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Bad design?

I recently extracted a floor heating thermostat that had probably been mis-installed and (as far as we can tell) never worked. The story about the electrician who did the job and left before testing the system is another sad and long one for some other thread.

The unit is (as far as I can tell) a triac that controls 120 VAC to an electric mat installed under a bathroom floor. It incorporates a ground fault sensing circuit so as not to electrocute one stepping out of the shower. Because of this, the unit has two line input wires (black and white) and two output wires to the mat (also black and white). See crummy ASCII art below. We suspect that the electrician initially mixed up the line and load connections (due to the way he left quickly without testing the system). I'm trying to figure out how one could damage a triac by swapping the Line/Load wiring. Due to the crappy color coding, this seems to be the most probable wiring error.

System (properly wired) as I envision the thermostat innards:

triac black |\\| black ----o-------+-----|/|---MMM-o---------+ | /|/| | | | / |GF CT | | | | | Line +------+ | Load |brains|------+ | +------+ | | | | | ----o----------+--------WWW-o---------+ white white

Most of the likely errors I can envision would simply prevent the unit from operating. Am I missing something?

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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c (velocity of light in a vacuum) = 1.8x10^12 furlongs per fortnight
Reply to
Paul Hovnanian P.E.
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If the triac gets zapped, it might turn into a very good 2 or 3 terminal wire.. The black box labelled "brains" should take that into account; adding a fuse or circuit breaker before the triac could them allow a fault open for safety.

Reply to
Robert Baer

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Your diagram appears to show the GF circuit breaker AFTER the thermostat/controller.

The Earth Leakage Circuit Breaker (ELCB or GFC interrupter) should be in the distribution box and the thermostat/controller should be wired between the ELCB and the in-floor heating element.

This is a typical installation document for in-floor heating as used on 240Vac in Australia

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which shows photos of a typical installation. Yours should be very similar.

Main in-floor heating page

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Reply to
Ross Herbert

Yes. This unit incorporates its own internal ground fault sensing for a fault in the mat. Based upon the observation that the unit has a GF fault LED that illuminates following a fault or test (test button not shown here), I figured it just shuts off the triac and the brains remain powered.

There's a mechanical on/off switch (also not shown) that is (probably) upstream of the triac in the hot leg to comply with UL requirements for a disconnect. This switch is not automatically opened following a GF, like a GFI receptacle reset is. It must be cycled manually.

My main concern is, given the color coding scheme shown above, could an electrician swap the two white wires or two black wires and cause a triac to fail? I don't know what is inside the 'brains'. I didn't open this thing up due to the fact that 1) it is expensive 2) in isn't mine and 3) we traded it in for a new one. All I know is that the electrician left the job site quickly, without informing anyone that the unit didn't work. Its possible that something else failed internally (maybe a fusible link).

The one error that I can envision is to connect source hot (black) to t'stst line black and source neutral (white) to t'stat load black. But I have yet to meet an electrician dim enough to connect a black to a white wire.

Since he didn't buy it (my friend did), he could have very easily said, 'Hey buddy, this mat you bought is junk.' We only discovered the problem later and, prior to isolating the problem to the t'stat, the fault could very well have been in the parts now embedded under an expensive tile floor. Fortunately, that was not the case and his failure to alert the customer is a subject for another thread on professional ethics.

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Paul Hovnanian	paul@hovnanian.com
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Procrastinators: The leaders for tomorrow.
Reply to
Paul Hovnanian P.E.

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