Answer / ibrahim s

4-20mA :-
* Easy to find the open circuit(because this
signal level starts from 4mA, if the circuit in open the
output shows 0mA, so we can find easily the open circuit).

* Easy 0 to 100% segregation
4mA - 0%
8mA - 25% 12mA - 50%
16mA - 75% 20mA - 100%

* All manufacturer following the same standard.

0-20mA :-
* If it starts form 0mA, not able to find open
circuit easily.

3-15mA :-
* Less linearity, because 3mA is 0%, 15mA is 100%
so the deviation b/w 3-15mA is lesser than 4-20mA, So that
it reduces the accuracy of the measurement.



So that 4-20mA is preferred over 0-10v

Answer / sukhiray

the lowest value is not 0 to create a difference in between
broken ckt or any other error in ckt and actual min current
value.
b'coz for broken ckt or any other fault in ckt the current
o/p will be 0. means no current. but when it is 4 , then it
is the min o/p of the ckt and the ckt is running properly
both is confirmed.
the upper limit is a conventional one, no such reason behind
it. but it is a value practically can be achieved by the
designer easily.
4-20 ma could be taken it is followed by all manufacturers.

Answer / rabindra ojha

main purpose is for live zero.in system voltage is 1 to 5
volt. used resistance is 250 ohm. at 1 volt and 250 ohm
resistance current is 4ma.like that when 5volt and 250ohm
resistance current is 20ma.so 4 to 20ma.

Answer / naren

1. to obtain live zero
2. noise reduction
3. no current drop
4. longb tranmission

Answer / wilson

Most people has got the answer correct for "why 4-20mA and
not 0-20mA". Yes, it is to prevent undetected failure of
circuit or instrument.

As for "why not 3 to 15 or anything". I assumed the
question is not "why not 3-15mA" because not one use 3-
15 "mA", if really you have to ask that, you might as well
ask why not 2.314 to 33.776 mA.

3 to 15 is used, but in pneumatic system and the unit is in
PSI. i.e. 0.2 to 1.0 bar. The reason for 3PSI (or 0.2bar)
is the same as 4mA, to prevent undetected failure.

Answer / giridharan

in this question i would like to add up my ideas

actually this 4-20 ma is fixed because of the main purpose
linearity of its output we can check whether its dead or not .

Answer / rajubisen1984

i read all answer of all abobe. max of u r correct for 0-20
mA but why not 3-15 mA
this is only bcoz max. of currenr transducer have the max
linearity in the range of 4-20.
again question raise that why higher limit is 20 mA
this is done because we need a large span range for good
analysis and 21.4 mA is the max. range of linearity in the
same way in lower side min range is 2.8 mA so its better to
use 4-20 mA

Answer / aravindan k

beyond 25 mA in live due any damage it cause spark, so the
they maintain below 25 mA it doesnt cause spark even
puncture or damage the live cable......
this is reason for maintain 4-20 mA. so we can use signal
at any environment

Answer / subash

from field to control room we use lengthy cables.due to the
resistance of the cable may voltage drop occur.hence we
always choose 4-20 ma over a 0 to 10v....if we select 0-20
ma then we can'n distingush between faut condition and
minimum scale....hence we choose 4-20 ma....

Answer / s.senthil

to find difference between live zero and death zero.

live zero means- 4mA

send me last 5 years questions asked in rrb chennai, for section engineer

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