API RP 552:1994 pdf download
API RP 552:1994 pdf download.Transmission Systems.
4.2 STANDARD ELECTRONIC SIGNAL RANGES
The wide variety of applications makes standardization of signal ranges difficult. Although the signal range of 4—20 milliamperes DC (maDC) has become the standard for process instruments, not all makes of electronic instruments with the same signal range can be intermixed. Electronic instruments designed to use the same input or output signal may not be compatible because of differences in signal voltages, load impedances, wiring techniques required, intrinsic safety, or other characteristics. ANSI/ISA S50.l, Cornpcitibilitv of Analog Signals for Electronic Industrial Process Instruinents, can be referenced for details on compatibility.
4.3 REGULATORY CODES AND
RECOMMENDED PRACTICES
Ills essential that those responsible for electrical design and installation in processing plants be thoroughly familiar with the current edition of ANSI/NFPA 70 National Electrical Code (NEC); the ISA S 12 series of standards; API Recommended Practice 500, Classification of Areas for Electrical Installations at Petrole urn Facilities; and API Recommended Practice 540, Electrical Installations in Petroleum Processing Plants.
4.3.1 Classified Areas and Listed Equipment
The presence of flammable liquids or vapors in an area necessitates classification of that area for the purpose of installing electrical equipment. API Recommended Practice 500 is a guide for determining classification. The three common classifications in a Refinery are as follows : Class I, Division 1: Class I, Division 2; and Unclassified. The criterion for a Division 1 area is that a flammable gas or vapor is likely to exist under normal operating conditions. The criterion for a Division 2 area is that a flammable gas or vapor is likely to exist only during an abnormal operating condition, such as failure or rupture of equipment. Locations that are not classified as Division I or 2 are unclassified. Unclassified areas are frequently referred to as nonhazardous areas.
Electrical equipment for a Division 1 location is normally designed to be explosion proof. This term means that the equipment enclosure is strong enough to withstand an internal explosion, and the enclosure joints are wide enough and clearances small enough so that flame will be quenched and will not propagate from the interior of the equipment to the surrounding atmosphere.The NEC specifically accepts pressurization in lieu of an explosion-proof enclosure. For guidance on how to provide approved purged and pressurized enclosures see ANSI/NFPA 496 “Purged and Pressurized Enclosures for Electrical Equipment” and ISA-S 12.4 “Instrument Purging for Reduction of Hazardous Area Classification.”
Another alternative to explosion proof equipment is Intrinsically Safe electrical equipment and wiring. Intrinsically Safe equipment is incapable of releasing sufficient energy, under normal or abnormal conditions, to cause ignition of a specific explosive atmosphere. The NEC also specifically mentions that Intrinsically Safe equipment and wiring can be installed in any hazardous location for which they are approved without regard to the NEC requirements for hazardous locations. For guidance in using Intrinsically Safe equipment, see NFPA 493, “Intrinsically Safe Apparatus in Division I Hazardous Locations” and ANSI/ISARP12.6 “Installation of Intrinsically Safe Systems for Hazardous (Classified) Locations.”
Equipment designed for Division 2 areas has no installed sparking contacts or parts hot enough to be a hazard, or has sparking contacts immersed in oil or contained in hermetically sealed chambers. When such equipment is not available, nonincendive or explosion-proof equipment is generally used.
For Class I Division 2 areas, “Non-Incendive” practices are acceptable. See ANSI/ISA S 12.12 “Electrical Equipment for Use in Class 1, Division 2 Hazardous (Classified) Locations.”
4.3.2 Grounding of Equipment Cases and Circuits
Grounding practices in instrumentation are of two types:
those concerned with personnel safety and those concerned with signal accuracy and dependability. Both types must conform to the NEC and the regulations of any local governing bodies. Details of grounding installations are discussed in Section 20.
5 Reducing Electrical Interference In Electronic Systems
Electrical interference is any spurious voltage or current from external sources that appears in the signal transmission circuit. When these voltages are excessive, signals are changed or cannot he detected.
