Terminal Blocks

Nov. 15, 2002
Planning for packaging and interconnections in the early phases of a design project eliminates the chances of poor design degrading long-term reliability.

Planning for packaging and interconnections in the early phases of a design project eliminates the chances of poor design degrading long-term reliability.

The integrity of system design depends on the reliability of conductors (wires, cables, and fiber optics), terminals, connectors, sockets, printed-circuit boards, backplanes, and the overall enclosure.

Terminal blocks require no wire preparation except stripping and are easily installed with a screwdriver. Terminal blocks accept a wide range of awg wire sizes, provide ready hookup of wires from different components, and ensure fast disconnection/reconnection during maintenance and troubleshooting.

Terminal bodies often are made from a copper alloy that has the same expansion coefficient as wire. This prevents loosening between metals with different thermal expansion rates and lower contact resistance. Corrosion, normally caused by electrolytic action between two different metals, is also eliminated.

There are two basic styles of terminal blocks, European and North American. European terminal blocks are physically smaller than North American blocks having the same voltage and current ratings. And unlike hardware made in North America, all mounting hardware for European terminal blocks follows standard DIN format. Blocks from one vendor can be installed on rails from another.

Terminal blocks in the North American style generally are vendor specific. Blocks and rail hardware from one manufacturer usually do not work with equipment from others.

Another difference between North American and European blocks is in connection techniques. European wire terminations follow what is called a dead-front configuration. Termination hardware is recessed within the plastic block housing, isolating electrically live parts. Maintenance personnel can touch live blocks with minimal chance of shock hazard or electrocution. North American blocks employ a more open design.

The simplest terminal blocks, called electrical blocks, merely join wires or cables. These devices accept wire sizes ranging from about 12 awg to 500 mcm with a maximum voltage rating of 600 V. The blocks either snap into a metal rail or are screw mounted on the panel of the control enclosure.

There are three styles of DIN rails: C (also called G or asymmetric), flat (also called symmetric), and mini rail. Flat and C rails are used in systems rated to 600 V. Terminal blocks on mini rails handle voltages to 300 V.

Special terminal blocks combine simple terminations with other functions found in the control panel such as fusing and disconnects. Fusible terminal blocks are widely used and generally contain ferrule fuses rated at 3 to 5 A, although some devices contain up to 30-A fuses. Special terminal blocks may also incorporate circuits that perform visual annunciation, surge suppression, and voltage regulation.

Another type of terminal block is often called an electronic block. These devices typically work with smaller wire sizes common to electronic applications, 30 to 14 awg at 300 V or less. The blocks generally mount and solder onto printed-circuit boards. They transmit power or control signals to circuit components through solder pins that are mechanically connected to the clamping body.

Several wire entry angles and numerous pole and pin configurations are available. In addition, these devices can be obtained in two-tier versions. Two-tier blocks contain two sets of connection terminals stacked on top of each other, saving board space. The wire entries on the two-tier blocks are offset or staggered so that screwdriver access is facilitated to the lower tier, even if the upper tier is fully wired.

Special terminal blocks have been developed to provide a transition between discrete wiring of power devices to low-signal cables or controller connectors. The most common transition is to ribbon cables using IDC connections, or to D-subminiature connectors, or DIN harness assemblies.

Terminal block manufacturers now offer special housings that can hold small printed-circuit boards. These boards, in turn, can be connected to the block terminations. The configuration allows end users to define special functions on the terminal staff.

Creepage and clearance are terminal block distances that determine voltage ratings. Creepage is the path between terminals measured along the surface of the insulation. Clearance is the shortest through-air distance between terminals and from terminals to ground.

Accessories for terminal blocks include jumper straps for connecting adjoining contacts; channel clamps for holding sections in mounting channels; transparent plastic covers for protecting conductive parts while permitting inspection of the contacts and marking areas; and sectional fanning strips for connecting and disconnecting groups of wires.

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