NEC (National Electrical Code ), 2006
The National Electrical Code (NEC) details the positive attachment of light fixtures to suspended ceiling systems in Section 410.16:
Means of Support (C) Suspended Ceiling Framing member of suspended ceiling systems used to support luminaires (fixtures) shall be securely fastened to each other and shall be securely attached to the building structure at appropriate intervals. Luminaires (fixtures) shall be securely fastened to the ceiling framing member by mechanical means such as bolts, screws, or rivets. Listed clips identified for use with the type of ceiling framing member(s) and luminaire(s) [fixture(s)] shall also be permitted.
UL 1598, definition for a supporting device:
“ A supporting device shall support for 1 hour a load equal to four times the total mass to be supported under intended operating conditions. The load shall be applied in the direction of actual loading conditions. ….. There shall be no deflection or deformation either during or after loading that reduces electrical spacing or compromises safety.”