Maurice Janssen wrote:

On Fri, 20 Oct 2006 08:04:56 -0400, John Gilmer wrote:

"RBM" rbm2(remove wrote in message
...

Things like computers "need" to be grounded

Not at all.

They work just fine, thank you, with a "floating" ground. Often, however,
where these is a ground provided there is a network which provides a small
leakage path between BOTH power conductors and the chassis (ground). The
effect of this network would be to cause the chassis get a voltage on the
order of 55 volts. There is no shock hazard and the leakage of even
several of these systems is not likely to trip a GFCI.

There is a lot of "tradition" here.

When IBM started making PCs they had a ground. When folks starting making
audio/visual stuff include stereos and TVs, there usually wasn't a ground.
But both classes of equipment have user exposure to the "chassis." In the
audio/visual stuff its from the "RCA" female connectors. In the PC world
it's the connector shell including the mouse, keyboard, USB stuff, printer
and serial interface.


There's a big difference between A/V equipment and computers. Well, at
least in Europe, I don't know much about US regulations.
Most A/V stuff has reinforced or double insulation. That's why they
have a plug without ground. Computers (except some laptops) usually
have only 'normal' insulation. A single fault in the insulation can
create a connection between the live pole and the cabinet. Without a
grounded cabinet, that could be lethal. With grounded cabinet, you just
blow a fuse.

In case of fire or other damage, the insurance company can give you a
hard time, if they find out that you have class I equipment connected to
a socket without ground.

BTW: the 55 V you mentioned (115 V over here) is more than enough to
blow a serial or parallel port when connecting a grounded printer to a
non-grounded PC. For me, that's also a good reason to ground my PC.


I found it interesting to discover that some two prong "wall wart"
transformers have a high value resistor (in the order of 500K ohms)
connected between the wide plug blade and the secondary of the
transformer to drain off static charges from the chassis of whatever it
is they are powering.

That large a resistance could only deliver about 250 microamps of "shock
current" if the receptical it was plugged into was crosswired hot to
neutral.

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
"Rudeness is the weak man's imitation of strength."