How much better is steel casework vs aluminium (aluminium) for shielding? - diyAudio
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Old 24th April 2015, 12:56 AM   #1
kimbo is offline kimbo  Australia
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Default How much better is steel casework vs aluminium (aluminium) for shielding?

Most of my projects so far have been in steel enclosures, mainly standard 19 rack sizes.
My next project will probably be in an alloy case. Would I be correct in thinking that the Al would offer much the same emf shielding as steel, but will not be as effective as shielding any stray magnetic flux from the transformer or the transformer of any other case it sits on? Or does a steel case actually couple the stray flux to the case and therefore transmit it more?
Or is my long forgotten Uni Physics way off the mark?
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Old 24th April 2015, 01:21 AM   #2
Struth is offline Struth  Canada
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Hi Guys

All metals provide electrostatic screening if the chassis is grounded.

Steel is magnetically conductive but provides little in the way of magnetic shielding unless it is extremely thick - much thicker than any standard chassis sheet. Because of the magnetic conductivity, a steel chassis may channel the field of the PT to areas far away, as the noise radiates from edges. Eddy currents are not so much of an issue unless you are using the chassis to make signal grounds between circuit points - a definite NO-NO.

Aluminium is magnetically transparent so the field around the PT stays around the PT. There is also no shielding from external magnetic noise sources. The chassis element have to be much thicker to have the same strength as a given thickness of steel.

Stainless steel comes in several grades, some that are magnetic and will rust, and others that don't rust but are like aluminium as magnetically transparent. Stainless steel can be thinner than regular steel for a given strength.

Stainless gets stained despite its name. Aluminium needs a clear coat or anodising to keep from oxidising. Steel rusts unless it is plated. Painted steel eventually rusts. Where plating does not interfere with panel bonding, anodising and paint do. so the contact areas of the chassis pieces must be sanded clean if an electrical bond is required. Most builders rely on the pem nuts and bolts to provide this connection, but it is a crap-shoot.

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Old 24th April 2015, 01:32 AM   #3
kimbo is offline kimbo  Australia
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Thanks Struth for such a clear and fast reply.
Yep....having fun is what we do!
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Old 24th April 2015, 03:26 AM   #4
jcx is offline jcx  United States
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steel, or even better, "hot rolled"/soft or annealed low carbon steel is nearer to soft iron in mag perm

the high mag field conductivity of the iron (permeability) gives the opportunity to magnetically shield by diverting mag flux

so iron boxes are good for shielding circuits inside from external mag field
but can concentrate the field of internal mag flux like transformer leakage

that is dealt with by added iron partitions or in high sensitivity instruments by "box inside a box" construction that "shorts out" the leakage flux around the xfmr/PS rectifier current loops - air space and iron make for good mag flux diversion
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