[sdiy] Grounding & computers

Magnus Danielson cfmd at swipnet.se
Thu Dec 20 22:02:05 CET 2001

From: Jim Patchell <patchell at silcom.com>
Subject: Re: [sdiy] Grounding & computers
Date: Thu, 20 Dec 2001 07:37:52 -0800

Hi Folks,

>     Yikes!
>     One posiblility is there is a line filter that the mains goes through.
> Each side has a cap that goes to the chassis....this makes a voltage divider,
> and will give you 110v on the chassis...you really need that earth ground to
> prevent this.
>     Anyway, this is my guess.

I'd go even further... I'd simply state that this is the most common
case of getting a hot chassi. The reason why is however interesting.

The caps are chosen to not produce too much leakage... for 117 V, but
when you put the same filter in a 230 V system (which is what Europe
have these days) you do sense the leakage.

There is a simple enougth way to test this. If you have the box
unpowered and still sense the current, then it is the filter since it
is allways hot, regardless of the power-switch.

Despite this, I allways recommend the usage of these filters. They do
the rigth thing for most situations, so there is not real reason to
avoid it.

Also, one should if possible run with earthed contacts, there are
other benefits with doing that.


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