A gap between two curves at one current isn't coordination. A gap held across the entire range of possible fault current is.
When a fault occurs anywhere in a distribution system, more than one protective device is technically capable of clearing it — the branch breaker right at the fault, the feeder breaker upstream of it, the main upstream of that. Selective (or "selectively") coordinated protection means only the device closest to the fault opens, leaving every upstream device closed and every other load on the system undisturbed. Non-selective — cascading — coordination means an upstream device trips at the same time as, or even before, the downstream device, needlessly de-energizing a much larger portion of the system for a fault that only ever affected one branch.
In practice, coordination is checked by overlaying the time-current curves (TCCs) of every series-connected device on the same log-log plot and confirming the upstream device's curve stays above and to the right of the downstream device's curve — with enough time separation for the downstream device to fully clear the fault before the upstream device even begins to respond. The device closer to the fault has less curve to clear through, so it should always finish first. The catch is that available fault current at any given point in the system isn't one fixed number — it ranges from the minimum fault current the system can deliver there up to the maximum. A pair of curves has to hold its margin across that entire range, not just at whatever single value happened to get checked.
The failure mode is subtler than two curves that simply overlap everywhere. Two curves can be cleanly separated right at a breaker's rated current — the one value a quick check tends to land on — and still converge or cross somewhere else along the same pair of curves, at a current the system is fully capable of delivering. A study that stops at one assumed value can sign off on a pairing that only looks selective.
Verifying selective coordination means overlaying TCCs and confirming adequate time and current separation at every point along the full range of available fault currentat that location — from the minimum the system can deliver up to the maximum — not just at one assumed value. That's exactly why NEC Articles 700, 701, and 708 mandate selective coordination specifically for emergency systems, legally required standby systems, and critical operations power systems. In an ordinary branch circuit, a non-selective trip is an inconvenience — a breaker resets, someone flips a switch. In a hospital's emergency panel, a single downstream fault that unnecessarily trips the upstream main isn't an inconvenience; it's a life-safety failure, taking out surgical suites, ICU equipment, and egress lighting that a fault on one branch circuit was never supposed to touch. Selective coordination is what keeps that outage confined to exactly the branch that failed — and nothing else.
Rated current is one point on the curve — and often not even the current a real fault will produce there. Available fault current at any given point in a system varies with utility source strength, system configuration, and how many other sources are online, ranging across a whole band of possible values. A curve pair has to hold its time margin across that entire band, not just at the single value a check happened to land on — two curves that are cleanly separated near rated current can still converge or cross further along the same pair, right in the range of currents a real fault is likely to deliver. Coordination studies exist specifically to check the full range, not to spot-check one number and call it done.
Explains why selective coordination means only the protective device closest to a fault opens — while every upstream device stays closed — and why that has to be verified across the entire range of available fault current, not just at one assumed value, especially on the emergency and critical-operations systems NEC 700, 701, and 708 govern.
Selective (selectively) coordinated protection means that when a fault occurs, only the device immediately downstream of the fault opens, isolating just the affected branch while every upstream device — feeders, mains — stays closed and every other load on the system keeps running. Non-selective, or cascading, coordination means an upstream device trips at the same time as, or before, the downstream device, needlessly de-energizing a much larger portion of the system for a fault that only ever affected one branch circuit.
Coordination is checked by overlaying the time-current curves (TCCs) of every series-connected protective device on a common log-log plot and confirming the upstream device's curve stays adequately separated — in both time and current — from the downstream device's curve. Critically, that separation has to hold across the full range of fault current the system can actually deliver at that point, from minimum to maximum available fault current, not merely at one assumed value such as the device's rated current. Two curves can look cleanly separated at rated current and still converge or cross elsewhere on the same pair, right in the range a real fault is likely to produce.
NEC Articles 700 (Emergency Systems), 701 (Legally Required Standby Systems), and 708 (Critical Operations Power Systems) specifically require selective coordination between all protective devices, from the utility connection down to the load. That's because on these systems, a non-selective trip isn't merely an inconvenience — it's a life-safety failure. A single fault on one branch of a hospital's emergency panel tripping the upstream main can take down surgical suites, ICU equipment, and egress lighting that the fault never should have reached.
Overcurrent protection is about a single device correctly clearing a fault or overload within its own rating. Selective coordination is a system-level property — it's about which device among several series-connected devices clears the fault, ensuring it's always the one closest to it, so the rest of the system stays energized.
Available fault current at a given point in a system isn't fixed — it varies with utility source strength, system configuration, and which sources are online, spanning a range from a minimum to a maximum value. Two curves can be cleanly separated at rated current and still cross at a different current within that range, which is exactly the current a real fault might deliver.
NEC Articles 700, 701, and 708 require selective coordination for emergency systems, legally required standby systems, and critical operations power systems, respectively — covering the protective devices from the source all the way down to the load, because a non-selective trip on these systems has life-safety consequences.
Engineers typically use time-current curve (TCC) plotting and coordination software fed by a short-circuit study, which supplies the minimum and maximum available fault current at each point in the system so the full range — not just one value — can be checked against every device pairing.
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