Instrumentation and control of 4-20 mA normally favour a signal variety of less than 4 mA and more than 20 mA. Current values below 4 mA and above 20 mA are used for devices to indicate a fault such as a thermocouple burnout, faulty signal cable, low/high resistance cable, inability of transmitters or other sensor failures.
It is possible to configure the transmitter for small or strong fault sign. Unfortunately, companies use distinct signal concentrations to show error that avoids narrow integration and analysis of analog signals in single loop sensors, control structures, and security devices.
4-20mA Current Failure Alarm Limits:
Failure alarm limit for 4-20mA is above 20mA and below the 4mA. Some generators could use 3.75 mA while others could use 3.6 mA or less. Some transmitters may or may not use 21.75 mA while others use 23 mA. This incoherence of signal concentrations for the sign of error makes it hard for control strategies to take complete benefit of the data of error.
According to the standards, 3.8 to 20.5 mA is defined as a true (’ good ‘) measuring qualit where saturation is indicated by 3.8 to 4 and 20 to 20.5 mA. A < 3.6 mA or > 21 mA signal suggests a breakdown of the receiver (’ Bad ').
Note, however, that all serious and insignificant system mistakes are flagged the same manner so that the user is unable to say the distinction and that if any mistake happens it is flagged and the assessment value is not given.
For each test, FOUNDATION fieldbus and PROFIBUS-PA devices use electronic interaction with distinct status indicator including real-time flagged measurement legitimacy as ’ Good, ’ ’ Bad ’ or ’ Uncertain. ’