1. Inevitable accident
An inevitable accident is something which cannot be avoided by the taking of ordinary precautions. With some exceptions, as under the rule in Rylands v. Fletcher, the plea will form a good defence to actions in tort:
National Coal Board v. J. E. Evans & Co. (Cardiff) Ltd. 1951 - electric cable unknown under land.

2. Assumption of risk
A person who consents to run the risk of injury cannot maintain an action in tort against the person who causes that injury - volenti non fit injuria. This also applies where the law presumes that they have consented to do so. In particular:
a) No one will be allowed to consent to run the risk of illegal harm. Examples: boxing match with bare fists; contestants in a fight or a match intending physical harm.
b) Mere knowledge of the risk need not necessarily amount to consent to run it: Smith v. Baker 1891 - employee hit by a stone having dropped from a crane.
However, outside the master-servant relationship, knowledge of the the existence of a risk coupled with a continuation of the acitivity to which it is incident, may sometimes be treated as equivalent to consent to incur it. But this is not always so: Dann v. Hamilton 1939 - co-riding woman knew of driver's intoxication, but no consent to negligence.
c) Where a person tries to avert the danger created by another, consent is no defence as long as the plaintiff acted under a clear moral duty, not being meddlesome or foolhardy: Haynes v. Harwood 1935 - policeman stopping horsewagon.
However, a special relationship to the person at risk is needed, i.e. professional duty (police, firefighters, doctors) and close contractual relationship (servants' duty of fidelity to their masters). The law encourages the here, but dislikes the busybody.

3. Self-defence
Similar rules as in criminal law will apply here.

4. Necessity
This is an acceptable defence if through the defendant's a act something worse is averted:
Southport Corporation v. Esso Petroleum Co. Ltd. 1956 - shipper releasing oil on plaintiff's land.
But it's rather narrowly applied; no defence in the following cases:
Rigby v. Chief Constable of Northhamptonshire 1985 - police setting a house on fire (psychopath).
Southwark London Borough Council v. Williams 1971 - homeless couple entering a house.

5. Statutory authority
Where the law expressly permits an activity, there is no wrong:
Vaughan v. Taff Vale Railway Co. 1860