Heating circuits

Claim 1

In a heating circuit comprising input terminals for connection to an AC supply, a first elongate conductor connected between said input terminals to effect heating, a second elongate conductor substantially coextensive with said first conductor, and elongate separating means that separates said first and second conductors and is in contact with each of said first and second conductors, said separating means having a relatively high impedance at least at normal working temperatures, and said separating means being of such a nature that in the event of overheating said impedance will drop to a relatively low value and/or it will melt to permit contact of said first and second conductors whereby said impedance will drop to substantially zero, the improvement comprising: a first half-wave rectifier means and a first resistor connected in series between one end of said first conductor and one end of said second conductor to permit current to flow through the first half-wave rectifier means, the first resistor and said impedance when the AC supply is of a first polarity; a second half-wave rectifier means and a second resistor connected in series between the other end of said first conductor and one end of said second conductor to permit current to flow through the second half-wave rectifier means, the second resistor and said impedance when the AC supply is of the other polarity; and circuit interruption means responsive to current flow through one or both of the first and second resistors indicating that said impedance has dropped in value as a result of overheating, said circuit interruption means acting to disconnect the heating circuit from the AC supply.