Are explosion-proof and flameproof the same thing?
Abstract
Same principle, different systems. "Explosion-proof" is the North American term (UL 1203, Class I Division 1); "flameproof Ex d" is the IEC and ATEX term (IEC 60079-1). Both contain an internal explosion, but standards, tests, wiring and markings differ.
They use the same principle but belong to two different certification systems. "Explosion-proof" (often written "explosionproof") is the North American term for an enclosure that withstands an internal explosion and stops it igniting the gas outside; it is certified to standards such as UL 1203 for Class I, Division 1. "Flameproof", type of protection "d", is the IEC and ATEX term, certified to IEC 60079-1 and marked Ex db. The construction rules, tests, wiring methods and markings differ, so a product certified under one system is not automatically accepted under the other.
Short answer
- Same idea: both let the gas ignite inside a strong enclosure and cool the escaping gases in narrow joints so the flame cannot get out, while keeping the outer surface below the ignition temperature.
- Different documents: explosion-proof to UL 1203 (US) or CSA C22.2 No. 30 (Canada), installed under the Division system; flameproof to IEC 60079-1 (and its US and Canadian zone versions), installed in Zones.
- Different gas groups: Class I Groups A, B, C, D versus IIC, IIB, IIA. IIC corresponds to Groups A and B together, IIB to Group C, IIA to Group D.
- Different wiring: Division explosion-proof systems use threaded conduit with sealing fittings; IEC flameproof installations normally use certified flameproof cable glands, or conduit with seals.
- Where accepted: an IEC
Ex dbproduct needs a North American listing to be used in the US or Canada; a UL explosion-proof product needs ATEX or IECEx certification for Europe and IECEx countries. - Language trap: many people use "explosion-proof" for any Ex equipment. An Ex e or Ex i product is explosion-protected, not explosion-proof.
What each term means
US federal regulations (29 CFR 1910.399) define explosion-proof apparatus as enclosed in a case able to withstand an explosion of a specified gas or vapour inside it, prevent ignition of the surrounding gas by sparks, flashes or that explosion, and run at an outer temperature that will not ignite the surrounding atmosphere. NFPA 70 Article 505 and IEC 60079-1 describe the flameproof enclosure in almost the same way: it withstands an internal explosion of a mixture that has penetrated it and prevents transmission to the outside.
In the IEC system the letter "d" is followed by a level of protection: db gives EPL Gb for Zone 1, dc gives Gc for Zone 2, and da exists only for catalytic sensors of portable gas detectors. North American Division equipment has no such levels; "explosionproof" means suitable for Class I, Division 1 for the groups marked. UL 1203 (edition 6) also covers explosionproof equipment for Class I, Zone 1, Groups IIA, IIB and IIC, so the term is not strictly limited to Division areas.
Where the two systems differ
| Point | Explosion-proof (North America, Division) | Flameproof Ex d (IEC, ATEX, zone) |
|---|---|---|
| Main standard | UL 1203 (edition 6, 2023); CSA C22.2 No. 30 | IEC 60079-1; UL 60079-1 and CSA C22.2 No. 60079-1 for zone use in North America |
| Area system | Class I, Division 1 and 2 | Zone 1 and 2 (db), Zone 2 (dc) |
| Gas groups | A (acetylene), B (hydrogen), C (ethylene), D (propane) | IIC, IIB, IIA |
| Typical marking | Class I, Div 1, Groups C, D, T4 | II 2 G Ex db IIB T4 Gb; in the US Class I, Zone 1, AEx db IIB T4 Gb |
| Wiring | Threaded rigid conduit; sealing fittings, for example within 18 inches of the enclosure where the NEC requires them | Certified Ex d cable glands (barrier glands where required) or conduit with seals |
| Threaded entries | Mostly NPT | Metric or NPT only (IEC 60079-1:2014), with the thread type and size identified on the enclosure, the nameplate or in the instructions |
Test methods differ in detail too: reference pressure measurement, overpressure tests, permitted gaps and the mixtures used for non-transmission tests. A 2005 review by the US Mine Safety and Health Administration (MSHA), comparing its own explosion-proof rules for mining equipment with IEC 60079-1, found different test pressures, gap limits and pass criteria, although it judged the safety factors of the two non-transmission test methods essentially equivalent. The point for users is not which is better, but that the certificate must match the system used at the site.
Using equipment across the two systems
In the US. The NEC accepts zone equipment in Division areas only within limits. The 2008 NEC text (501.5), for example, allowed equipment listed for Zone 0, 1 or 2 in Division 2, but only equipment listed for Zone 0 in Division 1; check the edition adopted locally. A flameproof AEx db motor listed for Zone 1 is therefore not enough for a Division 1 location; the area must be reclassified into zones, or the motor needs a Division 1 explosion-proof listing.
In Europe and IECEx countries. A UL-listed explosion-proof enclosure is not ATEX equipment. It needs EU-type examination under Directive 2014/34/EU, or an IECEx certificate where IECEx is accepted, before it can be installed in a Zone 1.
Dual certification is common: many manufacturers certify the same design to both systems and show both markings on the nameplate. Each marking applies only in its own system.
Example: A project in Texas specifies a junction box for a Class I, Division 1, Group D area. An Ex db IIB T6 Gb box with only IECEx and ATEX certificates cannot be accepted, even though it is flameproof. A box listed as explosion-proof for Class I, Division 1, Groups C and D (or dual-certified) is needed, installed with conduit seals as the NEC requires.
Common mistakes
- Calling every Ex product "explosion-proof". Increased safety, intrinsic safety and pressurisation are not explosion-proof designs and have other installation rules.
- Assuming IIB equals Groups B, C, D. Group B is hydrogen, which is IIC. IIB corresponds to Group C only.
- Mixing wiring methods. Fitting a Division conduit system to an IEC gland entry, or the reverse, needs certified adaptors and the correct thread type; see thread adaptors on Ex d entries.
Related on excodex
- Ex protection concepts at a glance: d, e, i, p, m, o, q, n and t
- Flameproof enclosures (Ex d): how flamepaths stop an internal explosion
- North American hazardous location approvals: NEC 500/505/506, UL, CSA and FM
- What is the difference between explosion-proof and intrinsically safe?
- Standards spaces: IEC 60079-1, NEC 500 to 506
Standards and further reading
- IEC 60079-1:2014, equipment protection by flameproof enclosures "d" (text incorporated by reference in US federal regulations): law.resource.org
- UL 1203 (edition 6, 2023), explosion-proof and dust-ignition-proof electrical equipment for hazardous (classified) locations: webstore.ansi.org
- NFPA 70 (NEC) 2008 edition, 501.5 on zone equipment in Division locations (text made public by Public.Resource.Org): archive.org
- NFPA 70 (NEC), Articles 500, 501 and 505; summary of Class I conduit sealing rules (2023 NEC, 501.15): nfpa.org resource
- US Coast Guard OCS National Center of Expertise, Drill Down 25 (material groups) and 26 (protection techniques): dco.uscg.mil, dco.uscg.mil
- US OSHA, 29 CFR 1910.399, definition of explosion-proof apparatus: osha.gov
- MSHA, Part 6 equivalency review and comparison of MSHA and IEC explosion-proof enclosure requirements (December 2005, archived): webarchive.library.unt.edu
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.