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1.0 INTRINSIC SAFETY Hazardous atmospheric mixtures include all explosive or ignitable air mixtures involving gases or vapors at an atmospheric pressure and with ambient temperatures between zero and 120°F. The order of ignitability of materials generally corresponds to the National Electrical Code groupings. The workable categories and test materials used as typical for each are: Group A: acetylene (8.7% by volume) Group B: hydrogen (21.0% by volume) Group C: ethylene (7.8% by volume) Group D: methane (8.2% by volume) The ignition capability of an electrical circuit is determined by the electrical energy available and the manner in which such energy is released. Energy may be released in the form of a spark, by resistive heating effects or a combination of the two. There are three basic mechanisms by which electrical energy may be released in the form of spark discharge: discharge of a capacitive circuit, interruption of current in an inductive circuit and make -break of a resistive circuit. The minimum ignition energy for any flammable mixture is the smallest amount of energy released as a spark and sufficient to ignite the mixture at 0 psig. The most easily ignited air mixture is that mixture of a flammable material in air, which requires the minimum amount of energy for ignition. The flammables are usually designated in percent by volume in air. Normal operating conditions include maximum supply voltage and the extreme environmental conditions which fall within the ratings given for the specific equipment under investigation. Abnormal operating conditions usually refer to any two mechanical or electrical faults occurring in combin- ation. The faults are independent and include accidental damage to, and failure of, components or wiring. Intrinsically safe electrical equipment and associated wiring are incapable of releasing sufficient electrical or thermal energy under normal or abnormal operating conditions to cause ignition of a specific hazardous mixture in its most easily ignited concentration in air. The flammable material may be a gas or vapor. Factory Mutual approval is based on examination and tests of samples of production -quality equipment and inspection of manufacturing and quality -control facilities. Of particular consideration are the adequacy of design and workmanship, uniformity and depend- ability of production, effectiveness of quality control, functional suitability, assurance of availability of service and replacement of parts. Installation of intrinsically safe annunciators makes it mandatory to maintain complete isolation between the field contact wiring and any other potential source of voltage. General-purpose external horn relays, horns and bells can be used if wiring is isolated, provided they all use the power from the intrinsically safe power supply. If the power to the horns is to be supplied from a source other than the intrinsically safe power supply, isolation of the other voltage source must be maintained by using the Model KV -700 Horn Relay or any other approved horn relay. To be completely assured of an intrinsically safe installation, all equipment used must be approved by an agency, and the installation, including the wiring, plus all the contact inputs, must meet requirements of isolation to avoid any failures that may occur in the system. Note: Ronan Engineering Company does not accept the responsibility of the installation of intrinsically safe equipment. 2.0 SPECIAL CAUTIONARY NOTES - INTRINSICALLY SAFE ANNUNCIATORS Before applying power and beginning the test procedure, it is important to review all the elements of the annunciator system, including the annunciator cabinet itself, to verify that each component meets the requirements of the National Electrical Code for the area in which it is installed. Particular attention must be paid to reviewing the selection of pushbuttons, horn relays, horns and bells, to assure that they are of an appropriate classification. 2.1 Alarm Contact Inputs Each alarm contact of the intrinsically safe system must be brought to the alarm contact inputs, labelled "1A" and "1 B," as the discrete pair of wires. The common practice of running one A(+) wire to many field contacts is not permitted if intrinsic safety is to be preserved. This is due to the nature of the barrier design, in which only a limited current, insufficient to actuate more than one B input, is made available to the field wiring through each A terminal. The jumpering of A terminals must be avoided, as it also defeats the current -limiting properties of the barrier. Each active input may be wired to a contact that either opens or closes with an alarm condition. Selection of the input contact type is accom- plished on each module by the position of a resistor -type jumper, identified as "NO" and "NC" for the normally open and normally closed positions, respectively. Using this terminology, a field contact that opens with an alarm condition is termed a "normally closed" alarm input and, conversely, the field contact that closes with an alarm condition is termed a "normally open" alarm input. 2.2 Power Source The success of an intrinsically safe system depends upon a resistive barrier to limit the