A classified area is one where the atmosphere itself can be the fault. Cable selection in that area is not a choice anybody makes from a catalog, because the area does not set a cable requirement. It sets a protection technique. The technique settles the wiring method, and the wiring method is what finally names a cable.
Reading the code in that order matters, because the same physical hazard is described twice, under two naming systems that both live in the same book and do not translate into each other.
The same hazard, named twice
Article 505 states its own position in the first sentence of its scope, and the wording is worth taking at face value: the article covers "the zone classification system as an alternative to the division classification system covered in Article 500" for equipment and wiring in Class I, Zone 0, Zone 1 and Zone 2 locations where fire or explosion hazards may exist due to flammable gases, vapors or liquids.
An alternative, not a refinement. Articles 500 through 503 handle Class I, II and III locations by division. Article 505 handles Class I locations by zone, and Article 506 handles combustible dusts and fibers by zone as well.
The definition of an unclassified location makes the same point from the other side, because it has to list both systems to exclude both: Class I Division 1 and 2, Class I Zone 0, 1 and 2, Class II Division 1 and 2, and Class III Division 1 and 2. A list that long is only necessary when both systems are live.
The special-occupancy articles then write the two side by side, which is how a designer actually meets them: one hazard, two allowable names.
Both naming systems describe the same hazard
The zone system names its techniques with letters
In the division system, protection is largely expressed through the enclosure and the wiring method. The zone system takes the opposite approach and names the protection technique itself, giving each one a letter.
Letter | Technique, in the wording of 505.2 |
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m | Encapsulation, where parts that could ignite an explosive atmosphere by sparking or heating are enclosed in a compound |
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d | Flameproof, where the enclosure withstands an internal explosion without damage and without igniting the external atmosphere |
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e | Increased safety, applied to equipment that does not produce arcs or sparks in normal service |
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i | Intrinsic safety, where any spark or thermal effect is incapable of causing ignition under prescribed test conditions |
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o | Oil immersion, where equipment is immersed in a protective liquid |
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q | Powder filling, where igniting parts are surrounded by glass or quartz powder |
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p | Pressurization, where a protective gas is held above the pressure of the external atmosphere |
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n | A type of protection where equipment is not capable of igniting in normal operation and a fault capable of ignition is not likely to occur |
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The suffixes carry the zone, and the code states the mapping directly: intrinsic safety is designated ia for Zone 0, ib for Zone 1 and ic for Zone 2, while encapsulation is designated ma for Zone 0 and m or mb for Zone 1. One technique at different levels of assurance, told apart by two letters, and the letter in the marking is what a designer reads to learn which zone the apparatus was assessed for.
One technique removes the wiring method question entirely
Article 504 covers intrinsically safe systems, and its wiring method clause is remarkable for how little it restricts. 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 is handled separately, and separation is handled separately.
Article 504 repeats the point in its identification clause, as a note to the reader: wiring methods permitted in unclassified locations may be used for intrinsically safe systems in hazardous locations.
The reason is structural. Intrinsic safety limits the energy the circuit can deliver rather than building a barrier around it. If the circuit cannot release enough energy to ignite the atmosphere under the prescribed test conditions, the jacket, the conduit and the enclosure are not what stands between the atmosphere and an ignition source. They can be ordinary, because the protection is somewhere else.
For a cable order that is the most useful fact in the family. On an intrinsically safe circuit the cable can be the construction used outside the classified area, and any reason to specify something unusual has to come from somewhere other than the area classification.
The protection technique sets the wiring method
It does not remove the separation rule
Two circuits of different kinds still have to be kept apart. Conductors of intrinsically safe circuits shall not be placed in any raceway, cable tray or cable with conductors of any nonintrinsically safe circuit.
Four exceptions follow, and one is where a cable’s construction starts doing real work.
Exception | What it allows |
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1 | Separation of at least 50 mm (2 in.) with the conductors secured, or a grounded metal partition or an approved insulating partition between them |
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2 | Where either all the intrinsically safe conductors or all the nonintrinsically safe conductors are in grounded metal-sheathed or metal-clad cables capable of carrying fault current to ground |
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3 | Sharing with nonincendive field wiring circuits in a Division 2 or Zone 2 location, under the conditions of the separation clause |
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4 | The same sharing where intrinsically safe circuits pass through a Division 2 or Zone 2 location to supply apparatus in a Division 1, Zone 0 or Zone 1 location |
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Exception 1 carries a note that answers a common question: a sheet metal partition of No. 20 gauge, 0.91 mm (0.0359 in.) or thicker, is generally considered acceptable.
Exception 2 is the one to read twice. It lets a metal-clad or metal-sheathed cable, capable of carrying fault current to ground, stand in place of physical separation, and the code names the cable types it has in mind by pointing at the articles covering metal-clad cable and mineral-insulated metal-sheathed cable. So the wiring method may be ordinary, but where the cable is used as the separation its sheath or cladding is carrying a protection duty.
The label becomes the evidence
Here is the consequence nobody expects. Because the permitted wiring methods are ordinary, an intrinsically safe installation does not look like a classified one. Nothing about the cable or the conduit announces what it is for.
The code’s answer is identification, and it is unusually specific. Labels must be suitable for the environment where they are installed, with consideration given to exposure to chemicals and sunlight. The wiring must carry permanently affixed labels reading "Intrinsic Safety Wiring" or the equivalent, visible after installation, placed so that they may be readily traced through the entire length, appearing in every section separated by enclosures, walls, partitions or floors, with spacing between labels of not more than 7.5 m (25 ft). Circuits are also identified at terminal and junction locations to prevent unintentional interference during testing and servicing.
An exception covers circuits run underground, which may be identified where they become accessible after emerging from the ground.
Then the code says why, in a note that repays reading: without labels to identify the application of the wiring, enforcement authorities cannot determine that an installation is in compliance with this Code. A second note adds the maintenance reason, which is that identification in unclassified locations is necessary so that nonintrinsically safe wire is not inadvertently added to an existing raceway at a later date.
Color coding is permitted as well. Conductors may be identified as intrinsically safe where they are colored light blue and no other conductors of that color are used, and the same color may be applied to raceways, cable trays and junction boxes containing only intrinsically safe wiring.
So on this technique the label is not decoration. It is the evidence that the installation was designed the way it was designed, and a chain of labels at the right interval is a compliance deliverable in the same sense that a test record is.
Intrinsic safety permits ordinary wiring methods
Where the ordinary methods are constrained anyway
Nothing above means a cable in a classified area may be run without thought, because other articles still apply. The scope of Article 505 says so directly: all other applicable rules in the code apply to equipment and wiring in hazardous locations, as modified by Article 504 and by that article.
The marine terminal article is a worked example. Underground wiring there must be in threaded rigid metal conduit or threaded steel intermediate metal conduit, or, where buried under at least 600 mm (2 ft) of cover, may be Type PVC conduit, Type RTRC conduit or a listed cable. Where a cable is used it must still be enclosed in threaded rigid metal conduit or threaded steel intermediate metal conduit from the lowest buried cable level to the aboveground raceway connection.
And then the clause that catches people. Buried raceways and cables under defined Class I locations are considered to be within a Class I, Division 1 or Zone 1 location. Depth is not an exit from the classification. A duct passing beneath a classified area is classified for that portion of its run, and the wiring inside it has to meet the requirement for that location rather than the requirement for the ground it is buried in.
What to establish before a cable is specified
Item | Why it comes first |
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Which naming system the design uses, division or zone | The two do not translate, and the marking on every component has to match the system the design was drawn in |
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The area classification document | The code requires classified areas in industrial occupancies to be documented and available to those who design, install, inspect, maintain or operate the equipment |
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The protection technique, with its letter | The technique is what permits or forbids a wiring method |
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Whether the circuit is intrinsically safe, and its control drawing | If it is, the wiring method is unrestricted and the separation method and the labeling become the real constraints |
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The separation method, if the circuit is intrinsically safe | Distance, a partition, or a metal-clad or metal-sheathed cable capable of carrying fault current to ground |
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The identification scheme and label interval | On an intrinsically safe installation this is what a compliance check will be performed against |
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The cable itself | Last, because it follows from the technique, the separation method and the wiring method |
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FAQ
Does a hazardous location classification mean the cable has to be special?
Not by itself. The classification sets a protection technique, and the technique decides the permitted wiring methods. On an intrinsically safe circuit the code permits any wiring method suitable for unclassified locations, so the cable can be an ordinary construction. The constraints then reappear as separation and identification rather than as cable specification.
Are division and zone ratings interchangeable?
No. The code presents the zone system as an alternative to the division system rather than a conversion of it. A component marked for one system is assessed against a different scheme from the other, so the marking has to match the system the design was drawn in.
What does the letter after the protection type mean?
It is the zone claim. Intrinsic safety is designated ia for Zone 0, ib for Zone 1 and ic for Zone 2, and encapsulation is designated ma for Zone 0 and m or mb for Zone 1. The letter tells a designer which zone the apparatus was assessed for, and it is not interchangeable with the others.
Conclusion
A classified area does not ask what cable you want. It asks which protection technique applies, and then the technique answers for the wiring method. On most of the eight techniques that answer is restrictive and the cable is named fairly quickly.
Intrinsic safety is the exception. Because the protection is a limit on circuit energy, the wiring methods suitable for unclassified locations remain available, and the cable can be ordinary. What replaces the restriction is discipline in two other places: the separation between intrinsically safe and nonintrinsically safe conductors, which a metal-clad or metal-sheathed cable can satisfy if it can carry fault current to ground, and the identification, which the code requires in writing because an inspector otherwise has no way to tell the installation apart from an ordinary one.
Cable for classified and industrial installations
ZONG Cable builds industrial and control cable and states, before an order, what construction and what marking a cable will carry, which is the point at which a project either lines up with its area classification drawing or does not. Send us the naming system, the protection technique, the conductor count and the separation method, and we will set out the construction that satisfies them. The range sits under cable, the project process from specification to shipment is covered in the guide to custom cable projects, mechanical and environmental selection for plant environments is set out under industrial flexible cable, sealing where a cable crosses a rated boundary is covered in the guide to firestop and cable penetration, the certification position is listed under certifications, and private-label marking and labeling programs sit under OEM support.
Sources
- NFPA 70, National Electrical Code, 2011 edition, Article 505, Zone 0, 1, and 2 Locations. Read directly for this article: 505.1 Scope, including the statement that the article covers the zone classification system as an alternative to the division classification system covered in Article 500, for equipment and wiring in Class I, Zone 0, Zone 1 and Zone 2 hazardous (classified) locations where fire or explosion hazards may exist due to flammable gases, vapors or liquids, and its informational note referring to Articles 500 through 504 for the division classification system; 505.2 Definitions, including the definitions of encapsulation m as a type of protection where electrical parts that could ignite an explosive atmosphere are enclosed in a compound, of flameproof d, of increased safety e, of intrinsic safety i as a type of protection where any spark or thermal effect is incapable of causing ignition of a mixture of flammable or combustible material in air under prescribed test conditions, of oil immersion o, of powder filling q, of pressurization p, and of type of protection n, together with the notes stating that encapsulation is designated ma for Zone 0 and m or mb for Zone 1, that intrinsic safety is designated ia for Zone 0, ib for Zone 1 and ic for Zone 2, and that intrinsically safe associated apparatus designated by the same letters in square brackets is connected to intrinsically safe apparatus but is located outside the hazardous location unless also protected by another type of protection such as flameproof; the definition of unclassified locations as locations determined to be neither Class I Division 1, Class I Division 2, Class I Zone 0, Zone 1 or Zone 2, Class II Division 1, Class II Division 2, Class III Division 1, Class III Division 2, or any combination of them; 505.3 Other Articles together with its exception that all other applicable rules apply as modified by Article 504 and that article; and 505.4(A) Documentation for Industrial Occupancies, requiring all areas in industrial occupancies designated as hazardous (classified) locations to be properly documented and the documentation to be available to those authorized to design, install, inspect, maintain or operate electrical equipment at the location. https://archive.org/details/gov.law.nfpa.nec.2011
- NFPA 70, 2011 edition, Article 504, Intrinsically Safe Systems. Read directly for this article: 504.20 Wiring Methods, stating that 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, with sealing as provided in 504.70 and separation as provided in 504.30; 504.30 Separation of Intrinsically Safe Conductors, including (A)(1) In Raceways, Cable Trays, and Cables, the prohibition on placing intrinsically safe circuit conductors in any raceway, cable tray or cable with conductors of any nonintrinsically safe circuit, Exception No. 1 allowing separation of at least 50 mm (2 in.) with the conductors secured or a grounded metal partition or an approved insulating partition and its informational note that No. 20 gauge sheet metal partitions 0.91 mm (0.0359 in.) or thicker are generally considered acceptable, Exception No. 2 allowing grounded metal-sheathed or metal-clad cables capable of carrying fault current to ground with its informational note naming the cable types covered by the articles for metal-clad cable and mineral-insulated metal-sheathed cable as typical, Exception No. 3 allowing intrinsically safe circuits in a Division 2 or Zone 2 location to be installed in a raceway, cable tray or cable along with nonincendive field wiring circuits when installed in accordance with the separation clause, and Exception No. 4 making the same allowance for intrinsically safe circuits passing through a Division 2 or Zone 2 location to supply apparatus in a Division 1, Zone 0 or Zone 1 location; the exception in the sealing section stating that seals shall not be required for enclosures that contain only intrinsically safe apparatus except as required by 501.15(F)(3); 504.80 Identification, including the requirement that labels be suitable for the environment where they are installed with consideration given to exposure to chemicals and sunlight, 504.80(A) Terminals, 504.80(B) Wiring, requiring permanently affixed labels with the wording "Intrinsic Safety Wiring" or equivalent, located so as to be visible after installation and placed so that they may be readily traced through the entire length of the installation, appearing in every section of the wiring system separated by enclosures, walls, partitions or floors, with spacing between labels of not more than 7.5 m (25 ft), its exception for circuits run underground, and its informational notes that wiring methods permitted in unclassified locations may be used for intrinsically safe systems in hazardous (classified) locations and that without labels to identify the application of the wiring enforcement authorities cannot determine that an installation is in compliance with this Code, and that in unclassified locations identification is necessary to ensure that nonintrinsically safe wire will not be inadvertently added to existing raceways at a later date; and 504.80(C) Color Coding, permitting identification of intrinsically safe conductors where they are colored light blue and where no other conductors colored light blue are used, and the same identification for raceways, cable trays and junction boxes colored light blue that contain only intrinsically safe wiring. https://archive.org/details/gov.law.nfpa.nec.2011
- NFPA 70, 2011 edition, Article 516, Spray Application, Dipping, and Coating Processes, at 516.3 Classification of Locations. Read directly for this article: the assignment of the interior of any open or closed container of a flammable liquid and the interior of any dip tank or coating tank as Class I, Division 1, or Class I, Zone 0, as applicable, and the assignment of the interior of spray booths and rooms, the interior of exhaust ducts, and any area in the direct path of spray operations as Class I, Division 1, or Class I, Zone 1, as applicable. https://archive.org/details/gov.law.nfpa.nec.2011
- NFPA 70, 2011 edition, Article 515, Bulk Storage Plants, at 515.8 Underground Wiring and 515.9 Sealing. Read directly for this article: the requirement that underground wiring be installed in threaded rigid metal conduit or threaded steel intermediate metal conduit, or where buried under not less than 600 mm (2 ft) of cover be permitted in Type PVC conduit, Type RTRC conduit or a listed cable, and that where cable is used it be enclosed in threaded rigid metal conduit or threaded steel intermediate metal conduit from the point of lowest buried cable level to the point of connection to the aboveground raceway; and the statement that buried raceways and cables under defined Class I locations are considered to be within a Class I, Division 1 or Zone 1 location. https://archive.org/details/gov.law.nfpa.nec.2011
- ZONG Cable range. https://zh-zong.com/cable/
