8 - Protection concept ‘i’ principles

Abstract

This chapter explains the principles of protection concept “I”. The principle of intrinsic safety (IS) ensures that the levels of heat or the size of sparks that occur in an electrical circuit in contact with a flammable gas are limited to below the specified levels that may cause ignition. The use of the term “circuit” has important implications. Electrical energy can only produce heat or sparks when electricity is flowing. Since it can only flow in a complete circuit, it is the safety of the circuit that is of concern. The components of the circuit do not pose a threat unless electricity is passing through them, in which case they must be part of a “circuit.” Heat and sparks occurring in this circuit can be assessed for compliance with the standards for IS. The chapter explains the shunt diode safety barrier (hereafter referred to as a “barrier”), which comprises the component arrangement discussed above but in a self-contained package. It may be interposed easily between safe and hazardous area circuits. The chapter also presents associated apparatus. Such apparatus has the effect of electrically interfacing between the safe and hazardous areas. Barriers and isolators are known as “IS interfaces” and are self-contained devices that interpose between two circuits. Signals may pass in either direction or a limited amount of power is allowed to enter the hazardous area IS circuit. Certified interfaces usually conform to industry standard safety descriptions and are easy to match with certified apparatus, which themselves follow a standard form.