Rumor has it that the type of power supply chosen by Mr. Bently when developing his system used transistors which produced the most reliable output with a negative polarity (NPN vs. PNP or vice versa).
Similar threads have discussed the relative merits of the two types of transistors on control.com.
Thinking capitalistically, it doesn't hurt sales if the polarity of the source required is different from everything else, either.
Another train of thought is that by requiring a source with a different polarity than most other devices the proximity probes will usually be powered by a different source than the other devices, and this can have benefits when electrical noise, cross-talk, and other electrical gremlins pop up in industrial applications.
Thank you CSA for the reply.
Well, just yesterday I finally got a reply to my inquiry message from GE Energy "Technical Support Specialist".
His message says:
"Basically when Don Bently designed these systems many years ago the only transistors available at that time used negative voltage."
> Why Bently Nevada proximity sensor needs -24V DC instead of +24V DC
Why bentley nevada use negative voltage in proximitor?
Thanks to a bit of research performed by Sir Humphry Davy for the British Navy, we have a technology known as "cathodic protection." First developed to keep the copper hulls of British naval ships from corroding, this technology has been applied to protecting everything from oil rigs to gas pipelines to telephony cabinets. By keeping the cabinet frame at a more positive voltage than ground, corrosion is reduced and the life of the equipment is increased. Who would have guessed?