A more thorough explanation:

Public necessity

In tort law, public necessity is a defense that can be used against charges of trespass. It applies when a defendant interferes with a plaintiff's property in an emergency situation to protect the community or society as a whole from a greater harm that would have occurred if the defendant had not committed trespass. Public necessity serves as an absolute defense, and a defendant is not liable for any damages caused by his trespass.

For example, if a fire breaks out in a building and a firefighter breaks down a door to enter the building and put out the fire, the firefighter can use the defense of public necessity if the owner of the building sues for trespass. The firefighter's actions were necessary to protect the community from harm, and therefore the defense of public necessity applies.

Another example is if a police officer enters a private property without a warrant to stop a dangerous criminal who is hiding on the property. The officer can use the defense of public necessity if the property owner sues for trespass. The officer's actions were necessary to protect the community from harm, and therefore the defense of public necessity applies.

Contrast with private necessity, which applies when a defendant interferes with a plaintiff's property to protect his own interests or the interests of a small group of people, rather than the community as a whole.

See also necessity defense, which is a broader defense that can be used in criminal law as well as tort law.