Conventional transient suppression devices are of two types: those that divert impulse energy to ground by conducting current, e.g., the gas discharge tube (GDT), and those that block energy from coupling the protected circuit by limiting the current, e.g., the PPTC (polymer positive temperature coefficient) resettable fuse. The former can handle very high power but has slow reaction times. As a result, some power may leak past the device before it ionizes and damage the protected circuit. The latter has a faster response but cannot block high power levels.The transient blocking unit (TBU) combines both types of devices utilizing the benefits of each: a current conduction device across the line, followed by a current limiting device. The current limiting device is a relatively fast acting PPTC device, and the current conduction device is a GTD. When a transient voltage occurs, the GTD does not conduct immediately. With the rise of voltage, the current in the PPTC device increases. The increased current heats the PTC device, and its resistance rises exponentially. The current is limited to a low value, and the voltage keeps rising. When the voltage raises sufficiently, the GTD fires and conducts the current to ground leaving a low voltage across the line. The resettable fuse returns to it low resistance state. When the transient is over, the GTD stops conducting and normal voltage is restored across the line. Read more Read less