Why we use 4-20ma rather then 0-20ma in control loop give 2 reasons?

1. Live line detection, a small current flows so you'll know that you don't have an open loop.,

2. Zero detection circuitry has to be added to field transmitters,

3. Induced Eddy currents will cause false readings.

If you use0mA, meaning, when you have an open loop, you'll have the minimum value you set in the program.

4-20mA is also used to prevent sparking in your loop because of the relatively large DC voltage supplying that loop.4-20mA is a perfect match to a24V DC supply.

REASON FOR 4 mA

The reason for choosing 4 mA is when the transmitter were first introduced it was working on pneumatic pressure signal for transmission.

later electronic chips were-introduced those chips requires minimum of 3 mA of current to function.so, a margin of 4 mA is taken as reference 

if 0 is used instead of 4 mA,we cannot differentiate the actual 0 value of the sensor or the problem in the transmission of signal. for eg. cable is cut or damage of cable.

REASON FOR 20 mA

20 mA is used as maximum because the human heart can withstand up-to 30 mA of current only. so,as a safety point of view 20 mA is chosen

and the scaling of 4-20 mA is easy like

4 ma is known as the live current in a system.it states that even when the system is offline ,there is a minium current flowing through it.it states that the actual current is never zero.every measurement is caliberated into this standard format.

Wire break detection 

Induced eddy current wont effect at 4ma

1. loop check, because  if instrument give current less then 3.2 mA this mean that instrument has a broplem
2. to power instrument and operate it,need mA, so we can't operate instrument with 0 mA

one reason is that we will able to distinguish between dead zero and live zero if we use 4-20 Scale. Means if it is scaled to 0-20 ma and when there is no process input the transmitter will be showing  zero . It will also give 0ma when the transmitter fails.

Another factor it is easy to scale 4-20 MA to 1-5 v Using 250 ohm resistance.

4-20 mA in this case you can easily troubleshhot, wire break or supply disconnect

other case (0-20 mA) difficult to troubleshoot.

its esay to find open circuits in our loops .when we have wire break .give me 0 ma not 4ma this is the reason.and for 20ma is the safety current for workers 

4 mA is enough to supply the standing current needed for loop power and not enough to cause excessive power loss so we are using 4-20mA.

This is because in two wire systems, 4-20ma is easily detectable by the devices like multimeters.

the semmiconductor devices operate on 3ma so the current must be greater than 3ma.

4-20ma contains multiple of 2 so the calculation of current is easy in the network.