Electrical equipment in acetylene areas why acetylene is Group IIC, what IS/IEC 60079-1 flameproofing means, and when to pressurise or remove equipment instead

Every electrical device in an acetylene plant is a potential ignition source, and acetylene is about as easy to ignite as any industrial gas: EIGA Doc 123 gives its minimum ignition energy in air as about 0.02 mJ and its autoignition temperature as about 305 °C. That is why the electrical installation is one of the first things a PESO officer, an insurer or a customer's auditor examines. This page explains the classification acetylene falls into, what Indian documents require, why "flameproof" is harder for acetylene than for almost any other gas, and the two alternatives — pressurisation and segregation — that plant designers have used for seventy years.

Under the IEC 60079 system, adopted in India as IS/IEC 60079, flammable gases are grouped by how easily a flame passes through a narrow gap and how little energy ignites them. Group IIA covers propane and most solvents, IIB covers ethylene, and IIC — the most demanding — covers hydrogen, carbon disulphide and acetylene. Acetylene is classed IIC (NEC Group A in the US system) in both systems' reference tables. Apparatus marked IIC is accepted for IIB and IIA gases; apparatus marked IIA or IIB is not accepted for acetylene.

The temperature class sets the hottest surface the equipment may present. Acetylene's autoignition point of about 305 °C (IS 14814:2023 clause 4.5.2) puts it in T2, a 300 °C ceiling. EIGA Doc 225 states the useful corollary for a filling plant: acetylene equipment (IIC T2) is suitable for acetone, which needs at least IIA T1, and for DMF, which needs at least IIA T2, but equipment rated only for the solvents is not suitable for acetylene.

Sources: EIGA Doc 225 Table 1 and cl 4.4; IS 14814:2023.

The last line needs a correction. IS 15190 Part 2 was written in 2002 against IS 2148, the older Indian flameproof standard, and its "at least IIB" is a minimum too low for acetylene. The clause is often wrongly attributed to IS 8471, which contains no electrical clause at all. For acetylene, specify IIC. IS 14814:2023, the more recent document, refers to IS/IEC 60079-1:2014 without a group, which in practice means the group appropriate to the gas — IIC. The wider context of the code is on the IS 14814 explained page.

A flameproof ("Ex d") enclosure does not keep gas out. It lets gas in, contains an internal explosion, and cools the escaping flame through long, narrow flanged gaps so that it cannot ignite the atmosphere outside. The narrower the gap a gas can pass through, the tighter and more expensive those flanges must be. Acetylene's flame passes very small gaps — the quenching diameter at 1 atm is about 0.12 to 0.16 mm, as compiled by S. A. Miller (1965) — and the gap shrinks as pressure rises.

In 1957 that led British practice, under BS 229, to the view that the maximum safe gap for flanged joints was too small to justify certifying flameproof gear for the most demanding gases, acetylene and hydrogen, on the basis of type test. In well-ventilated dissolved acetylene compressing stations, gear certified to the next group down was accepted; in plants using compressed acetylene for chemical reactions, motors and switchgear were housed away from the plant or pressurised. In the United States, the Underwriters Laboratories hazardous-location list of 1963 showed under 2 percent of certified equipment rated for acetylene — mostly light fittings, conduit fittings and small switches up to 2.5 kW. Both points are as compiled by S. A. Miller (1965). Modern IIC flameproof motors, lights and junction boxes are available, but the range and price still reflect that difficulty.

Pressurisation ("Ex p"). The enclosure is purged with clean air or inert gas and then held slightly above atmospheric pressure so the surrounding atmosphere cannot enter. British plants using compressed acetylene, and some generating plants, enclosed motors, lighting and wiring and pressurised the enclosures and conduit with dry air at a few centimetres water gauge, as compiled by S. A. Miller (1965). The method depends on a reliable air supply and on an interlock that cuts power if pressure is lost — a fail-safe principle the plant already uses for its instrument-air trips.

Segregation. The C.P.I. code Safety in the Production and Use of Acetylene (Paris, 1968) recorded that many manufacturers installed all electrical equipment, lights and cables outside the rooms where acetylene might be free, behind a gastight wall, so that nothing inside needed to be flameproof. Motors driving boosters were to be outside the generator room unless officially recognised as safe in acetylene, with the shaft passing through a gastight gland. Daylight was preferred inside. This approach also satisfies the PESO 15 m rule for non-flameproof fittings by putting the fittings on the far side of a wall rather than at a distance.

Hazardous-area classification divides a plant by how often a flammable atmosphere may be present: Zone 0 continuously, Zone 1 occasionally in normal operation, Zone 2 only abnormally and briefly. In an acetylene plant the inside of the generator, gasholder and pipework is not an electrical zone at all — nothing electrical belongs there. Around hopper lids, screw-feed glands, compressor packings, drains and manifold leads, releases can occur in normal operation, so those areas are commonly treated as Zone 1, and the rest of a well-ventilated building as Zone 2; the actual zones and their extents must come from a hazardous-area classification study for the plant. EIGA Doc 123 gives the ventilation that underpins a Zone 2 judgement: at least 0.3 m³/min per m² of ceiling area, with gas detection alarming at 25 percent of the lower explosive limit and shutting the plant down at 50 percent. Because acetylene's relative density is about 0.91, close to neutral, it tends to drift upward in a still building, so detectors and ventilation outlets are placed high; the lighter than air page explains why floor openings are still needed.

A correct electrical installation does not deal with static or friction. The static electricity page sets out the data: Hashiguchi and Fujisaki (1958) found the energy needed to ignite acetylene by spark falling from 10.5 J at 2 atm to 0.56 mJ at 15 atm, so at manifold pressures any static discharge is enough. The 1965 compilation by S. A. Miller gives 1 ohm as the maximum resistance across flanged connections, checked annually; EIGA Doc 123 limits hoses and plant to 10⁶ ohms; IS 15190 Part 2 allows 10 kΩ between a filling lead and its manifold. Belts must be static-conducting. Findings in plants KIGL has audited have included unauthorised modifications inside motor control centres, which make it impossible to know whether an interlock will trip, and earth resistances measured above the 1 ohm target.