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4 Factors To Consider When Selecting a Sectionalizer

Sectionalisers as one of the most used for distribution system protection (on photo: HO SCALE "Sectionalizer" Pole; credit: Vertryn Electric via Flickr)

Distribution system protection //

Sectionalizers are (beside overcurrent relays, reclosers and fuses) one of the most used for distribution systemprotection.

A sectionalizer is a device that automatically isolates faulted sections of a distribution circuit once an upstream breakeror recloser has interrupted the fault current and is usually installed downstream of a recloser.

Since sectionalizers have no capacity to break fault current, they must be used with a backup device that has faultcurrent breaking capacity.

Sectionalisers count the number of operations of the recloser during fault conditions. After a preselected number ofrecloser openings, and while the recloser is open, the sectionalizer opens and isolates the faulty section of line.This permits the recloser to close and re-establish supplies to those areas free of faults. If the fault is temporary, theoperating mechanism of the sectionalizer is reset.

The ABB AutoLink 3-phase sectionalizer – Isolating device that automatically isolates the faulted section of the network whena permanent fault occurs.

Sectionalizers are constructed in single- or three-phase arrangements with hydraulic or electronic operatingmechanisms.

IMPORTANT // A sectionalizer does not have a current/time operating characteristic and can beused between two protective devices whose operating curves are very close and where an additionalstep in coordination is not practicable.

Typical distribution network with ABB’s AutoLink sectionalizers

Hydraulic operating mechanisms

Sectionalizers with hydraulic operating mechanisms have an operating coil in series with the line. Each time anovercurrent occurs, the coil drives a piston that activates a counting mechanism when the circuit is opened and thecurrent is zero by the displacement of oil across the chambers of the sectionalizer.

After a pre-arranged number of circuit openings, the sectionalizer contacts are opened by means of pre-tensionedsprings. This type of sectionalizer can be closed manually.

Electronic operating mechanisms

Sectionalizers with electronic operating mechanisms are more flexible in operation and easier to set. The loadcurrent is measured by means of CTs, and the secondary current is fed to a control circuit that counts the number ofoperations of the recloser or the associated interrupter and then sends a tripping signal to the opening mechanism.

This type of sectionalizer is constructed with manual or motor closing.

Electronic Resettable Sectionalizer (CRS) – credit: HUBBELL

Factors to consider //

The following four factors should be considered when (technically) selecting a sectionalizer:

1. System voltage,

2. Maximum load current,

3. Maximum short-circuit level, and

4. Coordination with protection devices installed upstream and downstream.

The nominal voltage and current of a sectionalizer should be equal to or greater than the maximum values ofvoltage or load at the point of installation.

The short-circuit capacity (momentary rating) of a sectionalizer should be equal to or greater than the fault level atthe point of installation. The maximum clearance time of the associated interrupter should not be permitted to exceedthe short-circuit rating of the sectionalizer.

Coordination factors that need to be taken into account include the starting current setting and the number ofoperations of the associated interrupter before opening.

Sectionalizer with surge arrester protection //

Sectionalizer with surge arrester protection (photo credit: arresterworks.com)

References //

Protection of Electricity Distribution Networks – Juan M. Gers and Edward J. Holmes (Purchase from Amazon)

ABB AutoLink resettable electronic sectionalizers