Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?

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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1 Answer

Answer :

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 example: 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 reduces 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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