Building a steel footbridge over a railway line and its catenary

Question

I'm doing an investigation about building a bridge over a railway line and its catenary system. I'm mainly interested in the electromagnetic environment on the bridge as it is affected by the presence of the catenary.

Is this an issue that I need to be concerned with?

I have no experience in either of these fields and that is exactly why I would like to get insight from people who do work in these particular fields. To answer some of the questions that have been raised in the comments section:

What order of distance, what voltage? AC/DC? | South Africa or ...? | I'd expect regulatory requirements for unrelated reasons would place you well outside EM interaction range.

Ideally I would like the bridge to be of reasonable height such that the pedestrians wouldn't mind taking the stairs. I have freedom to reduce the height of the catenary system for exactly this reason. It is a 25 kV AC catenary system. What is the standard EM interaction range?

What electromagnetic enviroment? You haven't specified any. Will this bridge parallel a power line? Be near a large radio transmitter? What makes you think there are electromagnetic issues here at all?

According to this Wikipedia article, AC high voltage lines "give off" electromagnetic radiation. The bridge is not close to anything really (no power line or large radio transmitter), it just goes over the train track and its catenary.

Answer 1

"standard EM interaction range" depends a lot on frequency, or its inverse which is wavelength. Your 25 kV catenary probably is 50 Hz or 60 Hz, same as your national grid. That means the wavelength is 6000 kilometers. That's far more than your dimensions.

Hence, you can ignore the electro-magnetic aspects and treat this as an electro-static problem. Arcing is the main risk. Here there are two concerns that need tackling:

1. Can you prevent catenary movement? Some bridges have isolators beneath them to keep the catenary away from the bridge.

2. if the previous mechanism fails, will it fail safely? If the catenary would hit the structure, there will be a rather spectacular light show and the catenary might weld itself to the underside of your bridge. That will result in some high currents. Will the people on the bridge itself be safe? Metal may not be the best surface choice.

Also keep in mind that dry air is a much better isolator than wet air/rain. It's not just water forming a direct path, raindrops already are an issue. Your bridge probably should be designed to direct rainfall away from the catenary.

Answer 2

This sounds like a pretty broad question, but to get you started on the power line issue specifically:

Where I live, in the US, OSHA requires 10 feet of separation between any uninsulated power line 600v - 50kV and any worker or equipment. This might be a good place to start. Don't forget to consider that the bridge will deflect downwards under its maximum loads, and the power cable will bounce around in windy conditions (and possibly when a train passes under.) These variables should be taken into account in your design. If the bottom of the bridge requires any kind of maintenance (repainting?) you may want to provide a separation distance that allows a crew to get access and work under it without getting within 10' of the power line.

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To be clear, the separation distance as required here is about preventing an arc from jumping across the air gap, since air is an imperfect insulator. This distance is not intended to address interactions with sensitive electronics or other EMF-sensitive devices. If that's what your concern is about, please specify what kinds of devices you need to be able to work. It may also be possible to detail a robust insulating material between the power lines and the bridge, although in an outdoor environment, you'd need to protect against water forming a conductive path around the insulator.