Ferrules ( English wire ferrules ) are used to the stripped ends of stranded wire to be protected, so that they in no damage to the individual wires of terminals can be connected. When using certain types of terminal, such as elevator terminals, no ferrule is required.
In the early days of electrical engineering, the screw terminal was used almost exclusively as a terminal . The attachment of rigid conductors is easy. On the other hand, in the case of stranded conductors, the individual wires yield under the pressure of the terminal, detach from the bond and the connection loosens. The screw remains rigid in its position and does not adjust to the decreasing clamping pressure. The bad contact promotes sparking and consequently the risk of fire or loss of connection. The live conductor that may slip out can now touch other parts and cause damage and danger. To prevent this, enveloping ferrules were invented that hold the stranded wires together and thus behave as easily as a rigid conductor.
In connection technology today, CAD enables the transition from screw terminals to increasingly optimized spring-loaded terminals . In versions with a manually operated opening mechanism, wire end sleeves are not only unnecessary, they are even counterproductive. Here, too, the individual wires of the strand give way under the contact pressure. In contrast to the screw terminal, the spring here ensures that the terminal is narrowed and therefore the clamping pressure does not decrease. This not only maintains the contact closure, but the individual stranded wires are optimally distributed in the terminal, so that there is a large contact surface and a smaller contact resistance. This is not the case with the rigid ferrule.
While the ferrule in the spring-loaded terminal still works suboptimally, its use is completely impossible with the insulation displacement terminal, which is also increasingly used . Overall, the use of ferrules is decreasing.
The wire end ferrules are to be selected exactly for the conductor cross-section and processed with the intended insert in the crimping tool . The requirements for crimp connections are specified in EN 60352-2: 2014-04. In addition to avoiding typical errors such as those that can arise during crimping, the tensile strength achieved by the connection is an essential quality feature. The individual work steps, beginning with the stripping process to the correct length without damaging the strands and the remaining insulation, sliding on a suitable ferrule and finishing with a suitable crimping tool, require careful craftsmanship.
Errors that can occur during crimping
- Crack formation on the long edges and stamp imprints
- The end sleeve bursts open
- asymmetrical crimp shape
- strong burr formation on the long edges
- Sleeve not filled in by the conductor
- Individual strands pushed back protrude from the collar
- Individual strands pinched off
- Wire too short or pushed too far into the contact area
- Plastic collar damaged by crimp punch
- Conductor insulation not pushed into the plastic collar
- Wire end ferrule bent lengthways after crimping
Execution and variants
Wire end sleeves with protective collars (the colored ones in the picture) offer additional kink protection for the connected wire. The conical collar makes it easier, on the one hand, to insert the strand bundle into the sleeve, and, on the other hand, to insert the sheathed conductor into the terminal. The edges of terminal insulation and conductor insulation are prevented from getting caught. To connect two wires in one terminal, there are so-called duo wire end sleeves (in the picture the gray copy, which, however, also requires a special tool insert).
The color of the protective collar provides information about the maximum wire or conductor cross-section . There are three different, partly contradicting color codes. Color coding according to DIN standard 46228 is becoming increasingly popular in specialist retailers. In case of doubt, unused specimens can be identified by their inner diameter. Different lengths are offered for each cross-section.
|cross-section||French color code||German color code||DIN standard 46228||designation||Inside diameter|
|0.25 mm²||violet||Light Blue||yellow||0.8 mm|
|0.34 mm²||pink||turquoise||turquoise||0.9 mm|
|0.50 mm²||White||orange||White||E0506, E0508, E0510||1 mm|
|0.75 mm²||blue||White||Gray||E7506, E7508, E7510||1.2 mm|
|1.00 mm²||red||yellow||red||E1006, E1008, E1010||1.4 mm|
|1.50 mm²||black||red||black||E1508, E1510, E1512||1.7 mm|
|2.50 mm²||Gray||blue||blue||E2508, E2510, E2512, E2518||2.2 mm|
|4.00 mm²||orange||Gray||Gray||E4009, E4012, E4018||2.8 mm|
|6.00 mm²||green||black||yellow||E6010, E6012, E6018||3.5 mm|
|10.00 mm²||brown||ivory||red||E10-12, E10-18||4.5 mm|
|16.00 mm²||ivory||green||blue||E16-12, E16-18||5.8 mm|
|25.00 mm²||black||brown||yellow||E25-16, E25-22||7.3 mm|
|35.00 mm²||red||beige||red||E35-16, E35-25|
|50.00 mm²||blue||olive||blue||E50-20, E50-25|
In the designation, the first two digits encode the wire cross-section; the last two digits indicate the length of the metal sleeve in millimeters.
Cables that consist of single strands must be provided with wire end sleeves if the connection terminal is not approved for the connection of non-assembled stranded leads. When installing the stranded wire in an unsuitable terminal, individual wires often get caught on the edge of the terminal, are pushed back and, in the worst case, protrude over the insulated edge of the terminal. There is a risk of a short circuit and / or an electrical accident . In addition, there is an increased risk of corrosion if cables are not provided with ferrules . A ferrule is not required in elevator terminals, as the construction of such terminals effectively prevents individual wires from slipping out as well as shearing.
The soldering of stranded ends, which was often used in the past instead of wire end sleeves, is no longer state of the art in electrical installations. The reason for this is the solder's low compressive strength. In screw terminals, a soldered strand end deforms so much under the pressure of the screw over time (so-called "flow" ) that after some time there is no longer sufficient contact between the strand and the terminal. In extreme cases, an unwanted spark gap is created. The formation of oxide layers then causes the contact resistance to increase sharply, which leads to a high risk of fire. This development is also promoted by the high corrosion tendency of soldered connections. Soldering electrical connections is not expressly prohibited in the relevant standards, but VDE 0100-520: 2003-06 Section 526.2 states that soldered connections in power circuits should be avoided.
Machines are also available that automatically strip, attach and press sleeves. The sleeves are then usually linked in a manner suitable for machines. Few machines also work with loose tubes that are sorted and fed via vibratory conveyors .
In the industrial sector, crimpable end sleeves are also used instead of crimpable end sleeves. These are significantly more cost-effective, since strip metal can be used as a semi-finished product , which is pre-formed accordingly.
- Qualityfor industrial crimping. (PDF; 1.2 MB) Molex, December 23, 2009, accessed on November 27, 2012 (terms, procedures, error patterns, testing).
- Compare VDE 0100-520: 2003-06: Erection of low voltage systems - Part 5: Selection and erection of electrical equipment; Chapter 52: Cable and wire systems
- Note in VDE 0100-520: 2003-06 Section 526.2 - (analogously): “Soldered connections should be avoided in power circuits. If these are used, the connections must be made in such a way that the flow of the solder, mechanical loads and temperature increase in the event of a fault are taken into account. "