Tieing seperate 24 vdc com's together

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joepauley

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I was asked if the com's from two separate machines 24vdc power supplies could be tied together. They said they have done this in the past to make handshakes easier between the two machines without problems. In my experience this is bad, they should keep the two isolated from each other do to stray currents, noise, if one machine is shut down.

Anybody seen this in the code book?? or have any experience??
 
Re: Tieing seperate 24 vdc com's together

Are you talking about communications circuits or power circuits?

It is very common and often desirable to "ground" one leg (usually the negative) of the 24VDC power circuit the same as you would a 120V or 24VAC circuit fed by a control power transformer. It is not permissable to create a power circuit that uses the "hot" conductor from one source and the grounded conductor from a different source.
 
Re: Tieing seperate 24 vdc com's together

This would be for a 24 vdc control circuit. I like to ground the com/0-volts for short circuit path. What they were asking is if they have two seperate machines that have handshakes between them, could they tie the com/0-volts together for the handshake reverence??. They said they've done this in the past without any problems. I would always keep them isolated/separated. I think you end up with a ground loop if both supplies com/0-volts are grounded
 
Re: Tieing seperate 24 vdc com's together

I see no problem with connecting both 0V points together with a dedicated conductor which would effectively create a"single point" reference for communication wiring. Right now they are connected through "building steel" and it's resultant multiple ground paths each with a different potential.
 
Re: Tieing seperate 24 vdc com's together

In my experience bonding the power supply's 0v. to the chassis and therefore also the building's grounding system is common practice.

If you didn't do that you would definitely expect to find a voltage difference between the grounded chassis and the supply's 0v. terminal, that voltage would be noise.

Keeping supply voltages referenced to ground makes it so that the only voltage differences are intended supply voltages and other sources.

Edit: I agree Jim that an isolated 0v. conductor between supplies would reduce noise. But I also think it's overkill in most cases and especially with control circuits.

[ November 24, 2005, 06:24 PM: Message edited by: physis ]
 
Re: Tieing seperate 24 vdc com's together

Joe Keep in mind that current always returns to source (each power supply).
Grounding each power supplies negative conductor can be done but do it at one point only for each supply. Then run both the positive and negative to each load with insulated wires, this will keep the return path clean. This is just like a neutral in a 120/240 or 120/208 system after the first service disconnect or transformer. Don't use the grounding as a return current path for any loads or you will cause a ground loop to form and any current on the ground loop will cause data errors. Car manufactures had this problem with computers when they were first introduced to the automobile industry, all the current running through the chassis was corrupting the data signals. They learned to run separate negative conductors to each load and the problem went away.
If you noticed even the speakers in newer cars have a seperate return path but thats mostly because of the IC output amplfiers which will burn out if right & left negatives are connected together. :D
 
Re: Tieing seperate 24 vdc com's together

I am pretty sure UL 508 Industrial Motor Control Standard requires DC power supplies to be grounded.
If the DC power supply is not grounded, then a fuse is required in the + and the -.
 
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