CT disconnect terminal blocks come in screw, spring-cage or push-in connection technology.
By Harry Bentler, Phoenix Contact, GmbH & Co. KG, and Alan Sappe’, Phoenix Contact, CLIPLINE
Current transformers and potential (voltage) transformers are used to supply a reduced value of current or voltage to instrument circuits. They provide isolation from high-voltage systems, permit grounding of secondary circuits for safety and step-down the magnitude of the measured quantity to a value that the instruments can safely handle.
Current transformers (CT) supply a proportional current to the input of metering and relaying equipment. A standard CT secondary current is five amps to match the full-scale current rating of switchboard indicating devices, power metering equipment, and protective relays.
Switchboard indicating devices, power meters and protective relays are routinely tested to verify proper operation of current sensing elements. This testing is performed using high-current, low-voltage test equipment that provides a means of adjusting the value of current and also of measuring the operating time of the device under test.
Test and measurement CT shorting applications present a challenge because an open-circuited secondary can result in the development of a dangerously high secondary voltage. Therefore, a CT cannot have its circuit interrupted while current flows through it. To test the transducer or inject a calibration signal, however, the CT needs the circuit broken. Energized but unused CT circuits must be kept short-circuited.
Highly engineered test disconnect terminal blocks provide an interface to downstream measurement and protection systems. These specialized test disconnects safely short the CT and disconnect all power leading to the transducer or measurement devices.
Disconnect terminal blocks designed with knife-style disconnect levers enable operators to perform a variety of switching tasks on instrument transformers relating to energy generation and distribution. These Deutsches Institut fuer Normung e.V.—the German Institute for Standardization— rail-mounted, modular disconnect terminal blocks can also be employed for billing in areas upstream to the supply meter, as well as in low-, medium- and high-voltage switching systems.
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| The extensive range of accessories for disconnect terminal blocks can be arranged according to utility needs. |
Clear and Easy to Understand
Several key features in a terminal block can help simplify switching tasks.
A knife disconnect lever can be pivoted and locked into the desired switching position using a screwdriver. Switching symbols on the terminal’s disconnect element make it easy to determine the exact switching positions. Additional switching locks on both sides of the disconnect point can prevent accidental misuse.
Triple-function shafts on both sides of the disconnection point allow the operator to customize the use of bridge, testing and switching accessories. Simply plug the standard accessories for test terminal strips into the terminal blocks and lock into place. Both the test sockets for the 4-mm safety measuring lines and the bridge bars for the current transformer short circuit can be positioned in the terminal block assembly as needed.
Because of the angled test socket geometry and the triple-function shafts, 4-mm safety plugs and measuring lines with contact protection in accordance with IEC/EN 60101-031:2002 can be used. CT disconnect terminal blocks are 8.1-mm-wide. The bridge bars for the current transformer short circuit are also pluggable and can be snapped into both sides of the disconnection point. The short circuit switching operation is securely connected to the screw terminal points with the use of a standard screwdriver.
Unlimited Potential
Preassembled bypass bridges can be used for potential distribution. This requires the operator to set up star points in the terminal block assembly so that they are pluggable. Using bridgeable protective earth (PE) terminal blocks also makes it easy to ground one of the secondary current transformer connections in the assembly. Chain bridges eliminate complicated wiring tasks and save time during installation.
Application-oriented accessories can further simplify the task. For example, a partition plate that matches the terminal block profile offers visual separation for the terminal blocks and a park option for plug-in bridges. This allows the operator to store push-in bridges for CT shorting as well as bridges for future system expansions in the terminal block assembly. A transparent cover profile and sealable cover profile carrier secure the assembly against tampering. Modular test adapters can be configured for system testing.
CAE Support for Terminal Block Rail Assemblies
Planning software can make it easy to design and build a switching system. The software can help the designer select the correct parts, such as terminal blocks, interface and power supply modules, surge protection devices and automation solutions. System and circuit diagrams created in computer-aided engineering systems such as Eplan, Elcad, Ruplan, E3 or Promise, as well as data on terminal points, cables and markings, can be imported.
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| The safety measuring lines are touch-proof and securely contacted. |
Software with 3-D product data offers true-to-scale configuration diagrams of the terminal strips. The data can also be used in 3-D mechanical systems for mechanical collision tests. Terminal strips can be generated quickly, easily and without error. Some programs also include macros for a variety of measuring transducer connections.
Some terminal block systems are compatible across all connection technologies. Terminals have the same compact external contours regardless of the connection method—screw, spring-cage or push-in. Using switchable disconnect terminal blocks with matching profile as feed-through and PE terminal blocks increases the design engineer’s options. Standardized bridging, marking and testing accessories add to the system’s flexibility, cutting storage, logistics and installation costs
CT terminals are attractive to the power supplier sector. New functions can significantly reduce the number of wiring and installation tasks inside switching systems. When used with a flexible terminal block system, these products result in a major increase in efficiency, quality and system availability.
Harry Bentler works in product marketing terminal blocks with Phoenix Contact, GmbH & Co. KG in Blomberg, Germany. Alan Sappe’ is the product marketing lead specialist at Phoenix Contact, CLIPLINE in the U.S. He’s based in Harrisburg, Penn.
On the Net: http://phoenixcontact.com/meseries
