Why field instruments operating voltage is 24 Vdc? 1

[rajeshkolappan40]

Why field instruments operating voltage is 24 Vdc?

please explain

Regards
Rajesh.M

 

[moltenmetal]

High enough voltage so that sufficient power can be delivered at modest current.

Low enough voltage that there is no electric shock risk during live maintenance- much safer than 120 VAC.

But not low enough to make circuits intrinsically safe without resorting to a device such as a Zener barrier.

 

[danw2]

When an earthquake scrams your nuclear reactor and then the Tsunami floods your back-up generators, you can scavenge 12V auto batteries from the cars in the employee parking lot, hook up a couple batteries in series and get your instruments back on-line.