It Always Takes Connections

IDC Might Be Quicker to Assemble, but if You Have to Do Fault-Finding and Maintenance, Then It Is Very Difficult to Use

By Joe Feeley, editor in chief

There are several surefire ways to start a technology argument about an industrial automation and controls subject.

One way is to make a strong statement about Linux’s value and viability as an industrial-strength, real-time operating system. It doesn’t much matter what side you take. Just throw it out there and a passionate, food-for-thought fight will break out.

Another is to argue the superiority of one terminal block technology versus another. So, we’ll briefly review the technologies, add a few innocent statements of use, and ask you to add your 2₡ via e-mail, phone, a rock wrapped in a letter through our front window, or whatever your choice of delivery might be.  As you can see by our web poll on the subject, it’s a connection house divided.

Screw clamps use a screw to secure the wire in the terminal block. Wires must first be stripped and then inserted into the wire entry hole running perpendicular to the screw. Tightening the screw-clamp assembly pulls the wire securely into the current bar. With enough torque, the wire becomes embedded in the tin-plated copper current bar. To further promote good contact, a grooved surface on the current bar helps break any oxide film present on the wire.

Spring-clamp or tension-type connections offer seemingly faster installation than screw-clamp terminal blocks do. Spring-type terminal blocks use a metal spring to hold the wire tightly against the current bar. The wire is stripped, the spring is held open, the stripped wire is inserted, and the spring is released. The wire entry hole in these blocks is on top, which further simplifies installation and reduces space requirements. Spring-type terminal blocks have an upper limit, specified by the vendor, as to the size of wire they can hold securely, and multiple size wires generally can’t be held by one clamp.

The insulation displacement connector (IDC), often called the quick connector when it first appeared in the U.S., has seen steady adoption, although some potential users still have reliability concerns. This connection allows insertion of unstripped wire into the entry hole where the contacts in the block pierce the insulation through to the metal to make a connection.

“I’ve worked in the industrial machine environment for more than 20 years and have used everything from screw terminals to milspec connectors to insulation displacement,” Ray Opie, engineering director at Solidica in Ann Arbor, Mich., told us the last time we raised the issue.  “IDC might be quicker to assemble, but if you have to do fault-finding and maintenance, then it is very difficult to use. The best compromise I’ve found is the IEC-style, spring-loaded terminal. The terminals are faster to assemble than the screw-type, don’t vibrate loose, but are much more field-serviceable.”

Around the same time, we heard this from Steve Clayton of Motor Control Engineering in Mandan, N.D. “We built a test panel to mount on a piece of road equipment. It did not do anything. We just had a few 120 Vac and 24 Vdc circuits with lamps on them. We used cage-clamp and IDC terminals. At the end of one year, 19% of the IDC terminal connections had failed and 0% of the cage clamps had failed. By failed, I mean the circuit was open completely or intermittently."

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