Why Does RCD Trip with Earth Leakage Fault Downstream of UPS?

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Discussion Overview

The discussion revolves around the behavior of a Residual Current Device (RCD) or Ground Fault Circuit Interrupter (GFCI) when an Earth leakage fault is induced downstream of a Uninterruptible Power Supply (UPS). Participants explore the implications of the UPS's operational mode (normal vs. bypass) and its design (dual conversion) on the tripping of the RCD.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes a setup where an Earth leakage fault downstream of the UPS causes the RCD to trip, questioning the true dual conversion nature of the UPS.
  • Another participant suggests that the GFCI may trip due to common-mode load changes caused by winding-to-winding capacitance in the UPS, and proposes testing with different fault conditions.
  • A participant notes that in bypass mode, the RCD trips as expected, but in normal mode, the trip is unexpected, indicating a need for further investigation.
  • One participant speculates that the UPS may loop the ground and neutral circuits, which could lead to an unbalanced current in the RCD, causing it to trip.
  • A participant acknowledges that their specific setup does not bond the Earth and neutral, but recognizes that in another related scenario, they are bonded upstream of the UPS, prompting further consideration.

Areas of Agreement / Disagreement

Participants express various hypotheses about the behavior of the RCD and UPS, with no consensus reached on the underlying reasons for the tripping behavior or the implications of the UPS design.

Contextual Notes

Some assumptions about the UPS's internal wiring and the nature of the Earth leakage fault remain unresolved. The discussion includes varying conditions under which the RCD trips, indicating potential dependencies on specific configurations.

qbnchopper
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Gents, Ladies,

I have a design where I have:

AC -> RCD -> UPS (dual conversion apparently) -> PowerStrip -> Equipment

If I induce an Earth leakage fault downstream of the UPS (i.e. at the powerstrip), the RCD upstream of the UPS trips.

The UPS is in the "normal" not bypass state, and the UPS is supposedly dual conversion.

I cannot understand how this can happen unless the UPS is not true dual conversion !

Can someone confirm or debunk my understanding here please ?

Q
 
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Another name for the RCD is GFCI, BTW. Does the GFCI trip if the UPS is in bypass mode too? I agree that if it really has switching power supply transformer isolation between the AC Mains with the GFCI, and the load where you introduce the ground fault, that the GFCI should not see anything different, other than a slight change in its AC load (no common-mode load change).

On second thought, it might see a common-mode load change because of the winding-to-winding capacitance of the switching transformer. Does it only trip the GFCI if you introduce a hard fault to ground (like with a wire)? What if you use a 10kOhm resistor instead? Try some resistor faults right at the GFCI to see where its sensitivity threshold is, and then do the same downstream of the UPS. It may just be the capacitive common-mode transient from a hard wire fault downstream of the UPS that is tripping the GFCI.
 
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Berkeman,

Thanks for your reply ! I new that the RCD was called something else - but couldn't remember ...

Being an Aussie - RCD = Aussie; GFCI = American ? :-)

In bypass mode, the RCD/GFCI trips as expected.

In normal mode, the RCD/GFCI trips (not really as expected - but noting your second though!)

I am using a tool to induce the Earth leakage fault - it allows me to change the mA-erage to see how quickly an RCD responds. I came across this situation when I was testing to see if the UPS had anything in it to cope with a Earth leakage fault (i.e an experiment) ...
Anyway, I'll look at the tool and also try as you suggest ...

Thanks !
 
Glad to help, qbnchopper, and welcome to the PF. It's a great place.

I'd really be interested in the results of your experiments. Your initial test results are a bit non-intuitive for me as well.
 
The UPS loops the ground circuit thru to the UPS outlet.
I'm not entirely certain, but I think they also loop the neutral line thru.
If so, then at some point in a normal mains circuit the neutral and ground lines have a common bond.
Then a leakage current from the UPS hot to ground will loop back thru the neutral to complete the circuit. The unbalanced current in the RCD (GFCI) neutral will cause the breaker to trip.
 
Notime,

Thanks and Interesting ... in my case the Earth and neutral are not bonded ... however, on another problem that I am working that is related, earth/neutral are bonded upstream of the ups ... have to have a think about this ...

Q
 

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