The use of thermal fuses is a safety feature that is very important. They are thermally coupled to the protective elements (metal oxide varistors) and have the function of disconnecting the protective circuit in the event of thermal overload. When performing their protective function, varistors can age and become pre-damaged. As a result, leakage currents in the order of several amperes can flow through these protective elements. The resulting heating can cause these elements to glow without a line protection fuse noticing, which can pose a major safety risk. Replacing a thermal fuse would not change this situation; the protective elements would also have to be replaced. This should not be carried out by a non-professional electrician.