Re: Bad design?

On Wed, 19 Dec 2007 18:56:32 -0800, "Paul Hovnanian P.E."
<paul@xxxxxxxxxxxxx> wrote:

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?

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 http://www.devi.com.au/DSOT&DSIG.pdf which
shows photos of a typical installation.
Yours should be very similar.

Main in-floor heating page http://www.devi.com.au/slab.htm
.