see all loop powered transmitters are capable of
withstanding 10 t0 36v dc ...as per the process requirement
it will vary.. insome industries they are using 12v dc
also.. 24 vdc is used because as per ISA there are some
standards wich will match other process reqirement like
24vdc relays for digital control also wecan use 24dc to
energize tis relays
It totally depends, 24 V DC 2 wire is not always applicable
every wehere some times it is 2 wire, 3 wire (2 cable for
power up & signal & 1 cable is for earthing), some times it
will 4 wire(2 for powerup & 2 for signal).
24 V Dc is for certain limit b'coz voltage will be drop in
that case w'll use 230v AC.
as per ISA standards 24 VDC is the minimum possible voltage
at which the spark will not come,we know our transmitters
will give max output current of 27 mA here they have limit
this current bcause the power we get is less than 1 watt
(P=VxI)648mW which is not capable of producing a spark thus
they have avoided spark generation by selecting 24 VDC and
limiting currnt to 27mA.this comes under extrisic safety.