Auto-reclosing, device 79, is the function that trips a breaker for a fault and then automatically closes it again a short time later, betting that most overhead-line faults are temporary and will have cleared themselves once the line is briefly de-energized. Two timing settings govern the cycle. Dead time is how long the breaker stays open before the reclose attempt; it must be long enough for the fault arc to extinguish but short enough to preserve stability. Reclaim time is the window after a successful reclose during which the recloser watches to confirm the fault is really gone before it resets and is ready to count a fresh sequence.
Dead Time & Reclaim Time in one line: In an auto-recloser (device 79), dead time is the interval the breaker stays open before it recloses, set long enough for the fault arc to deionize but short enough to keep the system stable. Reclaim time is the period after reclosing during which the recloser confirms the fault has cleared; if the line stays healthy for that window the sequence resets, but if the fault returns the recloser advances toward lockout.
The great majority of faults on overhead lines are transient. A lightning surge flashes across an insulator, a branch brushes a conductor, birds or wind bring conductors momentarily close - the fault arc strikes, protection trips the breaker, and once the line is de-energized the cause clears itself and the ionized air around the arc path recovers its insulating strength. If the breaker simply reclosed after a brief pause, the line would very often come back healthy, and the outage would last a second or two instead of requiring a crew to patrol and manually restore it. That is the entire premise of auto-reclosing.
A minority of faults are permanent: a broken conductor lying on the ground, a failed insulator, a tree fallen across the line. For these, reclosing onto the line just re-energizes the same fault, so the recloser must be prepared to trip again and, after a set number of attempts, give up and stay open. The intelligence of the 79 function is in distinguishing the two by trying: if a reclose holds, the fault was transient; if the fault returns immediately on reclose, it is treated as permanent and the recloser proceeds toward lockout rather than pointlessly re-energizing a persistent fault.
This trial-and-recovery behavior is why reclosers are so widely used on distribution and transmission overhead lines: they restore the large fraction of transient faults automatically and quickly, dramatically reducing sustained interruptions, while safely handling the permanent minority through a bounded sequence that ends in lockout.
Dead time is the open interval between the breaker tripping and the automatic reclose, and setting it is a balance between two opposing requirements. It must be long enough for the arc at the fault to fully deionize. When the breaker opens, the arc is extinguished, but the air along its path stays ionized and conductive for a short while; if the breaker recloses before that path has recovered its dielectric strength, the arc simply restrikes and the fault appears permanent even though it was transient. The deionizing time grows with system voltage, so higher-voltage lines generally need longer dead times to give the arc path time to recover.
Pulling in the other direction is system stability, which matters most on transmission lines connecting synchronous sources. While the line is open, the machines at each end drift apart in angle, and the longer the line stays open the further they diverge; reclose too late and the two ends may be too far out of step to pull back into synchronism, or the reclose imposes a damaging shock. Stability therefore pushes for a short dead time. On transmission the engineer chooses a dead time long enough to deionize the arc yet short enough to reclose before the machines drift too far - often favoring a single fast reclose attempt.
On distribution feeders, where there is usually one source and stability is not the governing concern, dead times can be more relaxed and multiple reclose attempts with progressively longer dead times are common. The first dead time is often short to restore transient faults quickly, with later attempts spaced further apart to allow more time for a stubborn transient cause to clear. In every case the dead-time value is a deliberate compromise: too short and it fails to clear the arc, too long and it risks instability or an unnecessarily prolonged outage.
Once the breaker has reclosed, the recloser needs to know whether the reclose succeeded before it treats the sequence as finished. That is the job of reclaim time - the window that starts at reclose during which the line must stay healthy for the recloser to declare success and reset. If the line remains fault-free for the whole reclaim time, the recloser resets its shot counter to zero and returns to its normal armed state, ready to begin a fresh sequence for any future fault. If a fault reappears before the reclaim time expires, the recloser does not reset; it counts the reclose as unsuccessful and advances to the next shot or to lockout.
Reclaim time has to be chosen carefully. Too short, and a recloser might reset between two closely spaced faults, effectively giving a permanent fault more reclose attempts than intended and stressing the system with repeated energizations. Too long, and the recloser stays in a partly used sequence for an extended period, so a genuinely new, unrelated fault later on gets fewer attempts than a fresh sequence would allow. The setting is picked so the reclaim window comfortably outlasts the protection and reclose timing of the sequence while still resetting in a reasonable time.
The overall sequence is described by how many attempts it makes. A single-shot recloser makes one reclose attempt; if the fault persists it trips and goes straight to lockout. A multi-shot recloser makes several attempts with defined dead times between them, and only after the last unsuccessful attempt does it lock out. Lockout means the recloser has concluded the fault is permanent: the breaker stays open, further automatic reclosing is inhibited, and human intervention is required to investigate and reset. Together, dead time, reclaim time, and the shot count define the full 79 cycle - long enough open to clear transient arcs, patient enough to confirm success, and bounded so a permanent fault ends cleanly in lockout rather than endless re-energization.
Dead time is the interval the breaker stays open after tripping, before the automatic reclose; it must be long enough to deionize the fault arc but short enough for stability. Reclaim time is the window after reclosing during which the line must stay healthy for the recloser to declare the reclose successful and reset its shot counter. Dead time governs when to reclose; reclaim time governs whether the reclose is judged a success.
It decides by trying. After tripping, it recloses; if the line stays healthy through the reclaim time the fault was transient and the sequence resets. If the fault reappears when the breaker recloses, the recloser counts that attempt as unsuccessful and moves to the next shot. After the final unsuccessful attempt in its sequence it locks out, treating the fault as permanent and requiring human intervention to reset.
A single-shot recloser makes one reclose attempt; if the fault persists it trips and goes straight to lockout. A multi-shot recloser makes several attempts, with defined dead times between them, and only locks out after the last unsuccessful attempt. Transmission lines often use a single fast reclose to protect stability, while distribution feeders commonly use multiple attempts with progressively longer dead times to give stubborn transient faults more chance to clear.
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