The most obvious difference between polymer self-repeating fuses and ordinary fuses is their self-repeating nature. Although both provide overcurrent protection, polymer reclosing fuses can provide protection for many overcurrents, whereas ordinary fuses, once blown, must be replaced in order for the circuit to function properly. Polymer self-repeating fuses behave somewhat like time-delay fuses in that both need to dissipate their own heat, but polymer self-repeating fuses do not dissipate heat according to I2t like time-delay fuses. Polymer self-repeating fuses differ from ceramic self-repeating fuses in their initial resistance, response time to faults, and size. Both are self-repeating, but polymer self-repeating fuses are faster due to their smaller size than ceramic self-repeating fuses with the same holding current. Polymer self-repeating fuses are used in conjunction with loadable overvoltage devices, often in communications applications. For many fault events, loadable overvoltage devices such as thyristors, gas discharge tubes or diodes can provide protection. Polymer cutoffs can protect these overvoltage protection devices during certain fault events, and of course polymer cutoffs can also provide overcurrent protection.

How Polymer Resettable Fuses Resume

Once the fault event is cleared and the polymer self-repeating fuse has had a chance to cool, it resets itself. Cooling allows the carbon black particles to come into contact and reconnect, which reduces resistance. Typically, the way to cool a polymer cutoff fuse is to cut off the energy supply to the protected device, which cuts off the fault current and allows the polymer cutoff fuse to cool. Polymer self-recontained fuses should be distinguished from bimetallic devices that are also capable of self-recontainment. Typical bimetallic devices will self-recovery even if the fault event is not cleared, thus switching between the fault event and the protected state that could damage the equipment. Polymer self-resetting fuses remain in a high resistance state until the fault event is cleared. The time it takes for a polymer self-resetting fuse to revert to a low-resistance state depends on a number of factors: the type of polymer self-resetting fuse, how it is mounted or secured, the ambient temperature, the internal cause of the action, and the duration of the action. In general, most polymer fuses will recover within a few minutes, although many will recover within a few seconds.
Polymer self-repeating fuses will not transition between normal and operational states until the fault event has been removed. When a polymer cutoff fuse operates, its resistance goes from low to high, with a small amount of fault current remaining in the high resistance state. This small fault current is enough to keep it in the high resistance state. When the fault is cleared, the polymer self contained fuse is cooled back to the low resistance state.

When does a polymer self-repeating fuse self-repeat?

Polymer self-repeating fuses self-repeat as a function of current, voltage and thermal fuse. Polymer self-repeating fuses will often begin to self-repeat at temperatures below 90°C (you could say that a polymer self-repeating fuse has already self-repeated below 80°C).