[GreenKeys] Re: Dovetrons and GFI

[gil smith]

Hi Roy:

At 10:13 AM 2/14/2005, you wrote:
>>....all it would take to trip a
>>GFI would be a small imbalance in the line currents.  This imbalance
>>could result from slightly-different values of these line-to-ground
>>caps in the filter.
>
>No, it results from the fact that there are (a number of) capacitors from 
>both line ("hot") to chassis and from neutral to chassis. This forms a 
>voltage divider if the chassis is not grounded.  This happens even if the 
>capacitance is exactly equal on both sides.


Hmm.  I've had a moment to think this GFI thing through a bit more.  My 
statement about unequal capacitances was wrong.  There are several scenarios:

1)  Dovetron chassis-ground IS NOT connected to earth-ground (aka 
line-ground, or safety-ground).  The emi-filter's capacitances from 
chassis-ground to each side of the line forms a voltage divider, but only 
in this case when chassis is not grounded via the line cord.  This is 
indeed why Doug saw about 60V between earth-ground and his ungrounded 
dovetron chassis -- an his audio input was referenced to this 60Vrms.  Keep 
in mind that the ac neutral line is strapped to currently-unconnected 
earth-ground back at the breaker box -- this 60V is appearing across the 
capacitance from chassis-to-neutral, which, in this case, is the same as 
chassis-to-earth-ground.

2)  Dovetron chassis-ground IS connected to earth-ground.  The earth-ground 
connection shorts out the chassis-to-neutral capacitance (since the neutral 
line is strapped to earth ground back at the breaker box, and ignoring the 
small ac voltage due to an IR-drop in the neutral).  So now, the full line 
voltage appears across the capacitance from chassis-to-hot, resulting in a 
small ac current flowing from hot to ground.  Two possibilities:

      2a)  If the Dovetron is is plugged into a regular outlet, there is no 
problem.

      2b)  If the Dovetron is plugged into a GFI outlet, it will trip due 
to the hot line
             having more current than the neutral.  GFIs are simply looking 
for
             equal current in both hot and neutral lines.  I don't believe 
that they
             sense current in the ground line, though if they do, it is a 
secondary
             protection -- indeed, many GFI-protected items (eg: hair 
dryers) do not have
             a ground line at all.

3)  A line-isolation transformer is used.  This breaks the copper connection
between the ac neutral line and earth-ground.    Two more possibilities:

      3a)  The Dovetron chassis-ground IS NOT connected to earth-ground.
             The chassis is now floating, so there should be no appreciable 
voltage
             between earth-ground and the ungrounded dovetron chassis.
             Connecting another device to the Dovetron (eg: a PC) will connect
             the Dovetron chassis to the chassis of the other device, which may
             be obviously floating (eg: wireless laptop on batteries), or 
may also
             be connected to earth-ground (eg: scope, or desktop PC powered
             through a normal ac outlet).  Probably ok this way, but an 
ungrounded
             chassis is still a safety risk for an internal wiring fault.
             The isolation transformer may or may not trip a GFI -- I could 
see how
             unequal winding capacitances to ground could result in unequal 
currents
             in hot and neutral, and possibly trip a GFI.

      3a)  The Dovetron chassis-ground IS connected to 
earth-ground.  Neutral is
             still isolated from earth ground, but the chassis now has a 
earth-ground
           for safety.  This would be a safe connection.  Once again, I'd 
say that
             the isolation transformer may or may not trip a GFI.




Vaux Electronics, Inc.
480-354-5556
(fax: 480-354-5558)
www.vauxelectronics.com