Absolute protection of a solid state relay from a shorted load or line condition requires more thought than simply providing a common circuit breaker or fuse in the circuit. Compared to electromechanical switching devices, the solid state thyristor switching elements used in the output section of a Solid State relay have very short thermal time constants. Consequently, extreme current levels and surges caused by load or line faults, even if only applied over extremely short time periods, may cause the thyristor devices to permanently fail. Standard fuses and circuit breakers simply cannot react quickly enough to prevent the fault current from exceeding the maximum levels that the thyristors can withstand. Fortunately for the system designer, solid state relay manufacturers provide within their datasheets a specification value that designates the maximum current vs. time that the thyristors can handle. This value is commonly listed as ”maximum I²t for fusing”, (amperes squared seconds). Equally fortunate is that fuse manufacturers have certain types of fuses that also carry an “I²t” value. These fuses are generally called “Semiconductor” or “Ultra Fast Acting”, and are specifically designed to completely open within their published “total clearing I²t” value.