A systems approach to functional terminal block design

Oct. 20, 2011
Consider where signals are processed in measurement, conversion and regulation systems.

Ideally, functional terminal blocks should be easy to use, provide secure connections and make it simple to bridge terminals, whether or not they are adjacent. By taking a systems approach to functional terminal block design, though, a single block can be adapted to different connection requirements.

Functional terminal blocks, such as those with knife disconnects or component terminals, are widely used for measurement, conversion and regulation (MCR) systems, so they are found in almost all industries where sensor signals are processed. In particular, this includes process engineering, where distribution or marshalling cabinets hold row upon row of knife disconnect terminals that are used to bundle and monitor signals coming from equipment in the field. The water and wastewater industries use test disconnect terminals for monitoring pumps and valves. In the mechanical and plant engineering fields, functional terminal blocks are harnessed to cut off signal circuits in order to simulate system states by feeding defined test signals from outside sources. Railway operators, too, use knife disconnect terminals to calibrate the signals throughout the railway network.

Most functional terminal block designs found on the market, however, leave a lot to be desired. With some, for example, bridge shafts are omitted in favor of the disconnect zone. This makes it impossible to distribute the signals using skip bridging. As a result, time and effort must be expended to wire the desired connections between non-adjacent terminals.

Another issue arises with certain functional terminal block designs where the bridge shafts are placed underneath the disconnect zone. In this case, the terminal housing must be broken open below the terminal so that a bridging line can be inserted. This limits the terminal block’s flexibility since it makes the removal of individual clamps from the bridging assembly impossible after the bridge has been installed. In addition, when the housing walls are poorly separated, it is uncertain whether there is adequate spacing for airflow and creepage. Ground faults caused by hairline cracks are a common problem in control cabinets.

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Test-disconnect functions are also a problem for poorly designed functional terminal blocks. The terminal disconnect zones can also restrict the current flow. In this case, the disconnect knives cannot carry the full terminal current that a feed-through terminal of the same cross-section can, and the terminal is unable to support its full capacity.

Finally, it is important to realize that most functional terminal block systems are not universal. In particular, they are often missing practical combinations of feed-through and disconnect terminals in double-level versions or blocks with ground (PE) or shield connections. As a result, these blocks take valuable cabinet space that could be used for future expansion. Non-universal blocks also mean that different sizes of accessories such as bridges, fuses and disconnect plugs must be purchased and kept on hand in case they are needed. Added complexity increases the odds of error.

Carsten Luckheide is terminal blocks product marketing representative at Phoenix Contact (www.phoenixcontact.com).

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