Explosive atmospheres: flying a drone where a single spark can be fatal

An explosion needs three things together: fuel (flammable gas, vapour or dust), oxygen and an ignition source. A gas burns only when mixed with air within a certain range (between the lower and upper explosive limits, LEL and UEL). In a plant, fuel and oxygen cannot be fully eliminated, so safety relies on keeping ignition sources out. A drone is a bundle of ignition sources.

The international IEC 60079 series and the EU ATEX directives classify places by how often and how long an explosive atmosphere is present: Zone 0/1/2 for gases, Zone 20/21/22 for dusts. Japan uses the same concept, calling the gas zones 0種場所, 1種場所 and 2種場所. Each zone sets the level of explosion protection required for equipment brought in.

Protection methods include intrinsic safety (Ex i), which limits circuit energy below what can ignite the gas; flameproof enclosures (Ex d), which contain an internal explosion; and pressurisation (Ex p), which fills the enclosure with clean air. Equipment must also match the gas group (IIA/IIB/IIC) and a maximum surface temperature (temperature class T1–T6). In Japan, explosion-protected electrical equipment used in hazardous locations must in principle pass type examination (型式検定).

You meet explosive atmospheres around equipment in process plants, at flanges and valves of plant piping, inside and on the roofs of storage tanks, in sewers where methane and hydrogen sulphide collect, and in mines with methane and coal dust. They are all places that are dangerous for people, which is why drones are wanted — and also where the preparation before a drone goes in is heaviest.

• Get the zone drawing first: overlay the flight route on the operator’s hazardous-area classification drawing. Takeoff, landing and battery swaps always happen in non-hazardous areas.
• Put it on the permit to work: a drone is often treated like bringing in an ignition source or electrical equipment. Record aircraft, batteries, flight area and time on the permit and fly only once approved.
• Test gas before entry, during flight and until the end: measure flammable gas (as % of LEL), oxygen, and hydrogen sulphide or carbon monoxide. Many sites stop work when flammable gas exceeds a set fraction of LEL (e.g. 10%) under their own standards.
• Decide abort criteria first: if a gas alarm sounds, where does the aircraft land? Plan recovery of a battery-powered aircraft that comes down inside a zone.
• Manage batteries: never use damaged or swollen packs, charge outside classified areas, and suspect thermal runaway in any crashed battery.

In Japan, the Industrial Safety and Health Act and its ordinance restrict electrical equipment in places at risk of explosion or fire, and explosion-protected equipment is subject to type examination. For drone use in petrochemical complexes and similar plants, the Fire and Disaster Management Agency, the Ministry of Health, Labour and Welfare and the Ministry of Economy, Trade and Industry have jointly issued guidelines (first edition 2019, Ver3.0 in 2022) covering how hazardous areas are treated and the conditions for operation. Outdoor flights are also subject to aviation flight rules. Check the aviation, occupational-safety and fire-service (hazardous materials) procedures separately.

In Europe the ATEX directives cover both equipment (2014/34/EU) and workplaces (1999/92/EC); internationally, IECEx runs a certification scheme for explosion-protected equipment. Related: oxygen deficiency & toxic gas, confined spaces, heat, world regulations.

• Flyable explosion-protected drones: wrapping battery, motors and radios in intrinsic-safety or flameproof protection breaks the weight and endurance budget; few aircraft have reached certification.
• Inside running Zone 0/1 areas: demand to look inside without shutting down is strong, but shutdown and gas-freeing remain the norm.
• Lag and bias in gas readings: detectors take time to respond and propeller wash stirs the gas, so where and when you measured may not be where and when the aircraft is.
• Recovery after a crash: who retrieves an aircraft that falls inside a classified area, and how? Many sites still have no procedure.

Lightweight intrinsically safe batteries and electronics have become practical, and certified aircraft fly routine rounds above Zone 2 areas. They carry gas detectors; on finding a leak they withdraw outside the zone themselves and report location and concentration to operators. Only inside Zone 0 tanks does the rule stay “shut down, empty, test first” — and a clear line like that is itself what keeps people safe. That is the editors’ outlook.