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Transmission Overview

Two-Way 115 kV, Type 2GL

SEECO's primary market focus is on transmission sectionalizing and we offer a wide range of group operated switches, interrupters and sectionalizing automation devices to serve the specific needs of transmission system designers. 

Phase-Over-Phase GOABSŪ

This switch configuration is designed specifically for mounting on a single pole structure with conductors oriented in the vertical plane, either phase-over-phase or phase delta.  Phase-over-phase switches provide very economical sectionalizing and circuit control at tap and tie switching points.  Mounted on a single pole structure in the line, their use can minimize line right-of-way requirements and significantly reduce the costs of land and equipment associated with conventional substation switching.  

Phase-over-phase GOABSŪ utilize side-break style switches, mounted horizontal upright on aluminum support structures.  The switch is designed for maintenance-free operation in severe environmental and operating conditions.  The operating effort to open and close the switch is minimal, even at higher voltages; no slamming is ever required.  The switch is positively toggled when in the fully closed position, so the switch stays closed under adverse conditions, such as excessive pole deflection, vibration, and galloping conductors, without the use of blade locks or other latching devices.

Tie and Tap Switching - the economics of 'bundling'

Two-way and three-way phase-over-phase switches employ a multiple-switch concept where individual switch pole units are arranged in an integral design, sharing insulators, interrupters, and other switch components.  Because components are shared, the cost of switching becomes proportionally less expensive when compared to other conventional switch configurations.

In addition, the combining of two or three switches into one 'bundle' provides a single location for tap or tie circuit control, whether local, remote, or automatic.  This facilitates the addition and use of automation equipment, such as motor operators, rtu's, and line sensors, which support switch operation through supervisory control or auto-sectionalizing.  Like the sharing of switch components, the sharing of automation equipment makes the cost of control proportionally less expensive.

Selection Criteria

System designers will typically use some combination of the following selection criteria to choose the configuration of phase-over-phase switch that best serves their application requirements:

  • One-way: Used for basic sectionalizing, breaking long lines into smaller, more manageable segments.

  • Two-way: Used for circuit control at through line tap and tie points, where switching is required on both sides of the through line but switching the tap line is not.  To drop or isolate the tap, both switches on the through line must be opened, breaking the feed through. 

  • Three-way: Used for circuit control at through line tap and tie points, where switching is required on both sides of the through line and the tap line.  Three-way switches allow the tap to be dropped or isolated while maintaining the feed through.

Phase-over-phase Center Vee

Center Vee switches can also be used for economical sectionalizing on a single pole structure, mounted phase-over-phase or phase delta and are an alternative to traditional one-way, phase-over-phase switches.  Center Vee switches are best used where the system designer's primary concern is to a) minimize the height of the pole structure and b) he or she can live with more limited interruption devices, such as arcing horns and single bottle interrupters [where load dropping is not required now or in the future.]

Center Vee switches are appropriate for circuits rated 46 - 138 kV.  For 161 and 230 kV applications, we strongly recommend use of the traditional one-way, phase-over-phase GOABSŪ, which have been demonstrated to be more stable and mechanically reliable at these higher voltages.

Horizontal Cross-Arm or Truss Mounting

Not all transmission switching applications are phase-over-phase (vertical) on a single pole structure.  For those applications where the group operated switch is mounted on horizontal cross-arms or a truss type structure, SEECO supplies a number of different switch configurations depending on the required phase spacing, clearance considerations and interrupter requirements. 

For customer convenience, SEECO can supply aluminum and galvanized steel truss structures for one, two and three pole applications.  We can also ship the group operated switch fully assembled on the SEECO-supplied truss structure, ready for immediate installation upon delivery. 

Interruption Devices

SEECO manufactures a wide range of interruption devices to meet your complete sectionalizing requirements.  These interruption devices are available for most transmission switch configurations and mounting applications. Please refer to the interrupter overview section for a more complete description of these devices.

  • High-speed, snap-out arcing horns are standard equipment on all transmission switching applications.  Fabricated from beryllium tube, the rod is tapered for high tip speed during the first cycle after separation.  They are suitable for interruption of limited amounts of line charging and transformer magnetizing currents. 

  • For interruption of substantially greater amounts of line charging current, SEECO has developed the High Velocity Whip (HVW), a spring loaded, stored energy arcing horn, which achieves a very large air gap within the first cycle after separation. 

  • For loop switching applications (parallel break), SEECO manufactures a single bottle interrupter.  The interrupter has a maximum interruption capability of 30 kV peak and 3000 amps.

  • For maximum sectionalizing flexibility, SEECO manufactures multi-bottle vacuum interrupters that provide a broader range of interrupting capability in a single device, including load dropping, line dropping and loop splitting. 

Sectionalizing Automation

In response to the growing industry interest in transmission automation, SEECO has developed an integrated suite of products that complement and support the automation of group operated switches.  The overall theme of these products is to facilitate the rapid identification and isolation of faults and the restoration of service. This can be accomplished in conjunction with an existing supervisory control system or through independent, automatic switch operation. 

SEECO automation products are compatible with any brand or configuration of group operated switch and include the following products operating individually or in combination:

  • Motor operators - controlled by remote supervisory systems or by local logic devices

  • Line sensors for monitoring and reporting transmission line status and condition, including voltage-only, current-only and combination current and voltage units

  • Local logic devices - local processing of line sensor output signals, which are compared to stored threshold parameters; enables independent automatic sectionalizing of switches

Please refer to the Automation and Control Overview webpage for a more complete description of our capabilities and devices.

Switch Construction Details

For more complete, detailed information on transmission switch product features and functions, please view the Construction Details page.



Three-Way 69 kV, Type 3GL


Center Vee 115 kV, Type CP


Factory preassembled 69 kV Side Break w/ multi-bottle vacuum interrupter on SEECO-supplied aluminum truss


Factory preassembled 69 kV Center Break w/ current sensors on SEECO-supplied aluminum truss


115 kV Voltage Sensor in auto throw-over application





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