Common is common?

it sounds really dangerous to me - don't confuse "common" and "Neutral". And, I'd suggest you do a search on the phrase "ground loop" so you understand why the commons for the various suppies were separated in the first place.

Man, I just HATE a comment like this. How should I do it? All the 110 VAC are grouped together. All the 5VDC are grouped together. All the 50 VDC are grouped together. These are all the return or common wire at 0 volts. I know the 110 is then tied to the neutral in one spot that goes back to the load center. What do I do with the DC?

Tell a person with a question the correct way. DON'T JUST SAY "> it sounds really dangerous to me"

Karl

I'd say that the correct way to do it is to rewire the same as it was. If commons were tied, tie them the same way (and in the same places). If they are separate, keep them that way. Is there some reason to do it differently?

Steve

Karl Townsend wrote:

DC Common should be tied together and to ground and to the case of the machine. The wire color should be white w/ blue stripe.

AC Neutral should not be tied to ground or the case anywhere on the machine. It should be bonded to ground at your panel somewhere. The Ground wire coming in with your AC should be tied to the case.

I designed a rig with a Sola 24 VDC power supply which is powered by 3 phase 460 VAC, no neutral. The 0 VDC(-) side is isolated, and is not tied to the frame. The old e.e. I had check my design said that was the correct way to set it up.

If it were me, I'd stick with the way the equipment was originally set up.

Pete Keillor

I see a couple votes for "do it like it was", and one vote for tie DC common to frame. The wiring on the control retrofit was a total abortion. Never seen such a mess in my life. The pile of wire looked like a big bowl of spaghetti. I got this fine lathe for just over scrap price because it would not run reliably for the business that owned it. One of the problems is way too many wires on one terminal on the terminal strips, plus no connector ends. The various commons had six or more wires twisted and stuck under one screw with some wires pulled out. I've found the common wire stuck under a screw on the machine frame in various spots - I know that's wrong.

So far, I've installed a terminal strip just for all the common wires and brought them to one terminal strip just for this purpose. Eliminated a lot of the mess this way. I put the power supplies for the DC near this strip. I'll take the suggestion and put blue tape on all the DC commons so I have white wire with blue ID.

Now, do I tie all these DC commons to anything other than the - terminal on the supply? One concern I have is "what if 110 VAC accidentally connects to a DC wire somewhere?" In this case you need a good path to neutral/ground so a fuse blows.

Karl

In my case, the DC commons (terminated on DIN rail connections) connect only to the - terminal of the supply, and are not connected to ground. I was advised that at least with this power supply, it would not function if I tied it to ground.

I distribute the + side through DIN rail mounted individual fuse holders with appropriate fuses. These have led's to show blown fuses. These are mainly powering 4-20 mA process instruments through Brad-Harrison Nano-Change connectors. Since this is a research rig, we can change the configuration and instrumentation by just plugging stuff in.

In my case, the 120 VAC on the rig is provided by an on-board transformer, also powered by 460. I feed most of the 120V stuff (heaters, mostly) through an equipment protection GFI circuit breaker (trips at 20 mA, IIRC) safety relays (tripped by overtemp logic or selector switch) and individually controlled SSR's for temp control. The GFI protected 120V commons come back to a bus that's grounded through the GFI breaker. The other 120 V commons connect to a grounded neutral bus.

If you sort all the wiring, shouldn't you be able to assure yourself you don't have errant 120V connections? In my case, any heater failures will go to ground, which is tied to the structure ground, and will trip the breaker.

Good luck with it.

Pete Keillor

If the DC is being generated inside the machine's cabinet, I would not send it's common out of the cabinet thru the AC wiring. Likewise, if there are two seperate DC power supplies, one for 50VDC and another for 5VDC, as indicated by two seperately powered transformers, I would give each their own terminal strip for common. If they are both made from a single multi tap transformer, I would put all the commons together. I admit that I am no expert but I wouldn't mix the AC wiring with the DC wiring.

Shawn

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That sounds like a power supply with "remote sense" -- an extra pair of wires going to '+' and '-' at the load (ideally twisted pair through a shield) to tell what the voltage is *at the load*, and to allow it to regulate at the load, with any reasonable voltage drop in the wires.

Yes -- this kind of setup could have problems if the '-' is grounded.

An alternative with similar caveats would be a power supply with no transformer, so there is some (partial or complete) connection through to the AC power line.

Enjoy, DoN.

OK, there are sound reasons to redo the wiring. Here are a couple of thoughts:

1. Do not tie the DC commons to neutral. I can't imagine this being useful, and can imagine it being a danger, especially since you mention some DC commons are tied to the frame. If you must connect the DC commons somewhere, make it ground, not neutral.
2. Keeping the same connectivity but re-routing the wire pattern can give you headaches otherwise known as ground loops. For instance, if you had a machine with a bunch of commons coming to one point (which was a big mess), and you changed it so they all daisy chained each other through the machine, you might introduce a problem. Before, all the currents flowed through their own wires to the common terminal. The change I described would have all the currents adding, creating voltage drops due to wire and connection resistance, with the last piece of wire carrying all of the current. Lots of interference and crosstalk that way. Neaten the wiring and improve connections (by not piling as many on one screw for instance), but keep the same setup. Picture what currents flow in a wire and try not to add other (new) currents to the same wire just for the sake of neatness.
3. Sometimes you need to spread the wires out to avoid crosstalk. Not clear this would be a problem in your case, though.

I think if you change how the commons are wired together (such as connecting DC commons to places they aren't currently), you are asking for trouble. This could range from minor problems to blowing power supplies.

Good luck, I suspect this will be a larger job than anticipated. Maybe it would be wise to clean up the obvious awful stuff and then stop and see how the machine works.

Steve

I don't think it's the remote sense type, DoN, just a good quality switching power supply. But then again, I'm no electronics whiz (BSChem). That's why I get the plant guru to check my work. The model I used is the Sola SDP 10-24-480, found on the following website.

Look down the page for the three phase models. I've used 13 of the SDP 20-24-480C models on some larger production rigs.

The plant guys like 120 VAC signal DIO circuits, and only use 24 VDC for 4-20 mA stuff. I like 24 VDC for both for my small rigs because I have to open junction boxes to program vfd's and comm modules, and don't like the 120VAC buttons and pilot lights behind me (plus I have

5 ft. runs, not 5000 ft). I think I finally convinced them on the last job, but only by not letting the central engineering group anywhere near it, and doing the entire design myself. The most knowledgeable plant guy was the one I got to review my design.

We have widely acclaimed engineering and construction folks if you're building a plant to make 2 billion pounds a year of something, but they tend toward legendary overkill and cost if it's a small project.

Pete Keillor

Keep the AC wiring completely seperate from the DC wiring and you will probably do fine. The DC wiring may or may not like being all tied together as some low voltage circuits take offense to high power control circuits being randomly tied to them.

-- Why isn't there an Ozone Hole at the NORTH Pole?

The correct way is to use the wiring diagram. M.K.

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[ ... ]

O.K. Your URL took me to mid-page, right where you wanted me to go.

It looks as though you are correct, though when I saw two "+" outputs and two "-" outputs, I thought that they were implementing remote sense. But it turns out to only be for ease of connecting multiple loads. It took a bit of digging, including downloading the PDF file to discover that.

It is interesting how Sola have changed. They used to specialize in constant-voltage transformers, and brute-force power supplies built around them. Now, they are making switching power supplies which (apparently) have no input transformer, since they will accept both AC and DC input, as well as auto switching between different input voltage ranges. (That might be the reason for not grounding either side of the output.)

:-)

That can be a problem, of course.

Good Luck, DoN.

. The various commons had six or more wires twisted and stuck

Sort of lends credence to the ground loop concept, yes?

Harold

Lots of offshore machines < USA here > use seperate ground and common wires. You see green < ground > and green with a stripe < common > . One is ground the other is common

On some sites, green is ground while green with yellow stripe indicates computer or direct connected ground, ie not routed through even so much as a junction box. Gerry :-)} London, Canada

About fifty years ago an American service man was moved to Berlin in Germany. He decided to rent there an house and shipped his belongings. One slight problem, the cord connector (plug) of the washing machine did not fit in the outlet. The old cord from the USA had in it a green conductor connected to ground and black(power) and white connected to the motor. He called an electrian to put a new plug on. He connected the black wire to the ground pin and the white and green wires to the other pins of the new plug. (At that time yellow was used as a ground wire and green as an acceptable color for power in Germany.) When his wife used the washer she got electrocuted and died. It was decided to standardize over the whole world the color for the ground wire: It must be for at least 30% green and the rest yellow and/or not more than 70% green and the rest yellow. A ground wire (also called earth) is directly connected to the frame or housing of equipment. It is NOT the same as 'neutral' or 'common' which in North America is white. HTH.