Electrostatic charging in ATEX environments — why bonding a tanker truck is not the same as ordinary earthing

The guide on ATEX explosion hazard and the guide on intrinsic safety (Ex i) cover how electrical equipment itself can avoid becoming an ignition source in a hazardous area. This article covers an ignition source that has nothing to do with electrical equipment: electrostatic charging of non-electrical parts (a plastic IBC, the shell of a tanker truck, a powder stream) — and the bonding discipline that specifically manages this risk, set out in IEC 60079-32-1.

How the charge arises

When a liquid, powder or gas flows along a surface (filling or emptying a tank, transport through a pipe, discharge through a nozzle), friction causes charge separation — a process entirely independent of any electrical supply, and one that therefore also occurs in an installation that is completely de-energised. Given sufficient charge build-up on an isolated conductive part (an unearthed tanker shell, a loose IBC container, a person without antistatic footwear), a spark discharge can occur once that part comes close enough to an earthed object — and in an explosive atmosphere, that spark can form an ignition source entirely independent of any electrical wiring or equipment.

Why this is a separate discipline

Regular earthing of an electrical installation (main earthing terminal, protective conductors — see the guide on the main earthing terminal) is aimed at diverting fault current in the event of an electrical fault. Electrostatic bonding is aimed at something different: removing a charge difference between two conductive parts, neither of which is part of an electrical circuit — for example the shell of a tanker truck and the filling installation it is connected to. Both require a connection to earth, but for a different purpose and with a different resistance requirement.

The resistance requirement from IEC 60079-32-1

For a bonding connection when loading or unloading a tanker truck or mobile tank, practical guidance states:

a wholly metallic connection (chassis, tank, pipework and fittings interconnected) of 10 Ω or less — a higher value indicates corrosion or a loose connection that should be investigated first;
an earthing clamp connected before the loading or unloading operation starts, and disconnected only after it has finished;
as a practical upper limit for a bonding connection in the broader sense (including through a surface, for example an antistatic floor), 1 MΩ — high enough to protect a person against electric shock in the event of an electrical fault, low enough to dissipate static charge within an acceptable time.

Practical relevance

For work with flammable liquids, powders or gases in an ATEX zone (tanker trucks, IBCs, silos, dust extraction), the earthing clamp should actually be connected before the operation starts — not as a formality, but as a functional check, preferably with continuity monitoring that blocks the operation the moment the connection is lost. This is separate from, and additional to, the regular ATEX inspection of the electrical installation itself (see the guide on periodic ATEX inspection).

Common mistakes

Assuming a tanker truck making metal-to-metal contact with a loading arm is automatically earthed — corrosion, paint, or an isolated coupling can electrically interrupt that apparent metallic connection.
Connecting the earthing clamp only after filling has already started, when the first seconds of a liquid flow can actually produce the greatest charge build-up.
Confusing electrostatic bonding with the regular main earthing terminal of the electrical installation (§542.4) — both are a connection to earth, but with a different purpose, a different resistance requirement, and often a physically separate connection point.
No periodic check of the earthing clamp and cable itself (corrosion, broken wire in the clamp cable), allowing the resistance value to creep above the practical limit unnoticed.