So a 500v megger has resistors built in that might total 100,000 ohms so the operator doesn't get shocked?
And without those resistors in the megger, it would put out a lot more current?
Well, the internal resistance isn't comprised entirely of resistors deliberately inserted in series.
Part of the internal resistance is the windings of the transformer which generates the high voltage (I'm talking about a modern, electronic type megger here).
Maybe another example would help: Look at a 12V automobile battery versus a 12V transistor radio battery. Both have an EMF, or voltage source, of 12 volts.
The difference is that the chemical structure of the lead-acid car battery gives it a much lower internal resistance than that of the equivalent-EMF radio battery.
The latter might have an internal resistance (caused by its chemical composition) of around 10 ohms. That means that even if you put a dead short on the terminals, the current is limited by that 10-ohm internal resistance (I = 12 / 10 = 1.2A).
But the car battery has a much lower internal resistance, maybe in the region of 0.01 ohms. Short the terminals of that and the current flow is 1200 amps!
