Find ohms of the original.
R = E / I, R = 240V / 20A, R = 12 ohms.
Now this is used on a 208/120V system.
The Neutral of 208/120V system is intentionaly connected to Ground and you noted 0 restistance to ground, thus the circuit at midpoint is being operated as on 120V system.
1/2 of the 12 ohms is 6 ohms.
I on the 120v as now operating is;
I = E / R, I = 120V / 6R, I = 20A on each of the two sections of the ground faulted 240, 4800W, 20A heater operated on the 208V system.