Lightening protection ?

Unless the steel structure is an antenna tower :-), I would simply use galvanic isolation on both the serial data and the power supply side.

The RS-485 works fine with a single twisted pair without separate signal ground with termination resistors at both ends of the bus, so there is not going to be any significant differential mode voltages on the line and the isolation can handle 500-2500 V common mode voltage.

One other pair could be dedicated for the +24V and 0 V and use isolated switched mode power supplies at each device. Ground the cable shield at a single point to avoid loop currents.

With transmitting antenna towers, you must be aware of the lightning rod current, which generates a huge magnetic field around the conductor and also about RF fields with possibly wavelengths of the same order of magnitude as the cable length.

Paul

Old steel bridge over salt water.

The bridge structure is the highest point for quite a distance(few km's) and has a center lift section. The electric motors for the lift section are the originals from the 1940's but may have been rewound in the 70's.

The AC power in the area is very noisy due to a lot of industrial and mining equipment. Bridge has 240VAC for lighting and the lift motors.

Thank you

Alex

Keep the cables and equipment in the shade or indoors will help with this. Unless you mean lightning in which case this will offer little protection.

David

You have the exact same problem that your telco has when connected to every other building in town and with overhead wires all over town. Use this application note figure to understand the principles:

Every wire (overhead or underground) connects to same earth ground before entering the building. No way around that necessary reality. Every wire in every cable connects to that ground either directly or via a protector. Transients earthed before entering a building do not overwhelm protection (ie galvanic isolation) already inside all electronics.

Yes you need galvanic isolation for the trivial transients. But that internal protection is easily overwhelmed without single point earthing.

Same is how the telco protects a $multi-million computer and operates during every thunderstorm without damage.

Consider, for example, this product that has that necessary earthing wire:

Each wire then connects 'less than 10 feet' to a single point earth ground if its earthing wire is properly connected.

In that above application note, the tower (ie other building) and building are separate structures. Each is earthed. Still, wire must be in contact with the other structures earthing before entering that structure.

Introduced is how damage is created and how damage is eliminated using the phone system as a similar example. See this one long post in the newsgroup comp.sys.ibm.pc.hardware.storage entitled "Phone line surge protection?" on 1 Apr 2007 in:

this is who we begin to understand why your telco need not shutdown and disconnect phone service during every storm. Why 911 emergency operators don't remove headsets and leave the room. This is how protection was installed even 70 years ago.