Tying 24VDC- to Ground

[Maj. Toxido]

I never leave any supply floating unless it is absolutely required by the attached devices.
In all of my panels, all control reference legs (115VAC Neutral, 24VDC 0V, 12VDC 0V, 5VDC 0V, are all tied to earth. I've never had a problem.

Count me in.

 

[keithkyll]

First, consider an AC transformer. 120VAC in, 24VAC out. You ground one side. Ungrounded, you are exposed to 120VAC through the capacitance of the transformer. You are also exposed if the transformer has a primary to secondary short. Grounding eliminates this, and also provides a means for the fuse to blow if there's a primary to secondary short.

A Switch-Mode Power Supply (the most common today) has the AC line directly connected to a full-wave bridge. 200 or 400 VDC is developed as the primary source. A switching device pulses the primary of a transformer with this voltage. The secondary is connected to the output.

Simply put, you are still connected to the AC line with a transformer as isolation.
The justification for grounding your outputs is the same. Capacitance is dealt with by other means, and the supply has other protections so your not as exposed as you would be with the simple AC transformer, but the safety issue remains.

Use a ground for safety.

 

[panic mode]

I never leave any supply floating unless it is absolutely required by the attached devices.
In all of my panels, all control reference legs (115VAC Neutral, 24VDC 0V, 12VDC 0V, 5VDC 0V, are all tied to earth. I've never had a problem.

me three... everything gets grounded unless it's absolutely nececary to do otherwise. big majority get's negative side grounded but we have equipment where positive is tied to ground. i absolutely hate to see -24V unless this is correct or what meter will show. if i measure between -24 and +24, i expect to see 48V. if not, that psu (or panel) is garbage and needs fixing....

 

[BobB]

NEVER!!! I use switchmodes and if the negative is tied to ground, isolation is lost.

 

[rsdoran]

me three... everything gets grounded unless it's absolutely nececary to do otherwise. big majority get's negative side grounded but we have equipment where positive is tied to ground. i absolutely hate to see -24V unless this is correct or what meter will show. if i measure between -24 and +24, i expect to see 48V. if not, that psu (or panel) is garbage and needs fixing....[/QUOTE]

I am confused on this statement, why would you have a PS that measures +24 and -24 to equal 48vdc? Why would a panel have a 48vdc power supply?

ANY 24 volt dc power will have +12vdc and -12vdc output, IF you tie one of those to ground then you will have a 24vdc to zero (common) connection, whether plus or minus depends on the connections.

Whether you measure + 24 or -24vdc depends on how you connect a meter, they are polarity sensitive.

IN SOME CASES a center tapped connection is involved with 3 wires i.e. +12vdc, COMMON, and -12vdc, you get 12vdc between common and either of the 12vdc points, whether + or minus is AGAIN an issue of connection, not source.

ITS RELATIVE.

Technically there are no standards, legal issues, or safety issues that require a 24vdc PS to be grounded, in many cases grounding a 24vdc PS can create issues i.e. induced voltages from AC sources etc.

NOTE: The term neutral should never be applied to reference the negative or common connection point of a DC supply.

 

[AutomaticLeigh]

Originally posted By BobB
NEVER!!! I use switchmodes and if the negative is tied to ground, isolation is lost.

I don't understand that. A switchmode achieves isolation the same way any other supply does - with a transformer (assuming it is actually an isolating type). How does grounding one side of the output affect isolation from the supply?

Given a free choice, I always earth the negative of a DC supply, or one side of a AC supply. Some customers ask for SELV (separated extra-low voltage) as one way to meet the safety requirements of the IEE regs, which the regs state must NOT be earthed. No, I don't know why either.

 

[rsdoran]

I don't understand that. A switchmode achieves isolation the same way any other supply does - with a transformer (assuming it is actually an isolating type). How does grounding one side of the output affect isolation from the supply?

Grounding eliminates the isolation to a point, you are at that point connecting it to an AC system i.e. you have created a path that AC current can flow on the DC wiring.

 

[mgibbons]

Technically there are no standards, legal issues, or safety issues that require a 24vdc PS to be grounded,

Interestingly, just about a week ago (6/7/06), UL issued essentially a modification to the 508A standard for industrial control panels.
The summary is here:
[url="https://ifs.ul.com/ifr/ifr.nsf/0/01966A5BCB0232408625718000021B06/$FILE/SOR01966A5BCB0232408625718000021B06.doc"]https://ifs.ul.com/ifr/ifr.nsf/0/01966A5BCB0232408625718000021B06/$FILE/SOR01966A5BCB0232408625718000021B06.doc[/url], and the very first item addresses the topic of this thread (for higher voltage primaries). I haven't seen the actual text of the revised spec, so I'm not quite sure how to interpret "...may require power supplies..." The existing Paragraphs 16.1 and 16.2 in my copy of the 508A spec refer to transformer secondary grounding, but not power supplies.

--Mike

 

[jcchagnon]

Proper wiring practices for automation panels is to ha an isolated ground buss connected to the incomming building ground and not just tapped off a bare copper that is run around the building.

The code requires that all class II transformers have one leg of the output side tied to ground (isolated buss in this case) DC supplies are not regulated in this way but nonetheless should have the 0V also grounded for safety reasons.

As stated previously, the 0V should be seperated from control neutral.

UL & CSA will require grounding if the 24V is going to field devices.

 

[BobB]

I don't understand that. A switchmode achieves isolation the same way any other supply does - with a transformer (assuming it is actually an isolating type). How does grounding one side of the output affect isolation from the supply?

Not sure but the guy who builds/resells the things told me that is the case. He is a smart cookie and an electronics/electrical wizz - will have to ask him why.

He also designs and builds all the battery chargers I use.

 

[davefinic]

I have sometimes grounded and sometimes left floating never had any problems either way although i find it easier now to ground for future fault finding

 

[yxannes]

I don't see how tieing 0V to the ground is wrong. It makes for easy troubleshooting in case of faults.

Concerns for isolation are unfounded IMO. If there is a badass surge or a direct lightning hit, no amount of isolation can stop it.

I once has a surge pretty bad that it blew my 250A fuses, my MOV, blew my SCR, blew my opto-coupler all the way to the step-down transformer. The PLC was miraculously spared.

It all depends on your application. If you are in an precision or a cleanroom industry, I'd say you work on surge-suppressing the incoming supply, plus investing in proper anti-lightning equipment and grounding.

 

[panic mode]

i work all the time with systems that have grounded one of psu output terminal or systems
that have more than one power supply. this might explain my intolerance for something
as basic as properly labeling power terminals.
voltage is potential difference. if in the panel wires are labeled 0VDC and +24 (or -24VDC),
i expect to see difference of 24V (result could be positive or negative value depending on reference
and meter polarity).
But if the terminals or wires are labeled +24 AND -24V, the difference should be 48V (+48V or -48V).
if it is not, someone either messed up labeling or installed wrong psu or one of the outputs is dead.
whatever the case, panel deserves attention because who knows what else could be wrong.
this is a good indicator that quote for service of such equipment should be doubled...
after all who knows what else you gonna find, 600VAC labeled as 6V or "please touch me"
or output labeled as input? this could/would create damage if one wires input device (such as
swith) to an output.

switching power supplies COULD be realised without isolation (just like non switching psu,
- the only difference other than oparating principle is efficiency).
nonisolated switching psu units seam to be quite rare. i've never seen 24V industrial psu
without isolation. the non-isolated switching psu units i saw so far are either dc/dc converters
or integral part of some consumer products (not standalone psu products).
only advantage of nonisolated units is tiny cost saving. if one decides to go cheap and use such unit,
he better RTFM or datasheet and make sure he is using it properly (or change job).
next thing you know, our chap will fry someone... isolation fails too, specially
if unit is subject to contamination, surges and high loads. grounding is good practice.
those who are afraid to ground DC output, never worked with 3-phase power supplies and never
saw what damage is possible when insulation on those fail. blowing some hardware such as
fuse or already malfunctioning psu are least of worries...

 

[CharlesM]

Both

I don't connect my 24vdc common (I label 0VDC) to ground. Unless I find out that some of my components are connecting the 0VDC to ground. Many times I don't take the time to figure out what is taking it to ground. When I find this I connect the common on the power supply to a good ground. At least I know where it is connected. I have seen problems both ways. Sometime you get noise from connecting to ground and sometimes it fixes a noise problem.

 

[elevmike]

It's generally considered good practice to ground the 0vdc terminal. If this induces noise then fix the ground.

Mark, we never have problems with getting a ground because it's required, and the customer wont get an elevator (in service) with out it. Period.

If the owner wont bring you a ground to the machine, then let some other idiot take the job & liability for it.