RobotArm (Customer) asked a question.
What is the different between VDC- vs GND?
Are they the same? Where should shield connect to?
Should I connect VDC- to GND?
RobotArm likes this.
- RBPLC (Customer)
They're not the same and depending on who you ask you'll get varying opinions about connecting VDC- to GND. This is often a debated topic in various forums. You "can" connect them together but in general I think it's a bad idea. There are various reasons for this but in my opinion it's typically better to leave the VDC- floating and not tied to ground. The same argument arises when talking about tieing 485's to ground in different locations. Shields are typically connected on the sensor end only, not both ends.
Expand Post- RobotArm (Customer)
So how would you do for 24VDC- and 5VDC- ?
For electrical enclosure, do you connect "Earth"?
- RBPLC (Customer)
Without referencing specific NFPA requirements, in general, the enclosure is connected to Earth ground. Concerning 24VDC- and 5VDC- (assuming you have two different power supplies in the same enclosure) they don't necessarily have to be tied together and can be floating independently. This often times leads to reference voltage issues on signal conditioners, PLC cards and the like and therefore it is common (no pun intended) to tie 5VDC- and 24VDC- together so that they are floating with reference to ground but not tied to ground. However, as daakers said, some would tie both 24VDC- and 5VDC- to earth ground.
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- kewakl (Customer)
It is all about context.
If you look at TTL/CMOS logic datasheets, the zero volt reference/PSU rail may be called out as GND citation (pg4)
Not to discount any of the opinions stated here (and elsewhere) but, as daakers said, it makes his (and my) troubleshooting faster.
In my capacitor process/test equipment, I do tie the zero volt rail of the 24V PSU to ground.
-- we support some older iterations of similar equipment that does NOT have the zero volt rail grounded, and we see 'phantom' voltage on the zero volt rail with respect to ground. There may be between 45-60VDC potential between the zero volt rail and ground at various stages in the process.
I have read various posts that say to not tie zero volt rail to ground because doing so could/would bypass the electrical isolation that the PSU provides from the incoming service power.
Expand Post
Without referencing specific NFPA requirements, in general, the enclosure is connected to Earth ground. Concerning 24VDC- and 5VDC- (assuming you have two different power supplies in the same enclosure) they don't necessarily have to be tied together and can be floating independently. This often times leads to reference voltage issues on signal conditioners, PLC cards and the like and therefore it is common (no pun intended) to tie 5VDC- and 24VDC- together so that they are floating with reference to ground but not tied to ground. However, as daakers said, some would tie both 24VDC- and 5VDC- to earth ground.