TT Electronics/Welwyn ULW series wirewound resistors overcome the problems associated with conventional flameproof resistors caused by the ceramic rod at the resistor's core acting as a heat sink for the wire element. This can delay fusing, resulting in high enough temperatures to fragment the coating and ionize the air near to where fusing occurs. If ionization occurs close to the cap edge and at a voltage peak in the mains cycle, it can initiate a momentary flashover outside the component body, releasing far more energy than is required to fuse the wire element. Although the opening of the circuit is safe for most applications, it is far from silent.
The devices are designed to withstand inrush and surge conditions. For example the ULW4-10RJ has an energy limit of 4.3J and a 1.2/50μs peak voltage limit of 1.1 kV. Fusing performance is characterized by typical fusing times of 8 seconds at 45W initial power (ULW2) and 10 seconds at 90W initial power for ULW3 and ULW4 resistors, although rapid fusing in a fraction of a second is achieved under mains short circuit conditions.
