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Shielding
« on: February 19, 2014, 04:14:18 AM »
Hi everyone, new to Mach forum. I am a retired toolmaker and i am going to build a cnc router i am not woried about the mechanical side of things but i am trying to get guidance on the issue of using shielded cable.
I have spent many hours trawling the forum for info but i need a but of guidance if possible.
As i see it when members talk about ground you mean ground on the bob. When you talk about earth you mean mains power earth. Is this ok or am i off the plank.

I was going to build my controller with all signal shield grounds going to bob ground.
Stepper motor shield cable grounded out on the steel enclosure.
All shields connected one sidded

Am i on the right track here or as we say off the plank.
If anyoneone can help please remember i am a mechanical not electrical pearson. Thanks in advance for any help and guidance i get.
Best wishes to all.
Billybob

Offline RICH

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Re: Shielding
« Reply #1 on: February 19, 2014, 04:40:41 AM »
Do a search and you will find a number of posts relative to grounding and noise.

Here is just one:
http://www.machsupport.com/forum/index.php/topic,26521.msg186910.html#msg186910

RICH

Offline Tweakie.CNC

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Re: Shielding
« Reply #2 on: February 19, 2014, 07:43:06 AM »
Quote
I was going to build my controller with all signal shield grounds going to bob ground.
Stepper motor shield cable grounded out on the steel enclosure.
All shields connected one sidded

Hi Bob,

Welcome to the forum.

That's exactly the way I would do it.  ;)
Just one point - take all shield earth connections to a single point on the steel enclosure (star configuration).

Tweakie.
KEEP SAFE !

Offline Fastest1

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Shielding
« Reply #3 on: February 19, 2014, 08:28:23 AM »
Some tie the "mains earth and the v- of the power supply into the star ground also. Plenty of debate.

   I know 1 thing, noise causes problems.
I want to die in my sleep like my grandfather, not like the passengers in the car! :-)

Offline Chaoticone

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Re: Shielding
« Reply #4 on: February 19, 2014, 04:24:09 PM »
Hello Bob, Welcome! You may find this topic helpful as well. It has links to some pretty good, short and fairly inclusive guides. Doing the research now is certainly the best time.

http://www.machsupport.com/forum/index.php/topic,25616.0.html

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!
Re: Shielding
« Reply #5 on: February 19, 2014, 06:12:22 PM »
Bob,
Shielding is very important and the best practice is to shield everything. I also try to keep all signal wires away from power wires when I layout a panel.  AC one side, DC the other, that kind of thing.

Something to understand about grounding.  By the National Electric Code ground loops, grounding everything to everything is a good thing.  That is because the intent of power grounding is to make sure that when something faults and hundreds or thousands of amps are flowing through the grounding system, no two points you could touch will ever have a voltage difference large enough to hurt you.  All electrical equipment leaks power to ground, some very small amounts some very large. So power grounding systems will ALWAYS have current flowing through them.  I have to laugh when electronic engineers tell you that you need to have the mythical 'clean' ground!

Control grounding is very different, as what we care about is not having a signal created, distorted, or destroyed by noise.  In control grounding of devices and cable shielding you want to make sure first that there is only one ground point that connects the control grounding to the electrical power ground.  Then you must make sure that every device ground and every cable shield connects to that ground point by just one path.  This sounds simple, however what usually trips people up is that manufacturers of devices like pressure transmitters will provide a ground terminal, and internally they connected it to the metal case. So if you connect your cable shield to that terminal and to the ground point at the other end you now have two paths, one through the cabinet, one through the cable shield, which is bad.  Another touchy spot is shielded cables passing through a junction box.  It is tempting to tie the in and out shields to a ground bar that the box may have.  However in junction boxes shield wires must be insulated the same as conductors. Otherwise you create multiple ground paths.

That's it, hope that helps.