A polymer cutoff consists of a polymer matrix and carbon black particles that make it electrically conductive. Since the polymer self contained fuse is a conductor, an electric current passes through it. When an overcurrent is passed through a polymer self contained fuse, the heat generated (I2R) will cause it to expand. As a result, the carbon black particles will separate and the resistance of the polymer cutoff will rise. This causes the polymer cutoff to heat up faster, expanding even more and further increasing the resistance. When the temperature reaches 125°C, the change in resistance is significant, resulting in a significant reduction in current. At this point the small current flowing through the polymer cutoff is sufficient to keep it at this temperature and in a high resistance state. When the fault is cleared, the polymer self contained fuse shrinks back to its original shape and reconnects the carbon black particles, thus reducing the resistance to a level with a specified holding current. The above process can be repeated several times.

How many times can a polymer cutoff operate at maximum voltage and inrush current?

Each type of polymer cutoff fuse has a specific operating voltage and is subject to a specific inrush current, and UL regulations require that polymer cutoff fuses must exhibit the PTC effect after 6,000 cycles of operation. The SN/SF polymer cutoffs used in communications equipment are specified to remain within the original range of performance parameters after as few as a dozen or as many as a hundred actuations at the maximum voltage. Designers should recognize that polymer self-repeating fuses are intended for protection, not for use in situations where their constant action is considered normal operation.

How quickly does a polymer cutoff fuse recover from operation?

The time it takes for a polymer cutoff fuse to return to its low resistance state after operation is affected by factors such as: the type of polymer cutoff fuse; how it is mounted or secured; the ambient temperature; and the cause and duration of the operation. In general, most polymer cutoffs will recover within a few minutes, although many will recover within a few seconds.

How long can a polymer self-repeating fuse remain in an operational state without damage?

UL specifies that polymer cutoffs must be held at maximum voltage for 1000 hours without losing their PTC characteristics. The longer a polymer cutoff remains in the active state, the more likely it is that its resistance value will not recover and thus may not meet its initial definition. The amount of time that each polymer self-reclosing fuse can remain in operation varies with the event and type of fault.