American Petroleum Institute

Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Zone 0, Zone 1, and Zone 2 - FIRST EDITION
 N RP 505

Annotation

This document 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 gas or vapor, 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 equipment;

d. classification for locations containing combustible dust, ignitible 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.

PURPOSE

The purpose of this recommended practice is to provide guidelines for classifying locations Class I, Zone 0, Zone 1, and Zone 2 locations at petroleum facilities for the selection and installation of electrical equipment. Basic definitions given in the 1996 edition of NFPA 70, the National Electrical Code (NEC), have been followed in developing this recommended practice. This publication is only a guide and requires the application of sound engineering judgment.