My MB 220d says:
the GP's have an input of approx 50-60 amps (though some w115's use a 50amp fuse, so input current must vary with year ) with a voltage drop of 0.9 volts per plug.
the GP's remain energized when cranking the engine.
too high of preglow current is a result of a gp failure or poor ground. or short circuit in system.
oil and/or carbon bridges are the leading cause of gp failure's according to mb manual.
(my favorite): there is an insulation material within each gp to prevent internal short circuit's. you can see from the filament end of the gp the "end" of one of the two chamber's containing the insulating material (non-specified as to what this substance is). if you see erosion at the end, imo, that is an indication to not trust the gp. any carbon buildup is a big danger sign. bad injector(s) with poor spray patterns are listed as major cause of gp failure.
too low pressure from the ip is a culprit in gp failure. ip timing set too far advanced is a culprit in early gp failure.
note, that all of the above refers to the loop filament type of gp. testing for short circuits in a cold gp can be problematic if the insulation material is cold, that is, the short doesn't show up until the gp is hot.
other piece of info: in the 220d combustion temps are around 800 degrees centigrade. in preglow, the filament is around 900-1000 degrees centigrade. the threaded bore of the head into which the gp threads must be cleaned meticulously. using the tool to clean the prechamber opening into which gp slides is very necessary.
sorry for the disordered post. i'm just quoting or paraphrasing specific points contained in the manual.