Intrinsic Safety in Hazardous Areas
Intrinsically safe sensors limit the electrical energy in a hazardous area circuit so that no ignition-capable spark or hot surface can form — even under fault conditions. Zener barriers and galvanic isolators enforce the energy limits between the safe zone and the explosive atmosphere.
Use this when…
- When installing sensors inside Zone 0, Zone 1, or Zone 2 classified areas where flammable gases, vapours, or dusts are present — the energy of any spark in the circuit must be insufficient to ignite the surrounding atmosphere
- When your process requires the sensor to remain powered during operation and you cannot interrupt the circuit for maintenance — intrinsic safety allows live work in the hazardous area because energy levels are always below the minimum ignition energy
- When regional regulations or insurance require ATEX-certified equipment for EU installations, or IECEx-certified equipment for international projects in South Africa, Australia, or the Middle East
Oil and gas refinery
Level transmitters inside Zone 1 hydrocarbon vessels use Zener barriers to cap loop energy at 1.2 W and 28 V, ensuring the 4-20 mA signal cable cannot deliver enough energy to ignite the vapour cloud even if the cable is short-circuited or open-circuited.
Grain handling facility
Proximity sensors on bucket-elevator take-up pulleys are classified Zone 21 (combustible grain dust). IS-rated inductive sensors with galvanic isolators prevent dust-cloud ignition from the sensor wiring, satisfying the EU ATEX Directive 2014/34/EU Category 2D requirement.
Intrinsic safety (IS) is an explosion protection technique that restricts the electrical energy available in a hazardous area circuit below the minimum ignition energy of the surrounding atmosphere — under both normal operation and specified fault conditions.
Hazardous areas are classified by the likelihood of an explosive atmosphere being present. Zone 0 is a place where an explosive gas atmosphere is present continuously or for long periods. Zone 1 means it is likely to occur during normal operation. Zone 2 covers locations where it is not likely in normal operation but may occur in abnormal conditions. The equivalent dust zones are Zone 20, 21, and 22.
The key hardware element is the Zener barrier, wired in the safe area between the control system and the field device. A barrier contains two or three Zener diodes that clamp voltage below the certified limit, a resistor that limits current, and a fuse. If a fault drives the supply above the Zener knee voltage, the diodes conduct and the fuse blows — preventing excess energy from reaching the hazardous area. Galvanic isolators use transformer or optical coupling and offer a higher safe-area voltage range, eliminating the need for a high-integrity safety earth.
Cable capacitance and inductance matter because stored energy in a cable can feed a spark after the source is disconnected. IS certification documents include maximum allowable cable capacitance (typically 83–150 nF) and inductance (typically 1–4 mH) — the installed cable parameters must stay within these limits.
Two major certification schemes govern IS equipment. ATEX (from the French ATmosphères EXplosibles) is mandated by EU Directive 2014/34/EU and is required for equipment placed on the EU market. IECEx is the International Electrotechnical Commission's voluntary certification scheme, accepted in South Africa (SANS), Australia, and over 50 other countries. Both schemes assess against IEC 60079-11 (intrinsic safety) and require third-party testing by a notified body or ExCB. Equipment carries a certification mark, an Ex code (for example Ex ia IIC T4 Gb), and a certificate number.
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