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Seems like any equipment in a metal case is protected from rfi

Looking atFaraday cages in Wikipedia
Here's a quote:

A Faraday cage operates because an external electrical field causes the electric charges within the cage's conducting material to be distributed such that they cancel the field's effect in the cage's interior. This phenomenon is used to protect sensitive electronic equipment from external radio frequency interference (RFI). Faraday cages are also used to enclose devices that produce RFI, such as radio transmitters, to prevent their radio waves from interfering with other nearby equipment. They are also used to protect people and equipment against actual electric currents such as lightning strikes and electrostatic discharges, since the enclosing cage conducts current around the outside of the enclosed space and none passes through the interior.

Why are we trying to quiet the rfi to and from our amps, transports, etc when the metal case already does it? 

kavakat1
terry9
2,354 posts
 
Mu-metal tends to lose its properties after it is bent, unless it is annealed. Same with many other magnetic exotics.
geoffkait's avatar
geoffkait
23,365 posts
 
That’s why you always use annealed mu metal, which all of it is these days, since annealing improves performance considerably. Why wouldn’t you anneal it? And why you wrap the transformer without putting sharp bends in the metal, easier I suppose for toroidal transformers than for square type. Just ask for Ultraperm.

terry9
2,354 posts
 
My point is that wrapping in mu-metal is not straightforward.

"Why wouldn't you anneal it?" Perhaps because most people don't know that they should?
glennewdick's avatar
glennewdick
1,772 posts
 

you can not isolate from all RF period lower Frequencies are easier. The sun bombards us with RF, every cell phone every wireless device is bathing you in High frequencies. shorter the wave length the smaller the holes in the cage need to be. I’ve been in shielded rooms on the Navy base that use 12" of various types of shielding the doors are 12" thick and the door seals have multiple copper wipers all around and they still let in some frequencies. so don’t worry too much focus on the ones that are audible.

oh Terry9 once you bend any metal you get micro fractures so when you anneal the metal, heating and cooling under control you remove the micro fractures. This is a point for wires to consider don’t bend them to much and too often.

geoffkait's avatar
geoffkait
23,365 posts
 
glennewdick
oh Terry9 once you bend any metal you get micro fractures so when you anneal the metal, heating and cooling under control you remove the micro fractures.

Mu metal is annealed by the manufacturer prior to shipping. That’s why bending annealed mu metal won’t produce micro fractures like unannealed mu metal. Think of it like cryogenic treatment, which is what it is. The molecular structure of the annealed alloy is much more homogeneous and less brittle and more ductile. Obviously for toroidal transformers no sharp bending is involved when wrapping the transformer. You just make a "hat box." Mu metal is quite soft and malleable so for square transformers use a cylinder shaped tool to form the alloy into gradual bends around the transformer, thus avoiding sharp bends. Problem solved!

Timely tip: TAKE CARE WHEN HANDLING MU METAL. Always use those special metal-cutting scissors with beveled edges that makes a blunt edge on the alloy. It’s ok to use gloves when handling mu metal. Otherwise you will suddenly realize your fingers are bleeding since mu metal edges are very very sharp.
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