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Connecting enclosure to earth

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edrean

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Hi,

I would like to shield my board from noise coming from the outside world. I have opted to use an aluminium enclosure to house my PCB. I am hoping that by connecting the enclosure to the mains' Earth that the aluminium will conduct the radiated energy into the ground.

Just for background, in South Africa our mains plugs have 3 pins. We call it Live, Neutral and Earth (I noticed some other countries have different names for it). Live and Neutral comes directly from the mains transformer outside. In the distribution board they will run through an earth leakage circuit breaker and will be distributed through various other breakers. From each mains wall plug, the Earth wire will usually run through an Earth bus bar in the distribution board directly to the physical ground (usually a copper rod in the ground). Any current that flows from Live to Earth will trip the earth leakage breaker.

Now, back to the enclosure. The enclosure has slots on its side walls where my PCB can slide into. My idea is to have bare copper strips on both edges of the PCB which will make contact with the enclosure when the PCB is inserted in the enclosure's slots. Both copper strips will then each have a trace running to a central point on the PCB where I will connect the Earth wire. My theory is that this will provide a direct path to ground from the aluminium enclosure in order to catch and conduct any radiated energy into the ground and away from my circuits (obviously also any energy radiated from my circuit as well). It will also serve as a safety mechanism in case of the Live coupling to the enclosure.

It has occurred to me, though, that by connecting the Earth to both edges of the PCB I am effectively creating a loop through the enclosure metal. Is this a problem?
 
Hi,

I would like to shield my board from noise coming from the outside world. I have opted to use an aluminium enclosure to house my PCB. I am hoping that by connecting the enclosure to the mains' Earth that the aluminium will conduct the radiated energy into the ground.

Just for background, in South Africa our mains plugs have 3 pins. We call it Live, Neutral and Earth (I noticed some other countries have different names for it). Live and Neutral comes directly from the mains transformer outside. In the distribution board they will run through an earth leakage circuit breaker and will be distributed through various other breakers. From each mains wall plug, the Earth wire will usually run through an Earth bus bar in the distribution board directly to the physical ground (usually a copper rod in the ground). Any current that flows from Live to Earth will trip the earth leakage breaker.

Now, back to the enclosure. The enclosure has slots on its side walls where my PCB can slide into. My idea is to have bare copper strips on both edges of the PCB which will make contact with the enclosure when the PCB is inserted in the enclosure's slots. Both copper strips will then each have a trace running to a central point on the PCB where I will connect the Earth wire. My theory is that this will provide a direct path to ground from the aluminium enclosure in order to catch and conduct any radiated energy into the ground and away from my circuits (obviously also any energy radiated from my circuit as well). It will also serve as a safety mechanism in case of the Live coupling to the enclosure.

It has occurred to me, though, that by connecting the Earth to both edges of the PCB I am effectively creating a loop through the enclosure metal. Is this a problem?

It depends on many factors - easiest solution is only to connect it at one edge, and remove the problem entirely.
 
hi edrean.
Look at this pdf for pointers on grounding.
I prefer Star Single Point grounding in very electrical noisy environments.

If the mains supply in Tzaneen is anything like the mains in Port Elizabeth you have a problem..:rolleyes:
 

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Nigel Goodwin said:
It depends on many factors - easiest solution is only to connect it at one edge, and remove the problem entirely.
I'll do that. Thanks!!

ericgibbs said:
Look at this pdf for pointers on grounding.
Thanks for the pdf! I have read through it and I must say that I don't understand some of it. It will definitely become more useful as I understand more and more about electronics in future. My setup looks similar to slide 3-15. I just don't have any of the capacitors indicated. I assume the cap between the Live and Neutral wires is there to filter noise on the mains supply? Why are there caps from both mains wires to the chassis? Also for mains noise? What determines if you need these?

What I have tried to do with my board layout is something like the figure in slide 3-22. The difference being that mains Earth (which in South Africa goes to a copper rod in the ground) is not connected to my circuit's ground (0V). Should I connect mains Earth and my circuit ground (0V) together? Will that not cause multiple paths to ground?

ericgibbs said:
If the mains supply in Tzaneen is anything like the mains in Port Elizabeth you have a problem..
I assume the mains in Port Elizabeth is noisy. I don't know, it might be worse in Tzaneen :) . Port Elizabeth is a city, while Tzaneen is more of a rural town.
 
The primary thing is the impedance to ground; more than an ohm is trouble. First, the box if all joints have been electrically bonded, forms a Faraday shield so all charges will accumulate on the exterior of the box. What you do inside becomes relatively unimportant. You can connect multiple points on your board through separate wires to a common ground point in the box where your external ground connects. This forms what is called a “single point ground” and is used to bond critical equipment like telephone switching gear.

Your greatest source of trouble will be anything that sticks out of the box since it can act like an antenna to conduct unwanted signals into the shielded area. Cords, switches, lights, somebody’s thumb in the wrong place could act as a noise pipe if not all grounded and isolated from the sensitive parts of your project.
 
Thanks for the pdf! I have read through it and I must say that I don't understand some of it. It will definitely become more useful as I understand more and more about electronics in future. My setup looks similar to slide 3-15. I just don't have any of the capacitors indicated. I assume the cap between the Live and Neutral wires is there to filter noise on the mains supply? Why are there caps from both mains wires to the chassis? Also for mains noise? What determines if you need these?

What I have tried to do with my board layout is something like the figure in slide 3-22. The difference being that mains Earth (which in South Africa goes to a copper rod in the ground) is not connected to my circuit's ground (0V). Should I connect mains Earth and my circuit ground (0V) together? Will that not cause multiple paths to ground?
hi,
The noise on a mains feed can be balanced or unbalanced.
That capacitor configuration reduces most type of mains noise.

I would also go the 3-15 route for connecting 0V's together, the Ground/Earth is your copper rod.
Use a single Gnd connect point for both slide 3-15 and slide 3-22


For ref, you can buy filtered mains sockets for panel mounting, they can also include the small chokes in addition to the caps.

This is the type is an example Farnell dont stock them.
https://uk.farnell.com/roxburgh/rid0142h/filter-inlet-1a-emc-faston/dp/1428675
 
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Gary B said:
Your greatest source of trouble will be anything that sticks out of the box since it can act like an antenna to conduct unwanted signals into the shielded area.
There will be wires running into the enclosure. This is unavoidable in this project. There's even a quarter-wave whip antenna running into a 433MHz receiver (the antenna is well grounded, I think). However, I believe no part of my project is THAT sensitive to noise.

ericgibbs said:
Use a single Gnd connect point for both slide 3-15 and slide 3-22
Just to be perfectly clear. I will connect the 0V (-) from the bridge rectifier to the mains ground/earth. Then I will connect the grounds coming from the different circuits at a central point to the mains ground/earth. Then I will connect the enclosure/chassis also to the same central point to the mains ground/earth. That right??

Another thing that comes to mind. What if something happens and the mains Live conducts to the chassis. For an instant before the circuit breaker trips there will be mains power going into my circuits' ground. Will that not damage the components?
 
There will be wires running into the enclosure. This is unavoidable in this project. There's even a quarter-wave whip antenna running into a 433MHz receiver (the antenna is well grounded, I think). However, I believe no part of my project is THAT sensitive to noise.


Just to be perfectly clear. I will connect the 0V (-) from the bridge rectifier to the mains ground/earth. Then I will connect the grounds coming from the different circuits at a central point to the mains ground/earth. Then I will connect the enclosure/chassis also to the same central point to the mains ground/earth. That right??

Another thing that comes to mind. What if something happens and the mains Live conducts to the chassis. For an instant before the circuit breaker trips there will be mains power going into my circuits' ground. Will that not damage the components?

hi,
Consider the path the mains LINE short circuit current would take.?
As a safety consideration, without the chassis grounding the chassis would be at the LINE voltage and any person touching it would ground it.!!!


EDIT:
when you say the antenna is grounded , do you mean the 0V common or has it got its own connection to Earth ground.????
 
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ericgibbs said:
Consider the path the mains LINE short circuit current would take.?
OK, so if the mains Live conducts to the chassis it will conduct charge directly through the chassis and out to main ground/earth (shortest path)? So it will not kick up the potential (cause a spike) in the PCB ground?
 
ericgibbs said:
when you say the antenna is grounded , do you mean the 0V common or has it got its own connection to Earth ground.????
I guess it's the common I am talking about. The antenna connector has two conductors. One conducts the signal and the other is connected to ground on the PCB.
 
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OK, so if the mains Live conducts to the chassis it will conduct charge directly through the chassis and out to main ground/earth (shortest path)? So it will not kick up the potential (cause a spike) in the PCB ground?

If the short does produce a momentary voltage drop from the point of contact with chassis and the chassis grounding point, how is that voltage going to appear on the pcb's.??

The only possible interference would short radiated em pulse, the pcb decoupling should take care of that.

If the project is controlling a 'critical' process there are centres where you can submit the completed project for testing to the local EM and Health & Safety standards for approval.
 
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