Answer / carl_ellis

There is no national or international standard for 24Vdc in
industrial instrumentation and control.

The use of 24Vdc has evolved out of common practice and has
seen global use and acceptance as a 'de facto' convention,
as opposed to a formal standard. The acceptance goes
beyond loop powered transmitters, it goes into discrete
control as well.

The reasons are many.

1) 24Vdc is a low enough voltage to be considered 'safe'.
It is accepted as a voltage level that does not shock human
beings through contact with dry skin.

2) 24VDc is a high enough voltage to do real work with
currents that are practical.

3) An enormous array and variety of field instruments and
controls can be designed and operated intrinsically safe at
24Vdc, not just loop powered transmitters.

4) Loop powered field instruments powered by 24Vdc can
drive 500-600 ohms of load resistance, enough for one valve
positioner (split ranging 2 valve positioners with a single
current output driven by 24Vdc is difficult).

5) Lead acid batteries are useable for battery back-up or
UPS supplies.

6) On the discrete side, 24Vdc is sufficient voltage to
break through oxide coating on switch and relay contacts

7) For discrete control, 24Vdc offers a tolerance band
around binary 'high's and binary 'low's of more than a
couple volts, which allows for operation in a moderate
level of electrical noise.

8) Statistically, 24Vdc devices have failure rates about
half of that of 120/240Vac actuation devices (relays and
solenoids). This a serious consideration for emergency
shutdown or SIS systems.

Carl Ellis
Measure First

Answer / rajesh.kon

always field(Process) instruments are communicated or
connected with PLC, DCS system.those PLC, DCS operating
current is 24 Vdc, so that instrument which are connected with
DCS, PLC system will be also using 24Vdc only.

What is defferent betwen ESP &PCP PUMP .HOW IT IS WORK?

0 Answers   Jindal,

---

how to do dynolab wind speed sensor loop test, how to simulate value,i have simulate mA 4-20mA but not responding in DCS faceplate, please kindly advice me any one.

0 Answers  

---

what is the standard of determining opening/closing time of Motor Operated Valve?

1 Answers  

---

what is the difference between active & reactive power?

0 Answers  

---

what is the input of digital counter during the the calibration time?

0 Answers  

---

What is the difference between fiber optic cable & Ethernet cable

2 Answers   BEMCO, CCC, Pidilite,

---

what is Instrumentation?

5 Answers   RCL, TCS,

---

why use MMWC unit

9 Answers   Praxair, Saipem,

---

key phaser where is use and work

0 Answers   BAP,

---

Basic working principle of control valves

1 Answers  

---

WHAT IS THE DIFFERENCE BETWEEN DCS & PLC.

9 Answers   Reliance,

---

difference between accuracy and precision

6 Answers   Honeywell, ONGC,

---