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I have 2000 infrared proximity sensors along 100 meters, divided to 2 meter sections with an arduino reading them sending results by MAX485. For the power, I am supplying same 24v dc from each end of the line to prevent voltage drop, I don't have much space for a big wire, so I had to do that.
In every 2 meter section there is a small switching buck reducing voltage to 5 volts for arduino and other components including MAX485. This is a design for an industrial purpose where emf noise is present and the air is so humid and dusty, increasing the possibility of shorting on the PCB itself! I have already read other questions which were close to mine, but some points still not clear to me:

  1. In case of a short between 24v and 5v to the micro what will happen?! And how to prevent it?
  2. What about a short between 24v dc and rs485 data? How to prevent that? For the first point, even if it happens, in the worst situation I will lose my components on one section, but for the second one I guess all 50 sections are in danger.

I will use TVS diodes like SM712 which is really designed for transient and rs485 voltage levels, but I guess it can not help me for the short circuit. The 380v ac to 24v dc power supply has enough protection, and also a circuit breaker will be provided for the whole 24v line.

As I read in the same questions before, I will separate data and power cables, use earth connection, use different connectors for them, so the installation will be fool proof!

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  1. The micro will surely be destroyed. You can prevent it by either preventing the short in the first place or by adding series or shunt protection. For example, you could add an LDO regulator on each board that is capable of withstanding 24VDC on the input (series protection).

  2. Most RS-485 protection schemes are designed to handle transients, not a direct short to something like 24V so you would probably destroy a shunt protection device (typically a TVS array is used) if you add a series polyfuse type PTC or MOSFET based current limiter you can make a short to +24 survivable.

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