Answer Posted / abdus sattar mollah
initially,0-20ma of current is used as an transmitting
standard unless a objection was met.It was that:0 ma of
current used to indicate no signal/minimum signal from a
signal conditioner but if there is a faulty condition(such
as loop break),then also it will be show 0 ma of
current.Operator may think that there is no signal but
actually a fault has occured.So he cant distinguish between
them unless the plant is damaged.This is called TRUE ZERO.
So,after a reserach a new standard was proposed i.e 4-20 ma
of current.There was minimum interference in this
range,good quality transmittance,low power consumptions.So
this satndard is used.
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