common to ground? 2

[jag27]

is it necessary to physically connect the common on a 24V DC
to ground
what are the implication of that

is it a CE requirement??

 

[skiier]

Yes and No.
In Ontario here I use the "Ontario Electrical Safety Code", 23rd Edition, which I understand is basically the same as the NEC in the US of A.
There is no particular rule that I am aware of that states " you must gnd a 24VDC power supply". Instead this fact is stated in a roundabout way by means of the fusing required for your system. Basically, you must fuse all ungrounded conductors from a power supply, AC or DC (Rules 14-010 and 14-100). So in fact, if I may use some funny reasoning here, this rule states that if the conductor is "not ungrounded" than it must be "grounded". Are you following the logic here? Yes, this is the only way I know to explain it, sorry. So if you choose not to gnd your power supply you must fuse all the conductors from it - which basically doubles the quantity of fuses you need.
There are the exceptions of "Class 2" and "intrinsically safe" cct's however where you don't have to gnd any power supply conductors. In a Class 2 cct( Rule 16-200) a fault - or short cct - must be limited by code to 100VA. This means that the dc power supply must have an internal circuit or "clamp" to limit the output of the 24V supply to 100VA/24V=4.16A during a fault. If your power supply is greater than 100VA installing a fuse to limit the current to 4.16A only serves to "qualify" your cct as "Class 2" for wiring reasons. Again using the reasoning from above -if you install a fuse you must fuse all conductors or fuse one and gnd one. Get the idea yet? So conversely, if you don't have to fuse any conductors then why would you have to gnd any conductors?
If your power supply is greater than 100VA you pretty much have to gnd it. Check the NEC for "Class 1" and "Class 2" ccts and other applicable sections regarding fusing.

A gnded power supply is also easier to troubleshoot than an ungrounded system. I have seen too many J-boxes on machines that only had one conductor (+24VDC) to feed switches for PLC inputs. How do you check for power when you trouble shoot? Particularly if you are at a "2 wire" device such as a limit switch or something a long ways from the power supply. Do you drag a long test wire from the power supply common? I don't think so.
There are a lot of benefits to gnding your power supply. The number one benefit to gnding your power supply is safety of course. You greatly reduce the "potential" for shock hazards to occur. You may think "it is only a 24VDC power supply" but it is not the voltage that hurts you during a shock - it is the amperage. A current of less than 50mA thru the heart can kill you. Are you willing to bet that the "500W 24VDC" power supply you own can't kill you? And what about larger VA power supplies?
By gnding and fusing your power supply "and wiring system" - and I emphasise wiring system! - properly you ensure that that no hazardous shock conditions can occurr. For example, take the case of 2 24VDC devices sitting on 2 seperate metal tables next to one another - the tables being seperated physically and conductively and not gnded. Now if these two devices were fed from an ungrounded power supply and we had a simultaneous fault whereby one device shorted + to it's respective table and the other device shorted - to it's respective table what do you think would happen next? You wouldn't notice anything until you straddled both tables with the palms of your sweaty clammy hands and got a nice "wallup" from your power supply!
By gnding your power supply you simply blow a fuse when a "hot" touches gnd and no one is electrocuted.
I suppose if you really don't want to gnd your power supply you could use expensive "gnd fault" protection. It's your money.

 

[jag27]

when you say ground ur power supply, do you mean ground it on the DC side or the AC side

thanks