And if they didn't trespass in the first place? (not a rhetorical question either)
Does a rotten board on a porch, which could cause injury from someone falling through, predicate the need for a sign?
The owner knows it's there and avoids stepping on it until it's repaired. He warns guests to his property.
Should he also warn the thief coming to rob him unawares?
These are good questions
1) In tort law (civil, not criminal law),
- If the trespasser is expected, then the land owner has a duty to warn of ARTIFICIAL dangers, but not natural dangers on the land. In this case the trespasser might be considered known or anticipated and the step probably an artificial danger. So in your hypo, he may very well need to warn the trespasser of the step.
- However, if the trespasser is not known or anticipated, the land owner has no duty to him except to void willful infliction of harm.
2) In criminal law, dangerous or deadly devices are per se illegal. Most likely the electrified sign would need a warning sign. However, in your hypo, since the broken step is probably not there purposely to defend the property, the step most likely is not illegal.
If the person is not a trespasser, then different rules apply which are not relevant to the electrified sign-situation at hand