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5.4 Field Wiring in Hazardous Locations Nonincendive FTAs (current limiting) Some of the Field Termination Assemblies (FTAs) that are used in the High-Performance Process Manager subsystem have resistors in the output circuits to limit the current available to the field terminals. These output circuits have been examined and certified by Factory Mutual as being Nonincendive. This means that if the field wires are accidently opened, shorted, or grounded and the HPM is operating normally, the wiring will not release enough energy to cause ignition in the specified flammable atmosphere. Table 5-3 is a list of the analog input, analog output, and digital input FTAs that have Nonincendive outputs. Also, when digital output circuits of a digital output FTA are current and voltage limited to suitable levels by the user, the digital output FTA can also be considered Nonincendive. Cable and load parameters (entity parameters) To ensure that the field circuits are incapable of igniting a specified flammable vapor, the size of the cable and load parameters must be known and controlled. Table 5-3 provides the maximum permissible values of the parameters for each of FTAs that are listed in the table. Electrical code approval In general, field wiring in Division 2 hazardous locations must be done according to local codes; however, in some jurisdictions, Nonincendive wires need not conform to the normal Division 2 wiring rules, but can use wiring methods that are suitable for ordinary locations. See ANSI/ISA S12.12, the section “Electrical Equipment For Use In Class I, Division 2 Hazardous [Classified] Locations.” Current limiting resistor value The value of the resistors on the listed FTAs were selected to assure worst case short circuit currents in a hazardous area of less than 150 milliamps for normal operating equipment. According to NFPA publication #493, Intrinsically Safe Apparatus for Use in Division 1 Hazardous Locations, 150 milliamps from a 24 Vdc source is below the ignition threshold in a resistive circuit for gases in Groups A through D environments.