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Wednesday, October 5, 2016
Question:
HI Charlie,
I know Charlie does not get these questions, but his spirit lives on!
I recently made an installation of a 240volt baseboard heater controlled by a 2 pole Honeywell T410B which is supposed to provide in its words, "double line break". Now in the past that always meant to me that both side of the 240 volt single phase line was interrupted when the contacts of the thermal opened up on temperature rise. NOT in this case!!
The thermostat opens only 1 of the poles in this 2 pole version; it does the same things as the single pole stat. The only thing it does is open both poles when you turn the control knob all the way to the "off" position.
Isn't this a violation of the intent at least of having and using a 2 pole thermostat to control a 240 volt single phase heater? I can understand that the off position is acting as a disconnect for the heater as there should be one as I understand the code, but I also expect a 2 pole thermostat to open up both legs of the applied voltage.
In my humble opinion as an electrical engineer and former holder of a master license that there is virtually NO difference between the single pole and double pole thermostats sold by Honeywell. Except for the disconnect part of the devices.
How did these get an approval from Underwriters Lab?
Kenneth Lynes
A
Answer:
Hey Kenneth thanks for your question and the kind words.
Thermostats do not have to open all ungrounded conductors of a circuit as stated in 424.20(B). They can be used as the disconnecting means if they meet the conditions in 424.20(A) including the marked off position that does open all ungrounded conductors. I can't speak for the manufacturer but the product does "break both lines" when it is in the off position. I suspect they can be listed because they comply with the NEC.