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Protective control arrangement for duty cycle controlled sheathed heating elements

  • US 4,465,924 A
  • Filed: 01/07/1982
  • Issued: 08/14/1984
  • Est. Priority Date: 01/07/1982
  • Status: Expired due to Term
First Claim
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1. A protective control arrangement for a heating unit of the type comprising the concentric assembly of a resistive heating element embedded in an insulating material and enclosed within a metallic sheath, the insulating material in its normal state being characterized by a very high resistance and in its combined molten and plasma state being characterized by a relatively low resistance, the resistive element being adapted for energization by an external power supply, the heating unit being susceptible to a discontinuity developing in the resistive element when energized which can result in an electrical art at the discontinuity, the arc being effective to reduce the insulating material proximate to the arc to its relatively low resistance molten and plasma state to create a relatively low resistance current path from the discontinuity to the sheath and ultimately rupture the sheath;

  • the arc and rupture if unchecked propagating along the length of the sheath, the period of propagation being the time between initiation and termination of such propagation, the insulating material having associated therewith a nominal recovery time required to allow the resultant relatively low resistance molten and plasma insulating material to return to a relatively high resistance state upon extinguishing the arc by interruption of power to the heating element, said protective control arrangement comprising;

    power level selection means for enabling the user to select a power setting for the heating unit from a plurality of available power settings;

    switch means adapted to be operatively connected to the heating element for selectively coupling the heating element to the external power supply;

    control means responsive to said power level selecting means and operative to control the duty cycle of the heating element for at least one of said power settings by periodically opening said switch means to decouple the heating element from the power supply for idle periods of predetermined duration, the duration of said idle periods and the period between said idle periods depending upon the power level selected by the user; and

    timing means operative to monitor the time between decouplings, which time depends upon the power level selected by the user;

    said timing means being reset by said control means in response to the occurrence of each power control decoupling;

    said control means being responsive to said timing means and further operative to interrupt power to the heating element for a recovery period of predetermined duration whenever the time between decouplings exceeds a predetermined maximum time, said predetermined maximum time being selected to confine the duration of the period of propagation within predetermined limits, said recover period being at least equal to the nominal recovery time;

    whereby the propagation of a relatively low resistance current path from the heating element to the metallic sheath through the insulating material along a substantial portion of the length of the heating unit resulting from the occurrence of a discontinuity in the heating element is prevented.

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