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Article 504

Intrinsically Safe Systems

  1. Scope. This article covers the installation of intrinsically safe (I.S.) apparatus, wiring, and systems for Articles 500 through 516.Informational Note: For further information, see ANSI/ISA-RP 12.06.01-2003, Recommended Practice for Wiring Methods for Hazard‐ ous (Classified) Locations Instrumentation — Part 1: Intrinsic Safety.
  2. Definitions.•Different Intrinsically Safe Circuits. Intrinsically safe circuits inwhich the possible interconnections have not been evaluated and identified as intrinsically safe.•Intrinsically Safe Circuit [as applied to Hazardous (Classified)Locations]. A circuit in which any spark or thermal effect is incapable of causing ignition of a mixture of flammable or combustible material in air under prescribed test conditions.Informational Note: Test conditions are described in ANSI/ UL 913-2006, Standard for Safety, Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, and III, Division 1, Hazardous (Classified) Locations.•
  3. ARTICLE 504 — INTRINSICALLY SAFE SYSTEMS
  1. Application of Other Articles. Except as modified by this article, all applicable articles of this Code shall apply.
  2. Equipment. All intrinsically safe apparatus and associ‐ ated apparatus shall be listed.

Exception: Simple apparatus, as described on the control drawing, shall not be required to be listed.

504.10 Equipment Installation.

  1. Control Drawing. Intrinsically safe apparatus, associated apparatus, and other equipment shall be installed in accord‐ ance with the control drawing(s).A simple apparatus, whether or not shown on the control drawing(s), shall be permitted to be installed provided the simple apparatus does not interconnect intrinsically safe circuits.Informational Note No. 1: The control drawing identification is marked on the apparatus.Informational Note No. 2: Associated apparatus with a marked Um of less than 250 V may require additional overvoltage protection at the inputs to limit any possible fault voltages to less than the Um marked on the product.
  2. Location. Intrinsically safe apparatus shall be permitted to be installed in any hazardous (classified) location for which it has been identified.Associated apparatus shall be permitted to be installed in any hazardous (classified) location for which it has been identi‐ fied.Simple apparatus shall be permitted to be installed in any hazardous (classified) location in accordance with 504.10(D) .
  3. Enclosures. General-purpose enclosures shall be permit‐ ted for intrinsically safe apparatus and associated apparatus unless otherwise specified in the manufacturer’s documenta‐ tion.
  4. Simple Apparatus. Simple apparatus shall be permitted to be installed in any hazardous (classified) location in which the maximum surface temperature of the simple apparatus does not exceed the ignition temperature of the flammable gases or vapors, flammable liquids, combustible dusts, or ignitible fibers/flyings present. The maximum surface temperature can be determined from the values of the output power from the associated apparatus or apparatus to which it is connected to obtain the temperature class. The temperature class can be determined by:
    1. Reference to Table 504.10(D)
    2. Calculation using the following equation:

[504.10(D)]

T  Po Rth  Tamb

where:

T = surface temperature

Po = output power marked on the associated apparatus or intrinsically safe apparatus

Rth = thermal resistance of the simple apparatus

Tamb = ambient temperature (normally 40°C) and reference Table 500.8(C)

In addition, components with a surface area smaller than 10 cm2 (excluding lead wires) may be classified as T5 if their surface temperature does not exceed 150°C.

504.20 Wiring Methods. Any of the wiring methods suitable for unclassified locations, including those covered by Chapter 7 and Chapter 8, shall be permitted for installing intrinsically safe apparatus. Sealing shall be as provided in 504.70, and sepa‐ ration shall be as provided in 504.30.

504.30 Separation of Intrinsically Safe Conductors.

  1. From Nonintrinsically Safe Circuit Conductors.
    1. In Raceways, Cable Trays, and Cables. Conductors of intrinsically safe circuits shall not be placed in any raceway, cable tray, or cable with conductors of any nonintrinsically safe circuit.Exception No. 1: Where conductors of intrinsically safe circuits are separated from conductors of nonintrinsically safe circuits by a distance of at least 50 mm (2 in.) and secured, or by a grounded metal partition or an approved insulating partition.Informational Note: No. 20 gauge sheet metal partitions0.91 mm (0.0359 in.) or thicker are generally considered accept‐ able.Exception No. 2: Where either (1) all of the intrinsically safe circuit conductors or (2) all of the nonintrinsically safe circuit conductors are in grounded metal-sheathed or metal-clad cables where the sheathing or cladding is capable of carrying fault current to ground.Informational Note: Cables meeting the requirements of Arti‐ cles 330 and 332 are typical of those considered acceptable.Exception No. 3: Intrinsically safe circuits in a Division 2 or Zone 2 location shall be permitted to be installed in a raceway, cable tray, or cable along with nonincendive field wiring circuits when installed in accordance with 504.30(B).Exception No. 4: Intrinsically safe circuits passing through a Divi‐ sion 2 or Zone 2 location to supply apparatus that is located in a Divi‐ sion 1, Zone 0 or Zone 1 location shall be permitted to be installed in a raceway, cable tray, or cable along with nonincendive field wiring circuits when installed in accordance with 504.30(B).Informational Note: Nonincendive field wiring circuits are described in 501.10(B)(3), 502.10(B)(3), and 503.10(A)(4).
    2. Within Enclosures. Conductors of intrinsically safe circuits shall be secured so that any conductor that might come loose from a terminal is unlikely to come into contact with another terminal. The conductors shall be separated from conductors of nonintrinsically safe circuits by one of the methods in(1) through (4).
      1. Separation by at least 50 mm (2 in.) from conductors of any nonintrinsically safe circuits
        Table 504.10(D) Assessment for T4 Classification According to Component Size and Temperature
        imageTotal Surface AreaExcluding Lead Wires Requirement for T4 Classificationimage<20 mm2 Surface temperature ≤275°C≥20 mm2 ≤10 cm2 Surface temperature ≤200°C≥20 mm2 Power not exceeding 1.3 W**Based on 40°C ambient temperature. Reduce to 1.2 W with an ambient of 60°C or 1.0 W with 80°C ambient temperature.
      2. Separation from conductors of nonintrinsically safe circuits by use of a grounded metal partition 0.91 mm (0.0359 in.) or thicker
      3. Separation from conductors of nonintrinsically safe circuits by use of an approved insulating partition that extends to within 1.5 mm (0.0625 in.) of the enclosure walls
      4. Where either (1) all of the intrinsically safe circuit conductors or (2) all of the nonintrinsically safe circuit conductors are in grounded metal-sheathed or metal-clad cables where the sheathing or cladding is capable of carrying fault current to ground
      Informational Note No. 1: Cables meeting the requirements of Articles 330 and 332 are typical of those considered acceptable.Informational Note No. 2: The use of separate wiring compart‐ ments for the intrinsically safe and nonintrinsically safe termi‐ nals is a typical method of complying with this requirement.Informational Note No. 3: Physical barriers such as grounded metal partitions or approved insulating partitions or approved restricted access wiring ducts separated from other such ducts by at least 19 mm (3∕4 in.) can be used to help ensure the required separation of the wiring.
      1. Other (Not in Raceway or Cable Tray Systems). Conduc‐ tors and cables of intrinsically safe circuits run in other than raceway or cable tray systems shall be separated by at least 50 mm (2 in.) and secured from conductors and cables of any nonintrinsically safe circuits.Exception: Where either (1) all of the intrinsically safe circuit conduc‐ tors are in Type MI or MC cables or (2) all of the nonintrinsically safe circuit conductors are in raceways or Type MI or MC cables where the sheathing or cladding is capable of carrying fault current to ground.
  2. From Different Intrinsically Safe Circuit Conductors. The clearance between two terminals for connection of field wiring of different intrinsically safe circuits shall be at least 6 mm (0.25 in.), unless this clearance is permitted to be reduced by the control drawing. Different intrinsically safe circuits shall be separated from each other by one of the following means:
    1. The conductors of each circuit are within a grounded metal shield.
    2. The conductors of each circuit have insulation with a minimum thickness of 0.25 mm (0.01 in.).
    Exception: Unless otherwise identified.
  3. From Grounded Metal. The clearance between the unin‐ sulated parts of field wiring conductors connected to terminals and grounded metal or other conducting parts shall be at least 3 mm (0.125 in.).

504.50 Grounding.

  1. Intrinsically Safe Apparatus, Enclosures, and Raceways. Intrinsically safe apparatus, enclosures, and raceways, if of metal, shall be connected to the equipment grounding conduc‐ tor.Informational Note: In addition to an equipment grounding conductor connection, a connection to a grounding electrode may be needed for some associated apparatus; for example, zener diode barriers, if specified in the control drawing. See ANSI/ISA-RP 12.06.01-2003, Recommended Practice for Wiring Methods for Hazardous (Classified) Locations Instrumentation — Part 1: Intrinsic Safety.
  2. Associated Apparatus and Cable Shields. Associated appa‐ ratus and cable shields shall be grounded in accordance with the required control drawing. See 504.10(A).Informational Note: Supplementary connection(s) to the grounding electrode may be needed for some associated appara‐ tus; for example, zener diode barriers, if specified in the control drawing. See ANSI/ISA RP 12.06.01-2003, Recommended Practice for Wiring Methods for Hazardous (Classified) Locations Instrumenta‐ tion — Part 1: Intrinsic Safety.
  3. Connection to Grounding Electrodes. Where connection to a grounding electrode is required, the grounding electrode shall be as specified in 250.52(A)(1), (A)(2), (A)(3), and (A)(4) and shall comply with 250.30(A)(4). Sections 250.52(A)(5), (A)(7), and (A)(8) shall not be used if any of the electrodes specified in 250.52(A)(1), (A)(2), (A)(3), or (A)
  4. are present.

504.60 Bonding.

  1. Intrinsically Safe Apparatus. Intrinsically safe apparatus, if of metal, shall be bonded in the hazardous (classified) location in accordance with 501.30(A), 502.30(A), 503.30(A), 505.25, or 506.25, as applicable.in accordance with
  2. Metal Raceways. Where metal raceways are used for intrinsically safe system wiring, bonding at all ends of the race‐ way, regardless of the location, shall be

501.30(A), 502.30(A), 503.30(A), 505.25, or 506.25, as applica‐

ble.

504.70 Sealing. Conduits and cables that are required to be sealed by 501.15, 502.15, 505.16, and 506.16 shall be sealed to minimize the passage of gases, vapors, or dusts. Such seals shall not be required to be explosionproof or flameproof but shall be identified for the purpose of minimizing passage of gases, vapors, or dusts under normal operating conditions and shall be accessible.

Exception: Seals shall not be required for enclosures that contain only intrinsically safe apparatus, except as required by 501.17.

504.80 Identification. Labels required by this section shall be suitable for the environment where they are installed, with consideration given to exposure to chemicals and sunlight.

  1. Terminals. Intrinsically safe circuits shall be identified at terminal and junction locations in a manner that is intended to prevent unintentional interference with the circuits during test‐ ing and servicing.
  2. Wiring. Raceways, cable trays, and other wiring methods for intrinsically safe system wiring shall be identified with permanently affixed labels with the wording “Intrinsic Safety Wiring” or equivalent. The labels shall be located so as to be visible after installation and placed so that they may be readily traced through the entire length of the installation. Intrinsic safety circuit labels shall appear in every section of the wiring system that is separated by enclosures, walls, partitions, or floors. Spacing between labels shall not be more than 7.5 m (25 ft).Exception: Circuits run underground shall be permitted to be identified where they become accessible after emergence from the ground.Informational Note No. 1: Wiring methods permitted in unclas‐ sified locations may be used for intrinsically safe systems in hazardous (classified) locations. Without labels to identify the application of the wiring, enforcement authorities cannot deter‐ mine that an installation is in compliance with this Code.
    Informational Note No. 2: In unclassified locations, identifica‐ tion is necessary to ensure that nonintrinsically safe wire will not be inadvertently added to existing raceways at a later date.
  3. Color Coding. Color coding shall be permitted to identify intrinsically safe conductors where they are colored light blue and where no other conductors colored light blue are used. Likewise, color coding shall be permitted to identify raceways, cable trays, and junction boxes where they are colored light blue and contain only intrinsically safe wiring.

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