Re: Some grounding questions
- From: "Phil Allison" <philallison@xxxxxxxxxx>
- Date: Tue, 4 Sep 2007 13:56:05 +1000
"Christian Walter"
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.
** 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.
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 <7.5kV.
** Destruction.
Sensitive inputs must have ESD protection - ie zeners.
2) Earth connection (Class 1 equipment): A safety connection to earth
exists. Any conductive component in the device has to be connected to the
earth to protect users from any internal faults.
** 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.
Safety: No safety problems because if a current flows over the earth
connection the ac main is cut off.
** 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
.
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