Answer / rohan g

0 to 20 vdc signal may detoriate over distance but current
signal will remain intact.
noise will not affect current signal at all but vdc signal
will get affected.
if equipment fails then also it gives 0 vdc signal in case
of 1 to 20 vdc.but not in case of 4 to 20 ma.
DCS can be viwed as bigger version of PLC.Since no i/p dcs
can support is more in case of dcs.Also dcs can have
operator interface but plc dosent have such.

Answer / narendra mishra

suppose 0 to 20ma is used where 0 for
min.process value and 20 for max.then if
feed back cable get shorten or erthed or
broken it wil show min process value.where
in case of 4 to 20ma it will show "bad".

Plc are used to handle specific area where
DI DO signals are required like burner
managment system.plc response time is in
milisec. And that of dcs is in microsec.

Max span can be found by taking it online
and interfacing it with HART.

Answer / himanshu apte

4-20 mA current has the highest linearity with the field
instruments. Same linearity cannot be found in other
current options like 0-20mA or any others. In addition to
that, a cable failure in case of 0-20mA will not be
detected if the signal is already at 0 mA.

PLC have a very small scan time i.e. @ 50-100 milliseconds,
which is useful in Emergency shutdown systems..Controlling
algorithms are available in modern PLCs, but fine tuning of
controllers can be better achieved with DCS. DCS has a scan
time in the range @ 500ms, which is why it cannot be used
in ESD systems. DCS can also handle more no. of I/Os than
PLC.

Answer / samir

4-20 ma is used to instrument because if there is an
instrument faulty of it not getting the power or damage then
it gives 0 ma signal so we can found that the instrument is
faulty so we can easily check the instrument.
if we use 0- 20 ma we can not identify from the signal that
the instruments is faulty or actual the low signal from the
instrument during the measurement.

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