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Oven isolation switch

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  • 07-02-2017 10:09am
    #1
    yop
    Moderators, Home & Garden Moderators, Regional Midwest Moderators, Regional West Moderators Posts: 16,724 Mod ✭✭✭✭


    Hi
    Oven is around 11 years old. On a 32amp breaker.
    Last Thurs oven wouldn't turn on. Looked at the board and nothing tripped.

    I took off the cover and the neutral on the feed was black. Plastic burnt.
    Nothing lose.

    I got a new switch and put it on Saturday morning, used the over Sat and Sunday, the grill a few times.
    Yesterday even went to put it on and oven gone again.

    Nothing tripped.

    Any thoughts?


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    #2
    Tuco88
    Registered Users Posts: 1,051 ✭✭✭Tuco88


    Anyone care to explain why it's the neutral the majority of the time, as seen on shower pull cords also.


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    #3
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,595 Mod ✭✭✭✭2011


    Tuco88 wrote: »
    Anyone care to explain why it's the neutral the majority of the time, as seen on shower pull cords also.

    I believe that it is the neutral about 50% of the time on single phase circuits.
    The heat generated is equal to the square of the current by the resistance.
    As the current is the same in the phase as the neutral a resistance (for example due to a poor connection) on either can cause heating.

    Three phase circuits are a little different. In some instances the neutral can see currents higher than phase currents due to harmonics. Harmonic currents from all three phases can combine on a neutral conductor which can cause heating and other issues.


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    #4
    homewardbound11
    Registered Users Posts: 909 ✭✭✭homewardbound11


    I would have thought a burning neutral would be a poor connection or on your case the resistance provided by the oven would be low in ohms, the resistance by the neutral line is very small but attributable to the overall resistance and therefore heat sinking some current at the switch .

    Would that be a fair assumption . I'm not an electrician to be up front .


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    #5
    yop
    Moderators, Home & Garden Moderators, Regional Midwest Moderators, Regional West Moderators Posts: 16,724 Mod ✭✭✭✭yop


    I looked at the switch I got Saturday and tbh I think the quality was poor, so I went in this morning and purchased a hager switch. Just looking at the screws and housing ( I might be raving!) but it just seems so much better.
    I also hadn't cut back the blackened cable, which I did the AM so I installed it there and its working now.
    Time will tell.


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    #6
    Risteard81
    Registered Users Posts: 2,567 ✭✭✭Risteard81


    2011 wrote: »
    Tuco88 wrote: »
    Anyone care to explain why it's the neutral the majority of the time, as seen on shower pull cords also.

    I believe that it is the neutral about 50% of the time on single phase circuits.
    The heat generated is equal to the square of the current by the resistance.
    As the current is the same in the phase as the neutral a resistance (for example due to a poor connection) on either can cause heating.

    Three phase circuits are a little different. In some instances the neutral can see currents higher than phase currents due to harmonics. Harmonic currents from all three phases can combine on a neutral conductor which can cause heating and other issues.
    I think it's substantially more than 50% on single phase circuits. To my mind the reason is probably psychological with people perhaps tightening the phase connections better than the neutral, forgetting that it will be carrying equal current despite not having a voltage in reference to Earth. Also the neutral connections won't be tested e.g. with an Rp + Re test.

    Also, with three-phase circuits, whilst the neutral could be greater than the phases due to third harmonics etc. don't forget it can also be zero where the phases are balanced.


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    #7
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,595 Mod ✭✭✭✭2011


    homewardbound11 wrote: »
    I would have thought a burning neutral would be a poor connection or on your case the resistance provided by the oven would be low in ohms, the resistance by the neutral line is very small but attributable to the overall resistance and therefore heat sinking some current at the switch .

    Would that be a fair assumption . I'm not an electrician to be up front .

    Let's pretend that the neutral connection was 2 ohms and the current drawn was 25 amps, then the heat generated would be = (I^2)(R) = (25^2)(2)= 1.25 kW

    That is a lot of heat concentrated on a very small area!


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    #8
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,595 Mod ✭✭✭✭2011


    Risteard81 wrote: »
    I think it's substantially more than 50% on single phase circuits. To my mind the reason is probably psychological with people perhaps tightening the phase connections better than the neutral, forgetting that it will be carrying equal current despite not having a voltage in reference to Earth.

    There could be an element of that.
    But this is due to a lack of understanding/ training not a technical manufacturing or technical design fault.

    A competent electrician should not make that error!

    Also the neutral connections won't be tested e.g. with an Rp + Re test.

    Yes, this test would not detect that issue.
    don't forget it can also be zero where the phases are balanced.

    That is sometimes the case, but my point stands.
    I have seen first hand substantial fire damage caused by harmonics. As we move into an age with more VFDs, UPS and other nonlinear loads this is becoming more and more of an issue.


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    #9
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Risteard81 wrote: »
    don't forget it can also be zero where the phases are balanced.

    Ye learn something new every day.


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    #10
    homewardbound11
    Registered Users Posts: 909 ✭✭✭homewardbound11


    I'm as a novice as yop here but on a 32 amp cb,The wall isolation switch would be rated for 40 amp . So are we saying the resistance of the neutral copper line is paramount to the current returning at the switch .
    One thing idisnt realise here and I hope I got it from other posters correctly is ... in ac circuits is it possible to have current of significant amperage without significant voltage .
    I'm assuming the heater element in a oven might be a 100 ohms or less . So as a voltage divider the voltage at the isolation switch would be at most 1/100 of that . So is it possible to deliver 25 or 32 amps with the voltage dropped by 100th i.e. A couple of volts .

    Hopefully you guys can help me also with tips query . I'm really only familiar with dc circuits


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    #11
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,595 Mod ✭✭✭✭2011


    homewardbound11 wrote: »
    I'm as a novice as yop here but on a 32 amp cb,The wall isolation switch would be rated for 40 amp . So are we saying the resistance of the neutral copper line is paramount to the current returning at the switch .
    One thing idisnt realise here and I hope I got it from other posters correctly is ... in ac circuits is it possible to have current of significant amperage without significant voltage .
    I'm assuming the heater element in a oven might be a 100 ohms or less . So as a voltage divider the voltage at the isolation switch would be at most 1/100 of that . So is it possible to deliver 25 or 32 amps with the voltage dropped by 100th i.e. A couple of volts .

    Forget about volt drop, resistances and ratings of devices for a minute.

    Think of it this way: If 10A flows down the phase (live) to the cooker then 10A will flow back on the neutral (assuming there is no earth fault).
    Once current flows through a conductor the heat generated in the conductor is the current squared by the resistance of the conductor.
    This will happen whether the conductor is a neutral or phase regardless of volt drop and anything else in the circuit.


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    #12
    FishOnABike
    Registered Users Posts: 4,490 ✭✭✭FishOnABike


    yop wrote: »
    I looked at the switch I got Saturday and tbh I think the quality was poor, so I went in this morning and purchased a hager switch. Just looking at the screws and housing ( I might be raving!) but it just seems so much better.
    I also hadn't cut back the blackened cable, which I did the AM so I installed it there and its working now.
    Time will tell.
    This may be why the first replacement failed. The blackened cable and conductor could have left a high resistance poor contact causing heating and failure again.


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