Is a flameproof enclosure still safe if one cover bolt is missing?
Abstract
No. Every cover bolt is part of the certified flameproof design, which was pressure and flame tested with all fasteners fitted. One missing, loose or wrong bolt means the enclosure no longer matches its certificate. Fit the specified bolt before relying on the enclosure.
No. You cannot rely on a flameproof (Ex d) enclosure with a missing cover bolt. The cover bolts hold the flamepath gap closed under the pressure of an internal explosion, and the enclosure was tested and certified only with all of its specified fasteners in place. A missing, loose or wrong bolt is a fault that has to be corrected before the enclosure is relied on.
Short answer
- The type tests of IEC 60079-1 (explosion pressure, overpressure, flame transmission) are made with the screws and nuts the manufacturer specifies. The certificate covers that configuration only.
- IEC 60079-1 (2014 edition) sets no general minimum number of cover bolts and no maximum spacing for Group II enclosures. Each design proves its own number by test, so "eleven out of twelve is close enough" has no basis.
- Replacement bolts must match the manufacturer's design: size, length, thread, and property class or minimum yield stress, which is marked on the equipment or stated on the certificate.
- A bolt that is too long can bottom in a blind hole and leave the cover loose; a bolt that is too short may not hold.
- IEC 60079-17 lists missing bolts as a defect visible at a visual inspection and asks for bolts to be of the correct type, complete and tight.
Why one bolt matters
During an internal explosion the pressure acts on the whole inside face of the cover. The bolts stop the cover lifting and keep the flamepath gap within its certified maximum. Remove one and the unsupported span of cover between the remaining bolts doubles at that point. The cover can deflect more there, the gap can open beyond the tested value, and the joint may no longer cool the escaping gas enough. The wall and fasteners were also overpressure tested, at 1.5 times the measured explosion pressure (4 times for enclosures without routine production testing), with the full set of bolts.
There is no general margin you can count on. The required number and spacing come from testing a specific design, and a 2005 comparison of IEC and US mining requirements noted that the IEC standards do not clearly define a maximum spacing between fasteners or a minimum number of them, unlike the US mining rules. Only the manufacturer's tested arrangement is known to work.
A missing bolt is also a symptom. It often means the cover was removed in a hurry, that other bolts may be loose, or that the threads in the body are damaged. Check the whole joint, not just the empty hole.
Choosing the right replacement bolt
| Check | Why |
|---|---|
| Size and thread | Must match the tapped hole in the body |
| Length | Too long can bottom in a blind hole; IEC 60079-1 requires at least one full thread to remain free at the base when tightened without a washer |
| Property class or yield stress | Marked on the equipment as a caution label or stated on the certificate; a weaker bolt may stretch under explosion pressure |
| Head type and material | As specified; IEC 60079-1 (2014 edition, 11.2) does not permit fasteners of plastic material or light alloys on flameproof enclosures |
| Washer and torque | As in the manufacturer's instructions |
IEC 60079-17 requires bolts and similar parts on which the type of protection depends to be replaced only by similar parts in accordance with the manufacturer's design, and any special bolts or tools to be available. Keep spare bolts of the correct specification on site for common enclosure types.
Example: During a visual inspection an inspector sees an empty bolt hole on the cover of an Ex db IIB T4 Gb motor starter. The equipment label reads that fasteners must have a minimum yield stress. The finding is recorded and the site procedure is followed while the fault is open. A technician fits a bolt of the size, length and class given on the label, checks that the others are present and tight to the specified torque, and confirms that the flange faces are clean and the gap is within the documented maximum. The inspector closes the finding.
Common mistakes
- Using any bolt from the stores because it fits the thread. Strength class and length matter.
- Leaving bolts out on covers that are opened often. Every bolt goes back every time.
- Tightening a long bolt until it stops. If it bottoms in the blind hole, the cover is not clamped.
- Fixing the empty hole with a self-tapping screw or a bolt and nut through a damaged thread. Damaged threads are a repair job under IEC 60079-19.
Related on excodex
- Flameproof enclosures (Ex d): how flamepaths stop an internal explosion
- Common Ex inspection findings and how to fix them
- How are flamepath gaps on an Ex d enclosure checked during inspection?
- Standards spaces: IEC 60079-1, IEC 60079-17
Standards and further reading
- IEC 60079-1:2014 (edition 7), flameproof enclosures "d": fasteners specified by the manufacturer, yield stress marking, blind holes, overpressure test; publicly available as incorporated by reference: law.resource.org. A 2026 edition has since been published.
- IEC 60079-17, inspection and maintenance (current edition 2023); the 2007 edition, adopted in India as IS/IEC 60079-17, contains the definitions of the inspection grades (missing bolts at visual inspection, loose bolts at close inspection), the inspection schedule items and the replacement rule paraphrased above: law.resource.org
- P. Thurnherr (convenor of IEC maintenance team MT 60079-14), presentation on IEC 60079-14 and IEC 60079-17 including the inspection grades (missing bolts at visual inspection, loose bolts at close inspection), IECEx International Conference 2018, Split: slides
- Comparison of IEC 60079 and US MSHA requirements for explosion-proof enclosures, including fasteners (archived MSHA report): 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.