PolySwitch™ is Littelfuse’s brand for Polymeric Positive Temperature Coefficient (PPTC) devices which are passive overcurrent protection devices. When a fault occurs, the PPTC device heats up, increases in resistance, and cuts off current flow; thereby helping protect the equipment from fault. Once the fault is removed, the PPTC device resets, returns to low resistance, and allows current to flow. Because of this type of operation, the devices are typically placed in series with the load.