Separate or join ac and dc grounds? 3

[rickp]

I am working with instruments that have the 120vac mains ground pin tied to the chassis. The 12vdc and 3.3vdc returns are also tied to the chassis. The dc is used for some detectors, solenoid valves, stepper motor, small fan, and logic. So there are some quiet and some noisy dc devices. In the past, I have preferred to keep the ac ground and dc returns separate. The instruments work ok, but I wonder if the detector signals could be helped a bit by separating the ac gnd and dc return. Any suggestions?

 

[VEBill]

It may depend on the details of your application, but *typically* all the grounds are eventually tied together at one "single point" ground.

And one must be fairly careful about the design and reliability of that point to avoid some subtle and 'interesting' failure modes. Things like your PS return current taking the wrong path and burning out some signal ground returns, that sort of thing.

 

[waross]

It is not good practice to use a code ground conductor for signal returns.
Better practice:
Complete the AC grounds to an AC grounding bus in accordance with code.
Complete the DC grounds to a DC grounding bus in accordance with code.
Connect the AC ground bus to the DC ground bus in accordance with code.
Things to avoid.
Do not equate or confuse a safety ground path with a signal return path.
Functionality:
An AC ground path intentionally causes a high current to flow to ensure rapid operation of the protection devices.
This current may be several hundred Amps or more. The fault current may reach into the thousands of Amps on circuits with physically short conductors.
The expected ground fault current will cause a voltage drop in the grounding conductor. This may often be over 50% of the circuit voltage to ground. On a 120 Volt circuit the if the grounding conductor is used as a signal return, the result may be to inject over 60 Volts from a very robust source into the signal return.

Safety.
The transient voltage rise during a ground fault may cause serious and dangerous touch potentials between AC chassis and DC chassis. Solution:
1> Connect the AC and DC chassis together with a bonding conductor at the chassis to avoid touch potentials between adjacent equipment.
2> Use a separate, isolated conductor for signal return. This may be grounded at ONE point, generally at the power source of the signal circuit, but must NOT form part of the AC circuit ground path.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[flexoprinting]

Hello Gents

Could this be what you mean? Many people confuse negative signal of power supply with ground which should not to be mistaken as same signal.
When trouble shooting a DC circuit your black or negative probe should be on negative of your power supply source and not the ground of cabinet.

When working on an automobile you can use the frame of the vehicle, not the same with machinery. Correct me if wrong please.

Chuck

 

[MikeHalloran]

You've probably already got a good schematic of power distribution within the overall device.

It's time to generate a corresponding schematic of ground distribution...



Mike Halloran
Pembroke Pines, FL, USA

 

[flexoprinting]

In my youth working on cars I had a difficult time grasping the difference between ground and negative DC
and often at the shop I have mechanics who have same misunderstanding so I will show them a power supply while
pointing out the terminals, AC in, black is hot white is neutral and ground to earth positive and negative voltage
to dc circuits. I enjoy the look on their face when they understand and get it.

Bills explanation of coarse is right on the money however a car guy would still struggle understanding it without
studying DC theory or getting a demonstration I think.

Chuck

 

[zappedagain]

What Bill and Mike said. For good signal integrity, make sure you don't introduce ground loops, where noisy ground/return currents can run on the same wires as your detector return currents.

So typically chassis/safety/AC ground is made at the controller. A shielded cable to the detector will have the shield connected to chassis ground. The detector circuit will have a DC-signal-ground/return conductor that only connects to AC ground in the controller.

Z

 

[DanEE]

As waross and VE1BLL posts point out, this topic can quickly turn into a very deep span subject and not so obvious issues that can turn up.

Hard to say from your post how large the span is of your grounding and bonding question really is. i.e. are the valves, detectors, etc contained within a single integrated product or are they going to be scattered throughout a large facility... is this is a one off application or a product you are designing that will need to meet a number of standards?

So far as signaling and sensor circuits, using differential signaling/balanced circuits that maintain good longitudinal balance can produce some impressive performance over distance in a larger system and have good immunity in a noisy environment. And as you already know, in-appropriate grounds can quickly make it all go wrong.

If your application spans a larger scope e.g. sensors in a large facility, then the telecommunications industry has volumes of standards, specs and installation practices to deal with grounding, bonding, transient protection, AC/DC common bonding as illustrated in this very good but very extensive scope document. Many topics not relevant to your question are covered but chapter 5 becomes applicable if your application is large facility in scope.

http://www.ronet.co.za/downloads/R56 Guidelines.pdf

Cell sites, PBX centers and data centers with their typical use of AC and 48 volt DC plants, extensive signaling circuits have your issue on the large scale.

SCADA e.g. for manufacturing automation and instrumentation would also have volumes of information that could be of value to you.

 

[rickp]

Thanks for all your thoughtful responses. I will start by making an explicit connection between the dc return and the ac ground and see how it works.

 

[MacGyverS2000]

Add appropriate filtering to the connection...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[MikeHalloran]

Don't go adding ad hoc inter-ground connection to a system that is working.



Mike Halloran
Pembroke Pines, FL, USA