Selecting the correct hazardous area protection method is one of the most consequential decisions an instrumentation engineer makes. Use the wrong protection method in a classified area and you create an ignition source in an environment that contains explosive mixtures. The two most common protection methods — intrinsic safety and explosion proof (flameproof) — work on fundamentally different principles, have different installation requirements, and are suited to different applications.

How Hazardous Areas Are Classified

Before choosing a protection method, the area must be classified. Two classification systems are in common use:

  • IEC/ATEX (Zone system): Used in Europe, the UK, and most of the world. Zone 0 = explosive atmosphere present continuously or for long periods; Zone 1 = likely to be present in normal operation; Zone 2 = not likely in normal operation, but may occur briefly. Gas groups (IIA, IIB, IIC) indicate the ignition energy required — IIC (hydrogen, acetylene) is the most sensitive and hardest to protect against.
  • NEC/Division system (North America): Division 1 = hazardous atmosphere present under normal conditions; Division 2 = hazardous only under abnormal conditions. Groups A (acetylene), B (hydrogen), C (ethylene), D (propane) correspond approximately to IEC groups.

The classification determines which protection methods are acceptable and which equipment certifications are required.

Intrinsic Safety: Limiting Energy

Intrinsic safety (IS) prevents ignition by ensuring that the electrical energy in the circuit is always too low to ignite the hazardous atmosphere, even under fault conditions. The concept is based on the minimum ignition energy of the gas or vapour present — if the circuit can never store or release more energy than the ignition threshold, no ignition is possible regardless of sparking or open circuits.

IS circuits are designed to IEC 60079-11 (ATEX) or UL 913 (North America). Key elements:

  • Zener barriers: Installed in the safe area (control room). A zener barrier limits the voltage and current that can flow into the hazardous area using zener diodes (voltage clamps) and resistors (current limiters). The barrier is connected to a certified earth (ground). Zener barriers are the traditional solution — inexpensive and reliable, but require a certified earth conductor and careful installation.
  • Galvanic isolators: An alternative to zener barriers that use transformer or optical isolation to pass signals between safe and hazardous area without a conductive path. Galvanic isolators do not require a certified earth and provide better noise immunity. They are more expensive but preferred for new installations.
  • Associated apparatus: Any IS-rated equipment in the safe area (barriers, power supplies) that forms part of the IS circuit. The combination of associated apparatus and field device must be assessed together — two individually certified components may not form a certified system if their parameters are incompatible.

IS allows Zone 0 and Division 1 installations with appropriate apparatus. It is the only protection method approved for Zone 0 in most standards. IS circuits are also the only type that can be worked on live (with appropriate procedures) without shutting down the process — a major maintenance advantage.

Intrinsic Safety Installation Rules

IS circuits have strict wiring requirements to maintain their integrity:

  • IS circuits must be segregated from non-IS circuits throughout the installation — separate cable trays, separate junction boxes, or blue-sheathed IS cables. Mixing IS and non-IS cables in the same tray destroys the IS protection.
  • Total loop capacitance and inductance of the field cable must be within the limits of the IS certificate. Long cable runs may require de-rating of the number of devices per segment.
  • All junction boxes, cable glands, and conduit seals in the IS circuit must be rated for the area classification, even though the IS principle means sparks inside them are non-incendive.
  • Entity parameters (Voc, Isc, Ca, La from the barrier; Ci, Li from the field device) must be compared and verified to be compatible before the system is installed. This calculation is the IS loop certification.

Explosion Proof / Flameproof: Containing the Explosion

Explosion proof (North American term) or flameproof (IEC term, Ex d) takes the opposite approach. Rather than limiting energy to prevent ignition, it accepts that an internal ignition may occur and contains it within a robust enclosure. The enclosure is designed to:

  • Withstand the internal overpressure from an internal explosion without rupturing
  • Cool any hot gases before they escape through flame paths (controlled gap dimensions) so they cannot ignite the external atmosphere

The design of flame paths (gap length, gap width) is precisely calculated and certified for specific gas groups. An enclosure certified for IIA (propane) may not be safe in IIC (hydrogen), because hydrogen can propagate through narrower gaps.

Explosion proof / flameproof enclosures are suitable for Zone 1 / Division 1 (Ex d) and Zone 2 / Division 2. They are not approved for Zone 0 alone — IS or pressurisation (Ex p) is required for Zone 0.

Explosion Proof Installation Rules

  • Cable entries must use certified Ex d cable glands or conduit seals that maintain the enclosure integrity. Standard cable glands are not acceptable in Ex d enclosures.
  • Flame paths (housing joints, threaded entries) must be inspected and kept free of damage. A dent or scratch on a flame path surface can compromise the certification.
  • Enclosures must not be opened while energised in a hazardous area without a hot work permit or without gas testing to confirm the atmosphere is safe. Unlike IS, Ex d equipment cannot be worked on live.
  • Any modifications to an Ex d enclosure (adding cable entries, welding) void the certification. Modifications must be made by the original manufacturer or a certified repair facility.

Choosing Between IS and Ex d

The decision depends on the application and installation context:

  • Choose IS when: Instruments are small signal devices (4-20 mA transmitters, thermocouples, RTDs, HART-capable devices). IS is ideal for transmitters because their inherently low power requirements fit easily within IS energy limits. IS also allows live working and FOUNDATION Fieldbus / HART communication in the hazardous area without special procedures.
  • Choose Ex d (explosion proof) when: Equipment requires more power than IS can supply — motors, solenoid valves, analysers, switchgear, heaters, lighting. Ex d is the standard for power equipment in hazardous areas. It is also used for junction boxes that terminate multiple non-IS circuits.
  • Avoid mixing protection methods in the same circuit: An IS field device connected through a non-IS junction box loses its IS certification. Every element in the IS circuit from the safe area barrier to the field device must be part of the certified IS system.

Other Protection Methods

IS and Ex d are the most common, but not the only protection methods. Purged and pressurised enclosures (Ex p) maintain a positive pressure of clean air or inert gas inside the enclosure, preventing the hazardous atmosphere from entering — used for large control panels and analysers. Non-incendive (Ex nA, Division 2 only) is a simplified form of IS for Division 2 / Zone 2 areas where the atmosphere is only present abnormally, with less stringent energy limits.