For proper coordination, the total clearing time of any downstream protective device must be below what percentage of the minimum melting curve of the fuse being applied?

The correct answer is that for proper coordination, the total clearing time of any downstream protective device must be below 75% of the minimum melting curve of the fuse being applied. This criterion is crucial in ensuring that upstream devices do not inadvertently trip during fault conditions that would only affect downstream devices.

When the clearing time of a downstream device is kept below 75% of the minimum melting curve of the fuse, it allows the fuse to effectively clear a fault before the downstream device operates. This is essential for maintaining selective coordination in the electrical system, meaning that only the device closest to the fault operates while the other devices remain functional.

If the clearing time exceeds 75%, the risk is that the fuse may not operate quickly enough to protect downstream equipment, leading to unnecessary outages and potential damage. Therefore, adhering to this percentage helps ensure a reliable and safe electrical system by allowing faster protection from faults while minimizing disruptions to the entire system.