In this simulation, the different placement of the current sensor between (?) and (ACM) does not affect the operation. In the physical world of actual parts, the placement matters a lot. Placing the current sensor in the ground lead as in (ACM), may be physically impossible because the ground is a plane and not a lead. Placing the current sensor in the hot lead as in (?) requires a current sensor with an isolated output. A resistor is a simple current sensor which senses both AC and DC current, but the output is not isolated from the current being sensed. A current transformer provides an isolated output and senses AC current, but cannot sense DC current. A Hall-effect sensor is isolated and can sense DC current, but is more complicated than the other sensors.
