Using the fuse manufacturer’s selective ratio tables, the needed ratio for selective coordination is 2:1. Which ampere combination may not selectively coordinate?

Using the fuse manufacturer’s selective ratio tables, the needed ratio for selective coordination is 2:1. Which ampere combination may not selectively coordinate?

• A. 60, 80, 200
• B. 60, 100, 400
• C. 60, 120, 360
• D. 40, 60, 150

Answer: (B)

Selective coordination means the upstream protective device must be sized and set so that the downstream device clears the fault first, typically aiming for a ratio near 2:1 between upstream and downstream. In the triple 60, 100, 400, the downstream device is 400 A while the upstream device is only 60 A. That creates a very large mismatch—about 6.7:1 downstream-to-upstream—which far exceeds what the manufacturer’s selective ratio tables allow for reliable coordination. The 60 A upstream cannot be expected to coordinate with a 400 A downstream device, so this combination is unlikely to meet the 2:1 requirement and may not selectively coordinate. The other options, while not perfect, do not exhibit as extreme a disparity between the farthest downstream and the upstream device, making them more likely to meet the table’s tolerances for selectivity.

Selective coordination means the upstream protective device must be sized and set so that the downstream device clears the fault first, typically aiming for a ratio near 2:1 between upstream and downstream. In the triple 60, 100, 400, the downstream device is 400 A while the upstream device is only 60 A. That creates a very large mismatch—about 6.7:1 downstream-to-upstream—which far exceeds what the manufacturer’s selective ratio tables allow for reliable coordination. The 60 A upstream cannot be expected to coordinate with a 400 A downstream device, so this combination is unlikely to meet the 2:1 requirement and may not selectively coordinate. The other options, while not perfect, do not exhibit as extreme a disparity between the farthest downstream and the upstream device, making them more likely to meet the table’s tolerances for selectivity.