A more practical, albeit more expensive, solution is to use DC circuit breakers (preferably two-or four – pole models in + and —) in lieu of fuses, as these devices allow a string to be shut down while under load. Like fuses, the higher the ratio between system current and nominal current, the sooner a circuit breaker will be tripped, i. e. there are characteristic shutdown curves for each type of circuit breaker, as illustrated in Figure 4.9 (kf ~ 1.45). Circuit breakers rated for extremely high currents often integrate magnetic instantaneous tripping mechanisms rated for 5 to 15 times ISN or the like. Such circuit breakers should be used, at a minimum, for arrays in relatively large solar generators. If only a unipolar circuit breaker is used, at least one disconnect terminal or a suitable PV plug should be mounted on the other side of the string.