I have an electric radiant heat system under tile in a basement bathroom. Nuheat thermostat with built-in GFCI, dedicated 20 amp circuit. Ever since I purchased the home the system trips the GFCI whenever it's energized. Turns out one mat has a little problem but it was hard to find. A regular Fluke DMM won't even show any continuity between either the hot to the mat and ground or the neutral to the mat and ground but a megger (at 500 volts) tells me there's about 50 kilohms resistance between either lead and earth. That works out to 2.5 mA which shouldn't actually be quite enough to trip the 5mA GFCI built into the thermostat. When I test current on the hot side versus current on the neutral side of the mat while it's "hardwired" just for troubleshooting purposes I see up to 15mA difference in current going in from the line and coming back out on neutral. I know what the right way to fix it is: rip up the entire floor and replace the mat(s). But I don't want to do that. I want install a floor thermostat without a GFCI built in and protect the circuit with a 20mA GFCI breaker or simply install a 20mA (class C?) GFCI receptacle before the thermostat to protect everything (I have a spot to easily install a receptacle.) Problem is, I can't find a GFCI receptacle that trips at 20mA. Are there any Eaton/Cutler Hammer GFCI breakers that have a 20mA trip rating or an adjustable trip setting? Thanks for reading.
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Can you "divide and conquer" to find the faulty mat segment, or must the mat be replaced as a single unit?– ThreePhaseEelAug 21, 2017 at 3:15
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Standard ratings for home use (at least on this side of Atlantic) are 30mA, 300mA, 1A. Find on EU makers sites such Siemens, BTicino, Legrand, at worst you'll have to install a little box to have a din-rail model installed on.– DDSAug 27, 2018 at 7:23
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I was under the impression that 5ma was the standard for residential GFCIs because that was "safe", and that safety was the reason for the GFIC in the first place. The fact that others exists suggests that I'm mistaken, but if safety is the issue do you really want to use a higher rated one in a non-industrial context anyway?– MickeyfAgain_BeforeExitOfSOAug 27, 2018 at 13:29
3 Answers
The reason you can't find it as a receptacle is that there is no legal use for them in North America. Class C GFCIs can only be used where there is a reliable grounding connection, but that precludes their use where you want to plug something into a receptacle, because there is no way to ensure you are not plugging in a 2 wire device.
What you might be able to do is to put a Class C GFCI breaker on that circuit and hard wire the heating mat. But that is assuming 1) there is nothing else on that same circuit and 2) the heating mat is 3 wire, not 2 wire. If it's only 2 wire you cannot use a Class C GFCI.
I do not know of adjustable GFCI'S but Equipment gfci's that trip at 30 ma are available from Siemens. I have used these on chemical actuators where I needed GFCI protection but had problems with the inductive kickback on deceleration causing standard 5-6 ma GFCI'S to trip but worked fine at 30 ma. There are other values but I remember them being closer to 100ma. Search for equipment gfci's and I believe you will find what you are looking for.
It seems that short of replacing the faulty heating pad, you're trading safety for operation (a sliding scale as the trip leakage current is increased from about 6 mA to a higher level or not monitoring it at all). A wet floor (wet feet) and compromised insulation may prove a recipe for trouble. Since GFCI's should trip at 4 ~ 6 mA, I'd consider the safest short-term solution would be to find (trial-and-error, factory picking) one that trips very close to the 6 mA upper limit. Otherwise, repair the floor.
