Answer / mariyappan

Because of dead zero and live zero concept.
Dead Zero: this is basically derived from ohm's law
V=IxR. In electronics circuit minimum 250 ohm resistance is using. so V=Ix 250. Substitute I=0,1,2,3,4 after substitute 0 u will get V=0 ohm,(1;V=250 ohm,2;V=500 ohm,3;V=750 ohm,4;4=1000 ohm or 1 V) so when 4 ma is coming u will get 1 V,8 ma=2v,12 ma=3 V,16 ma=4v,etc..... u will get linearity .thats why 4 ma is choosing in all analogue value.
Live Zero: if tx is failure that time also zero value is coming.thats why 0 is not choosing

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