Incoming AC generally doesn't have any ground. However, in 3-pin mains connections you have a PE (protective earth). This PE should be connected to chassis in case you have a metal enclosure. If the enclosure is non-conductive, it's possible to use only 2 wires and leave PE unconnected. Generally speaking, all metal chassis end up connected to PE wire in the outlet.
If you provide more details it may be easier to understand what is meant by "incoming AC fround".
