PLC input protection required

Can your experts here please recommend the practical circuit to protect plc input for digital (24 volts dc.) signal? The 50 level and status swithes will run outdoor ~1000 feet away which are very prone to electrical noise and lightning.

When you mention lightning you're talking some serious issues there.

My thought would be to use a fiber optic link between the components outside and the controls inside. Perhaps you could use ethernet based I/O to a converter such as this

I've never actually done this, perhaps other have and can comment.

A couple of ways;

Use remote i/o modules and connect to your PLC using screened cable and lightning arrestors.

Run each line in screened cable and fit a lightning suppressor to each line. Pheonix contact Trabtech for instance. However I would not reccommend using 24v dc signals with 1000ft of cable.

The best way would be to use a small PLC nearby to collect the inputs and connect to the main system with a field bus system such as devicenet. If interference is a big problem then perhaps consider fibre-optics.