Short answer: An intrinsically safe (IS) circuit cannot release enough energy, by a spark or by heat, to ignite the hazardous atmosphere around it. Article 504 covers how these circuits are installed (NEC 504.1). The apparatus and associated apparatus are installed according to the control drawing (NEC 504.10(A)). Any wiring method suitable for unclassified locations may be used (NEC 504.20), but IS conductors are kept apart from the conductors of other circuits (NEC 504.30(A)(1)). IS wiring is identified at terminals and along its route, and light blue may be used for IS conductors where no other conductors are colored light blue (NEC 504.80). Article 505 is the zone system for Class I locations: Zone 0, Zone 1 and Zone 2, from the most to the least time a flammable gas is present (NEC 505.5(B)(1)).
PSI lists Special Occupancies, Equipment and Conditions (6 scored NEC Knowledge and 3 scored Calculations questions) as a subject area of the Texas Journeyman exam. PSI does not name Articles 504 or 505 or say how many questions cover them. The Texas Journeyman Practice Pack has 1,000 original questions and 10 timed mock exams in the real two-part format, with step-by-step solutions for every calculation. Try free practice questions without signing up. Get the full pack for US$39 with one payment.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.
Where this fits
Classes, divisions, groups and Class I wiring with seals are on NEC 500 hazardous locations. Read that first. This page picks up two ideas that page only names: how an intrinsically safe circuit is wired, and how the zone system splits a gas location. Texas refineries, chemical plants and tank farms use both, so the questions are practical.
Intrinsic safety: the words
- Intrinsically safe apparatus sits in the hazardous location, such as a transmitter or switch.
- Associated apparatus is the barrier or isolator that limits the energy. It is usually in an unclassified area or protected by another method.
- Simple apparatus is a device that will not generate or store more than a small amount of energy, such as a thermocouple or a plain contact. It does not need its own listing when the rules for simple apparatus are met (NEC 504.10(D)).
- Control drawing is the manufacturer’s drawing that shows how the IS system must be connected. The installation follows it (NEC 504.10(A)).
Intrinsic safety: the installation rules
- Wiring methods. Any wiring method suitable for unclassified locations may be used for IS circuits. Sealing and physical protection still apply as the article says (NEC 504.20).
- Separation. IS conductors generally do not share a raceway, cable tray or cable with non-IS conductors (NEC 504.30(A)(1)). The exceptions are specific: secured physical separation, a grounded metal or approved insulating partition, or a grounded metal sheath or shield on one group of conductors. Check the exact condition before you pick an answer. In enclosures, IS terminals and wiring are kept away from other circuits by distance or a barrier.
- Grounding and bonding. Associated apparatus and cable shields are grounded as the control drawing requires (NEC 504.50), and the hazardous-location bonding rules apply (NEC 504.60).
- Sealing. Conduits and cables that must be sealed get seals, but those seals are there to keep gas from moving, not to contain an explosion (NEC 504.70).
- Identification. Terminals are identified as IS. Raceways, cable trays and other wiring methods carrying IS circuits are labeled so the labels stay visible and traceable along the run, at the spacing the section sets, so they are not confused with other wiring, and light blue may be used for IS conductors where no other conductors are light blue (NEC 504.80(A)) (NEC 504.80(B)) (NEC 504.80(C)).
The zone system for Class I locations
Article 505 lets a Class I gas location be classified in zones instead of divisions (NEC 505.1). Each room, section or area is classified on its own (NEC 505.5(A)).
| Zone | Flammable gas present | Rough division match |
|---|---|---|
| Zone 0 | Continuously or for long periods | Part of Division 1 |
| Zone 1 | Likely in normal operation | Part of Division 1 |
| Zone 2 | Not likely in normal operation, and only for a short time if it is | Division 2 |
Zone 0 is the strictest, and intrinsic safety is one possible protection technique where the equipment and system are suitable for that zone. On a lookup question, first identify the zone, then check the equipment’s protection technique, gas group and marking in Article 505. Zone gas groups are IIA, IIB and IIC, not the A, B, C and D letters used with divisions.
Worked example: can these share a raceway?
A cable tray holds a 4 to 20 mA IS loop from a level transmitter in a Class I, Division 1 tank area and a 120-volt control circuit for a pump.
- Is one circuit IS and the other not? Yes.
- What does the separation rule say? IS conductors do not share a raceway, cable tray or cable with non-IS conductors unless the article’s separation method is used, such as a grounded metal partition.
- Result. Put the IS loop in a separate cable tray, or confirm that a shared tray meets one of the separation exceptions, such as a grounded metal partition. Then identify the IS wiring as the identification rule requires.
Check yourself
- Which document controls how an IS system is connected? The control drawing.
- Can an IS circuit use a wiring method suitable for unclassified locations? Yes.
- Which zone has flammable gas present continuously or for long periods? Zone 0.
- What color may identify IS conductors? Light blue, where no other conductors are light blue.
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Tab the hazardous location chapter by idea
NFPA 70, National Electrical Code, 2026 Edition (softcover). Write “control drawing, separate, light blue” at the top of Article 504 and draw a three-row zone table next to 505.5, so you do not mix zones with divisions on a timed question. The Texas exam is open book. PSI allows a soft-bound NEC with highlighting, underlining and notes made in advance. Loose-leaf, spiral-bound and ring-bound copies are not allowed, and only permanent index tabs made or supplied by the NEC publisher are permitted; homemade and third-party tabs are not.
FAQ
What makes a circuit intrinsically safe?
It cannot release enough spark or thermal energy to ignite the specific hazardous atmosphere, under normal and specified fault conditions.
Can intrinsically safe wiring share a cable tray with power wiring?
Not unless the separation method in Article 504, such as a grounded metal partition, is used.
What is associated apparatus?
The barrier or isolator that limits energy into the intrinsically safe circuit. It is usually located outside the hazardous area or protected by another method.
What is the difference between Zone 1 and Zone 2?
In Zone 1 a flammable gas is likely in normal operation. In Zone 2 it is not likely, and if present it lasts only a short time.
Do seals on intrinsically safe circuits have to be explosionproof?
No. They keep gas from passing through the raceway or cable, but they do not need to be explosionproof.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70, National Electrical Code, 2026 Edition, Articles 500, 504 and 505
