Cathodic protection versus main bonding — why connecting a pipeline can defeat the protection it's meant to have

The guide on galvanic corrosion at earth electrodes covers an unwanted, passive galvanic couple that arises when copper and steel are unintentionally bonded together in the soil. This article covers the mirror image of that: a situation where a metal pipeline or tank is deliberately cathodically protected against corrosion, and where the very bonding connection that the installation standard requires for electrical safety — main bonding — can itself undermine that deliberate protection.

What cathodic protection does

Cathodic protection (CP) is a corrosion-protection technique for buried or submerged metal structures — gas pipelines, oil tanks, sheet piling, pipeline systems — that works by electrochemically making the metal to be protected into a cathode, so that it does not corrode itself. This is done in two ways:

Sacrificial anodes: a less noble metal (usually zinc, magnesium, or aluminium) is electrically bonded to the pipeline to be protected and preferentially corrodes itself, protecting the pipeline via the galvanic couple — the same underlying mechanism as the unwanted copper-steel couple from the related guide, but here deliberately and in a controlled manner applied.
Impressed current: a rectifier drives a direct current through the soil to the pipeline via dedicated, often inert anodes, keeping the pipeline at a sufficiently negative potential relative to the soil to suppress corrosion.

Why main bonding can defeat this

The guide on main bonding of extraneous-conductive-parts covers the installation-standard requirement that an extraneous-conductive-part entering the building — including metal piping systems — must be connected to the main earthing terminal, to prevent a touchable dangerous voltage arising between that part and other earthed parts in the event of a fault elsewhere in the installation. That requirement is entirely independent of, and takes no account of, the cathodic protection of that same pipeline.

The problem: as soon as the cathodically protected pipeline is directly, low-impedance bonded to the building's main earthing terminal — and thereby, via the installation's earthing arrangement, to a much larger, uncontrolled system of other earthed metal parts — a significant part of the protection current (or, with a sacrificial anode system, a significant part of the galvanic current) can leak away via that low-impedance connection into the rest of the earthing installation, instead of continuing to flow to the pipeline itself. The result is that the pipeline's protection potential is no longer kept at the intended level, and the pipeline can still corrode despite an otherwise correctly dimensioned CP system — while at the same time the building's earthing system unwantedly ends up carrying part of the impressed or galvanic current.

Note: this is not an argument for simply omitting the bonding connection — that would violate the touch-safety requirement of §411.3. The point is that both requirements (electrical safety via bonding, and corrosion protection via CP) can only be met simultaneously with a device specifically designed for that combination, not with a simple, direct through-connection.

The practical solution: isolating joint plus DC-decoupling device

The common solution combines two elements:

An isolating joint (insulating flange) in the pipeline itself, at the point where it enters the building, which electrically separates the cathodically protected outdoor pipeline from the part of the pipeline inside the building.
A DC-decoupling device (for example a polarization cell or a semiconductor-based decoupler) connected in parallel across that isolating joint, which presents a high impedance at normal, low voltages (the typical DC potential differences of a CP system) — so that the CP current does not leak away via the building — but which becomes conductive quickly enough during an electrical fault that would cause a dangerous voltage across the joint, so as to still meet the bonding and touch-safety requirement.

In this way the pipeline inside the building remains effectively bonded to the rest of the installation for safety purposes whenever needed, without the cathodic protection of the outdoor section being short-circuited via the building's earthing installation during normal, fault-free operation.

Practical relevance

At a building or site where a cathodically protected pipeline or tank enters the building — common at industrial installations, filling stations, gas distribution, or larger agricultural/utility sites — the design of the main bonding must be explicitly coordinated with the party responsible for the cathodic protection (often a specialized corrosion engineer, not the electrical designer): whether an isolating joint with DC-decoupling device is present or needs to be added, and whether the existing bonding connection is not already unknowingly bleeding off the CP current. A simple visual check (is the pipeline directly welded or clamped to the main earthing terminal with a cable, without an intervening device?) often already gives a first indication of a problem.

Common mistakes

Bonding the pipeline directly, low-impedance, to the main earthing terminal without an isolating joint, short-circuiting the cathodic protection via the building's earthing installation.
Omitting the bonding connection to preserve CP effectiveness — this may solve the CP problem, but violates the touch-safety requirement of §411.3 for extraneous-conductive-parts.
Not consulting the party responsible for CP when designing or revising the main bonding, so a change to the electrical installation unknowingly disables a CP system that has been functioning for years.
Applying a DC-decoupling device that does not meet the required conduction current and voltage for the touch-safety function — the component must both block the CP current during normal operation and become conductive quickly enough during an electrical fault.