RAST and Wire Harnesses
May 21, 2007
Back in the 1980s, European appliance manufacturers found themselves facing a host of problems stemming from the growing complexity of their products. In contrast to North Americans, European customers have long been willing to pay a premium for appliances that use less water and electricity. Unfortunately, the sensors, electronic controllers and electrical actuators involved in meeting these demands, resulted in growing quality issues in the form of crossed and mismatched wires.
To solve this problem, appliance OEMs, wire harness makers and wire connector manufacturers, decided to create a new block-terminal standard, along the lines of what automotive manufacturers had been using for some time. The result was a connector standard called Raster Anschluss Teck Technik, or RAST, which roughly translated means “pitch connection plug technology.”
As part of this process, the appliance industry also hit on the idea of implementing insulation displacement connector (IDC) technology in its harnesses as a means of both improving quality and enabling more efficient assembly.
In contrast to a crimped connector, an IDC connector incorporates preloaded internal contacts that cut through a wire’s insulation without having to strip the wire first. An IDC connector’s contacts also engage a wire using far less force than is required when crimping. As a result, assemblers don’t have to worry about damaging or cutting threads during the terminating process. Nor do assemblers have to be as concerned about problems with the connectors working loose as the stressed wire and crimp materials relax over time-a particular problem when working with smaller-gauge wires.
Because IDC terminals simply snap shut over a wire or are terminated in a single step using hard tooling, they naturally lend themselves to high-speed, fully automated assembly and testing. In fact, depending on the model of connector, more than 30 wires can be terminated and then tested simultaneously. Presented with these kinds of efficiencies, Western European appliance titans like Electrolux, Bosch Siemens and Whirlpool suddenly found themselves with a very good reason to not outsource their wire harness work to countries in Eastern Europe, Asia and Latin America.
In codifying their RAST standards, European manufacturers created two basic categories. The first, RAST 2.5, refers to multiple-wire connectors with a 2.5-millimeter pitch, or spacing. This category is used primarily for low-power control circuits. The second, RAST 5, defines the parameters for connectors with a 5-millimeter pitch. In practice, this type is used primarily for higher-amp power circuits.
At its most basic, a RAST connection is comprised of a header receptacle mounted on, say, a motor housing or a controller PCB, and a block connector terminating anywhere from one to a couple of dozen wires. Although the wire termination portion of each connector model varies depending on the manufacturer, the “plug” side, or connector face for all RAST connectors is standardized. Therefore, a header from, say, Tyco Electronics (Harrisburg, PA) can be used with a connector from Molex (Lisle, IL), and vice versa.
In addition, the connector faces, which incorporate latching features to ensure a solid assembly, are all manufactured with RAST-standardized keying and color features. This means a connector that is supposed to be plugged into a controller board won’t be mistakenly attached to a washing machine turbidity sensor. Similarly, the same RAST connector that a manufacturer uses to plug a wire harness into an electric motor won’t be used to create a connection to a pressure switch.
It’s this kind of inherent error-proofing that has made believers out of European appliance makers, among which the use of RAST is now almost universal.
This is not to say that IDC is a piece of cake. In fact, while simple in concept, creating a reliable IDC connector requires some very precise engineering. Manufacturing tolerances must be extremely tight if each connector’s contacts are to effectively pierce each wire’s insulation and create a good circuit.
Then there is the cost of the equipment required for implementing fully automated, in-line test and assembly. Turnkey systems from wire-processing companies like Komax (Buffalo Grove, IL), Schleuniger Inc. (Manchester, NH) or Tyco Electronics represent a substantial investment. In most cases, an effective system is the result of experience and close cooperation with terminal manufacturers.
Still, given the flexibility and potential cost savings available through the implementation of IDC and RAST, it is worth the effort in the right settings-both in the appliance industry, and beyond. It is interesting that the RAST standards can also accommodate conventional crimping. However, over the years, IDC has steadily gained momentum, and with European manufacturers targeting China and India as prime targets for market growth, RAST standards are fast becoming the norm worldwide.