A sectionalizer is a protective device on power distribution lines that automatically isolates a faulted section of the electrical grid so the rest of the system stays energized. Unlike circuit breakers or reclosers, a sectionalizer does not interrupt fault current on its own. Instead, it works as a counting device, tracking how many times an upstream protective device (usually a recloser) has tripped in response to a fault. Once it reaches a preset count, the sectionalizer opens during a brief dead-time window, permanently disconnecting the problem section.
How a Sectionalizer Works
To understand a sectionalizer, you first need to know what a recloser does. When a fault occurs on a power line, a tree branch touching a wire, for instance, the upstream recloser detects the surge of fault current and briefly opens the circuit. It then automatically recloses to restore power, checking whether the fault has cleared on its own. Many faults are temporary, and this approach keeps the lights on for most customers.
The sectionalizer sits downstream of the recloser and monitors these open-close cycles. Each time the recloser trips open (creating a brief dead period of roughly half a second), the sectionalizer registers a count. If the fault is temporary and clears after one or two recloser operations, the sectionalizer resets its count and stays closed. Nothing happens.
But if the fault is permanent, the recloser keeps cycling. After the sectionalizer has counted a preset number of interruptions, typically two, it opens during the recloser’s next dead-time pause. This happens about one second after the recloser opens. Because the sectionalizer trips while no current is flowing, it never has to break fault current itself. That’s the key distinction: it’s a switch with a brain, not a breaker.
Once the sectionalizer opens, the recloser closes one final time and successfully restores power to the rest of the line. Only the section downstream of the sectionalizer stays de-energized, which is exactly the section with the permanent fault.
Why It Can’t Break Fault Current
A sectionalizer is built as a switching device, not an interrupting device. It lacks the arc-quenching mechanisms found in circuit breakers and reclosers. This makes it simpler, lighter, and significantly less expensive. The tradeoff is that it can only open when the circuit is already de-energized during the recloser’s dead time. If it tried to open while fault current was still flowing, the resulting electrical arc could destroy the device. This is why proper coordination with an upstream recloser or breaker is essential for the sectionalizer to function safely.
Where Sectionalizers Are Installed
Sectionalizers are placed on distribution feeders downstream of a recloser or relay-controlled breaker, often at branch points where a lateral tap splits off from the main feeder line. A single sectionalizer can effectively protect up to two or three miles of line. Utilities use them on taps serving neighborhoods, rural branches, or commercial areas where installing a full recloser would be too costly, but leaving the tap unprotected would risk wider outages.
They come in several physical configurations. IEEE standard C37.63, most recently updated in 2024, covers overhead, pad-mounted, dry-vault, and submersible models for systems up to 38 kV. Overhead units are the most common, mounted on utility poles along distribution lines. Pad-mounted and submersible versions serve underground distribution networks.
Typical Ratings and Settings
Distribution sectionalizers are rated for the voltage and current levels of the lines they protect. A common single-phase unit carries a continuous current rating of 300 amps at the 15 kV class, which covers most residential and light commercial distribution circuits. The actuating current, the threshold above which the device begins counting fault interruptions, is programmable across a wide range (as low as 10 amps up to 480 amps on some models). This lets utilities tailor the device to the specific characteristics of each feeder section.
The count-to-trip setting is also adjustable. Most installations use a count of two or three, meaning the sectionalizer opens after the recloser has tripped two or three times. Setting the count too low risks opening on temporary faults that would have cleared. Setting it too high means the recloser exhausts its own trip sequence before the sectionalizer acts, defeating the purpose.
Coordination With Reclosers
The relationship between a recloser and a sectionalizer is the core design principle. The recloser must always be set to attempt more operations than the sectionalizer’s count. If the sectionalizer is set to open after two counts, the recloser needs at least three or four reclose attempts before it locks out. This ensures the sectionalizer has time to isolate the faulted section before the recloser gives up and shuts down the entire feeder.
Multiple sectionalizers can be installed in series on the same feeder, each set to a different count. The sectionalizer closest to the fault trips first (at a lower count), minimizing the number of customers affected. This layered approach is one of the main tools utilities use to improve reliability metrics like the average duration and frequency of power outages across their service territory.
Why Utilities Use Them
Sectionalizers improve grid reliability at a fraction of the cost of additional reclosers. Because they don’t need to interrupt fault current, their internal mechanisms are simpler and require less maintenance. For a utility managing thousands of miles of distribution line, strategically placing sectionalizers on branch taps means that a permanent fault on one lateral no longer causes a prolonged outage for every customer on the main feeder.
The practical impact is straightforward: fewer customers lose power, and those who do lose it get restored faster. Optimal placement of reclosers and sectionalizers together is one of the most studied problems in distribution engineering precisely because the reliability gains can be substantial, reducing both how often outages happen and how long they last.