Why most of analog o/p devices having o/p range 4 to 20 mA and
not 0 to 20 mA ?
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Answer Posted / ekailash
4-20 mA is a standard range used to indicate measured
values for any process. The reason that 4ma is chosen
instead of 0 mA is for fail safe operation.
For exa.- a pressure instrument gives output 4mA to
indicate 0 psi, up to 20 mA to indicate 100 psi, or full
scale. Due to any problem in instrument i.e. broken wire,
its output reducess to 0 mA. So if range is 0-20 mA then we
can differentiate whether it is due to broken wire or due
to 0 psi.
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