Importance

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Abstract

An electric immersion heating element dry switch-on or boil dry protector unit for an electrically heated water boiling vessel comprises primary and secondary bimetals (31,32) responsive to the heating element head temperature and each arranged for actuating a respective one of primary and secondary switching contacts (11,12) connected in the line and neutral sides of the heating element supply via respective push rods (38,44). The secondary bimetal (32) is arranged to break later than the primary bimetal (31) in a dry switch-on or boil dry situation giving rise to excessive element temperatures, and to reset before the primary in a reducing temperature situation. The secondary bimetal (32) and associated switch contacts (12) provide back-up protection in the event of failure of the primary protection on account for example of welding of the primary switch contacts (11). A neon warning lamp connected across the secondary contacts (12) will be lit only if the secondary contacts open with the primary contacts closed and thus indicates failure of the primary protection. An advantageous manually releasable latching arrangement for the primary and/or secondary switching contacts is also provided. An additional bimetal subject to heating by steam vented from the vessel operates at least one of the switch contacts when water boils in the vessel to terminate the heating operation.

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First Claim