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Old 13th October 2007, 12:15 PM   #1
diyAudio Member
 
Join Date: Dec 2003
Location: brussels
Default plastic cases and grounding - shielding

hello everybody,

i got some questions about plastic cases.

1) since a plastic case doesn't shield, is that a problem for hifi ? i suppose it is, but can i turn it into shielded one ? like putting alu-foil on the inside ?

2)what about safety issues like getting shocked ? i mean since there is no mass on the case do i have to put all the parts like switches on the earth of the connection ? i suppose that every part should be on the earth then (at least that can be touched by hand)

3)this is a diffucult question to explain.
i have also a psu in a plastic box, the powercord (240v) is shielded. But the box has another powercord (12-0-12v) it has a shielded foil around it, but where do i have to attach the shielding to ? i thought maybe attach it to the shieliding of the 240 cable shielding without making contact to the earth ?

thanx a lot
friendly greetings
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Old 13th October 2007, 03:10 PM   #2
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Join Date: Mar 2007
Location: Canandaigua, NY USA
1) Yes. If you look inside commercial products with plastic cases (or turntables with plastic plinths) you'll see everything from adhesive copper and aluminum tape, to aluminum clad cardboard, to nickel based spray on coatings. If you need shielding, you need shielding.

2) Unless the item is a stand alone battery powered device, no circuit connected metal should be able to be touched by the user. You can use plastic knobs and switch designs that are completely insulated.

3) Not sure. Is the LV supply transformer isolated with a transformer that meets current creep, clearance, and bobbin design rules? If the transformer uses separate bobbin sections for primary and secondary, you can probably get away with keeping the output ground local to the power supply and not connecting it to power cord safety ground.

I'm no expert, but what you want to know is buried in the EU safety regs that allow one to use the CE mark on their products. US UL regs will say something similar. Everybody who builds electronics, even for a hobby, should read through that stuff.

edit/addition- a good multilayer circuit board with a decent ground plane can often work just fine in a plastic box, but IMO a conductive box is still better.
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Old 13th October 2007, 03:15 PM   #3
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Hi,
all exposed conductive parts must be connected to safety earth.
That includes the RCA input sockets and the speaker terminals if you can touch the metal parts.
If the speaker terminals are enclosed, can you touch the speaker end of the metal terminal?

What if the PSU transformer or it's mains cabling allowed mains voltage to touch the secondaries?
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regards Andrew T.
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