Industrial Process Automation-Control Systems Engineering

We use 4-20 mA signal for intrinsic safety purposes, as 4-20 mA signal is weak, it never cause fire / spark in-case of short circuit or Puncture. why we use current signal?,most of us know, the simple reason is to prevent the losses due to voltage drop if we are transmitting the voltage signals over long distances ,the effective wire resistance could change the actual value we use 4-20 mA because the strength of the noise signal is always less than 4 mA. by transmitting the current signal we can greatly reduce the voltage drops across the wire. In hazardous areas where field instruments are installed and where the signal through marshaling rack is coming and going moreover this electronic signal is changed into 3-15 psi instrument air which then operates the process valves located in field the minimum range of this signal is kept 4 mA so that the operation personnel in control room may differentiate between low or no signal means 4 mA is a low signal and 0 mA is no signal(means