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API RP 505: 2018 [paper]
API Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Zone 0, and Zone 2
The purpose of this recommended practice (RP) is to provide guidelines for classifying locations Class I, Zone 0, Zone 1, and Zone 2 at petroleum facilities for the selection and installation of electrical equipment. Basic definitions given in the 2014 edition of NFPA 70, the National Electrical Code (NEC), have been followed in developing this RP. This publication is only a guide and requires the application of sound engineering judgment.
NOTE Recommendations for determining the degree and extent of classified locations Class I, Division 1, and Division 2 are addressed in API 500, Recommended Practice for Classification of Electrical Installations at Petroleum Facilities Classified as Class I, Division 1, and Division 2.
Electrical installations in areas where flammable liquids, gases, or vapors are produced, processed, stored, or otherwise handled can be suitably designed if the locations of potential sources of release and accumulation are clearly defined. Once a location has been classified, requirements for electrical equipment and associated wiring shall utilize applicable publications that apply this document such as NFPA 70 (NEC), CSA C22.1 (Canadian Electrical Code), or API 14FZ.
NOTE There are other publications that establish area classification criteria that may differ from those in this document. Refer to those documents for installation requirements, protection techniques, and wiring methods that align with the area classification criteria applied.
This RP applies to the classification of locations for both temporarily and permanently installed electrical equipment. It is intended to be applied where there may be a risk of ignition due to the presence of flammable gases, flammable liquid produced vapors, or combustible liquid produced vapors mixed with air under normal atmospheric conditions. Normal atmospheric conditions are defined as conditions that vary above and below reference levels of 101.3 kPa (14.7 psia) and 20 °C (68 °F) provided that the variations have a negligible effect on the explosion properties of the flammable materials.
The following items are beyond the scope of this document:
a) piping systems used for odorized natural gas used as fuel for cooking, heating, air conditioning, laundry, and similar appliances;
b) catastrophes such as well blowouts or process vessel ruptures—such extreme events are not predictable and require emergency measures at the time of occurrence;
c) the suitability of locations for the placement of non-electrical incendiary equipment;
d) classification for locations containing combustible dust, ignitable fibers, or flyings;
e) installations underground in mines;
f) areas for the processing and manufacture of explosives; and
g) areas where the presence of flammable mist may give rise to an unpredictable risk and that require special consideration
Recommendations for determining the degree and extent of classified locations for specific examples of situations commonly encountered in petroleum facilities are given in Section 8 through Section 14. While it is important for area classifications at refineries, production and drilling facilities, and pipeline facilities to agree to some extent, there are differences in production, drilling, transportation, and refining facilities. Some differences include the process conditions, types and quantities of products handled, the physical size of typical facilities, and varying housing and sheltering practices.
API C50502, API Recommended Practice 505