Question(s) about understanding chassis ground in sheet metal enclosure

Thread Starter

kyleh04

Joined Jun 18, 2017
54
Hello,
I have designed multiple PCBs and I believe I am fairly knowledgeable in low power DC circuits. I am currently working on a project in which it will control multiple high power LED panels. The total current will be ~60A at 12V. I will use bus-bars on the PCB and it will be thick copper, so it should be fine in that regard.

My issue in understanding is when it comes to the proper usage of chassis ground and how it relates to the safety of a high power DC system. This PCB will be mounted into a sheet metal enclosure. This enclosure will be in close contact (touching) with people constantly, so safety is paramount. Within this same metal enclosure, there will be a metal enclosed DC power supply, which will supply my power for the board. Similar to this, but higher power rated of course.

So, I see a few options:
  1. Isolating the power supply and the PCB from the enclosure with plastic or another insulator
  2. Grounding the power supply or the PCB to the enclosure.
  3. Grounding both the power supply and the PCB to the enclosure.
I feel that the best course of action is number 3.
This is my reasoning for this:
  1. If there is a loose wire from the power supply, it will short with the enclosure of itself, which shorts to the main enclosure, ensuring a breaker/fuse blows.
  2. Same as 1, but with the PCB.
Assuming my number 3 is correct, is my reasoning for it correct? If so, how/where do I ground them at? To the same location of the enclosure?
If not, which should I do, and why?
If they are both isolated from the enclosure completely, that also seems like a safe route, but I feel that if number 3 is also safe, it ensures that the worst case scenario is handled safely.

Thank you so much,
Kyle
 

MrChips

Joined Oct 2, 2009
30,986
12V x 60A = 720W

From where are you getting your power? One would assume that the power comes from AC mains.
Check your jurisdiction for AC mains connection. You may need certification.

In any case, mains EARTH is wired to the chassis.
AC LINE is wired to the fuse, followed by ON/OFF power switch, then the DC supply.
Return is wired to NEUTRAL.

What you do with the DC supply is up to you. You can leave it floating or you can connect one of the power output lines to the chassis.
 

MaxHeadRoom

Joined Jul 18, 2013
28,779
  1. Grounding both the power supply and the PCB to the enclosure.
I feel that the best course of action is number 3.
Assuming my number 3 is correct, is my reasoning for it correct? If so, how/where do I ground them at? To the same location of the enclosure?
Kyle
Any earth grounding in a metal enclosure the usual practice is to set up a star point ground termination.
All grounding and bonding goes to this point together with the service earth conductor.
Max.
 

Thread Starter

kyleh04

Joined Jun 18, 2017
54
12V x 60A = 720W

From where are you getting your power? One would assume that the power comes from AC mains.
Check your jurisdiction for AC mains connection. You may need certification.
I'm not sure I follow? I'm using the AC-DC power supply, with the AC power being connected by a simple 3-prong plug. Is that not a good way to do it?

In any case, mains EARTH is wired to the chassis.
AC LINE is wired to the fuse, followed by ON/OFF power switch, then the DC supply.
Return is wired to NEUTRAL.

What you do with the DC supply is up to you. You can leave it floating or you can connect one of the power output lines to the chassis.
What is the reasoning behind picking floating or chassis grounding?

Any earth grounding in a metal enclosure the usual practice is to set up a star point ground termination.
All grounding and bonding goes to this point together with the service earth conductor.
Max.
OK great, that makes sense.
 

MaxHeadRoom

Joined Jul 18, 2013
28,779
I'm not sure I follow? I'm using the AC-DC power supply, with the AC power being connected by a simple 3-prong plug. Is that not a good way to do it?
What is the reasoning behind picking floating or chassis grounding?
.
There are basically two schools of thought, one is to keep all systems isolated from each other as much as possible, the other is to bond all system commons and in most cases take this to earth ground star point.
I have always favored the latter where possible.
So far it has worked for me.
Max.
 

Thread Starter

kyleh04

Joined Jun 18, 2017
54
There are basically two schools of thought, one is to keep all systems isolated from each other as much as possible, the other is to bond all system commons and in most cases take this to earth ground star point.
I have always favored the latter where possible.
So far it has worked for me.
Max.
Ahh OK. So both the ideas that I had are common ways it's done, just different preferences. Thanks so much for your help
 

strantor

Joined Oct 3, 2010
6,816
I think there are at least two ideas flying around in here without intersecting.

1. Earth bonding for AC. Let's say you have a 120VAC power tool (like a hand drill) with a metallic case, as it was in the old days (the reason why we now have earth bonding). Without earth bonding inside the drill case (a 2 prong plug), if the line wire were to come loose inside the drill and contact the case, the case is now energized. Anyone who grabs that drill is a dead man or least going to have a bad day. So we went to 3 prong plugs with the case bonded to earth. Now if the line wire contacts the metal case interior, it shorts out and trips a breaker. Greatly reduced deaths and soiled pants. Not so much a big deal anymore since almost everything is plastic. But you will (or should) at least still see 3 prong plugs on anything with exposed metal.

2. Case grounding in 12V automotive systems. Almost everything in cars is terminated to the chassis. PCBs in Electronic modules are terminated to their metal cases, which are bolted to metal chassis. There are 2 reasons for this that I am aware of. (A) Noise shielding. Creating faraday cages around all the electronics because a car is a very electrically "noisy" environments, and (B) cost savings. There are very few (if any) ground wires running back to the battery. The car chassis itself serves as ground point for virtually all electric loads and the number of required wires is cut in half. 12V is not enough to warrant any kind of grounding for the sake of safety.

If your circuit is a non-automotive situation powered from an isolated 12V supply then there is no good reason to worry about any of this in my opinion. The only consideration would be EMI radiation if you have any high-power, high frequency switching going on, in which case you should connect the metal chassis to ground.

If you're using a non-isolated supply then again, the case should be grounded because a fault in the power supply could result in the case being energized at line voltage.

If in doubt, ground the case. And heck, why not the PCB too, just for good measure.
 

MaxHeadRoom

Joined Jul 18, 2013
28,779
If in doubt, ground the case. And heck, why not the PCB too, just for good measure.
My area of expertise has largely been in the motion control/CNC retro-fit industry where there is anything from 5vdc to high 3ph AC, so I have mainly adopted the methods outlined in the Siemens pdf.
I have also been a moderator on the CNCzone forum since it started, where you have a very large number of builders of small CNC machines, these are often amateur's in the field of machining and/or electro-electronics.
So you have all kinds of levels getting in to a field that mixes low level electronics and high voltage control, both AC and DC.
I would say at least once or twice a week there is someone complaining about spurious machine occurrences they cannot explain, usually false tripping of L.S. inputs etc, etc.
After showing some past examples of identical behavior and the suggestion to employ the star grounding technique, it often clears the problem up.
I have come to the conclusion that much of this lies in the manufacturers of the Many individual boards and units that go into the hobby side of the industry.
I suspect that the reason they state that their unit should be wired isolated is that then no blame can come back to their unit if anything goes wrong or is miss-wired.
That is my general take anyway.;)
Max.
 

MrChips

Joined Oct 2, 2009
30,986
I'm not sure I follow? I'm using the AC-DC power supply, with the AC power being connected by a simple 3-prong plug. Is that not a good way to do it?

What is the reasoning behind picking floating or chassis grounding?
What I mean is that in some countries, once you put something in a box and connect an AC power cord to it, it has to be inspected and approved by somebody such as UL if you want to commercialize the product.

One reason why bench power supplies are commonly not grounded is that you can reference the supply to any voltage. For example, if you connect the +ve terminal to GND you end up with a -ve power supply.
 

crutschow

Joined Mar 14, 2008
34,700
My personal preference is to connect the chassis to the main's safety ground and nothing else.
That way there's less chance of accidentally creating a circuit short to common.
But either way is fine.
 
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