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Answer Posted / 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
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