A thick film heating element or resistance comprises a main track (4) arranged to carry electric current during normal operation and an auxiliary track portion (26a, 26b) electrically coupled to said main track (4) by a bridge (28) of glass, glass-ceramic or ceramic material such that it does not carry current during normal operation, but wherein the configuration of the auxiliary track (26a, 26b) and the bridge (28) of glass, glass-ceramic or ceramic material is chosen such that at a predetermined temperature a leakage current between the main (4) and auxiliary (26a, 26b) tracks rises to such an extent that a failure current flows through a section of the main track (4).