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 <http://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. 2. 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 <mailto: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 <mailto: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 <mailto: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. 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