Newsgroups: sci.electronics.misc
Subject: Re: No-Ground detection circuit
References: <[email protected]>
"TBass"  writes:

> Hi,
> 
> One problem we have with our machines is that customers will plug them
> into circuits that don't have the ground connected to earth ground, or
> just be lazy/stupid and not connect it to earth ground. Either way,
> this causes us problems.
> 
> 90% of our support is phone support, so I can't look at the wiring that
> the customer has. What I want to do is put a light on the machine that
> indicates a proper ground. 

Why not putting similar construction as used on those mains outlet 
testers to your machine. Anyway you need to make sure that the test 
circuit does not cause dangerous leakage to the equipment 
case if it is not grounded...
Something like those commercial devices:
http://www.tripplite.com/shared/img/products/large/ct120.jpg

> I know that a GCFI works by comparing the
> voltage on N to the voltage on ground, and trips if they approach each
> other. In this case, however the ground would not be connected at all.

Your knowledge is not correct! 

GFCIs are implemented usually in such way that phase (live) andC
neutral wirespass through a sensor coil. If currents are equal there
is no net magnetic field in the coil. There should be 0A differential
between hot and neutral if there is no leakage to ground. If live ant
neutral currents are uneqal because some current is leaking to earth,
a voltageis induced in the coil and that activates a circuit
breaker. Simplest RCDs have just a toroidal transformer, the L and N
being monitored being fed throughthe middle, with the secondary
feeding a trip solenoid that trip the switching element. Some more
complicated ones have electronics in them to process the input signal.
 
> I have seen music products that have an LED for when the ground is
> properly connected. How does one detect when the ground is connected to
> earth ground vs. to nothing? 

One example circuit seen on web for grounding testing is at 
http://www.giangrandi.ch/electronics/neon/simple.html 
here the "Earth checker". 
This circuit has one problem if attached to the equipment, 
this can cause over 1 mA leakage current to equipment case 
if the equipment is connected to non-grounded outlet. 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/