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Test Interfaces for Power Systems

Safety Test Switch


The ST Switch is a test block for interfacing substation devices (protection relays, fault recorders, revenue meters, etc.) to the voltage and current transformers and to other equipment on the system side of a power grid.

The ST Switch uses disconnect pins to isolate the substation devices from the system side equipment. Once isolated, secondary injection can be performed using banana jacks on the front side of the test block.

Built-in status windows allow an operator to observe the opening and closing action of the internal contacts.

Key Features

  • Finger-safe
    There are no exposed metallic parts on the ST Switch, greatly increasing safety during testing.
  • Keyed Disconnect Pins
    Disconnect pins are keyed to the corresponding slots of the test block, preventing mistakes and errors during test.
  • Integrated Status Window
    Status windows in the front of the ST Switch indicate whether the internal contacts are open or closed.
  • Wire Terminals
    Wire terminals are replicated from the FT Switch and offer maximum convenience for technicians and engineers. A screw terminal and a stud terminal version are available.
  • Less than 2 mΩ internal resistance
    Thanks to the patented construction, ST Switches have an extremely low internal resistance. This decreases the burden placed on instrumentation transformers and also reduces heat dissipation inside cabinets and panels.
  • Ideal for Retrofit of Older FT Switches
    The ST Switch models are available in 10 and 14 pole configurations. 10-pole ST Switches have the same panel cutout as legacy FT switches. Even the terminal labeling is the same. That means that you can have the increased safety and efficiency from the ST Switch without having to change your engineering drawings.

Principle of Operation

Closed circuit
An ST Switch contains either 10 or 14 modular units (or poles) each allocated to a current, voltage, signal or trip circuit. Each circuit is connected through the block via two silver-plated copper contacts, pressed securely together by two pressure springs to create a highly conductive electrical connection. In this situation, a electrical connection is established between the device side (B) and the system side (A).
Open Circuit
To open one of these modular poles, a disconnect pin is removed from its inactive "parking" position and inserted between the silver-coated copper contacts (the "test" position). In this position, the device and system sides of the test block are isolated from each other. For current circuits, the disconnect pins include internal shorting bridges. When the pin is moved back to the "parking" position, the internal springs force the contacts back together, returning the circuit to its normally-closed position. There is no in-between position, completely eliminating the risk of accidents resulting from improperly closed circuits.


Current Measurement Probe
This special test probe allows the connection of current measurement device or a shunt. The AWG 13 (2.5 mm) connection cable has a length of 118.11 inch (3 meters). The test probe are available with c-hook terminals or banana plugs at the tip of the cable.
STS 19´´ Rack Plates
SecuControl offers metal plates for installation of ST Switch in 19” racks that come painted in various colors and with various cutouts for LTB test blocks, in standard heights of 2U or 3U.

Please consult our manual on the download section of this site for ordering information.



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