CABLE ARMOUR AND CABLE SCREENING: A SMALL DIFFERENCE WITH MAJOR CONSEQUENCES
During ATEX inspections, the termination of armoured cables is a frequent subject of discussion. Should the armour be earthed only at the “clean” end, or should it be connected to earth through the cable glands at both ends?
The answer begins with an important distinction: cable armour is not the same as a cable screen.
Metallic armour primarily provides mechanical protection. According to EN IEC 60079-14, it should normally be bonded to the equipotential bonding system at both ends through suitable cable glands or an equivalent connection. Where intermediate junction boxes are installed, the electrical continuity of this bonding arrangement should normally be maintained.
A suitable armour cable gland clamps the armour around its full circumference and provides electrical continuity. However, the mere presence of a metallic cable gland does not prove that a reliable bonding connection exists. Paint, corrosion, non-metallic gland plates, sealing washers or an incorrectly installed armour clamping arrangement can interrupt the electrical path.
Where a non-metallic enclosure is used, a separate earth tag and bonding conductor may therefore be necessary. The enclosure or gland plate must also be demonstrably connected to the equipotential bonding system. Cable armour must not be treated as a substitute for the protective conductor unless this has been specifically assessed and verified in the electrical design.
A conductive cable screen or shield can require a different approach. Particularly in intrinsically safe instrumentation and control circuits, the screen is normally earthed at one point only, usually at the non-hazardous-area end. In practice, this point is often referred to as the “clean earth”. Single-point earthing prevents circulating currents from flowing through the screen and adversely affecting intrinsic safety or signal quality.
This is where confusion arises: the earthing principle used for an Ex i cable screen is sometimes incorrectly applied to the mechanical cable armour.
Single-point bonding of cable armour can be incorporated into an engineered design, but it is not the standard arrangement. It must then be demonstrated that no dangerous potential difference can develop between the armour and the installation. At least one reliable connection to the equipotential bonding system must always remain. Any cable entry device intended to isolate the armour from earth should be installed in the non-hazardous area or in a location requiring EPL Gc or Dc.
The correct inspection question is therefore not simply:
“Is the cable armour earthed?”
The correct question is:
“Is the cable armour electrically continuous and reliably bonded to the equipotential bonding system in accordance with the installation design, without creating a dangerous potential difference or an incendive spark?”
The principle is straightforward:
Cable armour is normally bonded at both ends.
An Ex i cable screen is normally earthed at one point.
Confusing these two functions can result in an installation that appears correct but does not meet the electrical and explosion-safety requirements.
Normative basis: EN IEC 60079-14:2024, including the requirements concerning the earthing of conducting screens and cable-armour bonding.
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