Does every Ex enclosure need its external earth terminal connected?
Abstract
Not always. Exposed metal parts must be bonded, and the external terminal is the usual way. IEC 60079-14 also accepts secure metallic fixing to bonded steelwork or bonding through armour glands or conduit, and IS enclosures need bonding only if documents or static require it.
No, not in every case, but in most. IEC 60079-14 requires every exposed and extraneous conductive part in a hazardous area to be part of the equipotential bonding system, and the external earth terminal is the normal way to achieve that for a metal Ex enclosure. The standard also accepts other routes: a metal enclosure firmly fixed in conductive contact with bonded steelwork or piping, bonding through armour-clamping cable glands or metal conduit, and, for intrinsically safe equipment, no bonding at all unless its documents or static control require it.
Short answer
- The terminal exists because IEC 60079-0 requires it: electrical equipment with a metallic enclosure must have an external earthing or bonding facility, except equipment designed to be moved while energised on a cable with an earth conductor, or designed only for wiring systems (metal conduit, armoured cable) that make an external connection unnecessary.
- Whether you must connect it is an installation question decided by IEC 60079-14, the manufacturer's instructions and the site's earthing design.
- Not separately connected is acceptable only where the enclosure is firmly secured to and in conductive contact with structure or piping that is itself bonded. Paint, gaskets and loose brackets do not count.
- Metal enclosures of intrinsically safe or energy-limited equipment need not be bonded (public 2007 text), unless the equipment documentation requires it or bonding is needed to stop static charge building up.
- When the route is unclear, connect the terminal: it is cheap, it is easy to inspect, and many site specifications require it anyway.
Why bonding matters in a hazardous area
Potential differences between an enclosure and the steel next to it can cause sparks. They come from earth fault currents, induced and stray currents, lightning and static charge. In a safe area such a spark is harmless; in an explosive atmosphere it is an ignition source. Equipotential bonding ties all metal parts to the same potential so that no incendive spark can jump between them.
The external terminal is in addition to the internal connection for the cable's protective conductor. IEC 60079-0 requires the two to be electrically connected inside the equipment, and it sizes the external facility for a conductor of at least 4 mm² (public 2004 text). The protective conductor deals with fault disconnection; the external bond keeps the enclosure at the same potential as its surroundings.
What IEC 60079-14 allows
The public text of the 2007 edition (reproduced as AS/NZS 60079.14:2009) sets out the rules below. The 2024 edition still has a clause on protective equipotential bonding; check its wording for your project.
| Situation | Does the external terminal need its own conductor? |
|---|---|
| Metal enclosure on a bonded structure, firmly fixed, metal-to-metal contact | No separate conductor needed, provided the contact is reliable and lasting |
| Enclosure on a non-conductive support, painted bracket, rubber mounts or plastic strut | Yes |
| Cable armour or braid clamped in a bonding cable gland, or metal conduit system | Glands that clamp armour or braid can provide the bonding; follow the manufacturer's instructions for that equipment |
| Metal enclosure of intrinsically safe or energy-limited equipment | Not required, unless the documents ask for it or static charge must be prevented |
| Frames of doors and windows that are not part of the structure or installation | Not required if there is no danger of voltage displacement |
| Installation with cathodic protection | Not connected to the bonding system unless the system is designed for it |
Connections must be secure against self-loosening and made so that corrosion does not reduce their effectiveness. Neutral conductors never form part of the bonding system.
Practical case: a junction box on a painted bracket
An Ex e junction box is bolted to a painted steel bracket welded to a bonded pipe rack, and fed by armoured cable through a gland that clamps the armour. The armour is bonded at the switchboard end. Is the external stud needed?
The painted bracket is not a reliable conductive contact, so the "firmly secured" route is out. The armour gland may bond the box, but only if the gland and the box's internal arrangement are designed to carry the bond and the instructions say so. Most designers would simply run a bonding conductor from the external stud to the rack's bonding point: it removes doubt, and it gives the inspector a connection to check.
What inspectors look for
IEC 60079-17 inspection schedules (public 2007 text) include a check that earthing connections, including any supplementary earthing bonding connections, are satisfactory, for example that connections are tight and conductors are of sufficient cross-section. An unconnected external terminal is a finding only if the enclosure has no other valid bonding route. Typical real defects are a bond made under paint, a corroded lug, a conductor looped through several items so that removing one breaks the others, or a missing bond on equipment mounted on non-conductive supports.
Common mistakes
- Assuming the cable's protective conductor is enough for every case. It handles fault disconnection; the site's bonding design decides whether a separate external bond is needed as well.
- Treating "not separately bonded" as a general exemption. It applies only to the routes the standard names, and the instructions of the equipment may still ask for the external bond.
- Relying on mounting bolts through paint. Contact through paint or gaskets is not a bond.
Related on excodex
- Earthing, bonding and equipotential bonding in hazardous areas
- Common Ex inspection findings and how to fix them
- What bonding resistance is needed to control static electricity in hazardous areas?
- Is armoured cable mandatory in hazardous areas?
- Standards space IEC 60079-14
Standards and further reading
- IEC 60079-14, design, selection and erection of electrical installations (current edition 6.0, 2024); publisher preview: https://cdn.standards.iteh.ai/samples/103765/564de58c9bf24efdbf0187e8d3027dfc/IEC-60079-14-2024.pdf
- AS/NZS 60079.14:2009, identical public text of IEC 60079-14:2007, clause on potential equalization (superseded; check the current edition): https://archive.org/details/as-nzs.60079.14.2009
- IEC 60079-0:2004, general requirements, clause on connection facilities for earthing or bonding conductors (public text; check the current edition): https://law.resource.org/pub/us/cfr/ibr/inc/iec/iec.60079-0.2004.pdf
- IS/IEC 60079-17:2007, inspection and maintenance, inspection schedule items on earthing and bonding (superseded; check the current edition): https://law.resource.org/pub/in/bis/S05/is.iec.60079.17.2007.pdf
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.