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Over Temp protection

Discussion in 'Electrical Engineering' started by Sarah Fender, Jul 8, 2004.

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  1. Sarah Fender

    Sarah Fender Guest

    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
  2. Jimmie

    Jimmie Guest

    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.
  3. w_tom

    w_tom Guest

    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

    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.
  4. Dave M.

    Dave M. Guest

    You must be refering to an optoisolator?
  5. Jimmie

    Jimmie Guest

    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.
  6. Dave M.

    Dave M. Guest

    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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