[sdiy] Grounding check (OT)

[Czech Martin]

All metal parts of guitars etc. should be connected to protective ground.
Of course, the protective ground must work (be connected in the outlet, etc.).
This is even more true if e.g. tube circuits are used.

It is possible to have appliances which are isolated in a way
that their outer metal shell does not need to be connected
to protective ground. Two prong connector. This is true for
many synths and effect boxes and "hifi" gear.

The transformer and other primary voltage parts are speciall build
for this purpose in order to give safety.

But in this case the case is floating!
If you touch such a case very slighly, you have the following circuit:
earth ground->mains power->parasitic (transformer) capacitance->
case->body (high resistance)-> earth ground.
You can feel the high voltage on the case. Since the current is limited
by the small parasitice capacitance, the current is only unpleasant,
but not harmfull. If you touch harder the unpleasant feeling
will disappear. I think that the body resistance will drop
in this case, shorting the parasitic path. Or perhaps a larger
skin area is involved, thus the current density goes down.
Or both.

I have two metal IKEA lamps which behave in this way.
Also the electric blanket which my wife uses in the winter.
Unpleasant to touch her in this case (no joke).

The floating chassis makes products cheaper, because "earth loop"
currents are limited by the parasitic capacitances to mains.
OTOH, if one appliance is grounded, signal pathes will spread
this ground to other  appliances. Backdoor grounding.
If cables get longer and potential differences get higher
(perhaps different rooms in building) this cheap approach
of limited earth loops via floating chassis will finally fail.


m.c.



> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of
> dreichert at sympatico.ca
> Sent: Montag, 22. September 2003 21:51
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] Grounding check (OT)
> 
> 
> Hi all,
> 
> This is a bit off topic, but I have a bit of a problem (maybe!)
> 
> I've plugged my bass into a used effects processor I just got 
> and when I play I'm feeling a mild sensation in my hands that 
> makes me suspect that something's not properly grounded. 
> 
> How can I check this for sure? I tried measuring the strings 
> against the recepticle-ground with a voltmeter, which read 
> nothing (both AC and DC). Am I doing this right or are there 
> better checks?
> 
> Cheers,
> Dave
> 
>