Some grounding questions

Hello,

Today me and a friend took a look at an existing design and finally we got lost in a discussion about grounding issues and how to connect earth ground, signal ground, chassis ground and shields. We talked about the necessity of correct earthing, ESD issues with the different methods and the connection of cable shields for external I/Os. Maybe someone who is a bit more experienced than me can comment on this because I don't feel very safe with all my argumentations and thoughts.

First of all any equipment powered from AC mains will either be double insulated (showing two concentric squares) or will require an earth connector cause of safety reasons. I would like to start with double insulated equipment:

1) Double insulated (Class 2 equipment): No safety connection to electrical earth exists. Because the potential of my internal signal ground is not fixed it can take on an arbitrary value.

Safety: There are no safety concerns because no current will/can flow to any user touching the equipment in case of an electrical fault.

ESD: This is where I am a bit unsure. What happens for example if I would touch a possibly sensitive pin with an external testing equipment charged up to

Reply to
Christian Walter
Loading thread data ...

Hi:

A ground is merely an arbitrary reference point. There is no such thing as an absolute ground.

Reply to
Charles

"Christian Walter"

** Only true if the item is completely inside a plastic case AND has NO connectors.

Typical class 2 items have metal cases and connectors - so insulation or component failure can render them lethal.

** Ridiculous.

Class 1 items require all external metal parts be connected to AC supply ground via the plug.

Nothing inside the item must connect the AC supply lines to the case or AC ground pin excepting only a class Y, RF suppression cap.

** Nonsense.

Even a direct link from AC supply *neutral* to an earthed chassis will go un-noticed for years.

In this condition, the item is extraordinarily dangerous since a simple loose or missing earth connection will render the case live.

YOU need to do a LOT more thinking.

....... Phil

Reply to
Phil Allison

snip

If an object holding a net electrical charge is brought into contact with (or even close proximity to) an uncharged object, charges will flow and redistribute themselves and breakdown may occur. It matters not how many layers of insulation or isolation exist between the earth and the object. This is the ESD case. Normally, we ensure that our test leads are ESD safe when working on sensitive components.

If one lead of a power supply or charged capacitor is brought into contact with a component, any current flow will be limited by the capacitance of the other lead to other components. This is not ESD but simple current flow and possibly voltage breakdown.

Does that help?

Chuck

Reply to
Chuck

Even without an actual failure this can cause problems. With switch-mode supplies (like most modern AC-DC adapters) you get a leakage current that is is definitely unpleasant to touch, and can be quite disconcerting.

--

John Devereux
Reply to
John Devereux

"John Devereux"

** It can be more than "disconcerting" to the input device of some piece of audio or video equipment.

It can spell instant death - see:

formatting link

&

formatting link

...... Phil

Reply to
Phil Allison

True. Of course the leakage goes straight through any connected equipment. So its own outputs rise to high voltages - potentially destroying anything connected to them if a path is formed to earth.

--

John Devereux
Reply to
John Devereux

"John Devereux" "Phil Allison"

** Yes - the " coup de grace " can well be delivered to an item well down the chain from the one with the Y cap.

Damage is more likely to happen to an item which is unpowered at the time, too.

....... Phil

Reply to
Phil Allison

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.