Which ignition sources are most often overlooked in hazardous areas?
Abstract
The sources outside certified electrical equipment: static from people, plastics and low-conductivity liquids; friction in pumps, fans and couplings; hot process surfaces; stray currents when pipework is opened; and what maintenance and contractors bring in.
The ignition sources most often missed are the ones that are not on the electrical equipment list: static electricity (from people, plastic items and low-conductivity liquids), mechanical friction and impact in non-electrical machinery, hot process surfaces, and stray currents when pipework is opened. Close behind come the temporary sources that arrive with maintenance and contractors, and ignition sources hidden inside tanks and vessels rather than around them.
Short answer
- Static electricity: people on insulating floors, plastic buckets, liners, funnels and hoses, unbonded metal parts inside tanks (floats, sample cans), and flow or filtration of low-conductivity liquids such as toluene.
- Mechanical sources in non-electrical equipment: pumps running dry, fan impellers touching housings, failing bearings, slipping belts, couplings and gearboxes, tramp metal in mills.
- Hot process surfaces: steam and thermal oil lines, compressor discharge piping, engine exhausts and dryers, which carry no temperature class.
- Stray currents and cathodic protection: sparks when a flange, spool or meter is removed without a bonding bridge.
- Temporary sources: hot work, phones and tablets, vehicles, contractors' tools and lighting.
- Self-heating and compression: pyrophoric iron sulphide when vessels are opened, oily rags, rapid pressurisation of gas lines.
Why these sources slip through
Many assessments start from the electrical equipment list and the area classification drawing. Electrical apparatus then gets checked thoroughly (Ex marking, EPL, gas group, temperature class), because it has a certificate and a nameplate to check. EN 1127-1 lists 13 types of ignition source, and electrical apparatus is only one of them. The others have no nameplate: a steam line, a person, a plastic jerrycan or a loose float give the reviewer nothing to tick.
Non-electrical equipment is a second blind spot. The EU directives before ATEX covered only electrical equipment; non-electrical equipment with its own ignition sources (the ATEX Guidelines name gears, fans, pumps, compressors, mixers and brakes) has needed ATEX conformity only since 1 July 2003. Older plants still contain such machines that were never assessed for ignition risk.
A third gap is time. Ignition sources often exist only during start-up, cleaning, maintenance or abnormal operation, while the review looks at the plant in steady operation.
The overlooked sources and what to check
| Source | Where it hides | What to check |
|---|---|---|
| Static from people | Operators on insulating floors or in ordinary footwear, filling or sampling | Dissipative footwear and floors where required, earthing points, procedures |
| Static from plastics | Buckets, funnels, drum liners, hoses, bags | Material choice, earthing, ban on insulating containers for solvents |
| Static inside vessels | Floats, sampling cans, gauging tapes, isolated metal parts | Bonding of every internal conductor, fill rate, splash filling |
| Low-conductivity liquids | Filling through filters, high velocities, two-phase flow | Velocity limits, relaxation time after filters, bottom filling |
| Mechanical friction | Dry-running pumps, rubbing impellers, bearings, belts, couplings | Ignition risk assessment of non-electrical equipment, temperature and vibration monitoring, dry-run protection |
| Process hot surfaces | Steam, thermal oil, compressor discharge, exhausts | Surface temperatures against the autoignition temperature, insulation |
| Stray currents | Opening flanges, removing spools or meters, welding return paths | Bonding bridges before breaking pipework, welding return close to the weld |
| Temporary equipment | Phones, cameras, vehicles, contractors' tools and lights | Permit to work, gas testing, rules for portable equipment |
| Self-heating | Iron sulphide deposits, oily waste, dust layers | Keeping deposits wet or inert when opening, closed waste bins, housekeeping |
| Adiabatic compression | Fast pressurisation of lines, compressors drawing in vapour | Slow-opening valves, discharge temperature trips |
A documented case: the static source inside the tank
On 17 July 2007 a storage tank exploded at a solvent distribution facility in Valley Center, Kansas, during transfer of VM&P naphtha, a nonconductive flammable liquid, from a road tanker. The US Chemical Safety Board (CSB) concluded that the transfer equipment between the truck and the tank was most likely properly bonded and grounded. The most likely ignition source was a static spark inside the tank, at a loosely linked level float whose linkage could open a gap as the float moved; stop-and-start filling had introduced air and turbulence. The explosion hurled the tank about 130 feet (40 m), and about 6,000 residents were evacuated.
The CSB also found that none of the 62 safety data sheets it reviewed for such liquids clearly warned that they can form explosive vapour inside storage tanks. Its suggested interim measures included verifying that tank level floats are effectively bonded, pumping more slowly, inerting tanks and using anti-static additives. The lesson for reviewers is that correct earthing of the hoses and the truck does not cover isolated conductors inside the vessel.
How to make an ignition source review complete
- Go through all 13 types of EN 1127-1 for each piece of equipment and each activity, and record "not present" where that is the finding.
- Include non-electrical equipment and check that it has been assessed (ISO 80079-36 and ISO 80079-37 for equipment, the ignition risk assessment in the technical file).
- Look inside tanks, vessels, ducts and filters, not only at the area around them.
- Review every mode: start-up, shut-down, cleaning, sampling, maintenance and expected malfunctions. In Zone 1 sources must be controlled in normal operation and expected malfunctions; in Zone 0 also rare malfunctions.
- Walk the plant with operators and maintenance staff. Plastic buckets, unbonded hoses and missing insulation are found on site, not on drawings.
- Cover contractors and temporary work through the permit-to-work system.
Common mistakes
- "All our equipment is ATEX certified." Certification covers the equipment, not people, plastics, process heat or maintenance work.
- Bonding the outside only. Isolated conductors inside a tank can still spark.
- Assuming a temperature class applies to pipework. Only equipment has a T-class; process surfaces must be compared with the ignition temperature directly.
Related on excodex
- Ignition sources in hazardous areas: the 13 ignition sources and how to control them
- Static electricity as an ignition source: charging, discharges and bonding
- Non-electrical equipment in hazardous areas: Ex h, constructional safety c, control of ignition sources b and liquid immersion k
- Can stray currents and cathodic protection cause ignition in hazardous areas?
- Can a hot pipe or surface ignite a flammable gas without any spark or flame?
Standards and further reading
- EN 1127-1:2019, explosive atmospheres, explosion prevention and protection, basic concepts and methodology (the list of 13 ignition source types).
- ISO 80079-36 and ISO 80079-37 (2016), non-electrical equipment for explosive atmospheres; IEC TS 60079-32-1, electrostatic hazards guidance.
- US Chemical Safety Board, final report on the Barton Solvents explosion, Valley Center, Kansas, 2007: csb.gov
- European Commission, ATEX 2014/34/EU Guidelines, 6th edition (January 2026), on non-electrical equipment in scope since 2003: single-market-economy.ec.europa.eu
- HSE L138, DSEAR Approved Code of Practice and guidance, on identifying ignition sources and safe systems of work: hse.gov.uk
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.