dc power supply commons tie togther or leave seprate

[DH_control]

Hello All,
I have 4) 20 amp power supplies I am using in a production cell. The cell has 7 different remote control boxes and 1 main control box. All the power will be coming out of the main box going to the remote boxes.

The question is do I tie all the power supply commons together or do i need to seperate out each power supply and isolate the commons.

The issue brought up here is the terminal blocks can only 30 amps, my arguement is that as long as the commons are distibuted evenly over the group of terminal block (1 common on every other terminal block) and no one circuit has more than 15Amps on it tieing them together is the way to go.

All of the commons on all of the power supplies are also tied to ground.

Any thoughts??

Thanks for any input you can provide!!!

Dennis

 

[petersonra]

This is not an especially easy question to answer. There are pros and cons to doing it both ways, but in the end it probably makes little difference.

As a practical matter, they are already bussed together through the EGC.

one thing is that you have to be careful how things are wired to make sure that none of the common conductors or terminals gets overloaded. It is a lot easier to make sure of this if they are separated out.

 

[DH_control]

Thanks,
I knew we would have to monitor the terminals and keep them within sprecs but since i have 4 power supplies and 7 boxes I will have to do the same when i split them up anyway.

 

[jaggedben]

Funny that we just had a thread with a broadly similar question.
http://forums.mikeholt.com/showthread.php?t=173575

If you don't tie them together then it's not really a 'common', right?

Your theory about the terminal block rating doesn't sound quite right to me but I'd have to see the actual spec to form a real opinion. If you could potentially have 80A flowing through a 30A rated terminal, well, that seems like a large risk.

From your sparse description, I'd probably keep them separate. It's less risk, will aid in any trouble-shooting, and I'm not seeing the benefit of tying them together. More info might make me retract that opinion.

 

[Smart $]

DC circuits require overcurrent protection rated accordingly for circuit components, including terminals and conductors, just like AC crcuits. If your terminal blocks are set up as circuit terminals, the design should preclude any excess nominal current through any one terminal block.

 

[petersonra]

DC circuits require overcurrent protection rated accordingly for circuit components

where does it say that in the code anywhere?

 

[DH_control]

DC circuits require overcurrent protection rated accordingly for circuit components, including terminals and conductors, just like AC crcuits. If your terminal blocks are set up as circuit terminals, the design should preclude any excess nominal current through any one terminal block.

There is seperate fusing coming out of the power supplies so I assume that will be the protection needed.

 

[Smart $]

There is seperate fusing coming out of the power supplies so I assume that will be the protection needed.

Not good to assume.

The requirements for DC branch circuits are essentially the same as for AC branch circuits. In fact, for the most part, Code does not distinguish one from the other.

 

[jaggedben]

where does it say that in the code anywhere?

240.4

 

[SceneryDriver]

This is not an especially easy question to answer. There are pros and cons to doing it both ways, but in the end it probably makes little difference.

As a practical matter, they are already bussed together through the EGC.

one thing is that you have to be careful how things are wired to make sure that none of the common conductors or terminals gets overloaded. It is a lot easier to make sure of this if they are separated out.

Tie the commons (the 0V of each power supply) together. This will aid in troubleshooting, and will help out the next person, as they'll probably assume they're tied together as well. As long as only one side of the power supplies are tied together - they're not ganged, as some can be - there will be no overcurrent worry. each power supply can only source a given amount of current. Provided the positive side of each power supply is protected by overcurrent protection, there can be no overload. Assuming that you are using DIN rail terminal blocks, and that you're bussing them together with jumper strips, evenly distribute where you land the commons from each power supply across your bus. I'm actually doing exactly this on a project right now. As long as terminals are rated for the max output of your largest supply, you're fine.



SceneryDriver

 

[SceneryDriver]

This is not an especially easy question to answer. There are pros and cons to doing it both ways, but in the end it probably makes little difference.

As a practical matter, they are already bussed together through the EGC.

one thing is that you have to be careful how things are wired to make sure that none of the common conductors or terminals gets overloaded. It is a lot easier to make sure of this if they are separated out.

They're only bussed together through the EGC (green wire) if you tie one side of your power supplies to earth. Switchmode power supply outputs are, by design, isolated from the incoming AC and earth. As to if it's wise to leave 24VDC supplies floating in regard to earth is a debate that has been about flogged to death. I favor floating the 24VDC supply, as it keeps noise off the DC rails. I just spent yesterday troubleshooting induced noise on a 0-10V analog signal that resulted from the cable's shield being tied to ground in a panel that contained a VFD (notorious noise source). As someone once put it to me, "The green wire is the gutter where all the electrical crap flows back to the sewer. Why hook up a sensitive device to it deliberately?"


SceneryDriver

 

[Smart $]

Tie the commons (the 0V of each power supply) together. This will aid in troubleshooting, and will help out the next person, as they'll probably assume they're tied together as well. As long as only one side of the power supplies are tied together - they're not ganged, as some can be - there will be no overcurrent worry. each power supply can only source a given amount of current. Provided the positive side of each power supply is protected by overcurrent protection, there can be no overload. Assuming that you are using DIN rail terminal blocks, and that you're bussing them together with jumper strips, evenly distribute where you land the commons from each power supply across your bus. I'm actually doing exactly this on a project right now. As long as terminals are rated for the max output of your largest supply, you're fine.



SceneryDriver

You should not state such unless you are completely familiar with design details... and I don't believe you are.

 

[Smart $]

where does it say that in the code anywhere?

Get your head out of the box, Bob.

Field wiring is typically covered by the NEC, and OP mentioned main and remote control boxes. As I said, Code does not distinguish between DC and AC, in most instances. If the article, part, section, paragraph does not somehow differentiate between AC and DC, it applies equally.

(PS: Box as in UL listed equipment enclosure.)

 

[Smart $]

..."The green wire is the gutter where all the electrical crap flows back to the sewer. Why hook up a sensitive device to it deliberately?"

In many cases Code requires us to do so. It's basically why there is an Article 250 in the NEC.

Now I know from your other postings over time that you are probably referring, in most cases, to power limited stuff, such as you find in communication, control, and "Class" rated circuitry... but seemingly broad statements can be taken outside or beyond the context as stated... and I believe your [quoted] statement is one of those instances.

 

[petersonra]

DC circuits require overcurrent protection rated accordingly for circuit components, including terminals and conductors, just like AC crcuits. If your terminal blocks are set up as circuit terminals, the design should preclude any excess nominal current through any one terminal block.

Once again I ask where does it say this in the code????

You have a history of making assertions like this and when called on them refuse to answer, or answer questions that were not asked.

 

[Smart $]

Once again I ask where does it say this in the code????

You have a history of making assertions like this and when called on them refuse to answer, or answer questions that were not asked.

You won't find it in the Code. Code only distinguishes between Ac and DC when there are different requirements. Otherwise, Code applies equally to both AC and DC.

You have to apply common sense here. For example, a wire conductor does not depend on whether it is carrying AC or DC current for it to be a wire conductor.

 

[Smart $]

Once again I ask where does it say this in the code????

PS: It's called the National Electric Code...

You have a history of making assertions like this and when called on them refuse to answer, or answer questions that were not asked.

From my perspective, you are asking the wrong question... and expecting an answer other than what I provide.

I can't answer in a manner which is against my nature... unless I put effort into it, and I don't care to tax my brain on your behalf. The question has to be worded so I understand it for me to answer in the manner you want it answered.
:slaphead:

 

[petersonra]

PS: It's called the National Electric Code...


From my perspective, you are asking the wrong question... and expecting an answer other than what I provide.

I can't answer in a manner which is against my nature... unless I put effort into it, and I don't care to tax my brain on your behalf. The question has to be worded so I understand it for me to answer in the manner you want it answered.
:slaphead:

yet you made an specific assertion that the code requires a specific rating of OCPD for terminals and other components. I just asked you to provide that reference and as has happened in the past when you make such assertions you seem unable to do come up with an actual code requirement, and also seem unable to admit you made an assertion that may not be accurate.

 

[Smart $]

yet you made an specific assertion that the code requires a specific rating of OCPD for terminals and other components. I just asked you to provide that reference and as has happened in the past when you make such assertions you seem unable to do come up with an actual code requirement, and also seem unable to admit you made an assertion that may not be accurate.

Ohh I forgot... your name is Bob too... or is that two (2).

 

[Smart $]

Once again I ask where does it say this in the code????...

Perhaps the answer you seek is 110.3.... :blink:

PS: That's just to start with. The rest falls into place once you understand the concept that the Code in general applies equally to DC circuits.