Coordination of MCCB and high-voltage side protection device

Coordination of MCCB and high-voltage fuse
             When a power fuse (hereafter, PF) is used as the high- voltage side protection device, it must be coordinated with the secondary MCCB. In other words, in the overload range MCCB must always function first, and the PF must not function. In addition, the fuse element must not degrade over repeated overload currents.

        In actual use, the PF short-time tolerable characteristics curve (if unknown, the average welding characteristics curve reduced by 20% at the current axis can be interpreted as the short time tolerable characteristics) must be overlapped with MCCB operating characteristics curve (PF converted to secondary side or MCCB converted to primary side), and both must not cross at the overload range.

            If this method has already been considered, this may have been experienced, but it is difficult to achieve PF and MCCB coordination at the shaded section shown on Fig. 4. 32. In this case, the arrow shows where the instantaneous tripping current value can be adjusted. Coordination can be achieved by lowering this setting.

         However, MCCB instantaneous tripping current value has a difference from the symmetrical value. Actual combinations that take the difference into consideration and attain a favorable coordination relation. Coordination can and cannot be achieved indicate the setting dial for the instantaneous tripping current explained earlier. Coordination can be attained with a setting dial less than this number. If a number is not indicated, coordination can be attained with all setting dials. When considering the coordination with the PF and MCCB, the non-coordinated sections shown in Fig. 4. 32 is the overload range. The current in this range is usually generated after MCCB₂ unless there is a high-impedance short-circuit in the electric circuit between MCCB₁ and MCCB_2.

           Thus, coordination with the PF should be considered in the space between MCCB₂, and the non-coordination MCCB₁ must be tolerated in some cases.