Grounding boxes?

[Mr. C]

Time to build and live inside a Farady cage.

Lol oddly enough I do for at least 30 hours a week my lab is very well shielded.

Here is a very odd but strangely true scenario, earlier this year I was working on a new pre amplifier design and was finishing off its ac line filtration circuit during pre compliance testing. The previous week I had just upgrade the labs mains power circuits and lighting to LED's so feeling pleased with my efforts I set up the equipment for the EC stands required Cisper 16. Now for this you need a isolation transformer for the DUT and the ground plane.

After around 30m minutes of testing I was getting so real spurious readings really struggled to get my head around them. I ended up tearing down the set up and taking to measuring the individual instruments themselves just to try and eliminate the blasted common mode noise.

Turn out to be a rather expensive bench power supply causing a heap of CMN, no here is the strange thing, even with the unit connected via an isolation (which lifts the earth) transformer the noise was still present. So even after switching the unit off at the power switch the noise is still present???

Ok must be me going somewhat mad, so I took out the mains plug from the bench psu guess what STILL there

Now the scopes and analysers are power by one side of the mains circuit, the other power supplies, iso transformers dc loads and ac power supplies are powered by the other side of the CU.

Now one of the new LED lights is an overhead bench unit for intricate work it has a delightful SMPS though it is one the scope side of the mains, well taking this out of the socket removed all of the noise from the dc power supply even with the unit UNPLUGGED.

This is an example of radiated RF in the real world and the switching frequncy 67Khz you really could not make this up, you do need ot be on your toes when taking low level measurements.

A poxy SMPS caused almost a days worth of testing time loss, project completion time -1 day experience gained invaluable

[Barry]

Time to build and live inside a Farady cage.

Not if you want to use a mobile phone!

[Macca]

Not if you want to use a mobile phone!

You could leave your phone on a table next to the cage. If it rings, stick your head out to answer it.

[Barry]

I didn't think of that - I assumed the whole living space would be a Faraday cage!

[Macca]

I was assuming just a literal cage. Like you might keep a dog in. Only bigger. Big enough to stick a chair and a beer fridge in. Even if it was the whole building you'd have to leave it to go to the shops and that I suppose.

[walpurgis]

Is a Faraday Cage earthed or not? Never considered it before.

And, assuming it has polarity, what result would you get from another Faraday Cage inside it of opposite polarity?

[Barry]

Is a Faraday Cage earthed or not? Never considered it before.

And, assuming it has polarity, what result would you get from another Faraday Cage inside it of opposite polarity?

Doesn't have be, but it is probably more effective if it is earthed.

Why should a Faraday cage have a ploarity?

[Puffin]

https://www.youtube.com/watch?v=ve6XGKZxYxA

[walpurgis]

Why should a Faraday cage have a ploarity?

You tell me.

[Light Dependant Resistor]

Doesn't have be, but it is probably more effective if it is earthed.

Why should a Faraday cage have a ploarity?

Good question Barry
Depending on how a Faraday cage is viewed, ie is the charge to be contained inside the cage disallowing escape , or is the charge outside the cage protecting the interior. The classic view is the latter
but the former is equally poignant.

It would appear IMO there is benefit in a ungrounded cage with no stated polarity, as any charge trying to penetrate or escape as the case may be, will tend to seek a opposite potential to dissipate,
The fixture may then have much added complication, such as concern of adequate grounding related to the charge voltage and current. Also direction taken by any conductor prior to
actual grounding may lead to the faraday cage changing its ideal shape to include the grounding cable prior to actual grounding., thereby lessening its effect.

A everyday example is a coaxial cable used as speaker lead, if the inner conductor is used for connection, then the outer braid when floating is a faraday cage of sorts over
its length