Over Temp protection

Recently I had trouble with my spa overheating. Replacing the temp sensors fixed the problem but not before it ruined my spa cover, I suppose it could
have done more damage if I had not caught it in time. I have on hand a temp controled switch that closes at 105F, My intent is to connect it from one of the hot wires to ground through a resistor so it will generate a ground fault should the temp rise to 105F and trip the breaker. Is there anything wrong with doing this? How much current should I use to trip a 50 ampGFCI breaker. I have tried it succesfully with about 100ma. Is this more than I should be using..Sounds high to me but the 1.2k resistor is the only one I have on hand at this time. TIA Jimmie
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GFIs trip at 5ma or so. If you used 20-30ma it would give you a pretty good safety margin. If you have a neutral load you can actually connect your shunt from neutral to ground. That is probably safer than intentionally grounding a phase, even through a resistor.
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good
shunt
grounding a

The 240 heater does not cause any neutral current but the 120 pump motor does. Normally the pump should be running whenever the heater is on. During a failure condition the heater may be on with the pump off but this is extremely unlikely so I may consider this. Also breaker trips reliably with 10ma of ground currrent.
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On Thu, 08 Jul 2004 13:17:48 GMT, Sarah Fender put forth the notion that...

I'd run the power through a contactor, and use a bulb-type adjustable thermostat to de-energize the contactor over the desired set point.
--
Checkmate

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I do have a serious problem with that circuit. You have assumed GFCIs don't fail. And yet GFCIs are also victims of surges. Older style GFCIs would simply leave power always on when GFCI failed.
Much smarter is for the overtemperature thermostat to trip and open a power relay. Disconnect all power from spa AND isolate temperature sensor from spa as well - more protective isolation.
Based upon what I saw in another post, I suspect you have sufficient technical ability to do this better protection. Furthermore, suggest you might use a solid state relay (or something equivalent) so that galvanic isolation between sensor and AC electric is greater than 1000 volts.
Sarah Fender wrote:

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You must be refering to an optoisolator?

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The spa already has such a circuit much as you described. Trouble is it failed once and caused damage to my spa. May never happen agan. I just want another level of protection, new spa cover cost $400. Also I regularly test my GFCI breaker. I have decided tto isolate the thermostat from the line using a relay even though the thermostat is elctrically insulated from HOH and metal case underneath insulation has ground connection. Even with the possibility of theGFCI failing to trip as I know it may it should give me an increased amount of protection. I just wanted to make sure I wasnt adding an increased amount of danger.With the thermostat insulated ,grounded and isolated with a relay I dont feel I could be adding much of a shock hazard.

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I any case if the GFCI were to fail for at least it's intended use, it could be "lights out" for the bather anyway.
For the thermostat, I'd be much happier with radio frequency or opto-isolation, because of the distance between the led and phototransistor.
Don't do anything that has the remotest chance of failing with your hot tub.

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