If I have EE or a rake of equipment or several pieces of equipment plugged into a power strip that has a combined touch current that trips a GFCI, what can be done about that? Will an isolation transformer solve the problem?
Thanks, The Other Brian On Sat, Aug 27, 2022 at 6:38 PM Richard Nute <ri...@ieee.org> wrote: > > > I wouldn’t describe the phenomenon as “cancellation.” The touch current > is always present and must have a path to earth/ground. > > > > The equivalent equipment circuit: > > Y1 capacitor L-(line)-to-PE. > > Y2 capacitor N-(neutral)-to-PE. Capacitor value is 25x Y1 capacitor value. > > 4.6 volts N-to-(grounded)-PE. > > > > In the USA, N is connected to a ground rod at the building service > entrance. PE is connected to N at the breaker box. In the building, PE is > parallel to N, but is a non-current-carrying conductor except in the case > of a fault. > > > > The Y1 and Y2 capacitors are in series and comprise a voltage divider to > an open PE. Because the Y2 capacitor is 25x the Y1 capacitor, the > open-circuit voltage at the PE connection is very low compared the line > voltage (instead of the usual half the line voltage). > > > > Normal condition touch current path is from L to Y1 to PE (open) to a > 2,000-ohm resistor to ground, Touch current is calculated using Ohm’s Law > from the measured voltage across the 2,000-ohm resistor. The 2,000-ohm > resistor is (in essence) parallel to the Y2 capacitor. Some of the L-to-Y1 > current (not touch current!) returns to ground through the Y2-N-ground > circuit, depending on the parallel network of capacitance reactance and the > 2,000-ohm resistor. > > > > Reverse polarity (L and N reversed in the supply to the equipment) current > path is N to Y2 to PE (open) to the 2,000-ohm resistor to ground. Because > Y2 is 25x Y1, the touch current is much higher than normal polarity. As in > the normal polarity condition, some of the current (very small) returns to > ground through the Y1 capacitor. > > > > If the Y1 and Y2 capacitors are of equal value, the supply voltage is 120 > volts, and the touch current limit is 0.5 mA, the Y1 and Y2 capacitance > reactance is 238,000 ohms each. The Y2 capacitance is shunted by the > 2,000-ohm resistor and can be ignored as the voltage across the Y2 and > 2,000-ohm resistor is 1 volt. (The current through the 238,000-ohm > reactance is 4.2 microamps.) > > > > We have a parallel circuit to ground from the junction of Y1 and Y2 when > the PE is open and when touch current is being measured. One circuit to > ground is through the touch current measuring circuit. The other circuit > to ground (via the N) is through the Y2 capacitor. If the Y2 reactance is > small, a significant N current can be in that path to ground thereby > reducing the touch current, not a partial cancel of the touch current. > > > > Best regards, > > Rich > > > > > > *From:* John Woodgate <j...@woodjohn.uk> > *Sent:* Thursday, August 25, 2022 12:32 PM > *To:* ri...@ieee.org; EMC-PSTC@LISTSERV.IEEE.ORG > *Subject:* Re: [PSES] High Touch Current and GFCIs > > > > There is also a question in my mind as to whether there can be partial > cancellation of touch current. I suspect this is highly improbable in the > US, due to the distribution system ensuring that the neutral has a very low > voltage difference from the PEC. But in Europe, it's not inconceivable that > the neutral could be, say, 4.6 V relative to PEC and the neutral-to-PEC > capacitance 25 times that of L to PEC, so that half the L-to-PEC leakage > current is cancelled by the N to PEC current. > > > ====================================================================================== > Best wishes John Woodgate OOO-Own Opinions Only > www.woodjohn.uk > Rayleigh, Essex UK > It all depends > > > On 2022-08-25 19:12, Richard Nute wrote: > > > > > > I wish to make two points: > > > > 1. Kirchoff’s Current Law states that the sum of currents entering a > node equals the sum of currents leaving the node. The Law applies to > summation of leakage (touch) currents (e.g., through a 2,000-ohm resistor) > and to summation of protective conductor currents (through 0 ohms). In a > power strip protective grounding conductor, I’m assuming 0 ohms to ground, > so the current is slightly higher (1 to 10 % depending on the leakage > current limit and the voltage you are using) in the protective grounding > conductor than leakage (touch) current. > > > > See IEC 60990 for touch (leakage) current and protective conductor current > measurement procedures. > > > > 1. A GFCI measures the current difference between line and neutral > conductors, not current in the protective conductor. It nominally operates > at 5 mA. We assume (with a reasonable degree of accuracy) that leakage > (touch) current is 100% of the differential current measured by the GFCI. > It is possible, although unlikely, for some of the GFCI differential > current to find another return path than the protective grounding > conductor. > > Best regards, > > Rich > > > > > > *From:* Lfresearch Jose <000000734758d943-dmarc-requ...@listserv.ieee.org> > <000000734758d943-dmarc-requ...@listserv.ieee.org> > *Sent:* Wednesday, August 24, 2022 1:44 PM > *To:* EMC-PSTC@LISTSERV.IEEE.ORG > *Subject:* Re: [PSES] High Touch Current and GFCIs > > > > I have wondered about something similar. > > > > If I use a 6 way power strip, I’m assuming all the leakage currents for > anything plugged in sum. Is that correct? I recall getting a few trips when > I used a power strip and It’s only just twigged that might be why. > > > > Cheers, > > > > Derek. > > Sent from my iPad > > > > > On Aug 24, 2022, at 3:27 PM, Brian Kunde <bkundew...@gmail.com> wrote: > > > > If I have a rake of electrical equipment with a single power cord and a > combined touch current exceeding 6mA, and I plug the rake into a circuit > with a GFCI, will it trip? > > > > Thanks. > > > > The Other Brian > > - > ---------------------------------------------------------------- > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. 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