When the electrode is in steam a high resistance to ground (insert wall) is presented, and therefore a large signal is returned.
When the electrode is in water a low resistance to ground is presented and therefore a small signal is returned.
If no signal is returned or only a very small amplitude is returned then either a short circuit to ground is present or a wire has been broken or disconnected.
If a ground connection becomes broken or disconnected then a large signal will be returned (equivalent to steam). A separate circuit is used to detect the broken or disconnected wire.