Why the standard range of current signal for sensors is normally (4-20) mA instead of (0-20) mA ?

0-20mA signal is also available. But just like in pneumatic systems a signal of3-15psi ensures that line is not choke and3psi signal is assumption for minimum (zero) signal for operation. Similarly, a4ma signal for instrument is zero operation signal, but when we find any fault, and for troubleshooting, it ensures us that cable/wire is not broken. So we can carry further for troubleshooting without any doubt that cable is disconnected.

so as not to get confused if the transmitter is powered off

if it's powered of then signal0 and if no flow or pressure signal0

but if0 signal is4 ma then u know that it's all powered

باختصار انا بستخدم كنترول من4 الى20 ميللى امبير عشان اعرف الاشاره القادمه اذا كانت صفر ميللى امبير هتدينى4 ميللى واذا كانت صفر نتيجه خطأ ما هتدينى صفر وبالتالى هعرف انه يوجد خطأ اما اذا استخدمت من0 الى20 ميللى امبير الاشاره القادمه اذا كانت صفر فانا مش هعرف اذا كانت دى خطأ او صفر

To illustrate the difference between zero signal (corresponding to low limit value, then the signal shows4 mA), and zero current (when the current loop is interrupted or short circuit occur, then the signal shows0 mA).

simply to show the difference between zero curent which represent by4mA and when fault occured in system which will represent by0 A

Because we have to differentiate between Live Zero or any kind of breakage of wire.

Main reason for the range of current signal is 4-20mA, instead of 0-20 mA is because, to know if the sensor is faulty, or sending a low level signal (low pressure, low flow ..etc..)

The simplest reason is to know if transmitter is connected (4mA) or disconnected (0mA),

also minimum required power for transmitter to operate is (3.98 mA X Resistance of Transmitter)

The standard Range of signal transmission is 4-20mA instead of 0-20mA because :

1. The device (2 wire transmitter) itself required some power 3.98mA to operate. 

2.to differentiate between live zero & dead zero. Live Zero is the output of transmitter when the process parameter is zero (4mA) while dead Zero is the output of transmitter when the power supply is zero (O/P less than 4 mA)/

The logic behind the4-20mA is that 

when there is low value on the input variable, the transmitter output is4mA.

when there is high value is high20mA.

if the instrument works on the0-20mA. then if the power to instrument / output wiring is cutout  it is impossible to detect the wiring / power fault. in case of4-20mA type as power/wiring fault occurs the value of the instrument goes to the zero. that is the logic behind it.  

To illustrate the difference between zero signal , and zero current