late one : To earth, or not to earth..

[Magnus Danielson]

>>>>> "L" == LBucha5714  <LBucha5714 at aol.com> writes:

Hi All!

 L> Hi all,
 L> I'm just wonder what opinions are on a safety item that I purposely ignored
 L> for grounding reasons.  Do other people make the same choice I have?  See
 L> below.

 L> In a message dated 98-01-09 12:24:38 EST, tim at redragon.demon.co.uk writes:
 L> << So to summarise the mains earth is there for protection not as a refence
 L>  point (many places the neutral and earth are tied together at the mains
 L>  incomming point.
 >>> 

 L> Ok so I  just finished my power supply which IS mounted inside a metal chassis
 L> but I did NOT connect (to ground or anywhere else) the third wire (ground from
 L> 120VAC wall socket).   I did this because I wanted the chassis to float in
 L> order to help avoid ground loops when interfacing to commercially built
 L> equipment which (in most cases) does have grounded chassis.  And just for the
 L> record, the power supply unit is a Power One brand whose output is isolated
 L> from the AC  line.

First of all, "floating" is really a DC thing in most cases. That is,
you can take it to any DC voltage you would want within some limits,
but you usually have no clue what is happening in the AC range.
It is an common missconception that transformers isolate "totally", it
will typhically isolate the DC pretty well until you get sparks all
over, but in the AC you get a capacitive divider cursuit distributed
all over the primary side over to the secondary side. This can cause
pretty high AC diffs between the sides. I have seen about 50 Vrms
between signal earth and chassi earth in a box sitting there all by
itself on the bench. The impedance of this is pretty high, around the
Megaohm range, but large enougth to cause trouble.

Also, the power cord earth is protective earth which is meant to take
any current which escapes the normal cursuit to the chassi towards the
power cord before you get fried with it. So keep it in!
Now, the chassi earth need not to be strictly connected to power earth
unless you cary high voltages with low impedances to the power cord in
the box. Also, the signal ground may not be connected strongly to the
chassi. You can keep them pretty loosly connected within some limits.

I would have the chassi ground hooked closely or fairly closely to
protective ground. I would probably get that anyway.

I would keep the chassi ground and signal ground semi-loosly
connected, with something like 100 kOhms of resistance inbetween.
This is enougth to keep transformer AC components down while still
maintain a floating characteristic well enougth to avoid severe ground
loops.

 L> Certainly this breaks all UL (an others) rules for ground saftey.  But I'm not
 L> planning on selling this item to anyone and I have tested to ensure that
 L> indeed, the chassis IS floating.  It saves me from using a ground lift plug
 L> thing which achives the same end (and we all know that everybody uses those).

 L> So do you feel this is something that I should just be wary of and proceed?
 L> Or is this one of those "Hell Buck, I wouldn't be on the same stage with you
 L> and Twisted Metal Box of Doom" cases?

We have had cases where sound crue got 230V from arm to arm out of strange
devices, it was (and luckilly still is) a good friend of mine who got
this unplesant experiance at a gig... I don't think that you should
get into that problem, but I strongly recommend to fix the ground
since it is there for a reason. Also, if you every happend to go to a
230V country on a gig, don't use those outlaw El Cheapo 230V->115V
transformers with only one coil, if you connect them incorrectly you
got a very hot chassi so to speak... my friend started to mumble
something about that he NEVER where going to accept any other
transformers than those he supplied, and he got a big one in store...

 L> I'll ground the chassis if everybody yells at me.  But I'm thinking that many
 L> others out there may be doing the same thing.  I mean, it is safe unless the
 L> power supply fails severely and shorts the line to the chassis......

Well, yes... but it is also the wrong place to fix the thing you want
to fix. It is the signal to chassi which should be broken if not
loosly connected, not the chassi to protective.

Also, don't come around saying that your wiring and/or connacts
depends on chassi earth being the same as signal earth...
It is just a matter of doing things the propper way rather than saving
money and gaining a hummy and/or risky life.

 L> Comments invited.

I think I have stated my thing... 

Cheers,
Magnus