Stranded wire is the most popular choice for most electrical applications. Due to its superior flexibility and long flex life, it offers more routing options than solid wire. Stranded wire is also more durable. It stands up to vibration and sudden movement that can cause rigid solid wire to snap.
Stranded wire gains its flexibility from the way the bundles of wire in the strands are woven together. The integrity of the stranded bundles is further reinforced by the jacket of insulating material that surrounds them. However, the moment the insulation is stripped from the wire end, its strength is transformed into a source of weakness, as wire bundles unravel and individual strands are exposed. In the process, the potential for short circuits and failed connections rises dramatically.
Identifying shorts is time consuming. In the case of a typical panel or wire harness, this could mean testing hundreds of connections.
Issues With Bare Connections
With stranded wire, the old adage applies: The whole is indeed greater than the sum of its parts. The reverse, unfortunately, is also true. Once the insulation is removed and stranded wire begins to unravel, ease of installation is sacrificed, along with long-term electrical performance, durability and safety.
The problems with bare stranded wire connections begin the moment the stripping tool is put down and an attempt is made to place the bare end of the stranded wire into the terminal compartment. If care is not taken, the wire will begin to splay, making installation painstaking and time-consuming.
If the unraveling wire is successfully inserted into the terminal block, it is unlikely that the full electrical efficiency of the connection will be realized. If the strands have fanned out, not all of them will make contact with the connector and be available to conduct current. Furthermore, the individual strands, once isolated, can break easily. This is particularly an issue when stranded wires are used with spring clamp connectors.
Stripping the insulation and exposing the stranded wire to air also undermines the long-term electrical performance of the connection. Weidmuller technicians conducted a series of tests using our SAK 4 screw terminal and a 12 AWG ferrule in a climate-controlled cabinet. Over time, corrosion of the unprotected stranded wire produced an increase in contact resistance.
As the contact resistance rises, the temperature inside the connection increases, leading to higher current flow. Above 0.43 milliohm, this can lead to losses of insulation, short circuits and burning within the panel.
In a salty environment, contact resistance of stranded wire approaches its limit even more quickly.
Uninsulated stranded wire extending out of the terminal block can also increase creepage, or leakage, distance. Creepage is the shortest path between two conductive parts (or between a conductive part and the bounding surface of the equipment), measured along the surface of the insulation. An adequate creepage distance protects against tracking, a process that produces a partially conducting path of localized deterioration on the surface of an insulating material as a result of the electric discharges on or close to an insulation surface. Excessive creepage distance can heat up the connection and cause short circuits.
Under normal conditions, stranded wire provides superior vibration resistance compared to solid wire, based on the way the strands are woven together. When stranded wire is stripped, the exposed bundles of strands lose their coherence and begin to separate. When subject to vibration, these individual strands are susceptible to loosening the connection or breaking. In addition, strands can easily break when bent or stressed. In either case, the result is decreased current flow and, eventually, failed connections.
Taken together, using bare stranded wires at the point of connection not only degrades system performance, but also poses significant safety risks. Higher temperatures, short-circuits, and, in some cases, arc flashes can result.
Ferrules Make Solid Connections
During the 1960s, engineers realized that one way to overcome the deficiencies of bare stranded wire at the point of connection was to give the exposed section the virtues of solid wire. They did this by encasing it in a tin-plated soft-electrolyte copper (E-CU-57) ferrule, which is crimped in place.
Over the years, ferrule technology has been improved. In the 1970s, ferrules were introduced with a plastic conical cable entry, providing added safety and durability. In the 1990s, twin ferrules, allowing for the simultaneous insertion of two wire ends, were invented, and DIN 46228 was adopted to standardize the sizes, dimensions and testing of ferrules.
During installation, the conductor insulation must be pushed into the plastic collar, and the conductor should completely fill the ferrule sleeve. Depending on cross-section, the conductor should protrude as much as 0.5 millimeter from the ferrule sleeve.
Efficient Use of Resources
Ferrules are required in Europe for CE certification—and it is easy to understand why. They enable users to take advantage of the features of stranded wire without the problems caused by a bare-wire connection. The few seconds required to apply a ferrule to the wire-end are more than made up by the ease of insertion. Because the ferrule completely encases the stranded wire, the quality of the connection is far superior, and there is no possibility that the wire can break, even when used with tension clamps. No matter how many times the wire is removed and reinserted, the ferrule retains its shape and integrity.
The long-term electrical performance is also higher because ferrules—applied with the right crimping tool—form a gas-tight connection, shielding the wire from corrosion even in a salty environment. Analysis shows that stranded wire with ferrules demonstrate resistance over time that is similar to solid wire.
In addition, ferrules with plastic collars reduce creepage values as well as gas intrusion. This is particularly important in components like PLCs, where connection density is important. Stranded wire-ends protected by ferrules are also much more resistant to vibration and breakage at the connection.
In effect, ferrules produce a much more efficient and durable connection than can be attained with bare stranded wire, which results in significant savings over the life of the system. The standard size ferrule for a 16 AWG wire typically costs $0.05. If there are 1,000 connections in a panel, the total cost is $50. That is a fraction of what downtime, testing, repair and replacement costs can run in the event of a short circuit.
Most important, ferrules also provide a critical margin of safety for operators, reducing the potential for shorts and arc flashes.
Sourcing Ferrules Wisely
Although ferrules might seem like a commodity product, they are not. When you source ferrules, it is critical to select a supplier whose ferrules go beyond the immediate advantage of simplified installation to provide an efficient, durable and safe connection over the long term.
Accordingly, when specifying or purchasing ferrules, keep the following guidelines in mind:
Choose a manufacturer that adheres to the DIN 46228 standard. This standard describes the allowable dimensions and tolerances of the plastic collar and metal sleeve, ensuring the quality of the connection. The crimped connection is subject to a pull-out force test calibrated to the cross section of the wire. In addition, it provides a uniform color-coding system for ferrules with plastic collars, with a specific color for every cable cross section. Thanks to this system, engineers and technicians can immediately recognize different cables, minimizing confusion and increasing operational safety.
Select a manufacturer whose ferrules have UL approval. This is particularly important when using ferrules in UL-certified panels. All UL 508 panels must use UL-listed components.
Choose a manufacturer that offers crimping tools and contacts that are designed to work together, a recommendation contained in the UL 508 and DIN EN 60352-2 standards. Equally important, select a manufacturer that has subjected its crimping tools, ferrules and connection systems to rigorous testing. These tests include gas intrusion, vibration, bending and long-term connectivity testing, as well as pull-out tests for screw clamps, tension clamps and push-in connectors.
Make your purchases from a manufacturer that offers a broad range of crimping shapes (including trapezoidal, trapezoidal indent, square and hexagonal), ferrules (with and without plastic collars, as well as twin ferrules), terminal blocks, and other devices so you can return to the same supplier for all your connection needs and maintain a consistent quality standard.