Two protective devices at different points on a system are deliberately given different trip-time curves — not so both trip together, but so the closer one always trips first, leaving the upstream device as a backup only.
Protective relaying detects abnormal conditions — short circuits, overloads, and other faults — and automatically trips a breaker or other interrupting device to isolate the problem. Coordination is the deliberate practice of setting multiple devices' trip characteristics so, for any given fault location, only the closest upstream device trips, minimizing how much of the system loses power for any single fault.
A protective device's time-current characteristic curve shows how long it takes to trip as a function of how much current is flowing relative to its pickup setting — generally, higher fault current trips faster (inverse-time behavior). Each device on a system can have this curve shifted or shaped independently via its pickup and time-dial settings, which is the actual mechanism coordination is built from.
For a fault at a given point, the device electrically closest to that fault should always trip meaningfully faster than any device further upstream — a deliberate time margin, not a coincidence. This way, a fault on one branch circuit trips only that branch's breaker, rather than a much larger upstream breaker that would also de-energize every other unaffected circuit sharing that same upstream device.
The upstream device's slower curve isn't wasted — if the downstream (primary) device fails to clear the fault for any reason, the upstream device will eventually trip and clear the fault anyway, just after a longer delay and affecting a larger portion of the system. This layered backup relationship is what makes a properly coordinated system reliably fail-safe rather than fully dependent on a single device working correctly.
It means protective devices are set up so that for any fault, only the nearest upstream device trips — isolating the smallest possible portion of the system — while devices further upstream remain closed and continue serving unaffected loads.
By design — their time-current curves are deliberately set with lower time-dial or instantaneous settings than upstream devices, specifically so a clear time margin exists between when the closer device trips and when the farther device would trip for the same fault, ensuring the closer device acts first.
Miscoordination can cause an upstream device to trip before (or simultaneously with) the intended downstream device — a condition called "cascading" — which de-energizes a much larger portion of the system than necessary for a fault that should have been isolated locally.
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