Controlling Stray Current

Controlling Stray Current

Monitoring and Management of Trackside Stray Currents
James A Phipps, Paul Lambert (Mott MacDonald Limited) and Graeme Jones (C-Probe Technologies Limited)

Abstract

An unwanted, but nevertheless real, problem for railway structures comes from the risk and consequential effect of stray current interference inducing accelerated corrosion.

Stray currents may arise from the proximity of unbonded structures to nearby cathodic protection systems, leakage from overhead lines or from associated electrical facilities, such as signaling.

If unchecked the effects can be devastating. They can also be managed through efficient design and management methods.

Here, the authors recount two instances where novel approaches mixed with knowledgeable management procedures provided solutions and information to give peace-of-mind to important structural assets.

Introduction

Wherever buried metallic structures are situated near electrical sources, such as DC traction systems and cathodic protection systems, stray currents from non-insulated items may flow to and from these metallic structures.

The upshot of this is that current can enter the ground from the source, pass into the isolated metallic structure causing no apparent detrimental effect in the vicinity of the leak. However, if the ground resistance permits then this leakage current can travel for many metres or kilometres, along for example pipelines, before leaving the structure with localised and dramatic failure of the metal due to accelerated corrosion.

Identification of the after-effects can be extremely difficult especially if, as is normally the case, it is a long pipeline.

In corrosion terms, current enters the metallic structure as a cathode (sustaining the oxygen reduction or hydrogen evolution reaction, depending on the prevailing environmental conditions) and leaves an anode behind (causing metal loss) in search of a return to earth.

Bonding-in isolated steel to the protected structure (in the case of cathodic protection) or designing efficient return paths for the traction and catenary systems will prevent such effects.

But, how do we check we have achieved this efficiency of design and how do we mitigate if the design has unforeseen complications?

The following cases document two approaches taken by Mott MacDonald Materials and Corrosion Engineering in conjunction with specialist companies, such as C-Probe Technologies Ltd, experienced in tackling specific corrosion issues.