Littelfuse’s app note about using Polymeric positive temperature coefficient (PPTC) device as protection when fluorescent tube lighting no longer operate normally. Link here (PDF)
The electronic ballasts used in fluorescent lighting systems may experience high failure rates, due to the fact that the ballast manufacturer may not have considered all failure mechanisms and provided appropriate protection against the many different abnormal states that can result in end-of-life (EOL) issues. Although lighting manufactures have come to understand the importance of abnormal-state protection by adding a clause to the international IEC 61347-2-3 standard, many major semiconductor manufacturers provide no, or limited, EOL abnormal-state protection for their devices. In addition, the operation of most ballasts is now achieved through the self-oscillation of a discrete device, the bipolar transistor, which does not offer EOL abnormal-state protection.