Overcurrent Selective Coordination Screen

ScopeScreened the way each device type actually behaves. Fuses coordinate on a RATIO, and the ratio is a published test result for a specific fuse family rather than a general rule. Within a family, an upstream fuse at or above the published ratio will not open before a downstream one clears, at any fault current up to the interrupting rating -- which is why fuses coordinate all the way down and why the ratio table is the whole answer. Circuit breakers fail differently and that is the part worth understanding. A thermal-magnetic breaker's instantaneous element trips essentially without delay, so once the fault current exceeds the upstream breaker's instantaneous pickup, BOTH devices open and selectivity is lost no matter how far apart the ratings are: a 400 A upstream against a 100 A downstream looks like a comfortable four-to-one and coordinates not at all above the pickup. Coordination therefore holds only up to the LOW edge of that pickup band (Bussmann's 400 A breaker at 5x can trip at 1,500 A, not 2,000), and whether that is acceptable depends entirely on the available fault current at the downstream device. The breaker case has three fixes and they are all real work: raise the instantaneous setting if the breaker allows it and the downstream equipment survives the longer clearing time, move to a breaker with a short-time delay and no instantaneous -- which raises incident energy, so the arc-flash and coordination requirements genuinely conflict -- or use fuses. A screen, never a study: the manufacturer's published ratio tables and time-current curves, a full coordination study, and the engineer of record govern.

Formula or tablefuses: ratio = upstream_rating / downstream_rating, coordinated when ratio >= the family's published minimum, and then to the interrupting rating. breakers: pickup = upstream_rating x instantaneous_multiplier, the low edge of its band = pickup x (1 - tolerance), and coordination holds only up to that low edge (Bussmann, Selective Coordination, 2008: a 400 A breaker at 5x can trip at 2000 x 0.75 = 1500 A).

Source and editionSelective coordination screened by device type -- fuses against the manufacturer's published minimum ratio for the family (commonly 2:1), which holds to the interrupting rating, and breakers only up to the low edge of the upstream instantaneous pickup band, above which both devices open. Cited by name; the ratio and the instantaneous setting are the manufacturer's published values and are entered rather than bundled. A screen, never a study: the published time-current curves, a full coordination study, and the engineer of record govern.

Free accessBoth screens are comparisons on values the user reads off the manufacturer's published data; no ratio table or time-current curve is reproduced.

What governsEstimate. AHJ and licensed professional govern.

API field namesdevice_type, upstream_rating_a, downstream_rating_a, published_ratio, instantaneous_multiplier, available_fault_a, ratio, pickup_a, coordinated_to_a

• Fuse ratios are tested a published ratio for a specific family, not a general rule; met, it holds to the interrupting ratingfuse manufacturer coordination tables
• Breakers lose selectivity above pickup above the upstream instantaneous pickup BOTH open, whatever the ratioovercurrent protection practice
• The conflict is real removing the instantaneous to coordinate raises arc-flash incident energyovercurrent protection practice