GFCI Indicator Keeps Tripping

I have a 3000 watt UPS in my office with two computers attached to it. I went ahead and put two GFCI extension cords onto each unique circuit breaker
on the back of the UPS, and then attached the electrical outlets for each computer to each of those GFCI extensions. My thinking was that if someone accidentally cut a cord and created an electrical shock hazard, that the GFCI logic would immediately shut off just the affected equipment.
One of the two computer/monitors is shutting off about once every two days now. It's clearly the the GFCI logic because when I examine the GFCI indicator, all lights are turned off. I hit the Reset button on the GFCI, and the circuit comes alive again.
What would cause GFCI to trigger? I doubt that the cord or PC/monitor is compromised. Maybe this indicates a potential overload condition on this one circuit breaker on the UPS?
Alternately, maybe my assumptions behind using a GFCI *behind* a UPS is faulty? Maybe a GFCI can only work correctly when attached directly to the wall socket?
Finally, maybe the UPS itself just doesn't have an attachment to true ground, and I need to attach a separate grounding cable? How would I prove that the wall socket's ground is going to true ground?
--
Will
westes AT earthbroadcast.com
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Computers have a line filter that connects the phase and neutral to ground through a L/C network. Usually there is not enough current to trip a GFCI but if you string enough of them in parallel it might.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
     snipped-for-privacy@aol.com (Greg) writes:

Usual guideline in the UK is that each Class I (grounded) IT appliance is allowed to leak up to 0.75mA. When designing a circuit, the design leakage should not exceed 25% of the RCD(GFCI) trip rating. We commonly use 30mA RCDs, which gives a max design leakage of 7.5mA, which is a maximum of 10 Class I IT appliances on an RCD protected circuit. For a PC type computer, each system normally has two Class I IT appliances, the base unit and the monitor, so that's 5 computer systems per 30mA RCD circuit.
Substitute the RCD rating for whatever your RCD is rated at and try repeating the calculation.
BTW, in the UK (and probably all Europe), RCD's are required to trip at the trip rating of the RCD, and are required not to trip at half the trip rating of the RCD (which is quite different from the rating on a fuse or circuit breaker for example). If the GFCI rating is differently interpreted in the US, then you may need to adjust for that too.
--
Andrew Gabriel

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Assuming your GFCI is not defective:
You probably have a path/leakage to ground in one of your monitors. This possibly "barks" through the insulation value of some weakened part or possibly your UPS is putting out high voltage spikes and a protective element or poor insulation in the monitor is taking it to ground.
This implies that your UPS output is connected to ground or there would be no way for any leakage to occur and get back to the UPS, tripping the GFCI logic. It probably has to be by law unless you are in some medical establishment.
A GFCI will work anywhere the "hot" and "neutral" fully return through it. There has to be a bonded (to ground) neutral in front of the GFCI, but no after. If you have drywall, many times the installer,with his zippy router type cutter will nick the wires when cutting out the electrical boxes. The hot wire will get attention (blows fuses) but the neutral will not, until you come along and use a GFCI. Now your return (neutral) current is being shared with the ground conductor and voila! IN + OUT does not equal zero.
message

breaker
that
GFCI,
the
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Too much, too soon.It's an overkill both of them.According to local rules, I have a GFCI breaker (for the whole house) and that's all.No UPS.Your utility has lighting arresters in every substation.There's AVR (automatic voltage regulation) in your local HV/MV substation.The frequency and voltage is precise in the range of mHz and 5% fluctuation, respectively.Just connect your computers directly to the wall receptacle (like me) or ask your master electrician.
-- Dimitris Tzortzakakis,Iraklion Crete,Greece major in electrical engineering freelance electrician dimtzort AT otenet DOT gr


breaker
that
GFCI,
the
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Do you understand what a UPS does?
message

I
each
days
is
this
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.