Why use 4-20mA?

The 4-20mA current loop is a common method of transmitting sensor information in many industrial process-monitoring applications.

typically in systems monitoring pressure, temperature, pH, or other physical factors. These systems employ the familiar 2-wire, 4-20mA current loop, in which a single twisted-pair cable supplies power to a module and also carries the output signal.

Transmitting sensor information via a current loop is particularly useful when the signal has to travel long distances – 1,000 feet is not uncommon.

The use of basic two-wire technology makes the installation both inexpensive and simple to wire, maintain, and troubleshoot.

The loop’s operation is straightforward: A sensor’s output signal is first converted to a proportional current, with 4 mA normally representing the sensor’s zero-level output and 20 mA representing the sensor’s full-scale output.

A reading of 20 mA means that a valve, for example, is 100% open, and a reading of 4 mA means that it’s closed. Readings between the maximum and minimum values mean the circuit is controlling the valve.

Why is 4 mA chosen as the lower threshold representing the ‘off’ or ‘closed’ position? The design takes into account that there must be a means of representing a fault caused by an open circuit or a lost feed.

Therefore, a reading of 0 mA is essentially “reserved” to signal a fault in the system, while a reading of approximately 4 mA or 20 mA would indicate a properly working circuit.

Key among the advantages of 4-20mA current loop technology is that the accuracy of the signal is not affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device.

Even if there is significant electrical resistance in the line, the current loop transmitter will maintain the proper current, up to its maximum voltage capability.

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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,

  1. 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.
  2. 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.
  3. 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.

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