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TLDR

I originally asked "Minimum current level to trip a GFCI"? But the answers pointed out that my biggest problem in this question is that I shouldn't use a general-purpose DMM to find an electrically insulated component whose insulation is going bad. When insulation breaks down, there may be some problems that only show up in the presence of line voltage.

Insulated components should be tested with a meter designed to test insulation at line voltage (or higher). This kind of meter is commonly called a “Megger”.

Background

I have an espresso machine which just started tripping my kitchen GFCI as soon as I turn it on.

According to google, the most common thing that fails on my unit is the heating element; so, I checked the heating element with my Fluke DMM's continuity beeper. There was no beep from the tester, and DMM measurement display said "OL" (too large to measure). I went on checking a bunch of other things, but found no problem.

On a whim, I used the DMM ohm setting on the meter and checked the heating element resistance to ground again. The resistance from the heating element to the chassis ground is roughly 3.6 megohms (see picture below). As far as I'm concerned, any measurable resistance to ground is bad and should be fixed. I pulled the leads to the heater element and powered the espresso machine on again. The GFCI doesn't trip if I disconnect the heating element and turn the power on; and the GFCI trips when it's connected.

Answers to questions in comments

If you are tripping with normal equipment that has been UL tested (or some other ETL) then I would try a different GFCI as they can have issues with tripping to early (I had to replace one the other day). – Voltage Spike♦︎︎︎︎︎ 13 mins ago

Fair point, I tried another GFCI and it tripped with the heating element connected... this seems like a legit ground fault.

Is the GFCI separate from the overcurrent circuit breaker?

Yes, it's a typical residential setup... 20A breaker in the central box for the kitchen circuit and a couple of inline GFCIs in the kitchen

Did you measure heating element resistance between Live and Neutral too?

Yes... but as others have pointed out, I shouldn't be trusting a DC resistance measurement for (complex plane) 120V RMS @ 60Hz impedance.

Was the heating element disconnected from all other circuitry when you measured resistance to earth?

Yes and I also made sure the boiler was drained so I wouldn't be checking resistance across any "water path" between the frame of the boiler and the heating element.

Sometimes other components such as filter capacitors can be charging or leaking from the multimeter and that is why multimeter shows some high megaohms resistance.

It's hard to see in the picture, but I disconnected all boiler leads before measuring with the meter.

Measurements

Let's do some math. How much current is leaking?

120.0 VAC / 3.6 MOhms (measured with my Fluke DMM) = 60 microamps (RMS) leaking to ground from my espresso machine.

CAVEAT: As pointed out in the answers, I probably should not trust that 3.6MOhm Fluke DMM resistance measurement to be the same impedance for 120V @ 60Hz. The DMM uses low voltage DC to measure that 3.6MOhm result.

Lesson learned: My DMM's continuity checker can miss some problems.

I almost missed this problem because the path to ground didn't trigger my DMM's continuity beeper, and the DMM display showed "OL" when I was using the continuity beeper.

GFCI Question

This UL link claims that GFCI current protection threshold seems to be in the range of 4ma leaking to ground. So we also have an interesting situation... 60 microamps leakage is much lower than the expected GFCI trip level. I consider it possible that GFCIs are manufactured to trip at much lower levels than UL-guidance of 4ma... for instance perhaps the reality of injury or death lawsuits (from "allowing a small current leak to ground") may drive manufacturers to trip at much lower levels.

Overall, I'm glad my GFCI tripped, but I still don't quite understand everything. This is a Cooper SGF20W GFCI; I haven't found published test data on where it trips. What is common in the industry for the current threshold that GFCIs are actually designed to trip at?

test

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  • \$\begingroup\$ The continuity test on you meter will show continuity for a low resistance - one of my meters will beep for continuity with a resistance of 40 Ohms or less - Megohms definitely won't trigger it. (Most people wonder why a continuity beep doesn't mean Zero Ohms...) \$\endgroup\$ – Peter Bennett Dec 11 '20 at 20:23
  • \$\begingroup\$ If you are tripping with normal equipment that has been UL tested (or some other ETL) then I would try a different GFCI as they can have issues with tripping to early (I had to replace one the other day). \$\endgroup\$ – Voltage Spike Dec 11 '20 at 20:53
  • \$\begingroup\$ Is the GFCI separate from the overcurrent circuit breaker? Did you measure heating element resistance between Live and Neutral too? Was the heating element disconnected from all other circuitry when you measured resistance to earth? Sometimes other components such as filter capacitors can be charging or leaking from the multimeter and that is why multimeter shows some high megaohms resistance. \$\endgroup\$ – Justme Dec 11 '20 at 20:59
  • \$\begingroup\$ Aside: if your DMM is indicated a low battery the readings may not be accurate, though I would hope fluke is better than the cheapies \$\endgroup\$ – sstobbe Dec 12 '20 at 1:13
  • \$\begingroup\$ Good eye @sstobbe... the Fluke’s DC resistance from heater terminal to ground is roughly the same with a fresh battery... but as others have said, a general purpose DMM is the wrong tool for this job. \$\endgroup\$ – Mike Pennington Dec 12 '20 at 1:34
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You are making your insulation resistance tests using low voltage DC. It should be checked using a higher voltage which can cause the weak point in the insulation to break down.

There are special testers for doing this (e.g. Meggar)

The low voltage DC impedance could be very different than impedance at 120 Vac 60Hz

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    \$\begingroup\$ So you’re saying that low voltage DC impedance could be very different than impedance at 120 Vac 60Hz correct? Thank you for pointing that out. \$\endgroup\$ – Mike Pennington Dec 11 '20 at 20:24
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    \$\begingroup\$ @MikePennington Similar to how air is normally non-conductive but after a point easily conducts an arc. \$\endgroup\$ – DKNguyen Dec 11 '20 at 20:31
  • \$\begingroup\$ Yes the LV DC impedance can be different answer edited to address. \$\endgroup\$ – RoyC Dec 11 '20 at 20:43
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    \$\begingroup\$ Yes those units are expensive and intended for pro use. Here in the UK we have system of domestic equipment checking known as Portable Appliance Testing (PAT). The testers used for this vary in price from a hundred pounds or so to many thousands, the cheaper units will only give you a go-no go measurement for insulation resistance. The most cost effective method is what you are doing at the moment which is to plug it into a GFCI. If you wish you could validate the GFCI trip level by testing it with various resistors connected live-ground (carefully, e.g. inside an assemble-able plug). \$\endgroup\$ – RoyC Dec 11 '20 at 22:56
  • \$\begingroup\$ I'm not sure whether this point is stressed enough: it's not only the voltage but also the frequency. If you have a high enough capacitive coupling, there will be a leak an AC leak that is invisible at DC, regardless voltage. Hence it's important to look at the cause of the leak as an impedance (here capacitive) path, not a resistive path. \$\endgroup\$ – P2000 Dec 13 '20 at 21:09
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You are assuming that the leakage is through a resistive fault. It's more likely that at some high enough voltage the voltage is causing a breakdown across an airgap in the mineral insulation. The relationship between voltage and current will be highly non-linear and non-conducting until breakdown occurs. Due to the non-linearity Ohm's law no longer generally applies.

The breakdown voltage of air will be in the order of 3 kV/mm.

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    \$\begingroup\$ It can as well be moisture. Water gets orders of magnitude more conductive when heated. It is also profoundly non-linear under ~2V (electrolysis starts at ~1.2V) \$\endgroup\$ – fraxinus Dec 11 '20 at 22:24
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From this UL document:

A Class A GFCI trips when the current to ground has a value in the range of 4mA to 6 mA (reference UL 943).

In other words, it should not trip at less than 4mA. The trip time must be <= 25ms. They will not trip at 60uA or even 1mA because that would cause nuisance tripping from capacitive coupling or internal Y capacitors on some kinds of supplies.

You could further test the element with a Megger type unit, as would be done at the factory, however it sounds like you've got it narrowed down to the element.

If the shell of the heater has been breached by corrosion and water has gotten in, then it may behave quite differently at high voltage vs. low, with ionic conductivity dominating and the water getting quite hot in a small area very quickly (which increases conductivity exponentially by several percent per degree C).

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  • \$\begingroup\$ Is shell of heating element grounded? \$\endgroup\$ – user263983 Dec 11 '20 at 22:32
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    \$\begingroup\$ @user263983 Typically it would be, yes. \$\endgroup\$ – Spehro Pefhany Dec 11 '20 at 22:35
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    \$\begingroup\$ Since a GFCI is not supposed to trip below 4mA, I've sometimes wondered whether there should be a UL-recognized category of "smart switches" that would make use of a ground connection in place of a neutral, but would be designed never to pass more than 100uA or so through it, for installation in places where a ground is available but neutral is not. Some design care would be needed to make a switch's internal circuitry operate at such low current, but at 120V such a device would have 12mW available, which would be plenty to operate a small microcontroller. \$\endgroup\$ – supercat Dec 12 '20 at 17:53
  • \$\begingroup\$ @supercat The ground current through Y caps is already present in a lot of SMPS designs, so I don't know that it would be a problem requiring a special category. I'm pretty sure the flame detector in my furnace uses a ground current for functional purposes, for example. \$\endgroup\$ – Spehro Pefhany Dec 12 '20 at 18:34
  • \$\begingroup\$ @supercat That is exactly what many smart switches/timers/motion sensors/dimmers/etc. do. There are very specific rules for it, but basically devices used to just leak through the light bulb when "off" but many LEDs will blink or glow from so little current, that the old trick doesn't work any more. At the same time, older houses don't necessarily have neutral in the switch box (which is the best solution) - so under limited circumstances devices can be designed to use ground instead of neutral for very low current. \$\endgroup\$ – manassehkatz-Moving 2 Codidact Dec 13 '20 at 3:01
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As you note any measurable resistance to ground is bad and should be fixed (with fixed meaning heating element or whole device replaced). But the amount of leakage current/resistance measured will be wrong, as 9V battery in your DMM is obviously not as powerful as 120V mains.

Which is also why people warn you that DMM is not right tool for the job - even if DMM said there was infinite resistance to ground, it might be wrong, simple because it is not powerful enough to measure real-world scenario.

TL;DR - you can use DMM to prove the device is broken, but you can't use it to prove it is not broken

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Was that measurement when the element was cold?

One aspect of a heating element is that it's supposed to get hot. And heated metal expands, allowing things to touch that might remain unconnected when cold.

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    \$\begingroup\$ Yes this was a cold element but the gfci never tripped on the cold element before today \$\endgroup\$ – Mike Pennington Dec 11 '20 at 20:28

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