High pressure discharge lamp having overcurrent fuse protection

A high pressure sodium discharge lamp, comprising:

• (a) an outer envelope,
• (b) a discharge vessel disposed within said outer envelope and energizable for maintaining a discharge during lamp operation, said discharge vessel containing a discharge sustaining filling comprising sodium and mercury,
• (c) means for connecting said discharge vessel to a source of electric potential, said discharge vessel having an operating current during lamp operation and a short-term high current substantially higher than said operating current during lamp start-up, said operating current of said discharge vessel increasing over lamp life from a nominal operating current to an overcurrent higher than said nominal operating current,
• (d) starting means connected to said discharge vessel for generating a voltage sufficient for inducing ionization through said discharge vessel, and
• (e) an integral thick film resistor disposed within said outer envelope, said integral resistor comprising a ceramic substrate and first and second thick film metallic resistive elements deposited on said substrate,
• said second resistive element being connected electrically in series with said discharge vessel, said operating current flowing through said second resistive element and heating said substrate during lamp operation, said substrate having a temperature during lamp operation substantially controlled by said second resistive element and positively dependent on said lamp current, and
• said substrate and second resistive element being selected such that in response to said long term increase in lamp current, said substrate reaches a temperature such that said substrate cracks, breaking said second resistive element, and disconnecting said discharge vessel from said source of electric potential, said substrate not cracking and not disconnecting said discharge vessel during each lamp start-up.