (among others), the equipment providing power to the dragline may already have these sampling transformers available for use, thereby eliminating the need to connect to the HV current shunt.

Good luck and work with one hand behind your back.

Bruce