Definitions:

Intrinsically Safe Circuit: A circuit in which any spark or thermal effect is incapable of causing ignition of a mixture of flammable or combustible material in air under prescribed test conditions.

Control Drawing: A drawing or other document provided by the manufacturer of the intrinsically safe or associated apparatus that details the allowed interconnections between the intrinsically safe and associated apparatus.

Rules to follow are in Article 504.

Some important highlights:

Intrinsically safe systems must be installed in accordance with the control drawing.

Intrinsically safe systems can be installed in hazardous locations using ANY OF THE WIRING METHODS USED FOR UNCLASSIFIED LOCATIONS 

HOWEVER,

SEALOFFS MAY BE REQUIRED.  SEPARATION REQUIREMENTS MUST BE ADHERED TO.

BONDING MUST BE PER 250.100.

CIRCUITS, RACEWAYS AND CABLE TRAYS MUST BE IDENTIFIED "INTRINSICALLY SAFE".

Identification:
 Raceways, cable trays, and other wiring methods for intrinsically safe system wiring shall be identified with permanently affixed labels with the wording “Intrinsic Safety Wiring” or equivalent. The labels shall be located so as to be visible after installation and placed so that they may be readily traced through the entire length of the installation. Intrinsic safety circuit labels shall appear in every section of the wiring system that is separated by enclosures, walls, partitions, or floors. Spacing between labels shall not be more than 7.5 m (25 ft).