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Insulation displacement makes the connection

Insulation displacement makes the connection

Engineers are having to squeeze their designs to achieve higher volumes, productivity, and reliability while lowering costs. And the use of electrical connections is no exception.

One example of improved connection performance and productivity is the resurgence of insulation-displacement connector (IDC) technology. Insulation displacement involves mechanically forcing an unstripped, unprepared wire under moderate pressure into a V-shaped fork or tube. The inner edges of the fork or tube are blades that slice through the insulation as the wire is pushed between them (see figure). As the insulation is sliced or pushed aside, the bare wire is exposed and comes into contact with the terminating blades. The blades also penetrate any surface oxides on the wire.

The quick connections possible with insulation displacement technology cut installation costs, especially when large numbers of contacts must be made. The latest connector block designs use common screwdrivers to make a contact secure.

Three methods. IDC was introduced during the 1950's for the greatly expanding telecommunications industry, as a means to reliably and cost-effectively connect high volumes of wires in the smaller 30-20 AWG sizes. The technology remains a mainstay in telecom. Other industries are now examining the technology for larger size wires and are beginning to incorporate it into their applications.

Other wire termination methods require a great deal of pressure to ensure a gas-tight, vibration-proof connection. While high applied force and contact area of the metal body to wire is a measure of connection quality, high pressure can also drastically deform the wire while breaking through oxides formed on its surface so a highly conductive metal-to-metal contact is made. The oldest and most widely used method is the screw clamp, with a top range of 300 N (67.5 lb) of contact force and about 5-16 mm2 of contact area. The second most popular type of connection is the spring cage having 40-50 N (6-7 lb) of contact force and a contact area of 2 mm2. IDC style connections offer about 80-100 N (14-20 lb) of contact force and roughly the same contact area as the spring cage.

Other cons and pros. Screw clamps, and their high connection quality, take longer to terminate. Spring cages, while easier to terminate than screw clamps, offer lower contact force than either screw or IDC connectors. While IDCs are not as tried and tested in the industrial market segment, they are proven for PVC and PE insulated wires, with testing continuing on silicon-jacketed wires. Teflon wire IDC connections (a very small market) are not yet guaranteed.

IDC technology's greatest advantage is installation speed. An installer doesn't need to strip or crimp the wire, thus shaving 50-60% off installation time, studies have shown. The user simply inserts an unstripped wire into the block and, via a special tool, actuates a lever that forces the wire between the bladestermination complete. Further, one recent IDC connection design does not need any special tools to actuate only an ordinary screwdriver.

In addition to having a strong contact force, the IDC connections are also gas-tight and vibration proof. This is assured by the special design and metal composition of the fork or tube in which the wire is inserted. The metals used retain their high contact force for long periods of time and in various climatic conditions. By maintaining this high contact force, the connections keep gas from creeping into the contact area.

Specially designed strain relief mechanisms incorporated into the metal bodies also ensure the contact is not susceptible to vibration. The mechanisms help stabilize the wire and eliminate premature separation. In fact the insulation itself actually plays a big role in vibration resistance. Because it is only nicked at the point of contact, the remaining intact portion of insulation provides a good gripping platform for the wire when pulled or stressed.

Flat fork and surrounding tube contacts use installation force to cut through wire insulation and effect a positive, gas-tight electrical connection.

IDC reliability testing. What's involved in ensuring such features will make IDCs reliable? Spring characteristics of the contact material are one set of factors, and it is important that these properties remain stable over time. To validate the design, the connectors are subjected to very high temperature changes, which places considerable stress on the contact materials, simulating the passage of time. For example, the IDC connector blocks go through 100 cycles of being cooled down to -55C (-67F) and then quick heating to 100C (+212F). Prior to this test, the largest acceptable cross section wire is connected, or terminated, to the connector samples and the contact resistance recorded. At the conclusion of the 100 cycles, the same measurements are taken using the smallest possible cross section wire the contact resistance has to remain unchanged from the first reading.

Other reliability tests include vibration and corrosion. To monitor the vibration ratings of the blocks, they are subjected to several hours of high vibration -- a frequency sweep from 10 to 500 Hz and a maximum acceleration of 50 m/s2 (5g). Before, during, and after the test, the contact resistance is monitored and it must remain constant.

In the corrosion test segment, connections are exposed to a corrosive nitrogen dioxide-saturated damp atmosphere. Again, contact resistance is monitored before, during, and after, to ensure it remains constant. At the conclusion of the test, not only must the contact resistance be unchanged, but also the blocks must show absolutely no corrosion in the contact zone.

Bottom line. One final area to consider is the dollar saving that can result from using IDC technology. An applied cost analysis for the different types of connections shows an interesting trend. Although IDC blocks cost a bit more, a savings in the installation costs leads to an overall reduction (see table).


Installed connection costs compared

Connection Type

Screw

Spring

IDC

Block Cost

$1.23

$1.19

$1.27

Installation Time Per Block

41 sec

32 sec

15 sec

Seconds Per Hour

3600

3600

3600

Labor Rate Per Hour

$35

$35

$35

Labor Cost Per Termination

$.39

$.32

$.14

Total Cost Per Termination

$1.62

$1.51
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