We have railroad equipment with 12 and 24 volts direct current with negative ground. I have junior mechanics asking why a positive engine starter post is grounded to frame and it doesn't short out. I told him it was protected with diodes somewhere and acted as an arc suppressor, but I was just guessing. Does anyone know for sure or have information/schematics or something that explains this?
thanx
You have a positive ground system. Works just like the negative ground we have on cars today, except that the negative side is the insulated-isolated switched side, and the chassis is the system common back to the positive side of the battery/charging system. It is common in older diesel trucks, and was used in some vintage American autos, and lots of British ones. It's the same thing, only it is the opposite polarity.Your system probably uses two 12 volt battery banks which are switched by a series/parallel switch to make 24 volts across the starter coils when cranking the engine. After starting, it reverts back to parallel for your 12 volt lights and controls.
You don't have negative ground system. If the positive terminal is connected to frame, it's positive ground.
Same as negative ground system, just invert your thinking... :-)
There may indeed be arc-suppression diodes in the circuit, but this has nothing to do with a positive ground system. Same diodes may exist on a negative ground system. They will protect switch contacts equally on both types.
If you remember to install them correctly :-)
Installing them backwards because you aren't aware it's a positive ground system will let the 'magic smoke' out of them.
daestrom
Not a problem. I have lots of spare magic smoke from some of my past projects.
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