43 U.S. Code § 503 - “Unusual or emergency conditions” defined
The term “unusual or emergency conditions”, as used in section 502 of this title, shall be construed to mean canal bank failures, generator failures, damage to transmission lines; or other physical failures or damage, or acts of God, or of the public enemy, fires, floods, drought, epidemics, strikes, or freight embargoes, or conditions, causing or threatening to cause interruption in water or power service.
Source(June 26, 1948, ch. 676, § 2,62 Stat. 1052.)