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    #16
    Originally posted by German shepards View Post
    I think if you have a 3 phase heater and one fuse blows yes the other ones will blow since it’s in a single phase condition and the other 2 phases will try and carry the current
    Go study Ohm's and Kirchoff's laws.

    This is a resistance type load, resistance will remain about the same (maybe somewhat negligible differences if there is a temp change happening here but otherwise can assume resistance will still be same). An inductive load like a motor will have different impedance characteristics if a change like lost phase is introduced. But for a resistance only load you just lose whatever portion of the circuit that is directly connected to the lost input.

    Now it gets a little more complex with three phase, in particular if the heating elements are connected in a wye configuration and there is no source neutral connected to the wye point. I believe I mentioned this in an earlier reply but will do so again. In that situation presuming 480 volts supply volts, if you connected the heaters in a wye configuration they will see 277 volts across each heater presuming they are of all equal resistance and there is no source neutral connected to the wye point. Now if you open one of the three input leads that leaves you with only two equal resistances in series with one another and with 480 volts supplied. Because they are equal resistance half the supply volts will drop across each element - meaning they only see 240 volts in this condition instead of 277. that is actually going to result in a little bit of a drop in current across each remaining element compared to if all three inputs were there.
    I live for today, I'm just a day behind.

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      #17
      Originally posted by kwired View Post
      My experience has always been if one contact welds shut, contactor can not mechanically drop out and all three poles remain closed and the load never stops. ...
      Depends on the contactor. Those with what's called a "positive drive" armature will do as you describe; the moveable contacts are hard fixed to the armature, so if one of them welds, the entire armature stays in that position. Those with "floating contacts" will not; the armature returns to the normal position and any un-welded contacts still open, only the welded one remains closed. NEMA design contactors will have floating contacts. Cheap DP contactors (and now, "Safety Contactors) have positively driven contacts.

      That said, many many heater control systems use cheap DP contactors, so practically speaking, if one contact welds, they all stay stuck.

      __________________________________________________ ____________________________
      Many people are shocked when they discover I am not a good electrician...

      I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

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        #18
        Originally posted by Jraef View Post
        Depends on the contactor. Those with what's called a "positive drive" armature will do as you describe; the moveable contacts are hard fixed to the armature, so if one of them welds, the entire armature stays in that position. Those with "floating contacts" will not; the armature returns to the normal position and any un-welded contacts still open, only the welded one remains closed. NEMA design contactors will have floating contacts. Cheap DP contactors (and now, "Safety Contactors) have positively driven contacts.

        That said, many many heater control systems use cheap DP contactors, so practically speaking, if one contact welds, they all stay stuck.
        I agree NEMA contactors more likely to hold in every pole when only one is welded. I know I have seen NEMA contactors welded and all poles closed, but may have had more than one pole that was actually welded. Haven't really ran into all that many NEMA contactors that were welded in the first place.

        I have seen many DP contactors that are still a floating design, but apparently don't have enough float range and they still end up sticking with all poles closed. Maybe seen a few IEC contactors that done this also.

        I live for today, I'm just a day behind.

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