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Shielding components from EMI/RFI... Help please

A recent experiment with a product designed to reduce EMI/RFI left me curious about other ways to reduce EMI/RFI in my system. In the past ten days, I've stepped onto a slippery slope, at the bottom of which is surely some kind of insanity...

I've been experimenting with copper plates in an effort to absorb, deflect, diffract, and block EMI/RFI. I've tried copper plates under components, on top of components, and inside components.

This is the point where you tell me I don't know what I'm doing and I'm likely to short circuit something and/or electrocute myself. Consider me duly warned. This is also the point where you tell me to get some balanced interconnects, or at least to get some shielded interconnects for Chrissake. Consider me duly informed. Moving on...

I'm hoping you can help me make the most of this experiment, and help me avoid killing a component or myself. My strategy so far has been to:

1. Place copper plates at locations that generate a lot of EMI/RFI, e.g., components with switching mode power supplies or high frequency clocks. The system has a total of 3 SMPS and 3 clocks.

2. Place copper plates at locations that are vulnerable to EMI/RFI, e.g., under the amp, near the transformer.

3. Place copper plates inside noisy components -- in particular, my Meridian G68 preamp/processor. I've begun to build 2 partial Faraday cages, one for the SMPS, and one for the analog output stage.

4. Ground the copper plates either to the component chassis (when plates are used inside a component) or to an independent ground point (when plates are used above/below a component).

Has anyone tried this sort of thing?

Bryon
bryoncunningham
jea48
4,486 posts
 
Almarg, I've always wondered about the code to use only one ground rod as well. I have never heard an explanation.
02-14-12: Sarcher30


Sarcher30,

There is no limit of the maximum number of grounding electrodes (ground rods), only the minimum required.

What NEC does require is that they must all be tied electrically together. When all tied together they are considered by NEC as one grounding electrode.

Several 8ft ground rods is not necessary better than one 30ft deep driven ground rod. What matters is the rod/earth resistivity...... The lower the earth soil resistance the better. 3 ohms or less is considered very good.

I would bet very few houses even come close. NEC code says if the earth resistance is greater than 25 ohms the ground rod shall be augmented by one additional ground rod. The earth resistance could be 60 ohms but all NEC calls for is one additional rod. The earth resistance still could end up higher than 25 ohms...... NEC code is satisfied with the one addition rod.....

At least in my area, I have never heard of a residential electrical contractor ever checking the earth ground resistance. I have only seen it checked on industrial facilities. Special equipment is required for the test.

http://www.cpccorp.com/deep.htm

http://www.erico.com/public/library/fep/strike/LT0540.pdf
.
sarcher30
596 posts
 
Jim, Interesting article. Do you think it would be worth while to have a second ground rod installed? It seems that even an eight foot ground rod would be difficult to install without drilling a hole first.

Sean
bryoncunningham OP
890 posts
 
Thank you, Al, for your help. In light of Jim's advice, I've abandoned the idea of using a dedicated ground rod. Even though I live in Southern California where lightning is rare, there's no good reason to take the risk. It's really no sacrifice anyway, because I could not hear a difference between the enclosures grounded vs. ungrounded.

The improvement from adding the two additional enclosures (whether grounded or ungrounded) was perceptible but not dramatic. The big improvement came from adding shielding INSIDE the G68 (grounded to the chassis). With that in mind...

I've been reading that copper is effective at shielding high frequency RFI, but not particularly effective at shielding low frequency EMI. So I'm thinking about adding a layer of STEEL shields around the G68's power supply, bolted directly to the copper plates that are already in place.

bc
magfan
1,583 posts
 
Try this:

http://en.wikipedia.org/wiki/Mu-metal
bryoncunningham OP
890 posts
 
Question...

Looking inside the Meridian G68, I see that the power is grounded to the chassis at TWO points. A ground wire connects the IEC input to the chassis. A second ground wire connects the power supply to the chassis.

The unit has been modified extensively, both by me and by a professional modder, so I don't know how it was grounded when it came from the factory.

Is it okay for the power to be grounded to the chassis at two points? Or am I creating a ground loop?

Thanks,
Bryon
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