NR NR/L3/SIG/11761/D010 ISSUE 1 PDF

Introduction

The purpose of this Information Sheet (IS) is to assist in determining whether the presence of metalwork such as steel reinforcing in concrete, or cast iron tunnel walls, is likely to influence the operation of EBI Track track circuits, and lead to downgraded performance. It gives examples of types of construction which are known to have, or would be envisaged to have, an effect on the tuned area sufficient to cause problems with EBI Track set-up and commissioning, and to advise on ways of avoiding these problems.

The result of this potential effect is to lower the series inductance of the rails due to a loading effect, and to increase the series loss due to eddy currents flowing in the coupled metalwork. Overall the rail's series impedance is lower and with a more resistive phase. This actually reduces track circuit signal attenuation in the main part of the track, but also has a detrimental affect on the tuned area.

Within the tuned area the lower inductance and higher loss mean that the launch voltage of a transmit end tuned area is lower than it should be, and that the receiver input current is lower for a given receive end rail current than it should be. The overall result is that the maximum track length that can be successfully set-up is significantly reduced. Another affect of the increased loss is to reduce the resonated impedance of the tuned areas, which can make it more difficult to shunt the track.

Practical testing of the influence of reinforcing of various geometries and distances from the rails would be a major undertaking. Accurate measurements would be required on either a large, and difficult to adjust, test arrangement, or on a wide variety of different infrastructures covering the range of construction designs likely to be encountered in future.

An accurate theoretical evaluation of the problem is also problematical, as it would require sophisticated (and expensive) simulation software. Therefore it is not easy to predict whether a particular construction would cause a major problem or not. This document summarises the knowledge on the subject built from site work to date, as well as from some simple and rather crude calculations, in an attempt to assist in avoiding the problem on future projects.

Any feedback from experience on new projects would be welcome, and will be used to improve the accuracy and usefulness of this IS. It should be made via the Engineering Manager, Manufacturing Engineering, Bombardier Ltd, Plymouth.