Hi guys! First post here but long time lurker. I am a mechanical engineering working on my first complex free lance project designing and building a custom positioning machine that will see loads in excess of 200 lbs along a 7 foot vertical span. The structural and mechanical portion of my design came easy, as expected, but I am still uneasy about all the proper techniques when it comes to design and manufacture of the electrical panel (enclosure).

Some specs of the machine:
120V AC to (2) DC sources
DC1 is a 84V 12A unregulated source powering the DC motor driver and motor. DC2 will be a 24V 5A DC source for the motor controller, control circuitry, switches, pneumatics (for custom brake), lights, ect..

The question I have, even after months of intense research, is what are the best practices for grounding the separate AC & DC components INSIDE the enclosure (with Very limited real estate) and EXTERNAL DC sensors (10 of them)? This machine will be used in a public setting so safety is my number one concern.

Thanks!

 

I assume you are in the U.S.A.

In general, everything goes back to one point, and that point is connected to earth, or bond...the planet. You have to learn to think of the power wire and the neutral wire as a floating source. Neutral will seldom be zero volts because of the current flow through it creating a few volts on its way back to the circuit breaker panel. Only bond is really zero volts and no connection to bond intentionally carries current.

Each DC supply will produce its own current and have a "common" point where the current returns to the supply after it's done with the motors etc. The DC supplies and their loads form an island in the world which would work just fine if it never touched anything but itself, but a final connection is made from, "common" of that supply to bond. That wire never has any current in it unless something shorts out or burns up. It's only a reference point to keep the DC supplies from accumulating static electricity.

If you want a single rule for sensors, a twisted pair in a shielded cable (twisted, shielded, pair) keeps out noise and protects spectators. The shield is only connected at one end, the "common" point for the DC supply that is running it, unless there is a good reason not to. Otherwise, it will pick up current from magnetic fields and noise from radio stations and spew that into the brains.

There's a start. Anybody have some more points to make?

 

Wow Thank you! Great Job of explaining that! You actually hit on something That has been foggy in my mind. For whatever reason the neutral DC bond point(s) and sensor commons have been blurry in my mind.

Would I bond the 2 DC common connections to a Secondary bond point? If so, does that Bond point connect to the AC common/Neutral at another Primary bond point inside the enclosure on the same ground (bottom) plate? Also The 24V supply I purchased doesn't include a Common or neutral. Can I assume the supply chassis will do that job?

Cheers! Thanks so much for the input!

 

Wow Thank you! Great Job of explaining that!

That's why there is a, "Thanks button" in my reply window.

 

No common or neutral on the 24 volt supplies??? They won't run like that. I think you need to polish up your definitions of neutral and bond. The neutral bond connections don't exist. Neutral and Bond are two different places.

Neutral is the white wire. It's where the current goes when it's done working. The bond is the naked wire or the green wire. It is connected to the planet and never carries current except when something melts down or blows up. The case of your machinery is connected to bond. Your motor windings will never be connected to bond.

The 24 volt supplies will get bolted to steel, somewhere, and that is Bond. The 120 V (black) and the Neutral (white) wire have to be connected to the power supplies somewhere else because it is illegal to run neutral current through a Bond wire.

 

No common or neutral on the 24 volt supplies??? They won't run like that. I think you need to polish up your definitions of neutral and bond. The neutral bond connections don't exist. Neutral and Bond are two different places.

Neutral is the white wire. It's where the current goes when it's done working. The bond is the naked wire or the green wire. It is connected to the planet and never carries current except when something melts down or blows up. The case of your machinery is connected to bond. Your motor windings will never be connected to bond.

The 24 volt supplies will get bolted to steel, somewhere, and that is Bond. The 120 V (black) and the Neutral (white) wire have to be connected to the power supplies somewhere else because it is illegal to run neutral current through a Bond wire.

I like it, especially the bit in italics.

A further point is that connections to steel can corrode and loose the electrical continuity.
Some regime of periodic checking this should be implemented, bearing in mind that this steel case will be touched by the general public.

 

For construction, you should search, DIN rail, DIN terminals and wire duct. See post #3 here: http://www.boatdesign.net/forums/electrical-systems/din-rail-wiring-opinions-39529.html Picked at random.

Here's the deal. Wire to avoid "ground loops"

What that means in theory, is:
Multiple isolated commons up to a point)
Sensors should return to their own common. (Analog ground)
Shields should be connected at the originating end only case ground.
Shields help RFI
Twisting helps EMI

High current commons should be separated and isolated to a point.
Your allowed and should have multiple ones for each system.

Digital grounds should be separated to a point.

Now at ONE SINGLE POINT on the chassis in your box you can connect the commons together.

-----

Aside:
Building ground - Earth (This only carries fault currents)
Neutral - carries a return current
HOT - 120 VAC

The US uses a single phase or really a "split phase" 120-0-120 system where the center tap of incoming power transformer is grounded.

At only ONE point for the building is ground and neutral bonded together. You have to maintain that separation. Neutral stays separate from ground in your box. It will eventually be connected together, but not by you.

 

There are two schools of thought on keeping various supplies in a system galvanically separate, the other is to bond, where possible, all P.S. commons and bond these to a common earth ground star point in the enclosure.
I usually prefer the latter.
If you are concerned about wiring procedures, obtain a copy of NFPA79, Electrical Wiring of Industrial Machinery, it is based around the NEC.
Also this Siemens PDF is an excellent reference for equi-potential bonding and earthing.
http://www.automation.siemens.com/doconweb/pdf/840C_1101_E/emv_r.pdf?p=1
Max.

 

I like it, especially the bit in italics.

Sometimes you have to change gears suddenly when you realize you aren't achieving effective communication.

@Max...I have read NFPA books and NEC books and got confused as to whether one was a subdivision of the other or whether one is, "Included by reference" or statute, or something. Never did figure that out.

 

NFPA79 is specifically aimed at Industrial machinery control wiring, and is based on factors laid out in NEC, just a specific expansion of sorts.
For example, they show recommended enclosure layout, typical ladder style documentation and PLC examples.
When I first got into control systems design, I looked high and low as to this kind of documentation until I stumbled on NFPA.
Max.

 

A typical example question:
How many are aware of the colour given to a conductor that enters a control enclosure that has the possibility of being live when the enclosure disconnect is OFF.
Answer is different for N.A. and Europe.
Max.

 

Answer is different for N.A. and Europe.

They just don't pay attention to us!
or is it the other way around?

 

They just don't pay attention to us!
or is it the other way around?

Not sure
Just for those that wondered, it is Yellow for N.A. and Orange for Europe.
Max.

 

Good. In N.A. the orange wire is the third phase of a 3 phase power supply. I have orange tape in my truck, but I'll buy some yellow, now that I know what it's for.

 

What's red in NA?

 

What's red in NA?

In power wiring, a split phase 240 VAC cable will have black, white, red, and green. The white is common to both phases, as the center tap of the mains transformer secondary. The black and red are the ends of the secondary winding and are 180 degrees out of phase with each other. The green or naked wire is the bond, or earth ground.

In control wiring, the red is the power supply, right off the low voltage transformer. In electronics, red is usually DC+.

 

Red (plus Black and Dark Blue) is also one of a 3 phase supply.
For AC control circuits, in Canada Black, or Red Secondary AC circuit, Blue is DC control, (where possible I use Blue for +v and lighter Blue for Common).
Neutral can be either White or Gray.
Max.

 

Here is an example of #12s great post about bonding

Utility grounding is usually pretty standard stuff but when you have stacked and isolated supplies with many sources of EMI and noise sometime finding the best place to bond protection devices for common-mode signals can be tricky.

We had a circuit (similar to the image) problem with a bias supply (600vdc/2.8A) blowing up internal filter caps when the extraction supply (80,000vdc/100mA) arcs. Taming the power "ground" loop that was feeding the energy into the power supply was the task. We used large copper plates instead of bonding wiring to reduce reactance to very low values for the protection devices and connected them to V2 as the +80kv point was the 'ground' for the floating supplies instead of earth or frame ground.

http://www.freepatentsonline.com/6777686-0-large.jpg

 

And in some worlds (thermocouples), red is negative.

 

Red (plus Black and Dark Blue) is also one of a 3 phase supply.
For AC control circuits, in Canada Black, or Red Secondary AC circuit, Blue is DC control, (where possible I use Blue for +v and lighter Blue for Common).
Neutral can be either White or Gray.
Max.

Here it's blue and brown for 24vdc.

And to make it confusing, white and black seems to be standard for open collector sensor wiring.