In earlier stages as electronics were not developed compared to today’s modern technology, so 10-50 mA was reference standard used in industrial applications, but concerning safety & evolved technology 4-20 mA came into existence & widely accepted standard globally.The most important point concerning use of 4-20 mA is,
- 3mA is minimum current required for excitation of electronic components used in transmitter/ receiver circuit. For safe margin we took 4 mA for setting LRV of physical measurements.
- Using 0 mA is not advisable, as the wire loop might have broken or the wire might have been earthed, so we can’t differentiate whether the signal is faulty or not.
- With 30 mA of current our muscle contract & our body is susceptible to cardiac arrest. Thus, for safe margin we use 20 mA for setting URV of physical quantities.
A residual-current device ( RCD ), or residual-current circuit breaker ( RCCB ), is a device that quickly breaks an electrical circuit to prevent serious harm from an ongoing electric shock. Injury may still occur in some cases, for example if a human falls after receiving a shock, or if the person touches both conductors at the same time.